The Sveconorwegian orogen of southern Scandinavia: setting, petrology and geochronology of...

33 IGC excursion No 51, August 2 – 5, 2008

The Sveconorwegian orogen of southernScandinavia: setting, petrology and geochronology

of polymetamorphic high-grade terranesOrganizers:Jenny Andersson, Geological Survey of Sweden, UppsalaBernard Bingen, Geological Survey of Norway, TrondheimDavid Cornell, Göteborg University, SwedenLeif Johansson and Ulf Söderlund, Lund University, SwedenCharlotte Möller, Geological Survey of Sweden, Lund

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TABLE OF CONTENTS

Abstract ...................................................................................................................................... 5

Logistics ..................................................................................................................................... 6Dates and location ............................................................................................................................. 6

Travel arrangements......................................................................................................................... 6

Accommodation................................................................................................................................. 6

Field logistics ..................................................................................................................................... 6

General Introduction................................................................................................................. 7

Regional Geology ...................................................................................................................... 8The Eastern Segment ......................................................................................................................... 13The Idefjorden Terrane ...................................................................................................................... 15

Excursion Route and Road Log.............................................................................................. 17

Excursion Stops....................................................................................................................... 20Day 1 Transect across the Sveconorwegian orogenic front......................................................... 20

Introduction ................................................................................................................................................. 20Stop No 1.1: Lu-Hf geochronology of mafic cumulates, example from the old quarry of Taberg......... 20

Location ............................................................................................................................................. 20Introduction........................................................................................................................................ 21Description......................................................................................................................................... 21

Stop No 1.2: The Protogine Zone and the Transscandinavian Igneous Belt of the pre-SveconorwegianFennnoscandian craton ........................................................................................................................... 24


Stop No 1.3: Zircon formation during metamorphism and deformation of mafic rocks, example frompetrology and U-Pb chronology applied to a metabasic intrusion in the Protogine Zone....................... 25


Optional stop: Mafic magmatism and mafic dyke swarms along the eastern boundary of theSveconorwegian orogen.......................................................................................................................... 29


Stop No 1.4: Direct dating of Sveconorwegian folding in the southern Eastern Segment...................... 31Location ............................................................................................................................................. 31Introduction........................................................................................................................................ 31Description......................................................................................................................................... 33

Day 2 Eclogites, high-P granulites and charnockites..................................................................... 37Introduction ................................................................................................................................................. 37

Stop No 2.1: Charnockitisation and polyphase metamorphism in the Eastern Segment of the southwestSwedish Gneiss Region. Incipient charnockitization in discrete dehydration zones .............................. 38

Location ............................................................................................................................................. 38Introduction........................................................................................................................................ 38Description......................................................................................................................................... 40Söndrum zirconology......................................................................................................................... 40

Stop No 2.2: Högabjär: Ion probe zircon dating of polymetamorphic “Hallandia” gneiss..................... 42Location ............................................................................................................................................. 42Introduction........................................................................................................................................ 42Description......................................................................................................................................... 42

Stop No 2.3: Lilla Ammås: Decompressed Sveconorwegian eclogites .................................................. 46Location ............................................................................................................................................. 46

Introduction........................................................................................................................................ 46Description......................................................................................................................................... 46

Stop No 2.4: Buskabygd: High-grade tectonites in the Ullared Deformation Zone ............................... 48Location ............................................................................................................................................. 48Introduction........................................................................................................................................ 48Description......................................................................................................................................... 48

Stop No 2.5: On the occurrence of 1.4 Ga old charnockites in the Southwest Swedish Granulite Region;igneous or metamorphic charnockitisation - or both?............................................................................. 49


Day 3 Terrane boundaries and tectonic build up of the Sveconorwegian Orogen.................... 51Introduction ................................................................................................................................................. 51

Stop No 3.1: The Mylonite Zone: a major Sveconorwegian structural, metamorphic and lithologicalterrane boundary in the Fennoscandian Shield ....................................................................................... 53


Stop No 3.2: Age and emplacement conditions of the Chalmers Metagabbro ....................................... 58Location ............................................................................................................................................. 58Introduction........................................................................................................................................ 58Description......................................................................................................................................... 60

Stop No 3.3: Migmatisation in Stora Le Marsstrand graywackes driven by mafic intrusions. Compositedyke development and the origin of calc-alkaline magma series by back-veining and assimilation.Archaean and Early Proterozoic zircon xenocrysts in Mesoproterozoic crust........................................ 60


Day 4 Terrane boundaries and tectonic build up of the Sveconorwegian Orogen (continued)65Stop No 4.1: The fate of zircon in crustal processes: ion probe U-Pb-Th (SIMS) and ICP-MS REE andU-Th analyses guided by Cathodoluminescence imaging. ..................................................................... 65


Stop No 4.2: U-Pb, Sm-Nd, Lu-Hf geochronology of Mesoproterozoic mafic intrusions in theSveconorwegian Province....................................................................................................................... 68


Stops 4.3 and 4.4: The Idefjorden terrane west of the Oslo Rift ............................................................. 74Introduction........................................................................................................................................ 74

Stop No 4.3: Preserved Bouma sequences in amphibolite-faces metagreywacke, with garnet-amphibolite dykes................................................................................................................................... 75


Stop No 4: Pervasive amphibolite-facies garnet blastesis in HP amphibolite-facies conditions ............ 77Location ............................................................................................................................................. 77Introduction........................................................................................................................................ 77Description......................................................................................................................................... 78

References................................................................................................................................ 80

AbstractThe Sveconorwegian orogen in southern Scandinavia is the result of a collision betweenFennoscandia (the southwestern continental segment of Baltica) and another continent in lateMesoproterozoic time. The orogenic province is composed of five distinct Proterozoic gneisssegments that were displaced and reworked during a succession of compressional (andextensional) orogenic phases at between 1.14-0.96 Ga. Sveconorwegian orogenesisculminated in a continent-continent collision phase at 0.98-0.96 Ga that involved regionalscale high-pressure granulite metamorphism and local emplacement of eclogites in thesoutheasternmost part of the orogen. The principal lithotectonic units of the orogen areinternally separated by crustal scale deformation zones along which final crustalreconfigurations and tectonic adjustments took place at about 0.92-0.90 Ga. Today, thesouthwestern Fennoscandian shield areas offers exposures of an exquisite traverse through apartly deep-seated Precambrian continent-continent collision zone(s). The 33IGC pre-meeting excursion No 51, to the eastern part of the Sveconorwegian orogen, will involve atraverse from well preserved rocks of the pre-Sveconorwegian Fennoscandian craton acrossthe Sveconorwegian deformation front into the high-grade gneiss complex of the partlyparautochthonous Eastern Segment and further west into the Sveconorwegian allochthon. Theexcursion participants will be taken to well exposed high-grade metamorphic domains,including the Southwest Swedish granulite region that exhibits a polymetamorphic high-gradegneiss complex with charnockites, high-pressure granulites and tectonically emplacedeclogites. A special focus is set on the timing and character of metamorphism anddeformation associated with the orogenic evolution of the eastern part of the orogen.Different aspects on the age and character of protolith rocks in different parts of the orogenwill also be highlighted.

The excursion to the Sveconorwegian orogen aims to highlight the combination of fieldgeology, metamorphic petrology and different applications of geochronological-geochemicalanalytical techniques to constrain the timing and character of metamorphic andtectonothermal events in high-grade metamorphic complexes. The individual excursion stopshave been selected to include key localities that have been used to construct, characterise anddirectly date the P-T evolution and tectonic build up of this part of the orogen. A primary goalwith the excursion is to bring together structural geologists, metamorphic petrologists,isotope geochemists, geochronologists, and other geoscientists to combine their expertise anddiscuss how to model tectonic cycles and thereby, how to understand the crustal evolution ofour continents.

The excursion is prepared as a four days field trip arranged to cover three principal themesregarding the tectonic build up of this part of the Sveconorwegian orogen. (I) “Transectacross the Sveconorwegian orogenic front”, deals with the tectonic build up of the orogenicfront and the geochronology of structures and metamorphism related to the tectonic evolutionof the easternmost high-grade parts of the orogen. (II) “Eclogites, high-P granulites andcharnockites”, focus on the timing and tectonic setting of high-P and high-P-T metamorphicevents in the high-grade gneiss complex of the Eastern Segment. (III) “Tectonic boundariesand lithotectonic build up of the Sveconorwegian orogen”, concerns the age and tectonicstyle of metamorphic terrane boundaries and the crustal evolution of allochthonouslithotectonic units overlying the high-P rocks of the Eastern Segment.

Logistics

Dates and locationTiming: From morning on the 2nd – to the evening on the 5th of AugustStart location: Participants will be picked up at the Landvetter Airport,

Gothenburg from which pre-paid fee starts to applyEnd location: Participants are dismissed at the Gardemoen Airport outside Oslo

for their own return travel arrangements

Travel arrangementsThe excursion is organised as a 4 days pre-meeting field tour in the Sveconorwegian orogenof Scandinavia. The excursion starts in the morning of Saturday 2nd of August at LandvetterAirport, Gothenburg/Göteborg [one hour flight from any of the Scandinavian capitals (Oslo,Copenhagen or Stockholm). The town of Gothenburg can also easily be reached by train fromCopenhagen or Stockholm, a journey of 2.5 to 3.5 hours. The participants will be picked upby the excursion leaders at the Landvetter Airport (Gothenburg Airport). This is a smallairport, and participants will be picked up just outside the arrival gates. Transportation fromthere on will be done by mini buses. The Landvetter airport can also be reached by bothdomestic and international flights. Please refer to the web for further information athttp://www.lfv.se/templates/LFV_AirportStartPage____2570.aspx Airport bus from central Gothenburg takes about 30 minutes. During peak hours, air portbuses to central Gothenburg depart every 15 minutes. More information available athttp://www.flygbussarna.se The excursion ends in the evening of Tuesday 5th of August in Oslo.

AccommodationOvernight accommodation will be in reasonably priced guesthouses or hostels that willprovide basic hotel standard (no need to bring sheets or towels). Two nights (2nd through 4th

of August) will be spent at the Bråtadal hostel in Svartå(http://www.kulturgarden.com/index_eng.htm). It is an environmental friendly hostel, simplebut with an excellent organic food kitchen and located in a picturesque district in the centralpart of the excursion area. The third night will be spent at the charming Skäret guesthouse onthe traffic-free island of Styrsö in the Gothenburg archipelago(http://www.pensionatskaret.se/english/). Both guesthouses are rather small and we will bethe only guests during our stay there. The guesthouses also have room and equipment forevening seminars. All meals will be provided during the excursion. Dinner and breakfast willbe served in at the guesthouses where we stay overnight. Lunch and coffee breaks will bebrought along from the guesthouses, to be eaten outdoors if the weather permits.

Field logisticsDuring the excursion, transport on mainland will be done by minibus. On the third excursionday we will leave the minibuses on the mainland to take a ferry to visit the traffic free islandsof Vrångö and Styrsö (overnight at Styrsö) in the Gothenburg archipelago. Excursion stops donot involve long hikes but participants should have adequate footwear and suitable clothingfor walking in rainy weather and in rough and uneven terrain. Rock outcrops may be slipperyin rainy weather and always along shorelines. The weather in August is on average, warm andpleasant, with midday temperatures between 15 and 25°C and occasional showers.

http://www.lfv.se/templates/LFV_AirportStartPage____2570.aspx

http://www.flygbussarna.se/

http://www.kulturgarden.com/index_eng.htm

http://www.pensionatskaret.se/english/

General IntroductionThis pre-conference excursion will be held in the Sveconorwegian orogen of southernScandinavia, a tectonic counterpart to the Grenville orogen in Canada. Here, the shield areaexposes an exquisite traverse through a deep-seated Precambrian continent-continent collisionzone(s). The orogen is composed of several Proterozoic gneiss segments attached along thesouthwestern margin of the Baltica proto-continent during a succession of compressionalorogenic phases at between 1.14-0.96 Ga. A final continent-continent collision phase at 0.98-0.96 Ga involved high-pressure granulite metamorphism and emplacement of eclogites in thesoutheastern part of the orogen. The excursion participants will be taken to well exposed high-grade metamorphic domains, including the Southwest Swedish granulite region that exhibits apolymetamorphic high-grade gneiss complex with charnockites, high-pressure granulites andtectonically emplaced eclogitised units.

The excursion aims to highlight the combination of field geology, metamorphic petrology andapplication of different geochronological-geochemical analytical techniques to pin down themetamorphic conditions and the timing of tectonothermal events of high-grade metamorphiccomplexes. The individual excursion stops have been selected to show key localities used toconstruct, characterise and directly date the P-T evolution and tectonic build up of the easternSveconorwegian orogen in Scandinavia. Our purpose with the excursion is to bring togetherstructural geologists, metamorphic petrologists, isotope geochemists, geochronologists, andother geoscientists to combine their expertise and discuss how to model tectonic cycles andthereby, understand the crustal evolution of our continents.The outline of the excursion follows three main themes: (I, day one) Tectonic build up of the orogenic front; the transition between weakly tounmetamorphosed rocks of the pre-Sveconorwegian craton and the high-grade gneisses of theSveconorwegian Southwest Swedish granulite complex. (II, day two) Geochronology and setting of eclogites, granulites, and charnockites. (III, day three and four) Terrane boundaries and lithotectonic build up of the Sveconorwegianorogen.This pre-conference excursion is thematically linked to a symposia on “Geochronology andIsotope geology” held in the IGC 2008 meeting, sub-session entitled “Geochronology ofmetamorphic reactions and deformation in high-grade orogenic settings” (sub-section MPC-02). Convenors: Jenny Andersson, Bernard Bingen, David Cornell and Ulf Söderlund

Regional GeologyAt the end of the Mesoproterozoic, the Fennoscandian margin (the present day southwesternsegment of continent Baltica) was reworked by orogenic activity that resulted from collisionwith at least one other major continent, possibly Amazonia (Fig. 1; Hoffman 1991). Thisorogenic activity is attributed to Sveconorwegian orogensis, and is bracketed in time atbetween 1.14-0.90 Ga. Today, the imprint of Sveconorwegian orogenic activity remains as ac. 500 km wide, partly deeply eroded, orogenic belt in southwestern Scandinavia. Thisorogenic province is referred to as the Sveconorwegian orogen (Fig. 2). The orogen isdelimited in the east by the Sveconorwegian Frontal Deformation Zone (Wahlgren et al.1994), a tentative zone outlined by discrete brittle- ductile deformation zones that marks theeastern boundary for Sveconorwegian tectonothermal reworking in Fennoscandia. East of theSveconorwegian Frontal Deformation Zone are Palaeproterozoic rocks of the c. 1.92-1.81 GaSvecokarelian orogen and largely unmetamorphosed and undeformed rocks of the 1.81-1.66Ga Transscandinavian Igneous Belt (Fig. 3).

Fig. 1. Classical plate reconstruction at the end of the Grenvillian-Sveconorwegian orogeny,with the Sveconorwegian orogen restored to the right of the Grenville orogen (Cawood et al.2007). The map shows the first order tectonometamorphic correlation between the two beltsfollowing compilations and data by Rivers and Corrigan (2000), Rivers et al. (2002), Bingenet al. (2008c), and Söderlund et al. (2008a).

The internal parts of the Sveconorwegian orogen includes five principal lithotectonic units(Bingen et al. 2005) separated by roughly N-S-trending crustal scale deformation zones ofSveconorwegian age (Park et al. 1991; Andersson et al. 2002). The principal Sveconorwegianlithotectonic units are, from east to west, the Eastern Segment, the Idefjorden Terrane, theBamble Terrane, the Kongsberg Terrane and the Telemarkia Terrane (Figs. 2 and 3). Theseunits differ from one another in their Pre-Sveconorwegian as well as their Sveconorwegiancrustal evolution, regarding both timing and style of crustal growth, deformation andmetamorphism. Rocks affected by orogenic reworking of Sveconorwegian age are presentalso north of the Caledonian Front. Since these rocks were later reworked during theCaledonian orogeny (500-400 Ma) their tectonic relation to rocks within the Sveconorwegianorogen south of the Caledonian front is at present unclear.

Fig. 2. Situation map of southwestern Scandinavia showing the main lithotectonic units andshear zones of the Sveconorwegian orogen.

Sveconorwegian orogenesis resulted in widespread high-grade metamorphism, partial meltingand deformation of the Fennoscandian crust. Available data and observations provideevidence for a sequence of events covering at least 240 million of years, between c. 1140 and900 Ma, including both compressional and extensional tectonic events (Figs. 4 and 5; seereview in Bingen et al. 2008a; 2008c). The period prior to the onset of Sveconorwegianorogenesis is characterized by bimodal magmatism at between 1280 to 1140 Ma, variablyassociated with sediment basins. This magmatism is abundant in the western Sveconorwegianorogen, in the Telemarkia terranes (Laajoki et al. 2002), but extends also deeply into theFennoscandia craton (Söderlund et al. 2005). It is possibly related to a subduction off-boardthe Fennoscandian continent. The earliest dated Sveconorwegian high-grade metamorphism isrecorded between 1140 and 1125 Ma in the classical Arendal granulites in the BambleTerrane (Figs. 4 and 5; Smalley et al. 1983). This orogenic phase is referred to as the Arendalphase and may relate to a local collision or accretion at the margin of Fennoscandia. The main

Sveconorwegian orogenic event included regional deformation, metamorphism and partialmelting in both the eastern and western parts of the orogen between 1050 and 980 Ma. Thisorogenic phase is called the Agder phase and metamorphism related to this stage varies fromlower amphibolite facies to high-pressure granulite facies (Figs. 4 and 5; Bingen et al. 2008b;Söderlund et al. 2008a). Both the tectonic style and the metamorphic grade vary widelybetween the different lithotectonic units. The Sveconorwegian orogenic evolution included amajor compressional event at 980-960 Ma that resulted in high-pressure granulite and eclogitefacies metamorphism in the Eastern Segment of the southeasternmost part of the orogen,(Figs. 4 and 5; Möller 1998; Johansson et al. 2001). This event is referred to as the Falkenbergphase and reflects final convergence related to the main continent-continent collision. Late-Sveconorwegian gravitational collapse took place between c. 970 and 900 Ma during theDalane phase (Figs. 4 and 5; Bingen et al. 2006). Exhumation and unroofing of the deeplyburied crustal domains, extensional reactivation of major shear zones and post-collisionalmagmatism increasing in volume westwards characterize this stage. This post-collisionalmagmatism includes the classical Rogaland anorthosite complex at the southwestern end ofthe Telemarkia terrane (Schärer et al. 1996), that also associated with high- to ultrahigh-temperature granulite-facies metamorphism of the surrounding gneisses (Tobi et al. 1985;Bingen & van Breemen 1998; Möller et al. 2003). Final relative motion between theSveconorwegian terranes is estimated at about 920-910 Ma.

Fig. 3. Cummulative probability curves of geochronological data on magmatic events in thefive lithotectonic units of the Sveconorwegian orogen. Figure following Bingen et al. (2008c).

In terms of thermal and metamorphic evolution, two first order trends emerge from thepresently available data. (1) A record of high-pressure metamorphism is preserved in the eastof the Sveconorwegian orogen, in the Eastern Segment and Idefjorden Terrane, while low tomedium pressure metamorphism prevails in the west. (2) Sveconorwegian magmatism,including syn-collisional and post-collisional magmatism, increases tremendously in volumetowards the west, with a sharp increase in the western part of the Idefjorden terrane (the Flåand Bohus granite plutons). This first order zoning of the orogen is analogous to the magmaticand tectonometamorphic zoning of the Grenville belt of Laurentia (Fig. 1).

The 33 IGC excursion No 51 covers the eastern parts of the Sveconorwegian orogen whichincludes the Eastern Segment and Idefjorden terrane and the geology of these two units isreviewed below. The geology of the central and western parts of the Sveconorwegian orogen,including the Telemarkia, Bamble and Kongsberg terranes is reviewed in Bingen et al.(2008a; 2008c).

Fig. 4. Summary of geochronological data on high-grade metamorphism in theSveconorwegian orogen, following Bingen et al. (2008b).

Fig. 5. Summary of the distribution of metamorphism, magmatism and sedimentary basinsduring the Sveconorwegian orogeny. For each time slice, the conditions of metamorphism aresummarized in the pressure-temperature space. Figure following Bingen et al. (2008c).

The Eastern SegmentThe Eastern Segment is the easternmost lithotectonic unit of the Sveconorwegian orogen andit forms the parautochthonous basement (at least parts of it) of the orogen. It is composed of1.81-1.66 Ga orthogneisses of the same age and composition as largely unmetamorphosed andundeformed intrusives of the Transscandinavian Igneous Belt (Fig. 3; Connelly et al. 1996;Söderlund et al. 1999; Söderlund et al. 2002; Möller et al. 2007; Bingen et al. 2008c;Söderlund et al. 2008b). The northern part of the Eastern Segment, north of lake Vänern andthe Hammarö Shear zone (Fig. 2), is largely composed of penetratively to semi-penetrativelydeformed felsic plutonic rocks. Metamorphic conditions are in the amphibolite to greenschistfacies. Titanite ages date cooling after Sveconorwegian metamorphism at about c. 960 Ma,but preserved Paleoproterozoic igneous titanite are also found in these rocks (Söderlund et al.1999). Metamorphism and deformation attributed to pre-Sveconorwegian orogenic activityhave, so far, not been recorded in the northern parts of the Eastern Segment. The southern partof the Eastern Segment, south of lake Vänern and the Hammarö Shear Zone (Fig. 2), iscomposed of largely migmatitic orthogneisses commonly interlayered with amphibolite,garnet amphibolite and mafic granulites. Sveconorwegian metamorphism in the southernEastern Segment reached upper amphibolite to high-pressure granulite conditions. P-Testimates obtained from metabasic rocks in the region yield temperatures between 680 and770°C and corresponding pressures of 9-12 kbars (Johansson et al. 1991; Wang & Lindh1996; Möller 1998; Möller 1999; Söderlund et al. 2004). Some of the high-pressure maficgranulite boudins show evidence for being decompressed eclogites (Möller 1998, 1999).Metamorphic zircon from a number of localities brackets Sveconorwegian metamorphism andmigmatitization between c. 990 and 960 Ma (Figs. 4, 5; Falkenberg phase; Andersson et al.1999; 2002; Söderlund et al. 2002; Möller et al. 2007). Zircon inclusions in garnet provide amaximum age of 972 ±14 Ma for the ecolgite-facies metamorphism (Ullared locality;Johansson et al. 2001). Titanite U-Pb data range from 960 to 920 Ma (Connelly et al. 1996;Söderlund et al. 1999; Johansson et al. 2001). The internal parts of the southern EasternSegment is structurally characterised by large scale upright to moderately overturned E-W-trending folds with wavelengths of between c. 4-15 km, and sub-horizontal, undulatingcommonly south-vergent fold axis. The regional scale fold pattern is also easily recognised inhigh-resolution aeromagnetic anomaly maps (Fig. 6; Möller et al. 2007).

Due to a general penetrative Sveconorwegian overprint, pre-Sveconorwegian minerals have,as a rule, recrystallised or re-equilibrated. Consequently, little is known about the pre-Sveconorwegian tectonothermal evolution of the Eastern Segment. Robust mineral isotopesystems like the U-Pb system in zircon, however, have in places a preserved record of a pre-Sveconorwegian metamorphism. Regional scale migmatisation and in places gneissic layeringhave been dated at between c. 1460 and 1420 Ma (Söderlund et al. 2002; Austin Hegardt et al.2005; Möller et al. 2007). This event is referred to as the Hallandian event. There is increasinggeochronological, petrographic and structural evidence that the Hallandian event includedsubstantial high-grade reworking of the southern Fennoscandia in Mesoproterozoic time. TheHallandian event is partly co-eval with orogenic reworking attributed to the Dano-Polonianorogeny described from areas south and southeast of the Sveconorwegian orogen (Bogdanovaet al. 2008). The Hallandian event was followed at c. 1400-1380 Ma by intrusion of partlycharnockitic granite and syenitoid rocks, and contemporaneous charnockitisation of side rockgneisses around felsic dyke intrusions at about 1400 Ma (Hubbard 1975; Åhäll et al. 1997;Andersson et al. 1999; Rimsa et al. 2007).

The youngest rocks in the southern Eastern Segment are high-angle discordant pegmatitic andgranitic dykes dated at about c. 950 Ma (Möller & Söderlund 1997; Andersson et al. 1999;Möller et al. 2007). Structurally young metabasic dykes occur at high-angle discordance toveined gneissic fabrics in the country rocks but are themselves metamorphosed in the high-pressure granulite facies. The igneous emplacement age of these dykes is at present unknown.

Fig. 6. Aeromagnetic anomaly map of southern Sweden. Abbreviations: PZ=Protogine Zone,MZ=Mylonite Zone, GÄZ=GötaÄlv Zone, e=Sveconorwegian eclogite. Part of the airbornemagnetic map (total field) over SW Sweden. Data source: Geological Survey of Sweden. Mapcompiled by Leif Kero, Geological Survey of Sweden, Uppsala.

The Sveconorwegian Frontal Deformation Zone (Figs. 2 and 6; Wahlgren et al. 1994;Söderlund et al. 2004) delimits the Eastern Segment in the east. It forms a tentatively outlinedborder zone for the eastern extension of the Sveconorwegian orogen and is outlined bydiscrete brittle- ductile deformation zones that mark the eastern boundary for Sveconorwegiantectonothermal reworking in Fennoscandia. South of lake Vättern, the SveconorwegianFrontal Deformation Zone borders the eastern parts of the Protogine Zone, a prominentdeformation zone system that marks a conspicuous boundary between high-gradeorthogneisses of the Eastern Segment, and non-penetratively deformed and unveined rocks ofthe Transscandinavian Igneous Belt (Figs. 2 and 6). The Protogine Zone it self forms a c. 25-km wide structural and metamorphic transition zone composed of numerous discrete, sub-vertical, N-S-trending deformation zones. It played a central role for the tectonic juxtapositionof previously deeply buried rocks of the Eastern Segment in late Sveconorwegian time.

In the west, the Eastern Segment is separated from overlying Sveconorwegian allochthonousterranes by the Mylonite Zone (Figs. 2 and 6). The southern section of the Mylonite Zone is ashallowly west-dipping prominent deformation zone that forms a major lithological,metamorphic and structural terrane boundary (Andersson et al. 2002). The Mylonite Zonenorth of lake Vänern is interpreted as a sinistral transpressional thrust zone with an overalltop-to-the-southeast transport direction (Park et al. 1991; Stephens et al. 1996). The MyloniteZone is reworked as a normal extensional shear zone in late Sveconorwegina time (Berglund1997). Zircon U-Pb data in the southern section of the Mylonite Zone and its direct hanging-wall and foot-wall record amphibolite-facies metamorphism and migmatitization at between980 and 970 Ma (Larson et al. 1999; Andersson et al. 2002). This interval is equivalent,within error, to the age of high-grade metamorphism in the Eastern Segment. Indirectestimates for extensional tectonics along the southern sections of the Mylonite Zone areprovided by hornblende 40Ar/39Ar plateau ages between c. 920 and 910 Ma, a titanite age at c.920 Ma and a zircon age in a stromatic migmatite at c. 920 Ma (Johansson & Johansson 1993;Page et al. 1996; Scherstén et al. 2004).

The Idefjorden TerraneThe Idefjorden Terrane (Fig. 2) is made up of c. 1660-1520 Ma plutonic and volcanic rocks,associated with greywacke-type metasedimentary sequences (Fig. 3; Åhäll et al. 1998; Breweret al. 1998; Åhäll & Larson 2000; Bingen et al. 2001; Andersen et al. 2004; Åhäll & Connelly2008). The magmatic rocks generally have calc-alkaline and tholeiitic compositions, typicalfor supra-subduction zone magmatism. The lithologies show a general younging towards thewest. The Horred metavolcanic rocks (c. 1660-1640 Ma) are exposed in the southeastern partof the terrane close to the Mylonite Zone. The Åmål supracrustal rocks and the coevalGöteborg granite suite (c. 1630-1590 Ma) form a belt situated to the east of the c. 1590-1520Ma Stora Le-Marstrand Formation and the Hisingen plutonic suite. These lithologies wereassembled during the Gothian accretionary orogeny (Andersen et al. 2004; Åhäll & Connelly2008; Bingen et al. 2008a). Amphibolite-facies metamorphism and deformation associatedwith Gothian orogenesis have been constrained at about 1540 Ma (Connelly & Åhäll 1996;Åhäll & Connelly 2008; Bingen et al. 2008b). The 1660-1520 Ma "Gothian" lithologies wereintruded by the bimodal plutonic Kungsbacka suite between 1340 and 1250 Ma (AustinHegardt et al. 2007), and by Sveconorwegian post-collisional norite-granite plutons between960 and 920 Ma (Eliasson & Schöberg 1991; Scherstén et al. 2000; Årebäck & Stigh 2000;Hellström et al. 2004; Bingen et al. 2006). These include the Flå and Bohus plutons. West ofLake Vänern, the Gothian and Kungsbacka metaintrusive rocks are overlain by the poorlydated supracrustal Dal Group (Brewer et al. 2002).

The Idefjorden Terrane shows a general N-S to NW-SE Sveconorwegian structural trend. Itcontains several amphibolite-facies orogen-parallel shear zones, including the Ørje ShearZone (Norway) or Dalsland Boundary Zone (Sweden) and the Göta Älv Shear Zone (Fig. 2;Park et al. 1991). These shear zones are interpreted as transpressive thrust zones (Park et al.1991). One zircon date at 974 ±22 Ma records metamorphism in the vicinity of the Göta ÄlvShear Zone, and may record deformation along this shear zone (Ahlin et al. 2006).Sveconorwegian metamorphism is variable in the Idefjorden Terrane and ranges fromgreenschist-facies to amphibolite-facies and locally granulite-facies conditions. East of theGöta Älv Shear Zone and Dalsland Boundary Zone, the Åmål supracrustal rocks are wellpreserved and partly show greenshist-facies metamorphism only. In contrast, high-pressuremafic granulite boudins occurs in a high-grade gneiss complex located immediately south oflake Vänern (Gaddesanda locality, stop 4:2; Figs. 4 and 7; Söderlund et al. 2008a). The gneissprotoliths are dated at about 1.6 Ga (they are thus coeval with the Åmål supracrustals) and thehigh-pressure granulite-facies metamorphism is dated between 1050 and 1025 Ma (Agderphase; Söderlund et al. 2008a). West of the Göta Älv Shear Zone and Dalsland BoundaryZone, amphibolite-facies metamorphism is dated between c. 1040 and 1020 Ma, according tozircon and titanite data (Hansen et al. 1989; Austin Hegardt et al. 2007). West of the Oslo rift,high-pressure amphibolite-facies conditions are locally recorded (Hensmoen locality; Figs. 4,5; Bingen et al. 2008b). Monazite and titanite dates in these rocks range from c. 1050 to 1025Ma (Bingen et al. 2008b). High-pressure conditions are dated at c. 1050 Ma in a kyanite-bearing metapelite.

To the west, the Idefjorden Terrane is separated from the Telemarkia Terrane by thesouthwest dipping Vardefjell Shear Zone. It is characterized by amphibolite-facies bandedgneiss rich in amphibolite-layers and amphibolite boudins. The timing of amphibolite-faciesmetamorphism in the banded gneiss is estimated at c. 1010 Ma according to zircon data(Bingen et al. 2008b), implying that ductile deformation along the Vardefjell Shear Zone iscoeval or younger than c. 1010 Ma. Fabric-parallel titanite may record continued deformationat 985 ±5 Ma (Bingen et al. 2008b). The Vardefjell Shear Zone is tentatively interpreted as athrust zone.

Excursion Route and Road Log

Fig. 7. Simplified geological outline of the Sveconorwegian orogen in southern Scandinavia(southern Baltic Shield). Locations of excursion stops are shown in magnified inset to theright.

The excursion to the Sveconorwegian orogen of Scandinavia is organised as a four days fieldtrip arranged to cover three principal themes on the tectonic build up of the eastern part of theorogen. The first theme, “Transect across the Sveconorwegian orogenic front”, (day 1, stop1.1-1.4) deals with the tectonic architecture of the orogenic front and the geochronology ofstructures and metamorphism connected to the tectonic evolution of the easternmost high-grade parts of the orogen. The second theme, “Eclogites, high-P granulites and charnockites”,(day 2, stop 2.1-2.5) focus on the timing and tectonic setting of high-P and high-P-Tmetamorphic events in the high-grade gneiss complex of the Eastern Segment (theeasternmost, partly parautochthonous part of the Sveconorwegian orogen). The third topic,“Tectonic boundaries and lithotectonic build up of the Sveconorwegian orogen”, (day 3 and4, stop 3.1-4.4) deals with the age and tectonic style of metamorphic terrane boundaries andthe crustal evolution of allochthonous lithotectonic units that are overlying the high-P rocks ofthe Eastern Segment. The three different topics of the excursion will be presented in moredetail below in introductory sections preceding the descriptions of the excursion stops. Thelocations of the individual excursion stops are indicated on detailed maps included in thedescription for each stop (in addition to co-ordinates for the outcrops given in UTM Zo33 andZo32, Northern Hemisphere). A geological outline of the eastern part of the Sveconorwegianorogen, with the individual excursion stops indicated on the map, is given in figure 7.

Day 1. Transect across the Sveconorwegian orogenic frontThe excursion starts in the morning of the 2nd of August at Landvetter airport. We will drivehighway R40 to Jönköping and continue on highway E4 towards the south (towardsHelsingborg) to reach Smålands Taberg and Stop 1.1. Here we will look at classical out cropsof early Sveconorwegian mafic cumulates and discuss Lu-Hf geochronological andpetrological data, and ore geology of these rocks and the implications for the tectonicevolution of the Protogine Zone. We will continue southwards along highway E4 to the Hokvalley and Stop 1.2 where we will look at the onset of Sveconorwegian deformation inmetagranites of the Transscandinavaina Igneous Belt. These out crops are located in theeastern Protogine Zone that forms the boundary for penetrative and non-penetrativeSveconorwegian deformation in the Fennoscandian shield. Thereafter we will drivewestwards, across highway E4 and take road 152 towards Gnosjö to look at variouslymetamorphosed and deformed metabasic rocks in the westernmost parts of the ProtogineZone at Åker (Stop 1.3). This stop will highlight aspects on the tectonic evolution of theeastern boundary of the Sveconorwegian orogen and different techniques to directly datemetamorphic reactions in mafic rocks. Continue westwards along road 27 for an optional stopin the partly well preserved mafic dolerites (Hyperites) at Herrestad (Optional stop). Theseare examples of the mafic magmatism and mafic dyke swarms that occur along the easternboundary of the Sveconorwegian orogen. Continue westwards along roads 27 and 153 toreach Stop 1.4 where we will look at Sveconorwegian migmatitic gneisses at Oxanäset. Thesegneisses have been used for U-Pb-Th ion probe zircon analytical work intimately integratedwith geological and aeromagnetic bedrock mapping for direct dating of migmatisation andsynchronous conspicuous regional scale E-W folding characteristic for the internal parts ofthe southern Eastern Segment. We continue towards the west for dinner and overnight at theSvartrå, Bråtadal hostel. http://www.kulturgarden.com/index_eng.htm

Day 2: Eclogites, high-P granulites and charnockitesIn the morning, we drive southwards towards Halmstad to reach the harbour and the oldabandoned quarry at Söndrum (stop 2.1). Here we will look at incipient charnockitization indehydration zones in the well-exposed walls of the quarry as well as excellent coastalexposures of patchy charnockites along the shoreline. The locality will highlight aspects onthe geochemistry, isotope geochemistry and geochronology of charnockitisation andpolyphase metamorphism in the Eastern Segment. Continue to stop 2.2, and the abandonedquarry at Högabjär, just east of Halmstad, where we will look at key localities forpolymetamorphic gneisses used to define 1.44 Ga migmatisation, 1.40 Ga granitic dykeintrusion, and post-1.40 Ga folding in the Eastern Segment. We drive back towards the northalong highway E6 and at Falkenberg we turn eastwards at road 154 to Ullared. Here we willvisit localities with decompressed Sveconorwegian eclogites at Lilla Ammås (stop 2.3) andthe high-grade tectonites in the Ullared Deformation Zone at Buskabygd (stop 2.4). Theselocalities include out crops of former eclogite, deformed and recrystallised into granulitefacies mylonitic gneiss and intercalated with felsic gneiss. The outcrops are key localities forstudies of the petrology, geochronology and tectonic evolution (extent and mode ofemplacement) of relict eclogite mafic boudins in the Ullared Deformation Zone. Focus ofthese two excursion stops is also aspects on the tectonic role of the Ullared DeformationZone, and the timing and character of deformation in the eclogite-bearing parts of the zone.We continue towards the west on road 153 to Varberg to look at coastal exposures of 1.4 Gaold charnockites at Getterön (igneous or metamorphic charnockitisation - or both? stop 2.5).Drive back towards the east on road 153 for dinner and overnight at Bråtadal, Svartrå hostel,http://www.kulturgarden.com/index_eng.htm.



Day 3: Terrane boundaries and lithotectonic build up of the Sveconorwegian orogenDrive towards the west on road 153 to Varberg and then northwards on the old highway E6 toreach the Årnäs peninsula where rocks in the Mylonite Zone are exposed along the southernshore of the Klosterfjord (stop 3.1). Here we will look at rocks used to constrain the age andtectonic role of the Mylonite Zone; a major structural, metamorphic and lithologicalSveconorwegian terrane boundary that separates rocks of the parautochthonous EasternSegment from overlying allochthonous lithotectonic units in the west. We continuenorthwards on highway E6 to central Gotheburg and the university of Chalmers (stop 3.2).Here we will look at an about 1.3 Ga old metagabbro that have been in focus for studies ofdiapiric wall rock melts, ion probe geochronology of xenocryst zircon and metamorphictitanite and constraints of P-T conditions from cation partition thermobarometry. Drive to theharbour at Saltholmen to catch a ferry to the Vrångö island in the southern Göteborgarchipelago (stop 3.3). Here we will look at migmatisation of Stora Le Marsstrandgraywackes driven by mafic intrusions and composite dyke development and the origin ofcalc-alkaline magma series by back-veining and assimilation. Catch the ferry to Styrsö fordinner and overnight at Guesthouse Skäret, http://www.pensionatskaret.se/english/.

Day 4: Terrane boundaries and lithotectonic build up of the Sveconorwegian orogen(continued)In the morning, ferry to mainland and the Saltholmen harbour. Drive across Gothenburgtowards Stora Lundby and stop 4.1. Here we will look at metamafic rocks that have been infocus for ion probe U-Pb-Th (SIMS) and ICP-MS REE and U-Th analyses of zircon guidedby Cathodoluminescence imaging to explore the fate of zircon in crustal processes. Continuenorthwards on road 45 towards Trollhättan and Gaddesanda (stop 4.2). Here we will look athigh-P granulite facies metadolerites analysed for U-Pb, Sm-Nd, Lu-Hf isotopes to date andcharacterise Mesoproterozoic mafic intrusions and their metamorphic history in the Idefjordenterrane. Drive towards the west on road 44. At the intersection with highway E6, turn northtowards Oslo. Drive to Veme in the Hönefoss area to look at preserved Bouma sequences inamphibolite-faces metagreywacke, with garnet-amphibolite dykes in the western IdefjordenTerrane (stop 4.3). Continue to Hensmoen to look at pervasive amphibolite-facies garnetblastesis in HP amphibolite-facies conditions (stop 4.4). Drive back to Oslo. Excursion endsin Oslo. The minibuses will return to Gothenburg in the evening of day four, immediatelyafter the excursion and participant are welcome to follow us back to Gothenburg.

http://www.pensionatskaret.se/english/

Excursion Stops

Day 1 Transect across the Sveconorwegian orogenic front

IntroductionThe first day we will visit localities within and west of the Protogine Zone. The scope of theday is the tectonic role of the Protogine Zone; the border zone between unmetamorphosed tomoderately metamorphosed rocks of the Transscandinavian Igneous Belt in the pre-Sveconorwegian craton and high-P granulite facies rocks in the Sveconorwegian Orogen.Focus will be on structural and metamorphic terrane boundaries across the Protogine Zoneand geochronology and isotope geochemistry of metamorphic and igneous events associatedwith these structures. The first excursion stop is made within the c. 1.20 Ga Småland Tabergultra mafic body, which belongs to a suite of 1.22-1.20 Ga mafic to syenitoid and graniticigneous rocks located along the Protogine Zone. The magmatic activity confined to theProtogine Zone may reflect intracratonic tension in response to tectonic activity at thecontinental Fennoscandian margin and thus marks the onset of Sveconorwegian orogenicactivity. The second excursion stop is made within non-penetratively deformed rocks of theTranscandinavian Igneous Belt to look at the onset of Sveconorewgian deformation in theProtogine Zone. The third stop is made in the westernmost parts of the Protogine Zone, andimmediately west thereof, where detailed zircon geochronology and petrography ofmetamafic intrusions testify of the tectonic evolution of the eastern part of theSveconorwegian orogen. The third stop is an optional locality within a metamafic intrusionthat belongs to one (or several) generation(s) of mafic dyke swarms found in the easternmostparts of the Sveconorwegain orogen. Isotope geochronology and isotope geochemistry ofthese mafic rocks combined with petrography and field data are used for models of the crustalevolution of this part of the shield area. The forth ands last stop of the day is located wellwithin the lower western level of the Eastern Segment which is typically composed ofmigmatite gneisses intercalated with high-P granulite facies and upper amphibolite faciesmetabasic rocks. The structural grain is here dominated by large scale E-W to NW-SEtrending folding of the lithological and gneissic banding. The locality exhibits foldedmigmatite gneiss intercalated with garnet amphibolite boundins, and is a typicalrepresentative of the gneiss complex that compose the internal lower level parts of thesouthern Eastern Segment.

Stop No 1.1: Lu-Hf geochronology of mafic cumulates, example from the old quarry ofTaberg

Location Smålands Taberg (UTM Zo33 NH: 445138/6393423). Outcrop in the old quarry of Taberg.Drive from Landvetter airport towards Jönköping along highway R40. From Jönköping, takethe E4 highway towards the south (direction Helsingborg). Turn right at Torsvik and then lefttowards Taberg (road no 93).

IntroductionThe Smålands Taberg is an ultramafic intrusion located within the Protogine Zone. It belongsto a generation of c. 1.24-1.20 Ga old mafic, syenitoid to granitic intrusions located withinand along the southern parts of the Protogine Zone. These intrusions testify to the early onsetof Sveconorwegian tectonic activity in Fennoscandia. A simplified map of the mainlithologies at Taberg is given in Figure 1.1.1.Topics of interest:- Emplacement, petrology, and ore geology of the Taberg mafic cumulates – implications

for the tectonic evolution of the Protogine Zone- The use of the Lu–Hf apatite chronometer

DescriptionSmålands Taberg is famous for its Fe-Ti ore deposits (in magnetite-rich melatroctolite). Themineralization is depleted in incompatible elements which precludes the use of minerals(baddeleyite, zircon, titanite or apatite) commonly used for dating the emplacement ofigneous rocks. Patches of leucogabbro in the melatroctolite have REE patterns and initial Hfand Nd isotope compositions identical with the host melatroctolite (Fig.1.1.2.). Thesecharacteristics are conclusive evidences for a common parental magma such that theleucogabbro crystallized after fractionation of olivine and titanomagnetite; two major mineralphases in the melatroctolite.

The age of this ultramafic body was recently determined by Lu-Hf apatite chronology(Fig.1.1.3; Larsson & Söderlund 2005). Apatite and plagioclase separated from theleucogabbro plus a whole rock sample define a Lu-Hf isochron with a slope corresponding toan age of 1204 ± 2 Ma. This result falls into the lower group of a magmatic syenite-granitesuite in southern Sweden (Söderlund & Ask 2006). Clearly, the Lu-Hf isotope system offersan important technique for dating Si-unsaturated rocks that lack baddeleyite or zircon.

Guides: Ulf Söderlund (Geobiosphere Science Centre, Lund University). Literature: Larsson & Söderlund (2005)

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75

.341Taberg

Modified from Hjelmqvist 1950

PegmatiteMelatroctoliteGabbroAmphibolite (metagabbro)Gneiss graniteMyloniteStrike and dipFault

100mLeucogabbro inclusions

325

300

275

250

1

2

3

4

A1

Fig. 1.1.1. Simplified map showing the main lithologies at Taberg. Modified from Hjelmqvist1950. (Larsson & Söderlund 2005).

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Fig. 1.1.2. REE-diagram showing parallel trends of various lithologies that indicate acommon source. Figure taken from Larsson & Söderlund 2005.

Fig. 1.1.3. Lu-Hf isochrone diagram including three apatite fractions of strongly elevatedLu/Hf ratios (outside diagram). Note wr D7 (Melatroctolite) plot just outside the 1204-Maisochrone. Figure from Larsson & Söderlund (2005).

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Stop No 1.2: The Protogine Zone and the Transscandinavian Igneous Belt of the pre-Sveconorwegian Fennnoscandian craton

LocationHok valley, Hok manor. (UTM Zo 33 NH: 456991/6378481). Drive southwards alonghighway E4 from Jönköping towards Helsingborg. At Skillingaryd, turn east towards Hok.

IntroductionThe Transscandinavian Igneous Belt forms a roughly N-S trending magmatic belt that intruderocks of the Palaeoproterozoic Svecofennian Province east of the Sveconorwgeian orogen.South of Lake Vättern, the Transscandinavian Igneous Belt is mainly composed of 1.81-1.66Ga old granites, monzonites and quartzmonzodiorites. The more intermediate compositionscharacteristically contain mafic enclaves. Gabbroic rocks are sparse and typically showmagma mingling and mixing with their side rock along the contacts. E-W-trending beltsdominated by felsic volcanic rocks, mainly rhyolites, are also present. A transection across theProtogine Zone, from east to west, exposes a transition of non-metamorphosed to weaklymetamorphosed intrusives of the Transscandinavian Igneous Belt into high-gradeorthogneisses to the west of the zone. Major lithological changes are bound by deformationzones. The migmatitic orthogneisses in the southern Eastern Segment are of the same age andcomposition as rocks in the western parts of the Transscandinavian Igneous Belt. It issuggested that these gneisses are reworked equivalents to rocks of the TransscandinavianIgneous Belt of the pre-Sveconorwegian Fennoscandian craton and that these rocks, at least inpart, form the parautochthonous basement of the Sveconorwegian orogen.

DescriptionThe area of the Hok valley exhibit outcrops with near isotropic granite (Barnarp granite)typical for felsic intrusives of the Transscandinavian Igneous Belt of the pre-SveconorwegianFennoscandian craton. The Hok valley also exposes out crops in which the onset andprogressive development of Protogine Zone deformation can be studied. In this area, nearisotropic megacrystic granite with mantled feldspars (Barnarp granite) pass into shear zones

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with pervasively foliated (schistose) metagranite. These rocks also contain a conspicuoussteeply west plunging stretching lineation (290/60), defined by K-feldspar and quarts. RotatedK-feldspars and C-S-fabrics indicate a top to the east sense of shearing.(Description of the Hok Valley granites is based on unpublished excursion guide by Per-Gunnar Andreasson, Lund University, Lund, Sweden, “NorFa Field Seminar 1999, day 1 –the Protogine Zone”).

Guides: Charlotte Möller and Jenny Andersson (Geological Survey of Sweden) Literature: Andreasson & Dallmeyer (1995)

Stop No 1.3: Zircon formation during metamorphism and deformation of mafic rocks,example from petrology and U-Pb chronology applied to a metabasic intrusion in theProtogine Zone

LocationÅker (on the boundary between the Eastern Segment and the Protogine Zone). (UTM Zo 33NH: 440022/6359604). Drive south from Jönköping on highway E4 towards Helsingborg. AtSkillingaryd, c. 50 km south of Jönköping, turn right (west) at road no 152 towards Gnosjö.The Åker metabasite crops out in an about 20 m long road cut along the road.

IntroductionAt least three different generations of mafic intrusions occur along the eastern border of theSveconorwegian orogen emplaced at about 1.6, 1.4, 1.2 and 0.9 Ga respectively. The Åkermetabasic intrusion belongs to the oldest generation (c. 1.6 Ga old) and records petrographicand geochronological evidence of a prolonged igneous and tectonic evolution of thiseasternmost part of the Sveconorwegian orogen. The road cuts at Åker exhibit metabasicrocks in various state of reworking ranging from well preserved isotropic gabbro topenetratively oliated garnet amphibolite (Fig.1.3.2). The metamorphic reactions are associatedwith release of Zr and growth of metamorphic zircon (Fig.1.3.3). Four stages of zircon growthhave been recognised and directly dated the Åker locality (Fig.1.3.3). The behaviour androbustness of the Sm-Nd and Lu-Hf isotope systems during metamorphism have also beenstudied at this locality.

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Fig. 1.3.1. Sinplified geological map of the Åker area. From Söderlund et al. (2005).

DescriptionThe Åker metabasite is an irregularly shaped metamafic body in the westernmost part of theProtogine Zone (Figs. 7 and 1.3.1). Dis-equilibrium textures among minerals in a transitionzone between isotropic gabbro and strongly foliated garnet amphibolite allows for theidentification of metamorphic net transfer reactions, of which some involved release ofzirconium (Zr) and crystallisation of new zircon. The occurrence of secondary zircon in theleast metamorphosed sample is restricted to small anhedral grains in contact with magmaticilmenite. These zircons probably originated as a late-magmatic exsolution product duringcrystallization of ilmente or, alternatively, during metamorphism from breakdown ofbaddeleyite. In strongly metamorphosed samples, zircon growth was triggered by net-transferreactions in the presence of a fluid, involving breakdown of anorthite-rich plagioclase andilmenite, and crystallisation of garnet, titanite, zoisite, biotite and albitic plagioclase.Consumption of ilmenite is recognized as the main controlling mechanism for zircon growthas evidenced by the marked concentration of coronitic, small zircons proximate to resorbedilmenite grains. Ion probe spot dating of sub-angular, faintly zoned, zircon yield 1562±6 Maand dates the protolith of the intrusion whereas sub-rounded zircons in variably reworkedsamples yield ages at 1437±21, 1217±75 and 1006±68 Ma. The occurrence of three zircongenerations in reworked samples suggests that metamorphism was localised to rock volumeswith a pre-existing hydrated mineralogy – developed either as a late-magmatic product orduring an earlier metamorphic event. In the age spectra, secondary zircon giving older agesare more frequent than younger zircon. Furthermore, there is a tendency for a common

27

decrease in both zircon and ilmenite abundances from weakly to strongly metamorphosedsamples. These observations agree with the recognition of ilmenite as the controlling phasefor new zircon growth. We conclude that zircon growth during metamorphism may beinsignificant or even absent, in rocks where the Zr-bearing phases were exhausted duringearlier metamorphism. Ages of secondary zircon cannot be assumed to date peak-metamorphic conditions but are rather suggested to date the timing of particular net-transferreactions. These reactions may occur throughout a metamorphic cycle and may even, asdemonstrated here, control zircon growth during geological events separated by hundreds ofMyr.

Guides: Ulf Söderlund (Geobiosphere Science Centre, Lund University), and Charlotte Möller(Geological Survey of Sweden). Literature: Söderlund et al. (2004)

Fig. 1.3.2. Thin section from variably metamorphosed and deformed parts of the Åkermetabasite. Figure from Söderlund et al. (2005).

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Fig. 1.3.3. Upper: Large zircon (left) and baddeleyite (right) of magmatic origins. The otherfigures show how zircon occurs relative to other phases. Lower: Lower: U-Pb concordiadiagrams showing SIMS data of zircon analyses.

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Optional stop: Mafic magmatism and mafic dyke swarms along the eastern boundaryof the Sveconorwegian orogen

LocationHerrestad (Eastern Segment, Southwest Swedish Granulite Region). (UTM Zo 33 NH:432765/6341248). Out crops along old road cuts and small abandoned quarries at around theHerrestad village. Drive south on highway E4 from Jönköping towards Helsingborg. Turnright (westwards) at Värnamo and continue westwards on road no 27. At Kärda, about 5 kmwest of Värnamo, turn right and drive northwards towards Herrestad.

IntroductionAt some places, partly well-preserved mafic rocks occur also within the Eastern Segment.One such example is the Herrestad metabasite that crops out about 10 km to the west ofVärnamo. It belongs to the oldest generation of mafic rocks (about 1.6 Ga old) found withinand immediately west of the Protogine Zone. The Herrestad locality is a classical outcrop withhypersthene-bearing mafic rocks with dark coloured plagioclase typically occurring in theeastern part of the Sveconrwegian orogen (so called Hyperite dolerites). The Herrestadmetabasic rocks have been in focus for studies of the isotope geochemistry and agerelationships of mafic intrusions along the Protogine Zone (U-Pb baddeleyite geochronologyand Sm-Nd isotope geochemistry).

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Well preserved variety of the Herrestad metagabbro.

Description(Text modified from Vollert, 2006)

Several generations of mafic intrusions occur along the Protogine Zone dated at about 1.6, 1.41.2 and 0.9 Ga respectively. The Herrestad metabasite have an U-Pb baddeleyite age of 1574±9 Ma (unpublished data belonging to the geological survey of Sweden). It crops out as apartly well preserved gabbro that in places is gradually transformed into garnet amphibolite.The best-preserved parts of the gabbro occur as lenses within a more strongly deformedgarnet amphibolite. The mineral assemblage of the gabbro is olivine, plagioclase, ortho- andclinopyroxene and apatite. The olivine always has two-tired coronas in contact withplagioclase, probably due to deuteric reactions. The garnet amphibolite consists of amphibole,biotite and granoblastic plagioclase. The Herrestad metabasites show a slight enrichment inLREE, have week negative Eu-anomalies and La/Nb relations exceeding 1. The spreading ofthe samples in discrimination diagrams shows ambiguous result of magma genesis andtectonic setting. Nevertheless, taken together, the geochemical data suggest that the Herrestadmetabasites evolved from a tholeiitic magma in a continental rift or back-arc setting.Worth noticing is that some of the metabasites in the area deviate from the others in many ofthe geochemical discrimination diagrams. If an age of 1.57 Ga is assumed, all the metabasiteshave positive εNd-values, which indicates that they derived from a depleted mantle source.Some of them, however, have CHUR model ages, which may be due to a disturbed isotopicsystem, analytical errors or reflect that there are up to three different generations of maficintrusions in the area, intruded at about 1.6 Ga, 1.2 Ga and 0.8 Ga ago.

Guides: Leif Johansson, Ulf Söderlund (Geobiosphere Science Centre, Lund University), andCharlotte Möller (Geological Survey of Sweden).Literature: Johansson & Johansson (1990), Söderlund et al. (2005), Vollert (2006)

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Stop No 1.4: Direct dating of Sveconorwegian folding in the southern Eastern Segment

LocationOxanäset quarry (Sven Pettersson, Västbo lastbilscentral). (UTM Zo 33 NH:408436/6338681).

IntroductionRegional-scale, E-W-trending fold structures form a conspicuous structural pattern in thesouthcentral Eastern Segment. The folds are upright to moderately overturned, havesubhorizontal fold axes and wavelengths of c. 4-15 km. The fold structures form a spectacularpattern on magnetic anomaly maps (Fig. 1.4.1). Mapping in the region has shown that themigmatitic gneisses have been tightly folded along E-W-trending horizontal axes and garnetamphibolite layers have been stretched and boudinaged along the same E-W direction. Anearly fold phase has been identified as outcrop-scale, tight to isoclinal intrafolial folds, inplaces rootless. Small-scale folds are commonly south-vergent with subhorizontal E-W-trending fold axes. Another conspicuous structural feature characteristic of the high-graderocks in the region is strongly developed stretching lineations defined by strung-out mineralaggregates, partial melts or amphibolite bands. The lineations are subhorizontal (undulating),roughly E-W striking and subparallel to the fold axes of the regional and outcrop scale E-Wfolds. In general, the linear fabric is more strongly developed than the planar and locally aplanar fabric is lacking.

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Figure 1.4.1 Aeromagnetic anomaly map (top, total field) of the Oxanäset area, southcentralEastern Segment. Part of figure from unpublished internal Geological Survey of Swedenreport by Möller et al. (2005, SGU-rapport 2005:35).

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DescriptionThe Oxanäset quarry is located in the southern limb of a regional scale upright synform thathas a slightly west-plunging axis and a fold wavelength of about 15 km. The large-scale E-W-trending fold structure is characteristic for the southcentral parts of the Eastern Segment and itis easily distinguished on the magnetic anomaly map (Fig. 1.4.1). The connection between thelithological/structural and the earomagnetic map pattern is due to the generally highercontents of magnetite, and thereby higher magnetic susceptibility, in granitic gneissescompared to surrounding gneisses of tonalitic and granodioritic compositions (compare Figs.1.4.1 and 1.4.2). This connection has proven useful for tracing the lithological and structuralpattern of this part of the Eastern Segment.

The migmatite gneiss exposed at the Oxanäset quarry is a typical representative for the veinedorthogneisses that make up the bulk of the high-grade gneiss complex of southern EasternSegment. It is a light greyish red to reddish grey, fine to medium-grained rock, predominantlygranitic in composition. Remnants of a coarser-grained and in places uneven-grained, relictigneous protolith fabric are locally recognisable. It is penetratively migmatised, but theveining varies in intensity from discrete to, in places, intense penetrative stromatic layering.At outcrop scale, the migmatite gneiss has been tightly folded along west-trending fold axes.The axial planes to these folds are gently north-dipping. The Oxanäset migmatite gneiss isintercalated with cm- to dm-wide bands and lenses of amphibolite and with thick, up to 10metres wide, boudins of compositionally layered garnet amphibolite. Vein material has beeninjected into the boudin necks.

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Figure 1.4.2. Geological map of the Oxanäset area, southcentral Eastern Segment, simplifiedafter the geological map of the Jönköping county (scale 1:250 000). Part of figure fromunpublished internal Geological Survey of Sweden report by Möller et al. (2005, SGU-rapport 2005:35).

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Fig. 1.4.3. Electron microscope images of texturally composite zircon from the Oxanäsetmigmatite gneiss. Protolith zircon are typically backscatter dark (dark grey) while secondaryformed metamorphic zircon occur asbright grey domains. (a-b) Zircon from gneiss mesosome(unveined gneiss). The zircon contains well preserved igneous-zoned domains surrounded bythin secondary rims only. (c-d) Zircon from gneiss with folded leucosome. Image showincreased occurence of secondary zircon rims. (e-f). Zircon from syntectonic leucomoeshowing abundant occurence of secondary formed zircon domains.

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At least three structural generations of veins can be recognised at Oxanäset. The oldest onesare penetrative, mm- to cm-wide, fine- to medium-grained leucocratic veins that define astromatic layering concordant with the gneissic layering and which is folded at outcrop scale(pre-kinematic leucosome). A structurally younger generation of leucosome occurs as morethan cm-wide, locally pegmatitoid, veins that commonly are concordant to the gneissiclayering, folded, but in places also crosscut the fold structures (syn- to post-kinematicleucosome). These veins in places host porphyroblasts of magnetite or hornblende. Both typesof veins are commonly bordered by melanosome and are interpreted to have formed from insitu partial melting of the granite gneiss protolith. A third type of leucosome is diffusemedium-grained anatectic leucoratic segregations that crosscut or just blur the folded veinedgneissic structures. The leucosome that occurs in both folded and semi-discordant relations tothe folds demonstrates that migmatisation took place synchronously with folding along E-Waxes.

Zircon data from three variably migmatised gneisses at the Oxanäset quarry date igneousemplacement of the gneiss protolith at c. 1.67 Ga and migmatisation and feeding ofsyntectonic leucosome at about 0.97 Ga (Möller et al. 2007). Zircon in unveined stronglyfoliated granitic gneiss without distinct leucosome segregations occurs as well preserved c.1.67 Ga old igneous domains that are surrounded by thin metamorphic rims only (too thin toanalyse, Fig. 1.4.3). Zircon in a nearby veined gneiss with fine-grained grey palaeosome anddistinct, light reddish, penetratively folded pre-kinematic leucosome is also dominated by c.1.67 Ga old igneous zircon but have broader rims of secondary formed zircon, one dated atabout 0.97 Ga. A third sample of intensely migmatised, reddish grey, fine- to medium-grainedgneiss with up to 10 cm wide, coarse-grained syn- to post-kinematic pegmatitic leucosomeveins that are both folded and locally discordant to the folded veined gneiss structures havelarge volumes of secondary formed zircon dated at about 0.97 Ga (Fig. 1.4.3). The externalzircon morphology is similar in all three samples but the proportion of young 0.97 Ga oldsecondary formed zircon varies widely and clearly increases with the volume of leucosome.The intimate correlation between the increase in the volume of leucosome and the volume ofsecondary formed zircon, and a simultaneous decrease in the volume of igneous zircon, showthat the formation of the secondary zircon is linked to the migmatisation of the gneissprotolith. This event is dated at 0.97 Ga. As the 0.97 Ga old secodary zircon occurs inleucosome in semi-discordant folded positions in relation to the E-W trending folds, it directlydates the folding and the associated amphibolite facies stretching and boudinage. Byimplication, it sets the age for one of the regional deformation phases in the southern EasternSegment; the phase that resulted in upright to southwards overturned folds with E-W toWNW-ESE trending, subhorizontal axes, and associated stretching along the same direction.The present data set does not rule out early c. 1.4 Ga old metamorphism recorded in otherareas of the southern Eastern Segment. However, it shows that the principal high-grademetamorphic melting of the gneiss protolith at Oxanäset is Sveconorwegian in age.

Guides: Jenny Andersson and Charlotte Möller (Geological Survey of Sweden).Literature: Möller et al. (2007)

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Day 2 Eclogites, high-P granulites and charnockites

IntroductionLate Sveconorwegian metamorphism in the southern Eastern Segment reached high-pressuregranulite and upper amphibolite facies conditions. The bedrock is dominated by migmatiticorthogneisses intercalated with different generations of metamafic rocks. Minor bodies ofcharnockites also occur throughout the granulite region (the Southwest Swedish GranuliteRegion, lower tectonic level of the southern Eastern Segment) but are more common in thewest. The metamafic rocks occur as amphibolites, garnet amphiboltes and high-pressuregranulites. P-T estimates from metabasic rocks within this area yield temperatures between680 and 770 °C and corresponding pressures of 9-12 kbar (Johansson et al. 1991; Wang &Lindh 1996; Möller 1998; 1999). Metamorphic zircon dates the high-grade event at 0.98-0.96Ga (Andersson et al. 1999; 2002; Söderlund et al. 2002; Möller et al. 2007 and referencestherein). In addition, a dismembered unit of 0.97 Ga old eclogite occurs interfolded with theEastern Segment gneisses (Möller 1998, 1999; Johansson et al. 2001). The eclogites havebeen overprinted by retrogressing high-pressure granulite and amphibolite faciesmetamorphism and deformation, but this distinct tectonometamorphic unit evidence c. 0.97Ga thrust-related displacements within the deep-seated basement of this part of theSveconorwegian orogen.

The scope of day 2 is to study metamorphic and tectonic features of the granulite and eclogitefacies rocks found in the lower tectonic level of the southern Eastern Segment. The first andthe last stops of the day (Stop 2.1 and 2.5) will high-light the two different types ofcharnockitic rocks that are rather common in the west. The first stop (Stop 2.1 at Söndrum)includes two different examples of metamorphic charnockites formed from dehydration of anorthogneiss protolith. The first outcrop exposes examples of incipient charnockitization indiscrete dehydration zones associated with pegmatite intrusions. Surrounding outcrops hostexamples of non-zonal small irregular occurences of patchy charnockite, typically less than1m wide. Examples of eclogite and high-P granulite facies metamorphism will be studied inmetabasic boudins within the Ullared Deformation Zone, an eclogite bearing tectonic nappeinternal to the Eastern Segment.

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Stop No 2.1: Charnockitisation and polyphase metamorphism in the Eastern Segmentof the southwest Swedish Gneiss Region. Incipient charnockitization in discretedehydration zones

LocationThe old quarry at Söndrum, Halmstad (N.B. Long stop, 2-3 hours), (UTM Zo 33 NH:363027/6280420).

IntroductionMetamorphic charnockites are known from several localities in the western part of the EasternSegment. They are typically spatially limited to patches or narrow elongated zones (decimetreto meter scale) some of which may be linked to pegmatites. The most prominent zonalcharnockite is located at a quarry in Söndrum, near Halmstad (Fig. 2.1.1). This locality hasbecome a key locality for studies of mechanisms behind charnockitisation in theSveconorwegian granulite region. The geochemistry and isotope geochemistry of thetransition zones between unaltered rock and charnockite have here been studied in detail. Thisdata have, among other things, been used to model the role of advective fluid flow anddiffusion during localised, solid-state dehydration (fluid in or fluid out?). The transition zoneat Söndrum between granitic gneiss and incipient charnockite have also been investigated indetail in order to model the effects on different geochronometers caused by charnockitisationwith special emphasis on the U-Pb-Th and REE of zircon.

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Fig. 2.1.1. Zonal charnockite at Söndrum.

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Description (Text based on Harlov et al. 2006)A localized dehydration zone, Söndrum stone quarry, Halmstad, SW Sweden, consists of acentral, 1 m wide granitic pegmatoid dyke, on either side of which extends a 2,5–3 m widedehydration zone (650–700°C; 800MPa; orthopyroxene–clinopyroxene–biotite–amphibole–garnet) overprinting a local migmatized granitic gneiss (amphibole–biotite–garnet). Whole-rock chemistry indicates that dehydration of the granitic gneiss was predominantlyisochemical. Exceptions include [Y + heavy rare earth elements (HREE)], Ba, Sr, and F,which are markedly depleted throughout the dehydration zone. Systematic trends in thesilicate and fluorapatite mineral chemistry across the dehydration zone include depletion inFe, (Y + HREE), Na, K, F, and Cl, and enrichment in Mg, Mn, Ca, and Ti. Fluid inclusionchemistry is similar in all three zones and indicates the presence of a fluid containing CO2,NaCl, and H2O components. Water activities in the dehydration zone average 0.36, or XH2O =0.25. All lines of evidence suggest that the formation of the dehydration zone was due toadvective transport of a CO2-rich fluid with a minor NaCl brine component originating from atectonic fracture. Fluid infiltration resulted in the localized partial breakdown of biotite andamphiboles to pyroxenes releasing Ti and Ca, which were partitioned into the remainingbiotite and amphibole, as well as uniform depletion in (Y + HREE), Ba, Sr, Cl, and F. Atsome later stage, H2O-rich fluids (H2O activity >0.8) gave rise to localized partial meltingand the probable injection of a granitic melt into the tectonic fracture, which resulted in thebiotite and amphibole recording a diffusion profile for F across the dehydration zone into thegranitic gneiss as well as a diffusion profile in Fe, Mn, and Mg for all Fe–Mg silicate mineralswithin 100 cm of the pegmatoid dyke.

Söndrum zirconology (From Rimsa, Johansson, Whitehouse: Contrib Mineral Petrol (2007) 154:357–369 "The original publication isavailable at www.springerlink.com")The incipient charnockite formation at Söndrum was a zircon-forming process. Thedehydration event (i.e. charnockitisation) is dated to 1397 ± 4 Ma (2σ, MSWD = 1.7). Internalstructure, chemical and isotopic characteristics of zircon indicate that the granitic pegmatite inthe core of the incipient charnockite is a melting zone. Commonly observed bulk rock HREEdepletion in incipient charnockites is not caused by zircon dissolution but by involvement ofgarnet as a reactant in the dehydration reactions. Moreover, REE patterns of the newly formedzircon are HREE enriched, indicating non-concurrent growth and suggesting that the degreeof charnockite depletion in HREE might be controlled by the volume of newly formed zircon.Based on the results of a combined SIMS U–Th–Pb and REE study integrated with thecathodoluminescence and back-scattered electron imaging of zircon from incipientcharnockite in Söndrum, SW Sweden, it was concluded that: 1. The effect of the incipient charnockite formation on morphology, mineral chemistry and

U–Th–Pb isotopic composition in pre-existing zircon is insignificant. This is supported by(a) the identical internal structures of protolith zircon across the charnockite–gneisstransition; and (b) age determinations for oscillatory zoned (1671 ± 4 Ma; 2σ, MSWD =0.64) and recrystallised (c. 1450 Ma, lower age limit) zircon which are identical withprotolith ages of rocks elsewhere in the ES and consistent with regional metamorphism at1450 Ma.

2. Recrystallisation of magmatic zircon resulted in blurred and broadened primaryoscillatory zoning that is unrelated to charnockite formation. Regional metamorphismassociated with zircon recrystallisation, took place at ca. 1.45 Ga based on complete to

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nearly complete resetting of the U–Th–Pb isotopic system. Zircon recrystallisation wasassociated with the expulsion of the large radii trivalent LREE and Th.

3. The central zone in the core of the dehydration zone is a zone of melting, which formedsimultaneously with charnockitisation. This may imply that incipient charnockite inSöndrum was formed by dehydration melting.

4. Newly formed zircon in the migmatitic gneiss allow determination of a precise andreliable age of incipient charnockite formation in Söndrum at 1397 ± 4 Ma (2σ, MSWD =1.7).

5. Depletion of the charnockite in HREE is caused by involvement of garnet in dehydrationreactions. Steep HREE patterns of the newly formed zircon indicates non-concurrentgrowth during dehydration reactions and mass balance calculations suggest that the degreeof HREE depletion in charnockite might be controlled by the volume of newly formedzircon.

Guide: Leif Johansson (Geobiosphere Science Centre, Lund University)Literature: Harlov et al. (2005), Rimsa et al. (2007).

Figure 2.1.2. Cathodoluminescence image (above) and simplified cartoonof the same image(below) of a zircon from the Söndrum charnockite. Abbreviations: A=recrystallised zircon,B=recrystallisation front, C=bleached OZ zircon, D=rim contemporaneous withrecrystallisation/recrystallised zircon, E=newly formed zircon. Figure taken from Rimsa et al.(2007).

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Stop No 2.2: Högabjär: Ion probe zircon dating of polymetamorphic “Hallandia” gneiss

Location Abandoned quarry of Högabjär (UTM Zo 33 NH: 372108/6292794).

IntroductionThe geological map pattern in the south-westernmost part of the Eastern Segment (alsoreflected in the airborne magnetic anomaly map pattern) is the result of polyphase ductiledeformation (Figs. 2.2.1 and 2.2.2). The rocks at Högabjär illustrate the character ofregionally consistent deformation structures. Detailed U-Pb-Th ion probe data of complexzircon in different rocks at this locality provide a direct age of migmatisation and upper andlower age brackets for two younger deformation phases. The results demonstrate that thesouthern Eastern Segment have experienced Hallandian orogenesis and migmatisation at c.1.42 Ga, prior to Sveconorwegian tectonometamorphism.

DescriptionHorizontal and vertical surfaces in the abandoned quarry expose structural relations inmigmatite gneiss in three dimensions. The migmatite gneiss is fine-grained with medium-grained, greyish red, granitic leucosome that makes up c. 25-40 % of the rock volume. Theveins are oriented subparallel to one another and define a layering (with local isoclinal foldhinges) with an overall strike along NNE and a steep dip towards WNW. Red, medium- tocoarse-grained metagranite dykes, up to 0.5 m wide, occur with low- to high-angle discordantrelations to the migmatitic layering, but have been folded together with the host gneiss. Onhorizontal surfaces the structures appear complex with highly irregular leucosome pods. Thestructural relations are best illustrated on south- and north-facing vertical surfaces, roughlyperpendicular to the axial surface of outcrop-scale folds. The folds are tight, upright toslightly overturned, with axial surfaces striking NNE and dipping around 60° to the ESE. Inplaces leucosome material is located along the axial-planar fold limbs, possibly developedduring the folding. Fold axes plunge 20-50° to the SSW. A linear deformation fabric, orientedparallel with the axial-plane of the folds, has developed in medium and coarse-grained rockdomains, i.e., in leucosome and folded granitic dykes. The fabric is a stretching lineation

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defined by elongated and recrystallised mineral aggregates. It is, however, not parallel to thefold axis, but plunges c. 70° towards ESE. Late, crosscutting and undeformed dykes occur at afew places. They are generally up to 20 cm wide, fine- to coarse-grained, pinkish and have anisotropic mineral fabric. They strike around NNW and dips steeply towards WSW.

Fig. 2.2.1. Aeromagnetic anomaly map of the Högabjär area, southern Eastern Segmentshowing the location of the Högabjär quarry. Scale: 3cm=8 km. Light blue line show outlineof coast line.

Five samples were selected for U-Pb-Th ion probe analysis of zircon. The results are given indetail in Möller et al. (2007), and summarised below (see also Fig. 2.2.3). Two samples ofmigmatite gneiss, mesosome HB-1 and leucosome HB-2, were investigated to connect aspecific zircon generation to the leucosome formation and thereby obtain a direct age of themigmatisation. Igneous zircon in migmatitic gneiss (mesosome and leucosome) dates thecrystallisation of the protolith at c. 1686±12 Ma. The rock is thus a strongly migmatised anddeformed variety of the 1.73-1.66 Ga felsic intrusions that dominate the Eastern Segment. Themigmatisation is dated at 1425±7 Ma by a secondary zircon generation formed in theleucosome. The orientation of this leucosome defines a layering, but field relations clearlyshow that multiphase deformation has modified the original character and orientation of thismigmatite structure. It is emphasised that, at this locality, the migmatite age is not a direct ageof deformation but a bracket for the various deformation structures. One sample, HB-3, ofdeformed metagranitic dyke, discordant to the leucosome in the host gneiss but foldedtogether with the gneiss, was investigated with the aim of obtaining an upper age bracket forthe upright folding along SSW-plunging axes and for the ESE-stretching. The youngestgeneration of igneous zircon in this dyke (two crystals) was dated at 1394±12 Ma, whichprovides an upper bracket for these deformation phases. The absolute age of the fold phase is,

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however, not known and it may be as young as Sveconorwegian in age. The geologic mapsuggests that this fold phase has oriented the gneissic and migmatitic layering into aregionally relatively consistent NNE-SSW strike. It also demonstrates that the origin of thelayering at Högabjär is pre-Sveconorwegian (unless the 1394±12 Ma zircons in the dyke arexenocrysts). It is probable that the layered structure originated in connection with the 1425±7Ma migmatisation and was later modified by post-1394±12 Ma deformation.

Fig. 2.2.2. Cutting from the geologic map by Larsson (1956) of the area north of Halmstad,illustrating the fold interference patterns and the location of Högabjär. Different varieties ofgneiss are shown in pink, brown and orange colours (pink and brown are graniticcompositions). Metabasites, mainly garnet amphibolite, are shown in dark green. Blackstreaks mark occurrences of metadolerite. Circles mark augen texture, red and wavy lensesmark migmatitic structure. Scale: 1 cm = 6 km.

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Fig. 2.2.3. Analytical data from zircon in samples from Högabjär. Error ellipses are plottedat 2σ level. A) Concordia diagram showing zircon analyses from Hallandia gneiss mesosome(HB1) and leucosome (HB2). Analyses of 1.67 Ga protolith zircon (mainly CL-bright, BSE-dark and oscillatory) are shown in blue ellipses and analyses of secondary (mainly CL-darkand BSE-bright) 1.44 Ga domains in red. Mixed analyses (artefacts) are marked brown.

The ages of two unmetamorphosed, undeformed and crosscutting granite-pegmatite dykes,HB-4 and HB-6, set lower brackets for these two ductile deformation phases at 952±7 Ma and946±8 Ma. Both generations of granitic dykes (1.40 and 0.95 Ga) are leucogranitic incomposition and are interpreted to represent late- to post-orogenic melts.

Literature: Möller et al. (2007)

Guide: Charlotte Möller (Geological Survey of Sweden).

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Stop No 2.3: Lilla Ammås: Decompressed Sveconorwegian eclogites

LocationLilla Ammås. (UTM Zo 33 NH: 362777/6337180). (N.B. Long stop, 2-3 hours, includingthree different localities in walking distance).

IntroductionRetrogressed, partly kyanite-bearing, eclogite (“e” in the aeromagnetic map, Fig. 6) occur asan up to 2 km wide, dismembered unit along the Ullared Deformation Zone, in the southernpart of the Eastern Segment. Probable eclogites (however kyanite-free) have been found alsofarther north close to the Mylonite Zone (Austin Hegardt et al. 2005). The presence ofeclogite provides evidence of a continent-continent collisional setting in this part of theGrenvillian-Sveconorwegian orogenic belt, at c. 0.97 Ga. The kyanite-eclogites are alsoevidence of significant tectonic displacements, internal to the Eastern Segment, between adistinct, tectonically bound, eclogite-rich gneiss unit and the surrounding high-P granulite andupper amphibolite facies crust.

DescriptionLilla Ammås is a c. 2 x 1.5 kilometres large layered and lens-shaped mafic body thatpreserves slightly different varieties of decompressed eclogite. It is heterogeneouslyretrogressed and deformed, and set in felsic and deformed, variably mylonitic, gneisses ofgranitic origin.

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Figure 2.3.1. Upper left: Field appearance of kyanite-bearing garnet pyroxenite. Upperright: Compositional zoning profiles of large garnet in kyanite-bearing garnet pyroxenites[Fe/(Fe+Mg) ratios and mol.% of components]. Rim at kyanite to rim at plagioclase. Lowerright: Backscattered electron image of reaction texture in sapphirine-bearing rock in theUllared Deformation Zone. Key: garnet (Grt), amphibole (Am), plagioclase (Pl), kyanite(Ky), sapphirine (Sa), corundum (Co), quartz (Qtz), orthopyroxene (Opx), late-stage sericite(Se). Lower left: Photomicrograph of former kyanite eclogite. Garnet (lower right).Clinopyroxene (upper and left) has exsolution-like plagioclase inclusions and is rimmed bysymplectitic orthopyroxene + plagioclase (+/- amphibole). Central part of the photo showsreplacements after kyanite: symplectitic sapphirine and corundum intergrown with anorthitic(mainly) plagioclase. Yellowish brown grains are rutile.

The most well preserved domains consist of coarse-grained kyanite-eclogite that has beenpartly recrystallised into high to intermediate pressure granulite facies assemblages. Garnet isgenerally only slightly resorbed and form up to 2 cm large grains. Clinopyroxene is generallycoarse-grained, pale green and carry abundant micro-scale blebs of expelled plagioclase(andesine). Close to kyanite, clinopyroxene grains are rimmed by orthopyroxene + andesinesymplectite. Coarse-grained blue kyanite is well preserved or variably replaced bysymplectitic intergrowths of anorthite + sapphirine and anorthite + corundum. The presenceof kyanite demonstrates that pressures were above the stability field of plagioclase-out (<15kbar at 700º C).

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Another, generally well preserved, lithology is quartz-rich eclogite without kyanite. Also inthese rocks, secondary assemblages formed during high-T decompression and include coronasof secondary clinopyroxene and plagioclase between garnet and quartz. P-T estimatescalculated for these secondary granulite facies assemblages together with the reequilibratedgarnet rim yielded estimates around 760º C and 10.5 kbar. Certain layers, up to 2 dm thick,are coarse-grained quartz- rich rocks with kyanite, garnet and rutile. In these three rock types,2-4 mm garnet grains show a distinct prograde growth zoning (Fig. 2.3.1), including rimwardsdecreasing spessartine and Fe/(Fe+Mg)-ratios, and a corresponding increase in pyrope(maximum increasing from 25 to 49 mole-%). Grossular contents are essentially uniformexcept at the rim (250 microns or less) where it drops. The preservation of growth zoningimplies a short residence time at high temperatures. Up to 2 dm thick, fine-grained, dark-redgarnetite layers (with quartz and rutile) occur sparsely. U-Pb ion probe dating was carried outon zircon inclusions in garnet from quartz-bearing eclogite. This zircon is homogeneous, hasrelatively low contents of U and Pb, and yielded an age of 972 ± 14 Ma. In the three differenteclogite varieties described above, zircon was found as a common inclusion in clinopyroxeneas well as in well-preserved garnet and kyanite. Since garnet has prograde growth zoning thezircon age is regarded as the maximum age for eclogite metamorphism and is interpreted asdating the prograde metamorphism. U-Pb TIMS dating was performed on titanite inclusionsfrom the same rock. The age of titanite is 945 ± 4 Ma; it is interpreted to have beenisotopically reset and date cooling.

Guides: Charlotte Möller (Geological Survey of Sweden), Leif Johansson & Ulf Söderlund(Geobiosphere Science Centre, Lund University).Literature: Johansson et al. (1991), Möller et al. (1997), Möller (1998; 1999), Johansson et al.(2001).

Stop No 2.4: Buskabygd: High-grade tectonites in the Ullared Deformation Zone

LocationBuskabygd (UTM Zo 33 NH: 365811/6334218)

IntroductionSee stop 2.3

DescriptionOutcrops at the pond expose former coarse-grained eclogite, deformed and recrystallised intogranulite facies gneiss, locally mylonitic, and intercalated with felsic gneiss. Previouslycoarse-grained and kyanite-rich domains in the eclogite have been recrystallised into lightbluish domains consisting of sapphirine and plagioclase symplectite (Fig. 2.3.1). Thetectonites demonstrate that the eclogites were emplaced into high-intermediate pressuregranulite facies crust.

Guides: Charlotte Möller (Geological Survey of Sweden) and Leif Johansson (GeobiosphereScience Centre, Lund University).Literature: Möller (1999)

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Stop No 2.5: On the occurrence of 1.4 Ga old charnockites in the Southwest SwedishGranulite Region; igneous or metamorphic charnockitisation - or both?

LocationGetterön, Varberg (UTM Zo 33 NH: 331138/6333507)

IntroductionThe Varberg Granite-Charnockite plutonic suite contains the largest occurrence ofcharnockite in southern Sweden. It is composed of granite (the Torpa granite) and severaltexturally different varieties of charnockite. The most coarse-grained type, the Trönningenäscharnockite, is a dark greenish-brown rock with orthoclase, plagioclase, quartz, hornblende,clinopyroxene, orthopyroxene and garnet. Orthopyroxene and garnet occur mainly as rimsbetween Fe-Mg-minerals and plagioclase. Some orthopyroxene form thin exsolution lamellasin the clinopyroxene. The same mineralogy is found in more fine-grained varieties of thecharnockite. The coarse-grained type typically occurs as layers, irregular meter size bodies oras disrupted schlieren in the fine-grained charnockite (cf. Fig. 2.5.1). Single, 2-3 cm sizedorthoclase crystals often occur in the finegrained matrix. In a few places very fine grained,aplitic, charnockite form small (< 1 m wide) dykes in the coarser Trönningenäs charnockite.This intimate relationship between different varieties of charnockite shows that they allbelong to the same magma.

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Fig. 2.5.1. Intermingled coarse- and fine-grained charnockite in the Varberg Granite-charnockite plutonic suite. Photo (Leif Johansson) from south of the Varberg castle.

DescriptionThe contact between the Torpa granite and enclosed charnockite is not exposed but at leasttwo large (km size) coarse-grained charnockite bodies form inliers in the Torpa granite. Oneof the coarse-grained charnockitic inliers has been dated at 1.38 Ga (U-Pb zircon by Åhäll etal 1997). This age dates crystallisation of the magmatic mineral assemblage. A deformedgarnet – cpx rich variety from south of the Varberg Castle was dated at about 0.89 Ga by Sm-Nd on mineral separates (Johansson & Kullerud 1993). The obtained age is now, in the lightof 40Ar-39Ar ages of hornblende from the region regarded as too young. On the other hand itclearly suggest a high-grade Sveconorwegian metamorphic event affected the Varbergcharnockite. This is further substantiated by thin Sveconorwegian rims on zircon (Johanssonunpublished results) and thick rims on zircons in migmatitic varieties of the Torpa granite(Andersson et al. 2002).

Rocks of the Varberg Granite-Charnockite plutonic suite and related meta-intrusives (theTjärnesjö and Källsjö metaintrusions) all have bulk geochemical compositions characteristicfor meta-aluminous, alkali-calcic intrusions and generally occur as monzonite, quartz-monzonite or granite. The granitic compositions are typically low-silica granites (<70wt%SiO2) and true granitic compositions are rare among the Tjärnesjö and Källsjö suites. Thegeochemistry of the Varberg, Tjärnesjö and Källsjö metaintrusive suites suggests that theserocks formed from fairly dry magmas at relatively deep crustal levels. Field evidence ofmingling and hybridization between monzonitic and granitic members of these intrusions and

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the generally straight trend lines defined by major, trace and RE-elements plotted against SiO2is interpreted in favour of that magma mixing was an important mechamism for the formationof these rocks. Some members of the granite-monzonite associations contain relict igneouspyroxene. True granitic compositions of the Tjärnesjö and Källsjö metaintrusion are,however, as a rule pyroxene-free. Clinopyroxene is common in more intermediatecompositions of these rocks while ortopyroxene is extremely uncommon in all varieties. Sincethe Torpa, Tjärnesjö and Källsjö intrusions all have identical bulk geochemical properties,their preset different mineralogical composition is likely to reflect differences inSveconorwegian metamorphic recrystallisation (suggesting different metamorphic conditionsin the western and eastern crustal levels).

In the Varberg Granite-Charnockite plutonic suite deformation and retrogression of the coarsegrained charnockite lead to the formation of augen granite and ultimately to finely bandedgneisses without any preserved identifiable fabrics of the original charnockite. Duringprogressive deformation, clinopyroxene, orthopyroxene, garnet and orthoclase were replacedby amphibole, mica, plagioclase and microcline. The breakdown of single orthoclase crystalto fine grained mosaics of tiny microcline grains started along the margins of, and fractures in,the orthoclase megacrysts. This process was essentially static but appears to have beenfacilitated by deformation. The reduction of the grain size softened the matrix and allowedremnant orthoclase crystals to rotate rather than to be fragmented. This would explain why itis possible to find rounded orthoclase crystals also in extremely deformed gneisses that oncewas coarse-grained charnockite.

Guides: Leif Johansson (Geobiosphere Science Centre, Lund University), David Cornell,(Earth Sciences Centre, Göteborg university).

Day 3 Terrane boundaries and tectonic build up of the SveconorwegianOrogen

IntroductionThe Sveconorwegian orogen is divided into five distinct lithotectonic units by prominent lateSveconorwegian deformation zones. These deformation zones hosted continental scaleSveconorwegian tectonic movements and played a central role for juxtaposition and finaltectonic adjustments of crustal blocks in late Sveconorwegian time. Post-tectonic “stitching”magmatism across the terrane boundaries at 0.93-0.92 Ga, west of the Mylonite Zone, set aminimum age for the Sveconorwegian assembly of the principal gneiss belts (Schärer et al.,1996; Andersen, 1997; Eliasson and Schöberg, 1991). None of the deformation zones havethe explicit character of a continental suture zone; hosting crust of indisputable differentcontinental provenance (in this case Fennoscandian and non-Fennoscandian continental crust)or occurrences of ophiolitic rocks. Consequently, none of these deformation zones qualifies asan exotic terrane boundary. All of the large scale Sveconorwegian deformation zones,however, separate crustal blocks that have different lithological, metamorphic and tectonichistories, in most cases this is valid for both the Sveconorwegian and the pre-Sveconorwegianevolution. The prominent Sveconorwegian deformation zones that separate the principallithotectonic units thus all qualify as metamorphic, structural and lithological terraneboundaries.

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Fig 3.1.1. Simplified geological map of the Göteborg–Varberg–Skene region (modified afterAndersson et al. 2002). Major deformation zones: MZ, Mylonite Zone; UDZ, UllaredDeformation Zone; GÄZ, Göta Älv Zone.

Available data on the timing and character of Sveconorwegian tectonothermal events testifiesto a complex diachronous orogenic evolution (See Geological setting and Figs. 3 and 4above). The contrasts in timing and character for igneous and tectonothermal events olderthan about 0.93 Ga imply that both small- and large-scale displacements took place betweenand within the major gneiss belts. Research on the evolution and architecture of the lateSveconorwegian tectonic framework is fundamental for correlation of earlier igneous andtectonothermal events between the Sveconorwegian gneiss belts and the pre-SveconorwegianFennoscandian craton. For example, what were the relations between crustal units west of theMylonite Zone and the proto-Fennoscandian continent prior to the Sveconorwegian orogeny?The scope of day 3 and 4 is to study the boundary between the parautochthonous EasternSegment and overlying allochthonous lithotectonic units in the west (the Mylonite Zone, stop3.1) and to look at the structural, metamorphic and litological characteristics of theSveconorwegian and pre-Sveconorwegian evolution of crustal domains within the Idefjordenterrane.

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Stop No 3.1: The Mylonite Zone: a major Sveconorwegian structural, metamorphic andlithological terrane boundary in the Fennoscandian Shield

LocationGrässkär, Lerhuvudet, Årnäshalvön. (UTM Zo 33 NH: 328632/6342572). The southernmostsection of the Mylonite Zone exposed along the shoreline of the Klosterfjord.

Introduction(Text in part modified from Andersson et al., 2002)The Mylonite Zone defines a principal lithological terrane boundary between 1.81-1.66 Gaorthogneisses in the Eastern Segment and Gothian 1.66-1.53 Ga calc-alkaline meta-supracrustals and orthogneisses in overlying allochthonous lithotectonic units. The southernsection of the Mylonite Zone also defines a conspicous Sveconorwegian metamorphic terraneboundary between high-pressure granulite and upper amphibolite facies rocks in theunderlying Eastern Segment and rocks in the greenschist to amphibolite facies west thereof.The southern section of the Mylonite Zone is a branched shear zone system composed ofseveral individual ductile deformation zones (hosting proto-mylonites, mylonitic gneisses andin places true mylonites). The main lithological contact between the Eastern Segment andoverlying units is often drawn along the lithological boundary between safely identified rocksunits, such as the 1.4 Ga Torpa metagranite and the 1.7 Ga Skene gneiss in the EasternSegment, and the 1.66 Ga Horred metasupracrustal belt, the 1.59 Ga Bua gneiss and the 1.30Ga Veddige augen gneiss in the west. This lithologically bounded outline also coincides witha conspicuous metamorphic break between high-pressure granulite facies mafic rocksenclosed in the Torpa metagranite and Skene gneiss, and titanite-epidote amphibolites, inwhich garnet is rare or absent, in the Horred belt, the Bua gneiss and the Veddige augengneiss. The difference in crustal depths requires substantial vertical displacement in lateSveconorwegian time. The lithological break also suggests considerable lateral displacements,the magnitude of which is not known.

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Fig. 3.1.3. Complex zircon from orthogneisses in the southern section of the Mylonite Zone.Ages refer to ion-microprobbe U-Pb analysis with location of dated spot indicated. Skenegneiss and migmatised Torpa granite were sampled in the eastern Mylonite Zone (EasternSegment). The Bua gneiss was sampled just north of the Klosterfjord, in the Idefjordenterrane, west of the Mylonite Zone. Figure from Andersson et al. (2002).

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Penetrative gently undulating, west dipping gneissic layering is typical for deformationstructures along the southernmost section of the Mylonite Zone. Rocks within this part of theMylonite Zone are mainly veined and banded mylonitic gneisses. Truly mylonitic rocks arefound in places. The gneissic structures are in places overprinted by discrete shears with atop-to-the-west sense of movement (Berglund 1997). These late brittle-ductile deformationstructures were developed during retrogression to the greenschist facies, with chlorite, epidoteand other sheet silicates, and are interpreted to represent late orogenic extensionalmovements. Syn- to post-kinematic (late-Sveconorwegian?) pegmatites belong to thestructurally youngest generation of Precambrian rocks of the Sveconorwegian orogen. In thesouthern section of the Mylonite Zone, the late Sveconorwegian pegmatites occur as thinsheets parallel to the foliation planes, or as less than a metre to several metres wide, syn-kinematic dykes, that in places crosscut the regional deformation fabrics. Pegmatite dykes andsheets in the central parts of the Mylonite Zone are as a rule deformed, which supports aprolonged tectonic activity in the zone that post-dates regional deformation in the surroundingcrustal segments.

Geochronology of complex zircons in migmatised and banded orthogneisses along thesouthern Mylonite Zone have been used to obtain a maximum age for the partial melting andassociated penetrative ductile deformation in the zone (Andersson et al. 2002). Themorphology and high modal abundance of secondary zircon (25-50% of the total volume ofzircon), the absence of early- or pre-Sveconorwegian secondary zircon, and field relationsprovide evidence for that anatexis and associated penetrative ductile deformation in thesouthern Mylonite Zone took place at or after 970 Ma. A 920 Ma age for syn-tectonic titanitein the southernmost section of the Mylonite Zone is the youngest titanite age obtained so farin southwestern Sweden, and was interpreted to date late ductile extensional movements(Johansson & Johansson 1993). Young Sveconorwegian ages within the southern part of theMylonite Zone have also been obtained by 40Ar-39Ar dating of hornblende that dates coolingthrough 500°C at approximately 915 Ma (Page et al. 1996).

DescriptionDuring the walk along the Gräskär we will see coastal exposures with gradually increasingdeformation that transform a coarse grained variety of the Torpa granite into folded bandedgneisses. Within the gneisses there are lenses of high-pressure mafic granulites with preservedigneous compositional layering (similar to that we saw at Söndrum stop 2.1, Fig. 3.1.2). Thedeformation is strikingly inhomogeneous and also lenses of relatively well preserved Torpagranites occur embedded in the banded gneisses. Farther east at Lerhuvud, on the southernshore of Klosterfjorden, we enter a sequence of banded gneisses that are isoclinally folded andrefolded (Fig. 3.1.2). This gneiss unit is composed of both felsic and mafic layers and clearlydifferent from the banded gneisses formed by deformation of the Torpa granite. Within thegneisses there are small lenses of garnet amphibolites in more or less retrograded states withbeautiful pseudomorphs after garnet.What to see and perhaps points of discussion:- Deformation and retrogression of charnockite to gneiss and lenses of preserved mafic

granulites in foliated and mylonitized gneisses in the underlying Eastern Segment- U-Pb titanite and 40Ar-39Ar dating of late-Sveconorwegian retrogression and deformation

in the Mylonite Zone- Tracing protoliths to the mylonitic gneisses along the Mylonite Zone. - What is the tectonic role of the Mylonite Zone- Geochronology of migmatite gneisses along the Mylonite Zone – what are we dating?

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Fig. 3.1.2. Field photos from the Årnäs peninsula of rocks within the southern section of theMylonite Zone are exposed (photo Leif Johansson). Upper: Boudin of mafic granuliteenclosed in gnessic variety of the 1.4 Ga old Torpa granite. Lower: Stromatic migmatite withmafic intercalations from the southern shore of the Klosterfjord typical.

Guides: Leif Johansson (Geobiosphere Science Centre, Lund University), Charlotte Möllerand Jenny Andersson (Geological Survey of Sweden).Literature: Johansson & Johansson (1993), Page et al. (1996), Andersson et al. (2002)

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Fig. 3.1.4. U–Pb concordia and Th vs. U diagrams of ion-microprobe (NORDSIM) zircondata from orthogneisses in the Mylonite Zone. (A-D) Eastern Mylonite Zone (EasternSegment) Stromatic Skene gneiss (A and B) and Migmatised Torpa granite (C and D).Western Mylonite Zone (Idefjorden terrane) Stromatic Bua gneiss (E and F). Key: Filledsquares, igneous protolith zircon cores. Open symbols, newly formed and recrystallisedzircon: diamonds, euhedral prismatic crystals; squares, core domains in anhedral crystals;circles, anhedral round/oblate crystals; triangles, overgrowths (rims). Figure from Anderssonet al. (2002).

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Stop No 3.2: Age and emplacement conditions of the Chalmers Metagabbro

LocationChalmers University, Gothenburg. (UTM Zo 33 NH: 319629/6398264).

IntroductionMetamorphosed mafic rocks may often lack igneous minerals suitable for precise dating ofthe igneous emplacement. The metamorphic re-equillibration and/or recrystallisation disturbor completely reset the isotopic system of the mafic mineralogy and ages obtained thereforerather date the timing for a complete or partial metamorphic resetting. Mafic rocks arenevertheless important key rocks for unravelling the evolution of crust. Detailed mapping andpetrographical studies may, however, be used to find rocks that are co-eval with the maficintrusion and that contain minerals suitable for dating igneous emplacement such as zircon- orbaddeleyite-bearing rocks.

The Chalmers locality in Gothenburg is an example of how zircon-bearing granitic contact-metamorphic melts that back-intruded a zircon- and baddelyite-free meta-gabbro may be usedto directly date igenous emaplcement of the gabbro. The Chalmers metagabbro also belongsto the quite recently discovered sequence of about 1.3 Ga pre-Sveconorwegian intrusionspresent in the Idefjorden terrane of the Sveconorwegian orogen. The locality records insightsin the pre-Sveconorwegian history of crustal blocks west of the Mylonite Zone.

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Fig. 3.2.1. Map of Chalmers cmpus, outcrops, and tram line tunnel. (E. Bdg)=show thelocation of the Electronics buildning, where the investigated cutting shown in figure 3.2.2 wasseen.

Fig 3.2.2. A smoothly cut rock face during construction at Chalmers campus 1998, showingdark gabbro (A,C) intruded by light granitic dyke (B), diapir (D) and veins (E). The xenolithof gabbro (A) in the granitic dyke (B) has in turn sent out mushroom-like fingers into thegranite, showing that both rock types were liquid at the same time. The ladder is 0.5m wide.Photo by A. Scherstén, in Kiel et al. (2003).

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DescriptionThe mafic intrusion underlying Chalmers Technical University in Gothenburg was exposedduring construction for the new electronics building, and in new tram tunnels beneath thecampus. Diapiric melts of the country rock were observed in the gabbro as shown in Fig. 3.2.2and are interpreted as contact-metamorphic melts of ~1.6 Ga country rock gneiss which back-intruded the mafic magma. Metasomatising fluids from these diapirs triggered garnet growthand hornblende alteration at a late stage of magmatism. Thermobarometric calculations showequilibrium conditions of 800°C and 10kbar, reflecting an emplacement depth of 30 km.Xenolithic zircons in the diapirs were first partly resorbed, then experienced new growth atthe time of the intrusion. Zircon cores and rims both give concordant ion probe ages of 1332 ±7 Ma, the age of the intrusion, at which time xenocrystic zircon cores were totally reset due totemperatures above 900ºC. Titanites give an age of 988 ± 16 Ma reflecting Sveconorwegianregional metamorphism. The Chalmers intrusion was emplaced in a tectonic setting at 1.3 Gawhich was either anorogenic or a pre-Sveconorwegian rift environment. Subsequentamphibolite grade Sveconorwegian metamorphism largely reset the U-Pb system in titanitesbut did not cause either re-equilibration of the thermobarometric minerals or lead-loss inzircon.

Guide: David Cornell (Earth Sciences Centre, Göteborg University)Literature: Kiel et al. (2003)

Stop No 3.3: Migmatisation in Stora Le Marsstrand graywackes driven by maficintrusions. Composite dyke development and the origin of calc-alkaline magma seriesby back-veining and assimilation. Archaean and Early Proterozoic zircon xenocrystsin Mesoproterozoic crust.

LocationVrångö, south of fishing harbour. Göteborg southern Archipelago, reached by ferry. (UTM Zo33 NH: 307727/6385998)

IntroductionThe structurally and isotopically oldest rocks of the Sveconorwegian orogen, west of theMylonite Zone, are c. 1.66-1.59 Ga metasupracrustal rocks of the Åmål-Horred volcanic belts,and meta-grey wackes and meta-volcanic rocks of the Stora Le Marsstrand supracrustal belt.Excellent exposure along the coastal shore lines of the archipelago offers sites to study thepre-Sveconorwegian evolution of the alloctonous crustal blocks west of the Mylonte Zone. Atthe area of Vrångö in the southern Göteborg archipelago are exposures of calc-alkaline ignousrocks formed by melting of grey wackes of the Stora Le Marsstrand supracrustal suite. Theseexposures offers a clue to investigate the source of ancient crustal components and thereby tomodel the tectonic setting of Mesoproterozoic supracrustal units in the allochthonousIdefjorden terrane of the Sveconorwegian orogen.

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Close up map of the town of Gothenburg and surroundings (map from www.eniro.se) showingthe locality of Vrångö in the southern Archipelago of Gothenburg in relation to theSaltholmen ferry port.

DescriptionIn the whole southern archipelago (skärgård), typified by this well-exposed locality atVrångö, metagraywackes of the >1590 Ma Stora Le- Marstrand Formation are 'granitised' andretain little of their original structure apart from some colour-banding. Thick layers andboudins of banded amphibolite with minor calc-silicate bands in the sequence probablyoriginated as pillow lavas. An intrusive gabbro melted and mixed with the metagraywacke toform hybrid magmatic rocks and bimodal dykes and different stages of this process can beseen (Figs. 3.3.3-3.3.5).

http://www.eniro.se/

Fig. 3.3.1. Detailed geologial map of the locality at Vrångö by Jessica Hult.

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Fig. 3.3.2. Arial photo showing the locality at Vrångö in relation to the ferry landing

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Fig. 3.3.3 In the strain shadow of abanded amphibolite bouding (left),the metagraywacke (right) forms agranitic melt (centre).

Fig. 3.3.4. A molten gabbro dykewas first intruded by low-viscositygranite veins in a brittle manner,then the still-molten but higher-viscosity gabbro fragments beganto deform, resulting in thisbimodal breccia dyke.

Fig. 3.3.5. In a bimodal dyke, axenolith of banded amphiboliteis contained within bothgranitic and gabbro magma,proving that both magmas wereliquid at the same time.

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Thick 1550 Ma metamorphic rims developed on detrital zircons which became xenocrysts inthe hybrid magma, see Fig. 3.3.6. Some of the xenocrysts retained their ages up to 1872 Ma,but others experienced major lead loss at the time of hybridization. The rocks were allrecrystallised at amphibolite grade during a 1030 Ma Sveconorwegian metamorphic event,but this is not recorded by zircon at Vrångö. Titanite U-Pb and Sm-Nd dates in garnet at otherlocalities in the Western Segment reflect this event.

Zircons up to 2000 Ma old have been found in the Stora Le Marstrand Formation at localitiesnorth of Vrångö, which is one of the southernmost outcrops. Together with a 3.4 Ga zirconfound in a 915 Ma dyke at Älgön, these show that the graywackes were not derived entirelyfrom the ~1600 Ma granitoids of the Sveconorwegian Western Segment. An older crustal source to the west is envisaged.

Literature: Åhäll et al. (1998), Cornell et al. (2000; 2001), Åhäll & Connelly (2008)

Fig. 3.3.6. Electron microscope images of zircons from the hybrid rock at Vrångö showing theion probe dates on cores (interpreted as detrital grains) and rims, which grew in the hybridmelt.

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Day 4 Terrane boundaries and tectonic build up of the SveconorwegianOrogen (continued)

Stop No 4.1: The fate of zircon in crustal processes: ion probe U-Pb-Th (SIMS) andICP-MS REE and U-Th analyses guided by Cathodoluminescence imaging.

LocationStora Lundby (UTM Zo 33 NH: 339396/6412211).

IntroductionApplication of high-spatial resolution analytical techniques to identify and date complexgrowth zoning in zircon has proven a powerful tool to date events of igneous andmetamorphic crystallisation. The geological significance of an age obatined from a zircon rimor a core is, however, often not easilly interpreted. One key question is under whatcircumstances existing zircon may act as an open system. At the Stora Lundby locality,complex zircon in a migmatitic gneiss have been used to date the igneous and metamorphichistory of rocks in this parts of the Idefjorden terrane. The data was also used to investigate ifdissolution processes affected pre-existing zircon.

DescriptionThe Stora Lundby gneiss is a stromatic migmatite gneiss (Swedish coordinates, RT90:129150/641635), located some 4 km west of the Mylonite Zone, and belongs to the ÅmålSuite (Samuelsson 1978) (Fig. 4.1.1). The Stora Lundby gneiss has a strong NNW trendingfoliation diping 45˚ to the west and corresponds to the foliation in the Mylonite Zone.Leucosomes are concordant with foliation (i.e. stromatic), but sometimes isoclinally foldedwith axial planes corresponding to the foliation. The mesosome (foliated rock hostingleucosomes) is a metaluminous granodiorite with biotite as the main mafic mineral.Petrographic data suggests that the temperature just exceeded that required for the breakdownof muscovite to water-rich melt plus sillimanite at about 680ºC above 0.4 GPa pressure.

Zircon U–Pb cores in the mesosome (Fig. 4.1.2) are dated at 1605± 10 Ma, reflecting originof the protolith during a major 1.61 -1.59 Ga crust-formation episode. During a firstmetamorphism rims formed by dissolution and recrystallisation of existing zircon (grains 41,60 & 14, Fig. 4) and give a poorly constrained Sveconorwegian date of 1010 ± 50 Ma,probably reflecting the 1030 Ma event prevalent in the Western Segment. A secondgeneration of Sveconorwegian zircon, dated at 917 ± 13 Ma, grew as new needle shapedgrains (Grain 11, Fig. 4.1.2) in the leucosomes which are regarded as locally derived meltsaugmented from below. These grains show positive Ce anomalies characteristic of magmaticzircon, shown in Fig. 4.1.3. This late migmatisation and injection veining of the Stora Lundbygneiss is the same age as the Bohus Granite and Hakefjorden norite intrusion on Älgön alongthe northwest coast. Locally it may be related to normal fault movements along the MyloniteZone and the exhumation of the Eastern Segment. Considering the regional picture, this datashows that the Western Segment experienced at least two Sveconorwegian deformationevents, at 1030 and 920 Ma.

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Fig. 4.1.1 Road map to Stora Lundby with inset showing detailed geological map from(Scherstén et al. 2004). Arrow in marked box in road map indicates location of samplespot(road cut).

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Fig. 4.1.2. Cathodoluminescence (CL) and backscattered electron (BS) images of selectedzircon grains from mesosome and leucosome. Three ion probe spots are visible in the BSimage of grain 134, and a laser crater in the core of grain 60, the large rim of which waspartly destroyed by the laser. For grain 11, which is essentially CL-dark with a small CL-bright apatite inclusion, the contrast was set very high.

Figure 4.1.3. Chondrite normalised REE-profiles for mesosome grain 60a (metamorphicrim), 60b (magmatic core), leucosome grains 13a (needle-like grain,), and 103b (rim onxenocryst). Ce anomalies characterise magmatic zircon whereas metamorphic zircons do notshow them.

Guide: David Cornell, (Earth Sciences Centre, Göteborg University)Literature: Scherstén et al. (2004)

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Stop No 4.2: U-Pb, Sm-Nd, Lu-Hf geochronology of Mesoproterozoic mafic intrusionsin the Sveconorwegian Province.

LocationGaddesanda (UTM Zo 33 NH: 353365/6475045).

Introduction A suite of meta-mafic dykes occurs in c. 1.6 Ga migmatitic orthogneisses in the easternmostpart of the Idefjorden Terrane, immediately south of Lake Vänern. The imprint of granulitefacies metamorphism of the dykes was discovered only a few years ago during bedrockmapping of the Geological Survey of Sweden (SGU). Garnet porphyroblasts are common inthe gneisses and especially in rocks of intermediate to mafic compositions. Contacts betweendykes and gneisses are rarely exposed but regional mapping indicates a NNE-SSW trendingdyke direction, largely parallel with the structural grain in the area. The exact spatial extent ofthe mafic granulite dykes is not known, but mafic granulites have been observed some tens ofkm to the south. Still further south, rocks between the GÄZ and the Mylonite Zone are oflower metamorphic grade and in large parts metamorphosed at epidote-amphibolite togreenschist facies conditions only.

Little is known about the extent and character of the high-pressure metamorphism in theIdefjorden terrane. U-Pb, Sm-Nd and Lu-Hf geochronology and thermobarometry wereintegrated and applied to two granulite facies diabase dykes in the eastern Idefjorden terraneto obtain data on the timing for igneous emplacement and metamorphism. The initial Hfisotopic composition of secondary zircon was compared with that of the bulk sample, back-projected from the measured value through time to enhance the interpretation of theradiometric ages for the metamorphic mineral assemblages. The obtained data was used toconstrain the timing for dyke emplacement, now dated at about 1.3 Ga. This suggests that thedykes belong to the Kungsbacka bimodal suite located along the Göta Älv Zone (AustinHegardt et al. 2007). Metamorphism of the dykes was dated at about 1.05 Ga, which is about70-80 Ma older than the age for high-pressure metamorphism in the underlying EasternSegment. The data provide further evidence for a complex diachronous Sveconorwegianorogenic evolution.

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Fig. 4.2.1. Simplified map of the geology at around the Göta Älv Zone. From Söderlund et al.(2008a).

DescriptionTwo dykes located c. 5 km NNE of the NNE-trending Göta Älv Zone (the Lunden andHaregården dykes) have been variably affected by high-grade metamorphism. Söderlund et al.(2005) reported near-chondritic initial Nd (0 to -2) and Hf (+1) epsilon values for these dykes.U-Pb isotope analyses of baddeleyite grains in the Lunden dyke indicate and emplacementage of the dykes at ~1300 Ma. Thermobarometry of garnet and omphacitic clinopyroxenecoronas indicates high-pressure metamorphism at c. 15 kbar and c. 740 °C for the Lundendyke. Growth of polycrystalline zircon at the expense of baddeleyite took place at 1046 ± 4Ma. Identical Hf isotope composition of polycrystalline zircon and baddeleyite shows that thebaddeleyite-to-zircon transition took place before Hf equilibration between the othermetamorphic minerals and, hence the c. 1046 Ma age of polycrystalline zircon sets an upperage limit for the high-grade metamorphic event. The Haregården dyke is recrystallised into anequilibrated, granoblastic hornblende granulite-facies assemblage. The estimated P-Tconditions are c. 10 kbar and c. 700 °C. In contrast to the Lunden metadiabase, the Hf isotopecomposition of secondary zircon grains in this sample indicates growth of zircon at a timewhen Hf isotopic equilibrium between minerals was reached. Indistinguishable Lu-Hf andSm-Nd mineral isochron ages of 1027 ± 8 Ma and 1022 ± 28 Ma are interpreted to date thehigh-pressure granulite event.

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Fig. 4.2.2. Backscatter electron images of the Lunden and Haregården dolerites. FromSöderlund et al. (2008a).

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Fig. 4.2.3. Backscatter electron images of the Lunden and Haregården dolerites. FromSöderlund et al. (2008a).

Points of discussion:- P-T-t evolution of the Idefjorden terrane- How to use Hf isotopes for the recognition of discrete stages along the P-T path- Differences in age of metamorphism in different crustal domains in the Idefjorden terrane-

what do they mean?

Guide: Ulf Söderlund (Geobiosphere Science Centre, Lund University)Literature: Austin Hegardt et al. (2007), Söderlund et al. (2005; 2008a)

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Fig. 4.2.4. Right. U-Pb concordia diagram with age results of baddeleyite and polycrystallinezircon in the Lunden dyke. From Söderlund et al. (2008a).

Fig. 4.2.5. Lu-Hf isochron diagram of high-grade minerals in the Haregården dyke. FromSöderlund et al. (2008a).

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Fig. 4.2.6. Diagram showing identical 176Hf/177Hf of baddeleyite and polycrystalline zircon.Note the much lower Hf isotope ratios relative to the whole rock composition and that age.From Söderlund et al. (2008a).

Fig. 4.2.7. P-T diagram showing a possible metamorphic evolution of rocks in the area. FromSöderlund et al. (2008a).

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Stops 4.3 and 4.4: The Idefjorden terrane west of the Oslo Rift

IntroductionThe Idefjorden terrane extends to the western side of the Oslo Paleorift, where it has beencalled Begna sector (Fig. 4.3.1; Bingen et al. 2001; Åhäll and Connelly 2008). West of theOslo rift, the Idefjorden terrane is limited against the Telemarkia terrane by the Åmot-Vardefjell shear zone, a conspicuous NW trending, SW dipping, banded gneiss sequence. TheIdefjorden terrane includes the Veme complex, which is the object of the two proposed stops(Stops 4.3. and 4.4.). The Veme complex is made up of Mesoproterozoic greywacke-dominated metasediments associated with metaplutonic rocks (Bingen et al., 2001;Nordgulen, 1999). It resembles the Stora Le-Marstrand formation exposed to the east of theOslo rift (Brewer et al. 1998). It is intruded by the voluminous late-Sveconorwegian 928 ±3Ma Flå granite (Bingen et al. 2008a; b).

Fig. 4.3.1. Simplified geological map following Nordgulen (1999), showing the location of theFollum metatonalite pluton and the two proposed excursion stops at Hensmoen and Veme inthe Veme complex.

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The Veme complex is metamorphosed in amphibolite-facies conditions. It shhows adominantly SW dipping SW-SE trending attitude parallel to the Åmot-Vardefjell shear zone.This general structural grain is parallel to large scale Sveconorwegian structures, and isconsequently regarded Sveconorwegian in age (Park et al., 1991). The apparent stain in theVeme complex is variable. The northeastern limit of the complex against the Randsfjordcomplex is mapped as a shear zone, while the central part of the complex locally includesrock packages showing little apparent deformation.

One of the largest mappable metapluton hosted in the Veme complex is the Follummetatonalite. It yields a U-Pb zircon intrusion age of 1555 ±3 Ma (ID-TIMS data, Bingen etal. 2005), and can be considered part of the widespread 1.58-1.52 Ga Hisingen suite definedin the Stora Le-Marstrand formation in the Idefjorden terrane east of the Oslo rift (Åhäll andConnelly 2008). This pluton forms an elongate foliated body parallel to regional structures inthe Hønefoss area, and it is extensively used as a source of road gravel. It is made up ofmetadiorite to metatonalite and contains enclaves of metagabbro. The pluton defines a low- tomedium-K calc-alkaline trend. The metagabbro and metadiorite have geochemical signaturessimilar to oceanic volcanic arc suites, for example basalts from the Izu-Bonin-Mariana arc,and have largely positive εNd values of 3.2 to 6.1 (Bingen et al. 2004). The geochemical andisotopic data are consistent with a comparatively primitive volcanic arc setting, possiblyinitiated in oceanic environment. Greywacke-dominated sediment sequences associated withthe plutons probably represent fore-arc or back-arc basin sediments.

Stop No 4.3: Preserved Bouma sequences in amphibolite-faces metagreywacke, withgarnet-amphibolite dykes

LocationVeme, Hönefoss area (UTM Zo32 NH, 562600-6674400), bus stop along road 7 directionGol, map 1815 III

IntroductionThe Veme complex is interpreted as the northwestern extension of the Stora Le Marstrandformation west of the Oslo rift (cf. stop 3.3.). Though Mesoproterozoic lithologies areaffected by widespread Sveconorwegian deformation and amphibolite-facies metamorphism,some rock volumes escaped deformation. In Veme, a greywacke package displays well-preserved Bouma sequences characteristic of a turbidite mode of deposition. Detrital zirconsfrom this locality were analysed in Bingen et al. (2001), and show a restricted age spectrum,indicating that this sediment was not sourced from an evolved continent withPaleoproterozoic to Archean lithologies.

DescriptionThe locality exhibits a spectacular natural glaciated outcrop cleaned from soil cover for thepurpose of upgrading the road. The outcrop is more than 100 m long and of easy access.

Mesoproterozoic lithologies of the Veme complex are generally affected by penetrativeSveconorwegian deformation and amphibolite-facies metamorphism. Some rock volumesnevertheless escaped penetrative overprint. At the locality, a greywacke package displayswell-preserved syn-sedimentary structures reflecting a turbidite mode of deposition (Bouma

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sequence). The outcrop consists of fine-grained biotite-muscovite-bearing metasandstone beds(10-50 cm thick, 68%SiO2) interlayered with biotite-muscovite-rich schist beds. Well-preserved graded bedding is observed in some of the sandstone beds (Fig. 4.3.2). Local slumpstructures are preserved between some beds.

Fig. 4.3.2. Preserved Bouma sequence in metagreywacke sequence in the Veme complex atStop 4.3.

Detrital zircons from a sandstone sample collected in this locality were reported by Bingen etal. (2001). Eight detrital zircons range from 1673 ±16 to 1533 ±16 Ma with a frequencymaximum around 1560 Ma. The youngest available detrital zircon constraints deposition ofthe sediment to be younger than 1533 ±16 Ma. The locality is situated south of the 1555 ±3Ma Follum metapluton, and structurally above it. The sediment sequence is thus probablyoverlying the Follum metapluton. The restricted age distribution indicates that this sedimentwas not sourced from an evolved continent with Paleoproterozoic to Archean lithologies. Themain source probably corresponds to the volcanic arc lithologies exposed in the Idefjordenterrane.

The outcrop is folded and an axial planar schistosity is developed mainly in the schist beds.No garnet blastesis is observed in the outcrop, except along a few conspicuous “dykes”,variably oblique to the bedding but predating the last deformation phase (Fig. 4.3.3). These“dykes” are 0.2-2 m wide, and characterized by centimetre wide garnet phenoblasts in amatrix containing amphibole and titanite. They are associated with 2 to 10 cm widequartzofeldspathic veins. The dykes are obviously more mafic and more metamorphosed thanthe surrounding greywacke. Are these "dykes" representing paleofractures or paleofluidpathways leading to a metasomatic/ metamorphic reaction? Are they dolerite dykesmetamorphosed during intrusion? or are they dolerite dykes metamorphosed after intrusiontogether with the host greywacke? In the last interpretation, the outcrop illustrate that maficlithologies are significantly more reactive than the enclosing greywacke.

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Fig. 4.3.3. Deformed "dyke" in metagreywacke at Stop 4.3. The "dyke" is crosscutting to thebedding. It is characterized by amphibolite-facies garnet blastesis and segregation ofquartzofeldspathic material.

Guide: Bernard Bingen (Geological Survey of Norway).Literature: Bingen et al. (2001)

Stop No 4: Pervasive amphibolite-facies garnet blastesis in HP amphibolite-faciesconditions

LocationHensmoen, Hønefoss area, Parking lot at rest area along road E16, direction towards Bergen(UTM Zo32, 0568331-6677859, map 1815 III). An alternative parking place is in the parkinglot of an industrial building (close to UTM Zo32, 0569200-6677200).

IntroductionThe visitor is invited to examine the natural and artificial outcrop around the rest area andalong the road. The road section is oriented approximately parallel to the SE-NW trendingregional structure. The alternative parking place can be used to access the southeastern part ofthe section.

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DescriptionThe section is situated just north of the Follum metapluton, and structurally below it. Itconsists of a SW-dipping sequence of variably banded ortho- and paragneisses characterizedby pervasive amphibolite-facies garnet blastesis. Garnet is present in all lithologies, anddeveloped in amphibolite-facies conditions during the Sveconorwegian orogeny. Garnet hasgenerally a rounded habit and predates the last deformation phase recorded in the outcrop.

At and around the parking area, the main lithology is a biotite + garnet + amphibole augengneiss with evidence for migmatitization (leucosome strings and pockets, Fig. 4.4.1). Onetitanite fraction in the augen gneiss yields a 207Pb/206Pb age of 1040 ±14 Ma, and two otherfractions, with similar characteristics, an age of 1024 ±9 Ma (Bingen et al. 2008).

Fig. 4.4.1. Migmatitic garnet-bearing augen gneiss at Stop 4.4.

To the northwest of the rest area, variably banded paragneisses are interlayered withapparently less deformed garnet-amphibole granodioritic gneiss and mafic boudins. A fine-grained garnet-rich metapelitic gneiss containing a garnet + biotite + sillimanite assemblage isreported in the section. The granodioritic gneiss (SiO2 =64.9 %, K2O =4.4 %) has anassemblage of amphibole + garnet + biotite + allanite and probably represents one (or several)orthogneiss sheet(s). One sample of granodioritic gneiss was collected for geochronology(Bingen et al. 2008). Zircon cores with magmatic oscillatory zoning yield an age of 1496 ±12Ma (SIMS analyses), dating intrusion of the granodioritic protolith. Metamorphic zircon rimsare locally present and give a concordia age at 1091 ±18 Ma. Abundant titanite forming oblate

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disks aligned in the fabric of the gneiss defines an age of 1043 ±8 Ma (ID-TIMS data). Closeto the parking area along the road, a mysterious breccia including garnet-rich boulders anddiorite boulders is observed associated with a layer of foliated phenocryst granite.

Approximately 1 km to the southeast of the rest area, (accessible easily from the alternativeparking at UTM 0569200-6677200) a migmatitic mainly metapelitic gneiss package includelayers of kyanite-bearing gneiss (Fig. 4.4.2). The outcrop display a number of garnet-bearingamphibolite boudins, apparently less deformed than the surrounding gneiss. The kyanite-gneiss displays a pristine equilibrium assemblage of garnet + biotite + kyanite + K-feldspar +quartz attesting to high-pressure amphibolite-facies conditions. Equilibrium P-T estimatesrange from 1.00 Gpa-688°C to 1.17 Gpa- 780°C (Bingen et al. 2008). A sample of this rockcontains abundant monazite (Bingen et al. 2008). The main population of monazite yields anage of 1052 ±4 Ma (SIMS data) interpreted as the timing of Sveconorwegian high-pressureamphibolite-facies metamorphism. A distinctly younger population of monazite at 1025 ±9Ma is detected, as well as a few cores with an age of 1539 ±8 Ma. The age of 1539 ±8 Ma isequivalent to metamorphic zircon rims at 1540 ±16 Ma and 1540 ±7 Ma in two samples ofparagneiss east of the Oslo rift (Åhäll and Connelly 2008). It represents one of the fewavailable dates for amphibolite-facies “Gothian” metamorphism. If interpreted asmetamorphic, the monazite age at 1539 ±8 Ma implies that sedimentation of the paragneissesexposed at Hensmoen took place before 1.54 Ga, and consequently represents a possiblebasement to the Follum metapluton. Several phases of Sveconorwegian metamorphism arerecorded at Hensmoen between 1091 ±18 and 1025 ±9 Ma. Peak high-pressure amphibolite-facies conditions are probably best estimated by monazite in the kyanite gneiss at 1052 ±4Ma. A phase of “Gothian” metamorphism at 1539 ±8 Ma is detected in the sequence.

Fig. 4.4.2. Garnet-biotite-kyanite metapelitic gneiss at Stop 4.4. A rounded inclusion-poorgarnet phenoblast approximately 4 mm in diameter predates the last deformation phaserecorded in the rock.

Guide: Bernard Bingen (Geological Survey of Norway).

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