REE-assisted UPb zircon age (SHRIMP) of an anatectic granodiorite: Constraints on the evolution of...

Lithos 116 (2010) 153ndash166

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REE-assisted UndashPb zircon age (SHRIMP) of an anatectic granodiorite Constraints onthe evolution of the A Silva granodiorite Iberian allochthonous complexes

P Castintildeeiras a F Diacuteaz Garciacutea b J Goacutemez Barreiro a

a Dpto Petrologiacutea y Geoquiacutemica-Instituto de Geologiacutea Econoacutemica (UCM-CSIC) Facultad de Ciencias Geoloacutegicas Universidad Complutense 28040 Madrid Spainb Dpto Geologiacutea Facultad de Geologiacutea Universidad de Oviedo 33005 Oviedo Spain

Corresponding author Tel +34 91 394 4908 faxE-mail addresses castigargeoucmes (P Castintildeeira

0024-4937$ ndash see front matter copy 2010 Elsevier BV Aldoi101016jlithos201001013

a b s t r a c t

a r t i c l e i n f o

Article historyReceived 12 June 2009Accepted 25 January 2010Available online 2 February 2010

KeywordsSHRIMP UndashPb datingRare earth elements in zirconMagmatic arcNorthern Gondwana marginIberian massifAllochthonous complexesA Silva granodiorite

The A Silva granodiorite is a plutonic body intruded into the metasediments of the upper unit of the OacuterdenesComplex (Variscan belt NW Spain) These metasediments represent the middle section of a magmatic arclocated in northern Gondwana The A Silva granodiorite has been classically considered a late Variscangranite In this work new field mapping structural analysis and SHRIMP UndashPb zircon dating indicate thegranodiorite is significantly older However the data indicate a concordant age range between 540 and460 Ma and therefore CL images are not useful toward the interpretation of the geochronological resultsThis issue can be unravelled by using the hafnium and rare earth element composition of zircon in theassessment of the age In this way we determined that the age distribution was the result of lead loss ratherthan a real age scatter or inheritance and we could obtain a 206Pb238U crystallization age of 51028 (+157minus144)Ma using the TuffZirc algorithmThis age together with the well-preserved field relationships of the host rock permit us to interpret the ASilva granodiorite as multiple sheets intruded into a sequence of metatexitic host rocks after crustalthickening and subsequent decompression that developed coeval with partial melting during the lateststages of a regional extensional event Taken together with the underlying Monte Castelo gabbro (499plusmn2 Ma) the whole plutonic complex reaches 8 km in thickness and forms an antiformal stack structure in ashear parallel (NndashS) cross-section This structure could be responsible for previously described localizedgranulite facies metamorphismThe presence of a late Cambrian magmatic event has been widely reported in other areas of northernGondwana and it is related to the opening of the Rheic Ocean

+34 91 544 2535s) jugbusales (JG Barreiro)

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copy 2010 Elsevier BV All rights reserved

1 Introduction

The variety of PndashT conditions where zircon can grow either inmetamorphic or in magmatic rocks makes zircon perfectly suited tostudy the geochronology of tectonically active areas (Kellett et al 2009Mattinson et al 2009 Yui et al 2009 Fohey-Breting et al 2010) Singlezircon grains can record different geological processes that can be datedby means of an ion microprobe with the aid of cathodoluminescence(CL) images to recognize different growth zones (Vavra et al 1996Kempe et al 2000 Rubatto and Gebauer 2000 Corfu et al 2003)However traditional CL-assisted dating is insufficient in the correctlinkage between geochronological data and geological processes incomplex zircons from such active areas This is because zircon growthzones with similar CL features could correspond to different magmaticandormetamorphic events that can occur in a short time span Ifwe are

not able to distinguish eachmagmatic andmetamorphic event and linkthem with their corresponding age we end up with a spuriousprotracted age distribution In these cases we need an additionalcriterion such as the composition of zircon to make the correct inter-pretation of the ages

Analytical improvements together with a growing number ofanalytical facilities have made it easier to simultaneously obtain fromzircon both geochronological and chemical data especially the rareearth elements (REE) These improvements have led to the recognitionof certain compositional patterns useful to unambiguously link eachzircon growth zone with a specific petrogenetic process whether it bemetamorphic (Rubatto 2002 Whitehouse and Platt 2003 McClellandet al 2006 Chen et al 2010) or magmatic (Gagnevin et al in press)The petrogenetic information provided by the zircon composition isparticularly useful when interpreting a complex age dataset Thus theage spread observed in a sample could be attributable to a real agescatter an inherited component or to lead (Pb) loss (Coleman et al2004 McClelland et al 2006) Additionally using this methodology onmetamorphosed igneous rocks it is possible to distinguish non-obviousinherited (ie detrital zircon from a sedimentary protolith that are not

154 P Castintildeeiras et al Lithos 116 (2010) 153ndash166

abraded by transport) or metamorphic zircons from those grains thatare grown during magmatism

In the present paper we use this new approach (REE-assisted zircondating) to interpret the age distribution obtained in zircons from the ASilva granodiorite Additionally the well-preserved intrinsic contactsand the general weak deformation in the A Silva granodiorite make it agood tectonic marker that enables us to better understand therelationships among its intrusion and the development of the regionalfoliation the generalized top-to-the-NNW shearing and the partialmeltingprocesses operating in themetasedimentary country rocks Thisleads us to the recognition of a transitory stage of vertical decouplingbetween the upper and intermediate levels and the lower levels of thearc during its tectonic evolution

We also first show through field mapping structural analysis andUndashPb zircon dating that the A Silva granodiorite is older thanpreviously thought and it is related to a CambrianndashOrdovician arcdeveloped in the northern margin of Gondwana (Goacutemez Barreiroet al 2007) This arc drifted away from the continent giving rise tothe Rheic Ocean (Bozkurt et al 2008 von Raumer and Stampfli 2008Nance in press)

Finally we consider the general implications of our study for thecharacterization of the type timing and kinematics of the tectonicprocesses that contributed to the exhumation of the orogenic rootgenerated in the Gondwana continental margin during CambrianndashOrdovician times

2 Geological background

Preserved as megaklippen in the core of open synforms theallochthonous complexes occupy the highest structural position in theVariscan belt of the NW Iberian massif (see Martiacutenez Catalaacuten et al2009 and references therein) These complexes include a number ofallochthonous slices that appear well represented in the largest theOacuterdenes Complex (Fig 1) These slices have been grouped into threemain units named from bottom to top the basal units the ophioliticunits (representing the Variscan suture) and the upper units Thebasal and ophiolitic units record a westward oceanic subduction (DiacuteazGarciacutea et al 1999a Saacutenchez Martiacutenez et al 2007 Goacutemez Barreiroet al 2010) that evolved to continental subduction of the Gondwanancontinental margin during Devonian to Early Carboniferous times(Arenas et al 1995 Martiacutenez Catalaacuten et al 2007 2009)

The upper unit is tectonically emplaced above the ophiolitic unitand it presents increasing petrological evidence suggesting that it wasgenerated in an arc environment of the Gondwana continental margin(eg Abati et al 1999 2003 Fernaacutendez Suaacuterez et al 2003Andonaegui et al 2002 Puelles et al 2005) This arc was separatedfrom Gondwana during the Early Ordovician starting to drift towardsthe north at the same time of the opening of the Rheic Ocean (GoacutemezBarreiro et al 2007 Martiacutenez Catalaacuten et al 2009) During the EarlyDevonian the upper unit was emplaced over the Gondwanacontinental margin at the top of the Variscan pile (eg Diacuteaz Garciacuteaet al 1999a) This main tectonic process allows us to directly examinedifferent levels of an arc-derived terrane that would originally haverepresented an arc crust nearly 55 km thick according to paleopres-sure estimates (eg Gil Ibarguchi et al 1999 Arenas and MartiacutenezCatalaacuten 2002 Puelles et al 2005) This crustal ensemble is nowstrongly reduced to approximately 12 km in thickness and it can befurther divided into three additional units with contrasting tectono-metamorphic evolution exhumed from different depths and sepa-rated by extensional detachments that from bottom to top are(Martiacutenez Catalaacuten et al 2002) the high-pressure and high-temperature unit (HPndashHT with remnants of the lower crust andupper mantle) the intermediate-pressure unit with intermediate tohigh-temperature (IPndashIHT a section of the middle crust) and theintermediate-pressure unit with low-temperature (IPndashLT represent-ing the upper crust)

The HPndashHT upper units consist of high-pressure mafic to felsicgranulites eclogites and high-pressure gneisses with closely relatedultramafic massifs Most of the mafic rocks are metagabbros oftholeiitic composition Their geochemical signature has been com-pared to MORB (Gil Ibarguchi et al 1990) and geochemical studies onthe ultramafic rocks from the Cabo Ortegal outcrops are consistentwith the hypothesis that these rocks were generated in an arc setting(Moreno et al 2001 Santos Zalduegui et al 2002)

Fabric studies of the main foliation (eg Engels 1972 Aacutebalos1997) demonstrate that eclogite and granulite rocks underwent aregional intense ductile deformation during the highest pressureconditions followed by widespread development of an amphibolitefacies foliation related to their continued exhumation Later duringthe Variscan Orogeny progress isoclinal folds and thrusts underepizonal conditions (eg Martiacutenez Catalaacuten et al 2002 GoacutemezBarreiro et al 2007) reworked the whole ensemble

The age of the HPndashHT metamorphism in the allochthonouscomplexes is a matter of intense debate which will be summarized asfollows A Precambrian age was proposed based only on the tectoniccomplexity (eg Ribeiro et al 2007) On the basis of UndashPb zircon datasome authors have proposed a Late CambrianndashEarly Ordovician age (ca480ndash495 Ma Peucat et al 1990 Fernaacutendez Suaacuterez et al 2002 2007)whereas others favor a roughly Middle Devonian age (ca 380ndash390 MaSchafer et al 1993 Santos Zalduegui et al 1996 Ordontildeez Casado et al2001 Roger and Matte 2005)

The IPndashIHT (intermediate-pressure intermediate- to high-tem-perature) upper unit consists of a metasedimentary sequence locallynamed the O Pino unit which is dominated by staurolitendashkyaniteschists and sillimanitendashbiotite paragneisses (Abati et al 2003Castintildeeiras 2005 Goacutemez Barreiro 2007) Late Cambrian metaplu-tonic massifs such us the Monte Castelo gabbro and the Corredoirasorthogneiss occur near the base of the unit together with a number ofdifferently sized gabbroic and granodioritic plutonic bodies TheMonte Castelo gabbro cropping out to the west of the OacuterdenesComplex is the largest and is a massive two-pyroxene gabbro with atholeiitic character very similar to those of island arc tholeiites(Andonaegui et al 2002) Under the microscope textures vary fromgranular to intergranular and ophitic The presence of olivine and thecommon ophitic textures indicate a relatively shallow emplacement

The Corredoiras orthogneiss is a large granodioritic massif withminor tonalitic and mafic rocks that crops out in the eastern part ofthe Oacuterdenes Complex It exhibits a porphyritic texture with alkalifeldspar and plagioclase as porphyroclasts in a coarse-grained matrixThe Corredoiras orthogneiss and the related lithologies define a high-K calc-alkaline series and tectonic discrimination diagrams based onimmobile elements suggest a volcanic-arc setting (Andonaegui et al2007)

The Corredoiras orthogneiss has been affected by intense top-to-the-north shearing developing hectometre-thick shear zones withinthe body and in the basal part that brought it into contact with theunderlying HPndashHT Sobrado unit (Gonzaacutelez Cuadra 2007) Thestructure responsible for its final emplacement above the ophioliticunit is also a shear zone developed under greenschist facies conditionsand located in its eastern part where top-to-the-SE kinematic criteriahave been obtained (Diacuteaz Garciacutea et al 1999b)

Ages belonging to these two main plutonic bodies (ID-TIMS UndashPbin zircon 499plusmn2 Ma for theMonte Castelo gabbro and 500plusmn2 Ma forthe Corredoiras orthogneiss Abati et al 1999) confirm thatvoluminous tholeiitic and calc-alkaline magmatism had a peak ataround 500 Ma and was shortly followed by the development ofshear zones In the basal section of the Monte Castelo gabbro thisshearing was developed under granulite facies conditions andrecorded a pressure increase of 2ndash4 kb (Abati et al 2003) Theshearing was first indirectly dated using UndashPb in monazites from thehost high-grade metasediments that yielded an age between 493 and498 Ma (Abati et al 1999) More recently an age of 483plusmn4 Ma was

Fig 1 Geological map (a) and cross-section (b) of the Oacuterdenes Allochthonous Complex with their units and main lithologies Abbreviations AC1 A Coruntildea granodiorite porphiriticexternal facies AC2 A Coruntildea granodiorite central facies CD Corredoiras detachment F Fornaacutes HPndashHTmassif FD Fornaacutes detachment PCD Ponte Carreira detachment S SobradoHPndashHT massif XD Xesteda detachment

155P Castintildeeiras et al Lithos 116 (2010) 153ndash166

obtained on metamorphic rims of zircon from a mafic granulite usingthe SHRIMP (Abati et al 2007)

In the south of the study area (Fig 1) the Fornaacutes detachmentbrought the Fornaacutes and Arinteiro HPndashHT granulites from the footwallinto contact with the metasediments and gabbroic to granodioriticrocks of the hangingwall that belong to the upper IPndashIHT unit (GoacutemezBarreiro et al 2007) The hangingwall metasediments were equili-brated under medium-pressure and high-temperature conditionsdeveloping synkinematic leucosomes of granitendashgranodiorite com-position whereas the granulite rocks from the footwall underwentwidespread development of amphibolite facies foliation related totheir continued exhumation

The mylonitic fabrics related to the Fornaacutes detachment were datedby 40Ar39Ar laserprobe incremental heating experiments The plateau

ages obtained for hornblende cluster around 420 Ma (Goacutemez Barreiroet al 2006) which are interpreted as the minimum cooling age(below 525 degC) and the upper age-limit for the HPndashHT event itselfsupporting a polyorogenic origin for the upper units

The IPndashLT (intermediate-pressure low-temperature) upper unitoccupies the top of the upper unit and consists of a package ofmetapelites and metagreywackes where deposition folding andintrusion of mafic dikes occurred in a short time span from 520 to510 Ma (Fernaacutendez Suaacuterez et al 2003 Fuenlabrada et al 2010 DiacuteazGarciacutea et al 2010) The lower contact with the IP upper units is thePonte Carreira top-to-the-north extensional detachment (PCD inFig 1) Mylonitic muscovites from the PCD were dated by 40Ar39Arlaserprobe yielding a cooling age of 371 Ma (Goacutemez Barreiro et al2006)


3 The structure of the A Silva granodiorite

Field work led us to distinguish between the A Silva and the ACoruntildea granodiorites previously considered as a single plutonic body(Fig 1) The latter is a large granodiorite to granite intrusion with atrapezoid shape (in map view) which crosscuts upright regionalVariscan folds Two main petrographic types were recognized on thebasis of texture and modal composition following a classical normalzoning pattern (Bellido et al 1987) A porphyritic external faciesoccupies the largest area and is characterized by K-feldspar mega-crysts (AC1 in Fig 1) whereas coarse-grained non-porphyritic two-mica granite including fine- to medium-grained two-mica leuco-granite are located in the core of the pluton (AC2 in Fig 1) Magmaticfoliation identified by the alignment of feldspar megacrysts issometimes observed and the presence of a vertical tectonic foliationis restricted to its margins in the northern area associated withcataclastic rocks Thus the structural features of the A Coruntildeagranodiorite are similar to other Carboniferous granites in this partof the Variscan belt where they appear undeformed or locally affectedby a vertical SndashC type foliation related to strike-slip shearing (egIglesias and Choukroune 1980)

In contrast the A Silva granodiorite exhibits a coarse-grainedporphyritic texture with Pl and Kfs centimetre-scale megacrysts in acoarse-grained matrix with abundant biotite aggregates and micro-granite to tonalite enclaves thus giving it a restitic appearance Thegranodiorite also contains a number of elongate metasedimentaryenclaves the western contact being delineated by the presence ofseveral pieces of wall rock embedded in the pluton that are nearlyconcordant with the metasedimentary country rocks The A Silvagranodiorite exhibits a flat-lying SndashC type foliation that is folded intoan upright open antiform with a fold axis gently plunging to the NE(Fig 1) Its eastern limb is transected by the Xesteda east-dippingnormal fault (XD in Fig 1) which brought it into contact with the IPndashLT metasedimentary sequence of the upper unit

Metasedimentary country rocks of the upper IPndashIHT zone of theupper unit consist of amphibolite facies schists and paragneisseswhere the gradual transition from metatexite to diatexite can beobserved close to the A Silva granodiorite Regional foliation of themetasediments is marked by quartz- and micaceous-rich millimetre-scale stripes with microscopic evidence of having evolved from acrenulation cleavage S2 Frequently the S2 planes are associated tospaced C and Cprime shear planes producing a sigmoidal shape to thefoliation Commonly the metasediments appear as metatexitescharacterized by centimetre-scale melt segregations (leucosomes)along the foliation planes to form stromatitic migmatite but also withsmall pods of post-S2 leucosomes accumulated in dilated sitesdeveloped during shearing

Close to the A Silva granodiorite at its western and upper contactsan irregular diatexite envelope has been mapped (Fig 1) It ischaracterized by the disintegration of the S2 foliation from the hostrocks and the development of raft-rich schlieren and heterogeneousdiatexites with some ghost metatexitic layering preserved Massivediatexites can also be found which contain plagioclase and K-feldsparphenocrysts and a relatively uniform texture interrupted by thepresence of centimetre-scale clasts of stromatitic migmatite Thus theabove described field relationships suggest that partial melting tookplace synchronously with the final stages of the S2 foliationdevelopment subsequent shearing and granodioritic intrusion

Most of the A Silva granodiorite outcrops show a weak foliationdefined by the flattening of biotitic aggregates the incipientdevelopment of quartz and feldspar ribbons and the rotation offeldspar megacrysts An SndashC type foliation can be observed in its basalsheet and in narrow bands of the interior of the pluton where itacquires a gneissic fabric The stretching lineation on C planes issubhorizontal and it has a roughly N30W trend and allows us todeduce a top-to-the-NNW shearing To a lesser extent especially in

the southern end of the granodioritic massif the opposite sense ofshear has also been found

According to the geometry of the western contact field observa-tions and to the presence of several metasedimentary xenoliths the ASilva granodiorite consists of multiple laminar bodies that intruded asequence of metatexitic country rocks subsequently affected by top-to-the-NNW shearing and later folded into an upright antiform

Taken together with the underlying Monte Castelo gabbro and inabsence of detailed petrological and geochemical analysis it could beenvisioned that thewhole ensemble constitutes a plutonic complex thatreached in its present configuration near 8 km in thickness Though theinitial configuration of this complex is impossible to ascertain it can besuggested that the main lens-shape was acquired as a consequence ofbeing dismembered into a number of superposed tectonic sheetsaltogether depicting a duplex and forming anantiformal stack structurethat is represented in a composite section in Fig 2 This configurationcould be responsible for the pressure increase recorded in the granuliticshear zones located at the base of the Monte Castelo gabbro If thisinterpretation were correct the granulite facies metamorphismdescribed in the Monte Castelo gabbro (Abati et al 2003) would bethe result of a localized metamorphism originated by magmaticunderplating rather than the result of a regional metamorphismdeveloped during a crustal thickening episode

4 UndashPb REE and Hf SHRIMP analysis

41 Sample description

Two samples from the A Silva granodiorite A Silva-1 and A Silva-2were selected for SHRIMP analysis that includes UndashPb geochronologyand REE (rare earth elements) and Hf determinations in zircon Thefirst sample was collected near the A Coruntildea granodiorite in thenorthern outer area of the pluton to trace its extent as far as possibleThe second sample was selected from a type locality at the internalpart of the pluton

Zircon separation was carried out at the Universidad Complutense(Madrid) following standard techniques including crushing pulver-izing sieving Wilfley table magnetic separator and heavy liquid(methylene iodide) Zircons from both samples have similar char-acteristics they are colorless or light brown with scarce mineral orfluid inclusions Most of the grains are elongated dipyramidal prismswith simple habit and high length-to-width ratios less predominantand smaller in A Silva-2 In stubbier grains it is possible to recognizexenocrystic cores Big (01 mm wide 03 mm long) colored multi-faceted grains are common and they are usually broken Roundedzircons or grains with signs of surface abrasion are also present In thefinal mineral separate sulfide grains (mainly pyrite and chalcopyrite)and apatite are common Apatite is easily distinguished from zirconbecause of its duller surface caused by its lower refractory index andits bigger size

The zircon grains were handpicked under a binocular microscopeat the Stanford-USGeological Surveymicro analytical center (SUMAC)Zircon with the most transparent habit and highest length-to-widthratios were selected to avoid inherited and metamorphic grains asmuch as possible Two aliquots of A Silva-1 and one of A Silva-2 weremounted separately on glass slides with a double-sided adhesive in1times6 mm parallel rows together with some grains of zircon standardR33 (Black et al 2004) and set in epoxy resin After the resin wascured the mounts were ground down to expose their central portionsby using 1500 grit wet sandpaper and polished with 6 microm and 1 micromdiamond abrasive on a lapwheel Prior to isotopic analysis the internalstructure inclusions fractures and physical defects were identifiedwith transmitted and reflected light on a petrographicmicroscope andwith cathodoluminescence (CL) on a JEOL 5600LV scanning electronmicroscope Following the analysis secondary electron images weretaken to determine the exact location of the spots

Fig 2 Synthetic cross-section approximately parallel to the inferred tectonic transport direction showing the relationships between the main extensional detachments and theantiformal stack developed in the gabbroic and granodioritic plutonic complex Abbreviations as in Fig 1 Zircon UndashPb protolith ages are shown in black boxes zircon UndashPbmetamorphic ages in grey boxes and UndashPb monacite ages in white boxes Hornblende 40Ar39Ar ages underlined


Cathodoluminescence images of zircons from the first A Silva-1aliquot (Fig 3) display moderately luminescent oscillatory zoning Insome zircon grains internal zones appear truncated and surrounded by

Fig 3 Cathodoluminescence images for selected zircons from samples (a)

additional oscillatory zones and the internal zones may represent aninherited component Discontinuous poorly luminescent rims withfaint oscillatory zoning may occur in several grains Zircons from the

A Silva-1 first aliquot (b) A Silva-1 second aliquot and (c) A Silva-2


second A Silva-1 aliquot are slightly longer but show similar CL features(Fig 3) ie some possible inherited xenocrystic cores overlain byoscillatory-zoned zircon that is extended to the outer grain surface andusually gets darker and fainter to the rim Zircons from A Silva-2 aresmaller but they share the same CL characteristics (Fig 3)

42 Analytical techniques

UndashThndashPb REE and Hf analyses of zirconwere conducted on the BaySHRIMP-RG (sensitive high resolution ion microprobe-reverse geom-etry) operated by the SUMAC facility (Stanford University-USGSmicroanalysis center) during three analytical sessions in May andSeptember 2008

UndashThndashPb analytical procedures for zircon dating follow themethods described in Williams (1997) Secondary ions weregenerated from the target spot with an O2minus primary ion beamvarying from 4 to 6 nA The primary ion beam produced a spot with adiameter of sim25 μm and a depth of 1ndash2 μm for an analysis time of 12ndash13 min Data for each spot were collected utilizing five-cycle runsthrough the mass stations and the counting time for 206Pb wasincreased according to the Paleozoic age of the samples to improvecounting statistics and precision of the 206Pb238U age The isotopiccompositions were calibrated against R33 (206Pb238U=006716equivalent to an age of 419 Ma Black et al 2004) which was analyzedevery fourth analysis Calibration errors for 206Pb238U ratios of R33 forthe different analytical sessions were 091 064 and 049 (2σ)The calculated external errors were incorporated when data from allsessions and mounts were compiled together

Data reduction was carried out using Squid software (Ludwig2002) which follows the methods described by Williams (1997) andIreland and Williams (2003) and Isoplot software (Ludwig 2003)was used to create the graphs All the ages are reported based on206Pb238U ratios corrected from common Pb using the 207Pb methodThe Pb composition used for initial Pb corrections (204Pb206Pb=00554 207Pb206Pb=0864 and 208Pb206Pb=2097) was estimatedfrom Stacey and Kramers (1975) Analytical results are presented inTable 1 and plotted in Fig 4

La to Yb and Hf were measured concurrently with the UndashThndashPbanalyses as additional masses on each pass through the mass range Inthe second and third sessions two more peaks were included in theprocedure (Y and Lu) whereas Ce was not analyzed in the A Silva-1second session The concentration of U Th Hf and REEwere calibratedusing an in-house zircon standard (MAD see concentrations inTable 2) and are reproducible at 2ndash4 (1σ) except for La (15)because of its typical low concentration Pr was calculated from itsneighbor elements because CeH cannot be resolved from the Pr peakand contributes significantly to the counts at Pr141 given the lowabundance of Pr and the relatively much high Ce concentration

43 UndashPb results

The eighty-three analyses performed on 73 zircon grains suggestthat both A Silva samples are equivalent and the data is accordinglyconsidered together henceforth After an initial appraisal of the data acouple of analyses were ruled out in the discussion of the crys-tallization age because of their evident inherited nature (A Silva-1162) or high common Pb content (A Silva-2 5) Moreover somezircons from the second A Silva-1 aliquot exhibit a behavior thatdefies conventional schemes of interpretation as younger ages wereobtained for inner rather than outer areas of grains Several pos-sibilities were considered to explain this particularity including afailure in the calibration extreme Pb-loss in the zircon cores owningto radiation damage an analytical bias due to high U content in thezircon rims (Butera et al 2001) or heterogeneity of the zircon withdepth as the oxygen beam excavates the surface during analysisHowever none of these possibilities seem to apply in this case and we

decided to discard those analyses (1 2 3 5 8 13 and 15) in thesubsequent discussion The remaining 67 spots represent oscillatoryzones disregarding their luminescence avoiding evident xenocrysticcores and homogeneous non-luminescent rims The results show arough correlation between luminescence and U content which variesfrom 150 to 1750 ppm even though most of the zircons havemoderate U concentrations (lower than 400 ppm) The analysesdefine an apparent concordant age range between 540 and 460 Maalong concordia on a TerandashWasserburg plot (Fig 4) ThU ratiosdisplay a wide range of values 004 to 095 and show two distincttrends when plotted against the 206Pb238U age (Fig 5) Most of theanalyses define a gentle slope with a greater ThU range in youngerzircons whereas at least seven analyses from the second A Silva-1aliquot exhibit a steeper slope (analyses 51 71 9 10 11 12 and14) These seven analyses were obtained from central areas ofoscillatory zoning in apparently simple zircons (Fig 3) However inthe light of this plot it is likely that they are inherited zircons andtherefore will be omitted from any further discussion of the A Silvagranodiorite crystallization age Still obtaining an age with suchsmooth variation in the data is not straightforward and it requires acoupled assessment with the REE and Hf composition of the zircons

44 Zircon REE and Hf composition

Chondrite-normalized REE plots are presented in Fig 6 usingvalues from Anders and Grevesse (1989) modified by Korotev (1996)We have plotted sixty-one A Silva analyses together with zirconstandard R33 data for comparison In general all samples have REEpatterns characteristic of magmatic zircon (Hoskin and Schaltegger2003 Hanchar and van Westrenen 2007) In the A Silva zircons Lacontents are usually low ranging from 0003 to 05 ppm Highervalues (05ndash15 ppm) are commonly associated with high U contentsand could reflect metamictization of the zircon (Belousova et al2002 Hoskin 2005) CeCe positive anomalies are variable from 1 to21 with the lowest values largely influenced by light (L) REEconcentrations This anomalous Ce content is usually interpreted asthe result of the oxidation state of the original magma which favorsCe4+ versus Ce3+ (Hoskin and Schaltegger 2003) although fraction-ation of other minerals that predominantly take Ce3+ is also possible(Wooden et al 2006) The patterns show a pronounced negative Euanomaly (EuEu=002ndash005) with higher values in the analyseswith higher LREE contents Ce and Eu oxidation state is dependent onoxygen fugacity however there is no simple correlation betweenboth anomalies because plagioclase growth strongly influences Eu2+

availability in the magma Heavy (H) REE values show a variationbetween DyN=100ndash3000 and YbN=1000ndash10000 and the patternsreveal a uniform moderately fractionated shape with most of the YbGd values ranging between 10 and 20

Comparatively zircon standard R33 has lower and more homoge-neous La concentration (0005ndash025 ppm) CeCe anomalies arehigher with values ranging from 10 to 100 The negative EuEuanomaly is shallower and more homogeneous (sim030) HREE varybetween DyN=110ndash2100 and YbN=850ndash8000 and display a morefractionated and variable pattern than the A Silva zircons with YbGdvalues ranging between 10 and 40

5 Discussion

51 Petrogenesis of the A Silva granodiorite the zircon REE approach

Taking into account the zircon REE contents and various elementalratios some petrogenetic aspects can be suggested for the A Silvagranodiorite Wooden et al (2006) and Lowery Claiborne et al (2006)have determined that there are three excellent monitors of magmaevolution by fractional crystallization during zircon growth namelyYbGd ThU and Hf

Table 1UndashThndashPb SHRIMP analytical data for zircons from the A Silva granodiorite

Spot numberanddescriptiona

Common 206Pb()b

U(ppm)

Th(ppm)

ThU 207Pb corrected Uncorrected ratios 204Pb corrected ratios

206Pb238U age 206Pb238Uc 238U206Pb 207Pb206Pb 238U206Pbc 207Pb206Pbc

A Silva 1 Metagranodiorite (first aliquot) UTM 540003 4780000 4401 o 019 436 53 013 4726plusmn56 00761plusmn00010 1312plusmn13 00580plusmn16 1314plusmn13 00572plusmn182 o minus006 258 133 053 4896plusmn62 00789plusmn00011 1268plusmn14 00564plusmn20 1268plusmn14 00564plusmn203 o minus001 209 36 018 4950plusmn65 00798plusmn00012 1253plusmn15 00570plusmn22 1255plusmn15 00558plusmn254 o minus010 1748 84 005 5241plusmn57 00847plusmn00011 1182plusmn12 00570plusmn08 1182plusmn12 00572plusmn085 o minus006 252 50 021 5072plusmn65 00819plusmn00012 1222plusmn14 00569plusmn21 1224plusmn14 00562plusmn236 o 005 188 33 018 4968plusmn63 00801plusmn00012 1248plusmn14 00576plusmn20 1249plusmn14 00566plusmn227 o 019 380 42 011 4757plusmn57 00766plusmn00010 1303plusmn13 00582plusmn14 1305plusmn13 00573plusmn168 o 019 821 154 019 4792plusmn53 00772plusmn00010 1293plusmn12 00582plusmn10 1294plusmn12 00579plusmn109 o minus006 218 37 018 4802plusmn60 00773plusmn00011 1294plusmn14 00562plusmn19 1296plusmn14 00547plusmn2410 o 017 239 37 016 4744plusmn59 00764plusmn00011 1307plusmn14 00579plusmn19 1307plusmn14 00579plusmn1911 o 045 268 63 024 4742plusmn59 00763plusmn00011 1304plusmn14 00602plusmn16 1306plusmn14 00589plusmn2012 o 004 185 19 010 4768plusmn61 00768plusmn00011 1302plusmn14 00570plusmn20 1303plusmn14 00563plusmn2213 o 067 159 55 036 4741plusmn66 00763plusmn00012 1301plusmn16 00620plusmn22 1314plusmn16 00543plusmn5414 o minus037 184 72 040 4860plusmn62 00783plusmn00011 1282plusmn14 00539plusmn21 1283plusmn14 00532plusmn2315 o minus003 321 36 011 5048plusmn60 00815plusmn00011 1228plusmn13 00571plusmn15 1229plusmn13 00567plusmn1616 o minus007 242 44 019 4899plusmn63 00790plusmn00012 1267plusmn14 00564plusmn19 1267plusmn14 00564plusmn1917 o 016 147 66 046 4625plusmn62 00744plusmn00011 1342plusmn15 00575plusmn24 1342plusmn15 00575plusmn24

A Silva 1 Metagranodiorite (second aliquot)11 c 022 134 70 054 4873plusmn34 00785plusmn00009 1271plusmn11 00587plusmn30 1271plusmn11 00587plusmn3012 r 020 464 78 017 6392plusmn29 01042plusmn00008 957plusmn07 00626plusmn20 962plusmn07 00585plusmn3421 r 005 674 71 011 5192plusmn16 00839plusmn00004 1192plusmn05 00582plusmn13 1192plusmn05 00579plusmn1322 c 000 151 56 038 4964plusmn33 00800plusmn00009 1249plusmn10 00572plusmn39 1249plusmn10 00572plusmn3931 r 003 561 94 017 5261plusmn18 00850plusmn00005 1176plusmn05 00582plusmn14 1177plusmn05 00574plusmn1632 c 027 114 41 037 4860plusmn35 00783plusmn00009 1274plusmn11 00591plusmn31 1279plusmn12 00555plusmn464 o 009 495 56 012 5203plusmn20 00841plusmn00005 1188plusmn06 00585plusmn18 1190plusmn06 00573plusmn2151 r minus005 636 49 008 5395plusmn18 00873plusmn00005 1146plusmn05 00579plusmn14 1146plusmn05 00579plusmn1452 c minus003 232 52 023 5220plusmn27 00844plusmn00007 1186plusmn08 00576plusmn22 1186plusmn08 00576plusmn226 o 012 360 43 012 5088plusmn23 00821plusmn00006 1216plusmn07 00584plusmn22 1216plusmn07 00584plusmn2271 c minus006 500 444 092 5263plusmn19 00851plusmn00005 1176plusmn06 00574plusmn15 1177plusmn06 00571plusmn1572 r 000 612 54 009 5163plusmn17 00834plusmn00005 1199plusmn05 00577plusmn14 1201plusmn05 00563plusmn1881 c 019 386 45 012 5187plusmn21 00838plusmn00006 1191plusmn06 00593plusmn17 1191plusmn06 00593plusmn1782 r minus008 1155 83 007 5296plusmn12 00856plusmn00003 1169plusmn04 00573plusmn09 1169plusmn04 00570plusmn109 o 011 271 65 025 5394plusmn28 00873plusmn00007 1145plusmn08 00592plusmn22 1146plusmn08 00579plusmn2510 o minus007 425 162 040 5323plusmn21 00861plusmn00005 1163plusmn06 00575plusmn16 1164plusmn06 00566plusmn1811 o minus021 135 67 051 5204plusmn36 00841plusmn00009 1192plusmn11 00561plusmn29 1192plusmn11 00561plusmn2912 o minus031 348 226 067 5287plusmn23 00855plusmn00006 1174plusmn07 00554plusmn18 1175plusmn07 00544plusmn22131 c minus039 68 23 034 5022plusmn49 00810plusmn00013 1239plusmn15 00541plusmn45 1223plusmn17 00649plusmn90132 r minus013 417 54 013 5306plusmn25 00858plusmn00007 1167plusmn07 00569plusmn17 1170plusmn07 00550plusmn2314 o 007 298 128 044 5174plusmn24 00836plusmn00006 1196plusmn07 00582plusmn20 1196plusmn07 00582plusmn20151 c minus053 139 34 025 4964plusmn35 00801plusmn00009 1256plusmn11 00529plusmn32 1261plusmn11 00496plusmn48152 r minus002 594 41 007 5173plusmn17 00836plusmn00004 1197plusmn05 00575plusmn13 1197plusmn05 00575plusmn13161 r minus007 780 36 005 5273plusmn15 00852plusmn00004 1174plusmn05 00574plusmn12 1174plusmn05 00574plusmn12162 c inher minus042 87 39 047 6956plusmn60 01139plusmn00016 881plusmn14 00591plusmn33 881plusmn14 00591plusmn3317 o minus012 693 71 011 5195plusmn16 00839plusmn00004 1193plusmn05 00567plusmn13 1194plusmn05 00562plusmn1418 o 075 296 45 016 5119plusmn25 00826plusmn00006 1201plusmn07 00636plusmn26 1211plusmn08 00572plusmn4919 o 011 625 36 006 5135plusmn16 00829plusmn00004 1205plusmn05 00585plusmn14 1206plusmn05 00578plusmn1620 o 001 437 25 006 5070plusmn19 00818plusmn00005 1222plusmn06 00575plusmn16 1223plusmn06 00567plusmn1721 o minus020 337 58 018 5152plusmn22 00832plusmn00006 1204plusmn07 00560plusmn23 1204plusmn07 00560plusmn23

A Silva 2 Metagranodiorite UTM 538749 4775197 3541 o 010 242 41 018 4865plusmn13 00784plusmn00004 1274plusmn06 00577plusmn15 1276plusmn06 00564plusmn182 o minus021 345 55 017 5131plusmn12 00829plusmn00004 1210plusmn05 00558plusmn13 1209plusmn05 00560plusmn133 o 010 245 65 027 5117plusmn14 00826plusmn00005 1209plusmn06 00583plusmn16 1213plusmn06 00559plusmn254 o minus009 657 85 013 5199plusmn09 00840plusmn00003 1192plusmn03 00570plusmn13 1191plusmn03 00573plusmn145 hcPb 213 959 80 009 4612plusmn07 00742plusmn00002 1320plusmn03 00735plusmn08 1347plusmn03 00572plusmn276 o 014 379 51 014 4970plusmn11 00801plusmn00004 1246plusmn04 00583plusmn12 1246plusmn04 00584plusmn127 o 009 393 29 008 4870plusmn11 00785plusmn00004 1273plusmn04 00576plusmn13 1273plusmn04 00577plusmn138 o 013 339 20 006 4982plusmn11 00803plusmn00004 1243plusmn05 00582plusmn13 1244plusmn05 00578plusmn149 o 056 365 40 011 5103plusmn12 00824plusmn00004 1207plusmn05 00621plusmn13 1213plusmn05 00579plusmn2310 o 002 1516 55 004 5213plusmn08 00842plusmn00003 1187plusmn03 00579plusmn06 1188plusmn03 00575plusmn0611 o minus001 392 31 008 5179plusmn12 00837plusmn00004 1196plusmn05 00576plusmn13 1195plusmn05 00578plusmn1312 o minus005 324 67 021 5184plusmn13 00837plusmn00004 1195plusmn05 00573plusmn14 1195plusmn05 00570plusmn1413 o 020 176 47 027 4989plusmn16 00805plusmn00006 1240plusmn07 00588plusmn18 1241plusmn07 00583plusmn1914 o minus006 859 46 006 5092plusmn07 00822plusmn00002 1217plusmn03 00570plusmn08 1217plusmn03 00569plusmn0815 o 001 319 54 017 4945plusmn13 00797plusmn00004 1254plusmn05 00571plusmn14 1255plusmn05 00567plusmn1516 o minus007 332 44 014 5223plusmn13 00844plusmn00004 1186plusmn05 00573plusmn14 1185plusmn05 00575plusmn1417 o minus030 305 25 008 5105plusmn13 00824plusmn00005 1217plusmn05 00551plusmn15 1217plusmn05 00553plusmn1518 o minus015 276 42 016 4969plusmn13 00801plusmn00004 1250plusmn05 00559plusmn15 1251plusmn05 00552plusmn1619 o minus006 348 35 010 5191plusmn12 00839plusmn00004 1193plusmn05 00572plusmn13 1193plusmn05 00574plusmn1320 o 009 541 43 008 4966plusmn09 00801plusmn00003 1248plusmn04 00579plusmn14 1247plusmn04 00580plusmn14

(continued on next page)


Table 1 (continued)


Common 206Pb()b

U(ppm)

Th(ppm)



21 o 005 223 69 032 4860plusmn14 00783plusmn00005 1276plusmn06 00573plusmn17 1276plusmn06 00576plusmn1722 o 002 560 53 010 5237plusmn10 00846plusmn00003 1181plusmn04 00580plusmn11 1182plusmn04 00579plusmn1123 o minus001 592 65 011 5326plusmn10 00861plusmn00003 1161plusmn04 00580plusmn10 1162plusmn04 00575plusmn1224 o 076 771 96 013 5086plusmn09 00821plusmn00003 1209plusmn03 00636plusmn22 1217plusmn04 00583plusmn3025 o 010 321 26 008 4987plusmn12 00804plusmn00004 1242plusmn05 00580plusmn14 1242plusmn05 00579plusmn14261 r minus004 599 29 005 5173plusmn09 00835plusmn00003 1197plusmn04 00573plusmn10 1198plusmn04 00571plusmn10262 r minus016 575 92 017 5253plusmn11 00849plusmn00004 1180plusmn04 00566plusmn11 1180plusmn04 00567plusmn1127 o 005 894 46 005 5053plusmn08 00815plusmn00003 1226plusmn03 00577plusmn09 1226plusmn03 00577plusmn0928 o 003 359 40 012 5151plusmn12 00832plusmn00004 1202plusmn05 00579plusmn13 1203plusmn05 00572plusmn1429 o 000 308 101 034 5062plusmn12 00817plusmn00004 1224plusmn05 00574plusmn14 1224plusmn05 00576plusmn1430 o minus004 375 53 015 5041plusmn11 00813plusmn00004 1230plusmn05 00570plusmn13 1231plusmn05 00565plusmn1431 o 124 655 50 008 5064plusmn09 00817plusmn00003 1209plusmn03 00675plusmn09 1227plusmn04 00552plusmn2732 o minus002 238 38 016 4992plusmn14 00805plusmn00005 1242plusmn06 00571plusmn16 1243plusmn06 00564plusmn1833 o 020 195 27 014 5089plusmn16 00821plusmn00006 1215plusmn06 00591plusmn18 1217plusmn07 00577plusmn2134 o 003 388 37 010 4986plusmn11 00804plusmn00004 1243plusmn04 00575plusmn12 1243plusmn04 00574plusmn1235 o minus004 461 54 012 5224plusmn11 00844plusmn00004 1185plusmn04 00575plusmn12 1185plusmn04 00576plusmn12

All errors are 1sa Zircon characterization o = oscillatory zoning c = core r = rim inher = inheritance hcPb = high common Pbb Negative values denote reversely discordant analysesc Pb denotes radiogenic lead

A Silva 2 Metagranodiorite UTM 538749 4775197 354


According to Wooden et al (2006) for common magmatic suitesthe YbGd ratio which represents the steepness of the HREE patternshows a starting ratio of about 10 and it increases rapidly at relativelylow temperatures (b750 degC) This increase in the steepness can berelated to the fractionation ofmiddle (M)REE from themelt during thecrystallization of accessory minerals (mainly apatite and titanite) Incontrast ThU tends to decrease with decreasing zircon crystallizationtemperature showing the strongest change at higher T Hafniumconcentration in zircon is usually higher as fractional crystallizationprogresses and temperature decreases

Additionally the CeSm ratio typically rises with increasingfractionation (eg YbGd ratio) CeSm is preferred as a monitor ofmagma evolution rather than CeCe because it varies more regularlywhen plotted against a fractionation index (Wooden et al 2006)

We have plotted these ratios for the A Silva magmatic zirconstogether with those of zircon standard R33 analyzed in the samesessions for comparison In a ThU versus YbGd plot (Fig 7a) R33(diamonds) shows a reversely correlated asymptotic trend which istypical of a suite of zircons that have crystallized in an evolvingmagma(Wooden et al 2006) ie zircon chemistry reflects the degree of itshost rock fractionation (Belousova et al 2002) Contrastingly YbGd

Fig 4 Tera-Wasserburg plot showing distribution of SHRIMP zircon analyses from the ASilva samples Error ellipses are plusmn2σ

ratio for the A Silva zircons shows a limited variation from 10 to 20and most of the A Silva zircons have low ThU ranging from 004 to03 even though there is some scatter in the ThU ratio (sevenanalyses between 03 and 06) The A Silva zircons have significantlylow and constant Ce to Sm ratios compared to those of the zirconstandard (Fig 7b) The homogeneity in the fractionation indices hasimportant implications for the age assessment as it precludes thepossibility of a dominant zircon inherited component or the processof fractional crystallization in a long-lived magmatic chamber (seediscussion in Section 52)

Further petrogenetic information can be suggested using otherelemental ratios It is observed that in general terms metamorphiczircon has higher U concentration than magmatic ones whereas Ce ishigher inmagmatic zircon (eg Hoskin and Schaltegger 2003) Using avast zircon geochemistry dataset obtained from a variety of geologicalsamples the scientific staff from SUMAC devised a bilogarithmic plotof the UCe ratio versus Th concentration in which a 11 line separatesmagmatic from metamorphic zircons This plot is shown in Fig 7c forthe R33 standard and the A Silva zircon data Noticeably R33 plots inthe magmatic field conforming to a linear trend with positive slopewhereas the A Silva zircons plot in the metamorphic field inagreement with the anatectic character of the granodiorite

On a Hf versus EuEu plot (Fig 7d) the depth of the Eu anomaly inthe A Silva zircons stands out compared to the anomaly of the zirconstandard and is interpreted to be the consequence of coeval plagioclasegrowth which strongly fractionated Eu+2 from the melt

52 Crystallization age of the A Silva Granodiorite

The smoothvariation in ageobserved in theASilva zircons (from530to 460 Ma) can be interpreted in three different ways (1) It reflectsanalytical scatter (2) it is a true difference in age or (3) it is the result ofsome combination of Pb-loss and inheritance (Coleman et al 2004McClelland et al 2006) The first explanation analytical scatter cannotbe invoked to account for the 70 my range in age of the A Silva zirconbecause the age range is similar in both samples regardless of theanalytical session and there is no correlation between U concentrationand age The second alternative requires either a long-lived magmaticchamber (although 70 my is an unreasonable time span) or a sequenceof different geological processes in a short time span such asmetamorphism and magmatism However the homogeneity in thezircon fractionation indices such as YbGd ThU and Hf suggests thatzircon from the A Silva granodiorite grew during a single igneous event

Table 2Rare earth element (REE) and Hf data for zircons from the A Silva granodiorite

Spot number anddescriptiona

U Th Hf La Ce Nd Sm Eu Gd Dy Er Yb Lu Y ThU YbGd EuEu CeSm UCe

A Silva-1 (first aliquot)1 390 51 12311 0033 165 108 498 015 69 431 862 1469 na na 013 212 002 033 2362 234 130 9349 0435 516 420 988 056 89 328 535 837 na na 055 94 006 052 453 187 35 11586 0030 139 096 347 012 43 195 309 469 na na 019 110 003 040 1344 1566 81 14989 0201 148 081 360 014 59 429 720 1252 na na 005 212 003 041 10575 226 48 12077 0026 136 092 400 013 50 193 270 391 na na 021 78 003 034 1676 170 32 10882 0015 134 085 340 014 42 193 300 455 na na 019 108 004 039 1277 336 40 12316 0053 410 028 136 011 21 119 169 249 na na 012 121 007 302 828 742 149 11503 0478 339 152 591 019 81 445 709 1027 na na 020 128 003 057 2199 196 36 11011 0021 151 117 472 012 61 330 634 1035 na na 018 169 002 032 13010 211 35 10959 0021 138 102 452 011 59 331 623 1044 na na 016 177 002 030 15311 235 59 10183 0331 229 200 586 018 70 311 533 814 na na 025 117 003 039 10312 166 18 11397 0006 072 044 234 009 35 182 256 331 na na 011 94 003 031 23113 143 53 9949 0149 259 281 782 026 79 315 537 856 na na 037 109 003 033 5514 164 69 10049 0032 243 264 767 017 78 287 475 751 na na 042 96 002 032 6815 291 34 11883 0016 114 072 344 010 49 306 590 995 na na 012 204 002 033 25416 217 42 11107 0019 142 106 439 015 58 260 390 543 na na 019 94 003 032 15317 130 62 10590 0019 316 136 470 019 53 211 375 612 na na 048 116 004 067 41

A Silva-1 (second aliquot)11 OR-YC 130 72 10573 0024 na 275 775 020 77 285 477 771 140 2756 055 101 003 ndash ndash

12 OR-YC 401 70 12781 8660 na 591 682 046 63 283 379 535 87 2829 018 85 007 ndash ndash





4 483 58 13184 0135 na 111 492 009 64 372 703 1285 227 4055 012 200 002 ndash ndash

51 OR-YC Inher II 617 50 13635 1686 na 149 424 021 55 319 438 573 88 3283 008 104 004 ndash ndash


6 351 45 14076 0009 na 071 261 011 38 214 362 573 99 2393 013 149 003 ndash ndash

71 Inher II 491 460 11044 0063 na 593 1457 059 115 390 623 938 159 3707 094 81 004 ndash ndash

72 598 56 12791 0710 na 161 443 022 58 329 454 605 94 3474 009 105 004 ndash ndash












161 759 37 14474 0008 na 038 229 006 42 320 534 840 132 3613 005 201 002 ndash ndash

162 Inher I 85 40 9622 0008 na 058 124 071 10 31 64 160 37 387 047 166 063 ndash ndash

17 687 74 12758 0013 na 098 500 011 69 372 601 937 154 3829 011 136 002 ndash ndash

18 295 47 12853 0793 na 180 386 020 50 256 371 523 86 2441 016 105 004 ndash ndash

19 614 37 13680 0052 na 050 262 009 40 248 356 512 81 2518 006 127 003 ndash ndash

20 433 26 13644 0004 na 028 184 006 29 199 312 443 70 2109 006 155 002 ndash ndash

21 333 60 11940 0008 na 149 617 018 82 441 756 1200 204 4461 018 146 002 ndash ndash

A Silva-21 243 41 13179 0440 230 150 388 018 46 210 301 446 73 1922 017 98 004 059 1062 337 53 12363 0019 213 287 1000 043 109 376 314 290 39 2925 016 27 004 021 1593 244 63 11640 0545 302 234 750 025 79 331 533 830 142 3282 026 106 003 040 814 654 83 13553 0104 184 135 593 022 83 501 896 1442 238 5356 013 174 003 031 3555 HcPb 962 79 14884 15400 5098 2830 2608 359 132 625 828 1266 200 5195 008 96 019 195 196 377 50 12637 0016 133 094 406 012 55 291 476 746 124 2906 013 135 002 033 2847 388 28 13021 0422 136 054 253 009 40 282 505 778 124 3128 007 196 003 054 2868 334 20 13293 0009 057 031 161 005 26 167 242 340 53 1747 006 132 003 035 5919 358 39 12570 0108 117 096 461 014 59 288 396 541 86 2764 011 91 003 025 30510 1536 55 17130 0639 248 128 393 021 53 387 586 911 140 4041 004 172 004 063 62011 383 30 13051 0003 082 062 294 010 42 224 303 403 63 2295 008 97 003 028 46412 323 66 12128 0019 174 155 585 012 65 304 455 675 111 2920 020 104 002 030 18613 177 46 11651 0019 219 227 639 031 64 282 495 784 136 2929 026 122 005 034 8114 880 46 14078 0009 079 057 327 009 51 348 493 650 90 3477 005 129 002 024 111015 320 53 11946 0443 290 144 408 021 50 270 433 653 107 2660 016 130 005 071 11116 331 43 12204 0035 137 130 571 021 72 374 587 882 147 3687 013 123 003 024 24117 298 24 12689 0008 065 040 234 008 35 196 290 421 69 1963 008 119 003 028 45718 273 41 11899 0010 135 120 464 016 61 319 572 930 161 3270 015 152 003 029 20219 343 34 12370 0008 101 084 420 014 61 342 548 810 132 3575 010 132 003 024 33920 551 44 13079 0006 087 054 300 009 46 301 443 595 89 3065 008 128 002 029 63021 222 68 10810 0030 264 269 846 021 89 380 651 1038 180 3788 031 117 002 031 84



Table 2 (continued)



A Silva-222 544 50 12980 1594 284 112 289 009 42 270 409 572 89 2915 009 137 002 098 19123 577 63 11859 0161 163 195 573 017 71 372 554 814 132 3812 011 115 002 028 35424 772 95 13821 1622 568 304 658 049 70 401 631 933 145 3927 012 133 007 086 13625 319 25 13196 0008 086 078 355 017 44 165 150 167 25 1292 008 38 004 024 369261 603 29 13645 0007 063 024 172 005 29 233 364 485 69 2554 005 165 002 037 949262 576 91 12694 0047 240 322 1177 023 145 818 1508 2462 401 8796 016 170 002 020 24027 908 45 14994 0109 100 045 278 008 45 348 536 740 107 3651 005 166 002 036 91028 357 39 12656 0014 113 098 463 015 64 373 622 947 156 3776 011 148 003 024 31529 306 99 11662 0028 280 227 657 025 72 351 622 977 166 3576 032 137 004 043 10930 372 52 11778 0015 147 130 484 015 63 334 565 879 147 3463 014 139 003 030 25231 654 49 13086 0476 185 099 408 015 58 372 518 683 101 3633 008 118 003 045 35432 239 37 11444 0025 142 110 432 014 56 299 536 855 146 3086 016 152 003 033 16833 189 26 12360 0012 114 071 294 012 40 240 470 807 142 2698 014 203 003 039 16634 393 37 12408 0005 102 084 410 010 56 347 606 1014 171 3575 009 183 002 025 38635 453 53 12177 0010 162 130 574 020 77 393 584 826 135 3902 012 107 003 028 280

a Zircon description OR-YC old rim-young core Inher I inherited age Inher II spot interpreted as inherited (see Fig 5) HcPb high common Pb na not analyzed


with limited fractional crystallization hence precluding the possibilitythat the age range represents a true variation in age

The last option to explain the age dispersion involves a com-bination of Pb-loss and inheritance As shown in Section 2 the A Silvagranodiorite is generated from the melting of the O Pino metasedi-ments which has detrital zircons (Abati et al 2007) that record analmost continuous age distribution of magmatism (from 640 to510 Ma) and metamorphism (from 510 to 480 Ma) Even though weselected the cleanest and longest zircon grains during the final handpicking in order to avoid inherited zircons it is impossible todistinguish detrital grains that have experienced very little abrasionfrom those generated during the A Silva granodiorite crystallizationon the basis of morphology alone However excluding the oldestgrains with higher ThU ratios the homogeneity in the zirconfractionation indices suggests that the inherited component involvedin the analyzed A Silva zircon population is negligible

Thus themost probable process controlling thewide age distributionis Pb-loss Accordingly we used the TuffZirc method developed byLudwig and Mundil (2002) to obtain a reliable age from a datasetaffected by slightly positive and negative age biases The best ageestimate obtained is 51028 (+157 minus144)Ma (Fig 8) usingthe TuffZirc algorithm on a group of 60 analyses ranging from 530to 460Ma This age is the median obtained by pooling togetherelevenanalyses considering the largest set of internally concordantdatesthat are statistically coherent and it is interpreted as the best statisticalestimate for the crystallization age of the A Silva granodiorite Ages

Fig 5 ThU versus 206Pb238U age for the analyzed A Silva zircons See Section 43 forexplanation

calculated using this method are reliable provided the eleven selectedanalyses are cogenetic and unaffected by Pb-loss In this case we canargue the validity of these assumptions based on the zircon CL features

Fig 6 Chondrite-normalized rare earth element (REE) patterns for (a) 17 analyses fromthe A Silva-1 first session (b) 30 analyses of the A Silva-1 second session and (c) 36analyses of the A Silva-2 sample Light yellow fields represent the REE patterns for thestandard R33 included for comparison

Fig 7 (a) ThU versus YbGd plot (b) CeSm versus YbGd plot (c) Th versus UCe plot (d) Hf versus EuEu plot See Section 44 for explanation

Fig 8 Age distribution for the 61 magmatic zircons analyzed White bars analyses notconsidered in the TuffZirc calculation blue bars rejected analyses red bars analysesused to obtain the best age estimate See Section 52 for explanation


and geochemistry ie oscillatory zoning consistent with zircons grownin igneous rocks (Corfu et al 2003) and homogeneous fractionationindices

53 Constraints on the evolution of the Upper units

The new UndashPb zircon age supplied by the A Silva granodiorite(510 Ma) indicates that it is an old plutonic body enclosed within thewidespread CambrianndashOrdovician magmatism (520ndash495 Ma) Thismagmatism has been widely documented in the upper units of theallochthonous complexes (van Calsteren et al 1979 Peucat et al1990 Schaumlfer et al 1993 Abati et al 1999 2007 Santos Zalduegui etal 2002) and in other areas of the northern Gondwana margin (seeMurphy et al 2010 and references therein) In addition thepreservation of the original relationships with the upper unit hostrocks in its western and upper contacts makes this plutonic body asuitable place to examine key aspects of the CambrianndashOrdovicianpre-orogenic evolution of the continental margin of Gondwana Theupper units represent different crustal levels of the arc crust at thattime and record processes such as granite emplacement regionalextension and exhumation of the HPndashHT rocks situated below

Field relationships suggest that the A Silva granodiorite wasemplaced after crustal thickening in the IP unit which metamorphicpeak conditions for the IP units are estimated at 10 kb and 650 degC in

the O Pino schists (Castintildeeiras 2005) Subsequent decompression PndashTconditions have been estimated at 4 kb and TN650 degC in the HPndashHTunits (Goacutemez Barreiro 2007)


The laccolithicsill-like geometries shown by the A Silva granodi-orite are likely to be the result of their intrusion within a generalizedsubhorizontal kilometre-thick shear zone with top-to-the-NNWkinematics where positive feedback between melting and shearingmay occur This scenario promotes subhorizontal host rock displace-ments magma accumulation magmatic wedging and stopping thusgiving rise during progressive deformation to multiple intrusions ashas been documented in several cases (eg Brown and Solar 1998)

In a more regional context the emplacement of the A Silvagranodiorite and its diatexite envelope was followed by the intrusionof a number of gabbroic rocks (Monte Castelo gabbro) and smallplutons in the metatexitic country rocks that escaped to the mainregional deformation suggesting that this extensional episode oc-curred at 510 to 495 Ma

In addition the intrusion of diabasic dikes that cross cut theregional S2 foliation in the uppermost unit yield a UndashPb age of 510 Ma(Diacuteaz Garciacutea et al 2010) supporting the aforementioned minimumage for the extensional shearing in the IPndashIHT upper units

The plutonic complex formed by the A Silva granodiorite theMonte Castelo gabbro and minor intrusive bodies was determined inthe top-to-the-NNW shear zone giving rise to the piling of theplutonic sheets reaching at least the present thickness of 8 kmwithinan antiformal stack This thickening event could by itself beresponsible for the near isothermal pressure increase to the granulitefacies (from 6 to 8 kb) that has been recorded exclusively in the basalshear zones of this antiformal stack affecting the Monte Castelogabbro (Abati et al 2003) The timing of this localized compressionalevent was from 495 to 480 Ma (Abati et al 2003 2007)

These compressional structures and the related pressure increasehave not been identified in the remaining units and probably reflect atransitory stage of thickened middle crust that occurred after thecooling and crystallization of the gabbroic to granodioritic complexpromoting vertical decoupling within this orogenic lithosphere

In contrast in the HPndashHT upper units this compressional event hasnot been recognized but the available data demonstrate that eclogiteand granulite rocks underwent intense ductile deformation accom-panied by partial melting and followed by widespread developmentof an amphibolite facies foliation related to their continued exhuma-tion (eg Aacutebalos 1997 Goacutemez Barreiro et al 2006)

Later structures consist of more localized extensional detachments(BD XD CD and FD in Figs 1 and 2) developed in kinematic continuitywith the top-to-the-NNW shearing that affect all the upper units andeventually brought together the uppermost and HPndashHT units complet-ing the thinning of the upper units Hornblende from mylonitic fabricsrelated to the Fornaacutes detachment (FD) yielded exhumation ages for theHPndashHT units of around 420 Ma (Goacutemez Barreiro et al 2006) inagreement with other amphibolite facies retrogressed-foliation ages inHPndashHT upper units (425 Ma Dallmeyer et al 1997) According to thesedates the HPndashHT upper units underwent an unroofing event in thefootwall of an extensional shear zone with roughly top-to-the-northkinematics during a protracted time span indicating a large residencetime under lower and middle crustal conditions This situation couldsatisfactorily explain the scattered ages from 495 to 430 Ma in thezircons extracted from leucosomes (Peucat et al 1990 FernaacutendezSuaacuterez et al 2003 2007) and independently support the idea that peakmetamorphic conditions for theHPndashHTunitswoulddateback to510 Ma

The geodynamic framework proposed by several authors for thenorthern part of Gondwana similar to other large and hot orogensenables us to situate these processes ofmelting at themid-crustal levelsfollowed by magmatic underplating extension and exhumation of theHPndashHT rocks (Murphy et al 2006 Goacutemez Barreiro et al 2007Linnemann et al 2008 Martiacutenez Catalaacuten et al 2009) These authorshave suggested a geodynamic model that occurs diachronously alongthe northernGondwanamargin and consists of a LateNeoproterozoic toEarly Cambrian ridgendashtrench collision leading to the termination ofsubduction and the generation of a continental transform during the

Cambrian This event was followed by the individualization of acontinental ribbon that drifted away fromGondwana as the interveningRheic Ocean expanded According to Fuenlabrada et al (2010) andDiacuteazGarciacutea et al (2010) the accretionary processes recorded by the earlieststructures developed in the low-grade part of the upper unit wouldhave continued until 510 Ma This age providedbypost- to syn-tectonicmafic dikes marks the change to a period of north-directed extensionanatexis intrusion of arc plutonics andmafic dyking probably linked toridge subduction and a change to an extensional regime that leads tostrong thinning of the arc crust and the exhumation of the arc-rootwhich culminates in the opening of the Rheic Ocean

6 Conclusions

This study provides significant insight into the A Silva granodioriteand increases its value as amarkerwithin the tectonic evolution of theintermediate-pressure unit of the Allochthonous complexes of theIberian massif

The results fromeighty-three analyses performed in 73 zircon grainsfrom two samples define a near-concordant age range between 540 and460 Ma Taking into account a coupled assessment with the REE and Hfcomposition of the zircons we obtained a 206Pb238U crystallization ageof 51028 (+157minus144)Ma using the TuffZirc algorithm designed toobtain an age from a dataset affected by Pb-loss or slight inheritance

The A Silva granodiorite consists of multiple sheets intruded in asequence of metatexitic host rocks after an early stage of crustalthickening (up to 10 kb) and during subsequent decompression (ca4 kb) that developed coevally with partial melting in the final stagesof the D2 regional extensional event

Subsequent to its cooling and crystallization the whole gabbroicand granodioritic complex underwent a transitory compressionalstage within a generalized extensional scenario The complex wasaffected by top-to-the-NNW shearing thus developing a flat-lying SndashCtype foliation and forming an antiformal stack structure in a NndashSprofile The basal shear zones of this 8 km-thick duplex registered a 2ndash4 kb pressure increase between 495 to 480 Ma

Field relationships with the country rocks allow us to deduce thatcrustal thickening and the earliest stages of subsequent extensionwascompleted by Upper Cambrian time in the intermediate-pressureupper units of the Allochthonous complexes

Acknowledgements

Wewould like to dedicate this work to the memory of our belovedcolleague and friend Florentino Diacuteaz Garciacutea who sadly passed awaylast August

Pablo Gonzaacutelez Cuadra is kindly thanked for his assistance duringthe sampling Joe Wooden and the SUMAC staff at Stanford Universityare especially acknowledged for their help in operating the SHRIMPinstrument and in interpreting the results J Abati and R Arenas arealso thanked for their constructive comments of an early version ofthe manuscript The original manuscript has greatly benefited frominsightful reviews by B Murphy J Wooden and W Premo

Financial support for this research has been provided by Spanishproject CGL2007-65338-CO2BTE (Ministerio de Ciencia e Innova-cioacuten) This study is also a contribution to the IGCP-497 project ldquoTheRheic Ocean Origin evolution and correlativesrdquo P Castintildeeirass stayat the SUMAC facility was financed with a ldquoProfesores UCM en elextranjerordquo travel aid J Goacutemez Barreiro was supported by aMEC-Juande la Cierva Postdoctoral contract

References

Aacutebalos B 1997 Omphacite fabric variation in the Cabo Ortegal eclogite (NW Spain)relationships with strain symmetry during high-pressure deformation Journal ofStructural Geology 19 (5) 621ndash631


Abati J Arenas R Martiacutenez Catalaacuten JR Diacuteaz Garciacutea F 2003 Anticlockwise PndashT pathof granulites from the Monte Castelo Gabbro (Oacuterdenes Complex NW Spain)Journal of Petrology 44 (2) 305ndash327

Abati J Castintildeeiras P Arenas R Fernaacutendez Suaacuterez J Goacutemez-Barreiro JWooden J 2007Using SHRIMP zircon dating to unravel tectonothermal events in arc environmentsThe early Palaeozoic arc of NW Iberia revisited 19 Terra Nova pp 432ndash439

Abati J Dunning GR Arenas R Diacuteaz Garciacutea F Gonzaacutelez Cuadra P Martiacutenez CatalaacutenJR Andonaegui P 1999 Early Ordovician orogenic event in Galicia (NW Spain)evidences from UndashPb ages in the uppermost unit of the Oacuterdenes Complex Earthand Planetary Science Letters 165 213ndash228

Anders E Grevesse N 1989 Abundances of the elements meteoritic and solarGeochimica et Cosmochimica Acta 53 197ndash204

Andonaegui P Gonzaacutelez Cuadra P Castintildeeiras P Martiacutenez Catalaacuten JR Arenas RDiacuteaz Garciacutea F Abati J Saacutenchez Martiacutenez S 2007 Geochronology andgeochemistry of the Corredoiras orthogneiss (Ordenes complex Iberian MassifNW Spain In Arenas R Martiacutenez Catalaacuten JR Abati J SaacutenchezMartiacutenez S (Eds)The Rootless Variscan Suture of NW Iberia (Galicia Spain) Field Trip andConferenceAbstracts IGME 188

Andonaegui P Gonzaacutelezdel Taacutenago J Arenas R Abati J Martiacutenez Catalaacuten JRPeinado M Diacuteaz Garciacutea F 2002 Tectonic setting of the Monte Castelo gabbro(Ordenes Complex northwestern Iberian Massif) Evidence for an arc-relatedterrane in the hanging wall to the Variscan suture In Martiacutenez Catalaacuten JRHatcher Jr RD Arenas R Diacuteaz Garciacutea F (Eds) VariscanndashAppalachian Dynamicsthe Building of the Late Paleozoic Basement Geological Society of America SpecialPaper 364 pp 37ndash56

Arenas R Martiacutenez Catalaacuten JR 2002 Prograde development of corona textures inmetagabbros of the Sobrado window (Ordenes Complex NW Iberian Massif) InMartiacutenez Catalaacuten JR Hatcher Jr RD Arenas R Diacuteaz Garciacutea F (Eds) VariscanndashAppalachian Dynamics the Building of the Late Paleozoic Basement GeologicalSociety of America Special Paper 364 pp 73ndash88

Arenas R Rubio Pascual FJ Diacuteaz Garciacutea F Martiacutenez Catalaacuten JR 1995 High-pressuremicroinclusionsanddevelopmentof an invertedmetamorphic gradient in theSantiagoSchists (Oacuterdenes ComplexNW IberianMassif Spain) evidence of subduction and syn-collisional decompression Journal of Metamorphic Geology 13 141ndash164

Bellido F Gonzaacutelez Lodeiro F Klein E Martiacutenez Catalaacuten JR Pablo Macia JG 1987Las rocas graniacuteticas herciacutenicas del Norte de Galicia y Occidente de AsturiasMemorias del Instituto Geoloacutegico y Minero de Espantildea 101 157

Belousova EA GriffinWL OReilly SY Fisher NI 2002 Igneous zircon trace elementcomposition as an indicator of source rock type Contributions to Mineralogy andPetrology 143 602ndash622

Black LP Kamo SL Allen CM Davis DW Aleinikoff JN Valley JW Mundil RCampbell IH Korsch RJ Williams IS Foudoulis C 2004 Improved 206Pb238Umicroprobe geochronology by the monitoring of a trace-element-related matrixeffect SHRIMP ID-TIMS ELA-ICP-MS and oxygen isotope documentation for aseries of zircon standards Chemical Geology 205 115ndash140

Bozkurt E Pereira MF Strachan RA Quesada C 2008 Evolution of the Rheic OceanTectonophysics 461 1ndash8

Brown M Solar GS 1998 Shear-zone systems andmelts feedback relations and self-organization in orogenic belts Journal of Structural Geology 20 (213) 211ndash227

Butera KM Williams IS Blevin PL Simpson CJ 2001 Zircon UndashPb dating of EarlyPaleozoic monzonitic intrusives from the Goonumbla area New South GalesAustralian Journal of Earth Sciences 48 457ndash464

Castintildeeiras P 2005 Origen y evolucioacuten tectonotermal de las unidades de O Pino yCarintildeo (Complejos Aloacutectonos de Galicia) Nova Terra 28 279 (In Spanish)

Chen RX Zheng YF Xie L 2010 Metamorphic growth and recrystallization of zirconDistinction by simultaneous in-situ analyses of trace elements UndashThndashPb and LundashHfisotopes in zircons from eclogite-facies rocks in the Sulu orogen Lithos 114 132ndash154

Coleman DS Gray W Glazner AF 2004 Rethinking the emplacement and evolutionof zoned plutons geochronologic evidence for incremental assembly of theTuolumne Intrusive Suite California Geology 32 433ndash436

Corfu F Hanchar JM Hoskin PWO Kinny P 2003 Atlas of zircon textures InHanchar JM Hoskin PWO (Eds) Zircon Reviews in Mineralogy andGeochemistry 53 Mineralogical Society of America Washington pp 468ndash500

Dallmeyer RD Martiacutenez Catalaacuten JR Arenas R Gil Ibarguchi JI Gutieacuterrez Alonso GFarias P Aller J Bastida F 1997 Diachronous Variscan tectonothermal activity inthe NW Iberian Massif evidence from 40Ar39Ar dating of regional fabricsTectonophysics 277 307ndash337

Diacuteaz Garciacutea F Arenas R Martiacutenez Catalaacuten JR Gonzaacutelez del Taacutenago J Dunning GR1999a Tectonic evolution of the Careoacuten Ophiolite (northwest Spain) a remnant ofoceanic lithosphere in the Variscan Belt Journal of Geology 107 587ndash605

Diacuteaz Garciacutea F Martiacutenez Catalaacuten JR Arenas R Gonzaacutelez Cuadra P 1999b Structuraland kinematic analysis of the Corredoiras Detachment evidence for early Variscansynconvergent extension in the Ordenes Complex NW Spain International Journalof Earth Sciences 88 337ndash351

Diacuteaz Garciacutea F Saacutenchez Martiacutenez S Castintildeeiras P Fuenlabrada JM Arenas R 2010 Aperi-Gondwanan arc in NW Iberia II assessment of the intra-arc tectonothermalevolution through the UndashPb SHRIMP dating of mafic dykes Gondwana Research 17352ndash362

Engels JP 1972 The catazonal poly-metamorphic rocks of Cabo Ortegal (NW Spain) astructural and petrographic study Leidse Geologische Mededelingen 48 83ndash133

Fernaacutendez Suaacuterez J Arenas R Abati J Martiacutenez Catalaacuten JR Whitehouse MJJeffries TE 2007 UndashPb chronometry of polymetamorphic high-pressuregranulites an example from the allochthonous terranes of the NW IberianVariscan belt In Hatcher Jr RD Carlson MP McBride JH Martiacutenez Catalaacuten JR(Eds) 4-D Framework of Continental Crust Geological Society of America Memoir200 pp 469ndash488

Fernaacutendez Suaacuterez J Corfu F Arenas R Marcos A Martiacutenez Catalaacuten JR Diacuteaz GarciacuteaF Abati J Fernaacutendez FJ 2002 UndashPb evidence for a polymetamorphic evolution ofthe HPndash HT units of the NW Iberia Massif Contributions to Mineralogy andPetrology 143 236ndash253

Fernaacutendez Suaacuterez J Diacuteaz Garciacutea F Jeffries TE Arenas R Abati J 2003 Constraintson the provenance of the uppermost allochthonous terrane of the NW IberianMassif Inferences from detrital zircon UndashPb ages Terra Nova 15 138ndash144

Fohey-Breting NK Barth AP Wooden JL Mazdab FK Carte CA Schermer ER2010 Relationship of voluminous ignimbrites to continental arc plutons petrologyof Jurassic ignimbrites and contemporaneous plutons in southern CaliforniaJournal of Volcanology and Geothermal Research 189 1ndash11

Fuenlabrada JM Arenas R SaacutenchezMartiacutenez S Diacuteaz Garciacutea F Castintildeeiras P 2010 Aperi-Gondwanan arc in NW Iberia I isotopic and geochemical constraints to theorigin of the arc mdash The sedimentary approach Gondwana Research 17 338ndash351

Gagnevin D Daly JS Kronz A 2010 Zircon texture and chemical composition as aguide to magmatic processes and mixing in a granitic environment and coevalvolcanic system Contributions to Mineralogy and Petrology in press doi101007s00410-009-0443-0

Gil Ibarguchi JI Aacutebalos B Azcarraga J 1999 Deformation high-pressure metamor-phism and exhumation of ultramafic rocks in a deep subductioncollision setting(Cabo Ortegal NW Spain) Journal of Metamorphic Geology 17 (6) 747ndash764

Gil Ibarguchi JI Mendia M Girardeau J Peucat JJ 1990 Petrology of eclogites andclinopyroxene-garnet metabasites from the Cabo Ortegal Complex (northwesternSpain) Lithos 25 133ndash162

Goacutemez Barreiro J 2007 La Unidad de Fornaacutes Evolucioacuten tectonometamoacuterfica del SO delComplejo de Oacuterdenes Nova Terra 32 334 (In Spanish)

Goacutemez Barreiro J Martiacutenez Catalaacuten JR Arenas R Castintildeeiras P Diacuteaz Garciacutea FWijbrans JR 2007 Tectonic evolution of the upper allochthon of the Oacuterdenescomplex (northwestern Iberian Massif) structural constraints to a polyorogenicperi-Gondwanan terrane In Linneman U Nance RD Kraft P Zulauf G (Eds)The evolution of the Rheic Ocean From Avalonianndash Cadomian active margin toAlleghenianndashVariscan collision Geological Society of America Special Paper 423pp 315ndash332

Goacutemez Barreiro J Martiacutenez Catalaacuten JR Prior D Wenk H-R Vogel S Diacuteaz Garciacutea FArenas R Saacutenchez Martiacutenez S Lonardelli I 2010 Fabric Development in aMiddle Devonian Intraoceanic Subduction Regime the Careoacuten Ophiolite (North-west Spain) Journal of Geology 118 163ndash186

GoacutemezBarreiro JWijbrans JR Castintildeeiras PMartiacutenezCatalaacuten JR Arenas RDiacuteazGarciacuteaF Abati J 2006 40Ar39Ar laser probe dating of mylonitic fabrics in polyorogenicterrane of NW Iberia Journal of the Geological Society of London 163 61ndash73

Gonzaacutelez Cuadra P 2007 La Unidad de Corredoiras (Complejo de Oacuterdenes Galicia)Evolucioacuten estructural y metamoacuterfica Serie Nova Terra 33 254 (In Spanish)

Hanchar JM van Westrenen W 2007 Rare earth element behavior in zirconndashmeltsystems Elements 3 37ndash42

Hoskin PWO 2005 Trace-element composition of hydrothermal zircon and thealterationofHadean zircon from the JackHills AustraliaGeochimica et CosmochimicaActa 69 637ndash648

Hoskin PWO Schaltegger U 2003 The composition of zircon and igneous andmetamorphic petrogenesis In Hanchar JM Hoskin PWO (Eds) zircon ReviewsinMineralogy andGeochemistry 53Mineralogical Society of AmericaWashingtonpp 27ndash62

Iglesias M Choukroune P 1980 Shear zones in the Iberian arc Journal of StructuralGeology 2 63ndash68

Ireland TR Williams IS 2003 Considerations in zircon geochronology by SIMSIn Hanchar JM Hoskin PWO (Eds) Zircon Reviews in Mineralogy andGeochemistry 53 Mineralogical Society of America Washington pp 215ndash241

Kellett DA Grujic D Erdmann S 2009 Miocene structural reorganization of theSouth Tibetan detachment eastern Himalaya implications for continental collisionLithosphere 1 259ndash281 doi101130L561

Kempe U Gruner T Nasdala L Wolf D 2000 Relevance of cathodoluminescence forthe interpretation of UndashPb zircon ages with an example of an application to a studyof zircons from the Saxonian Granulite Complex Germany In Pagel M Barbin VBlanc P Ohnenstetter D (Eds) Cathodoluminescence in Geosciences SpringerBerlin pp 425ndash455

Korotev RL 1996 A self-consistent compilation of elemental concentration data for 93geochemical reference samples Geostandards Newsletter 20 217ndash245

Linnemann U Pereira F Jeffries TE Drost K Gerdes A 2008 The CadomianOrogeny and the opening of the Rheic Ocean the diachrony of geotectonicprocesses constrained by LA-ICP-MS UndashPb zircon dating (Ossa-Morena andSaxo-Thuringian Zones Iberian and Bohemian Massifs) Tectonophysics 46121ndash43

Lowery Claiborne L Miller CF Walker BA Wooden JL Mazdab FK Bea F 2006Tracking magmatic processes through ZrHf ratios in rocks and Hf and Ti zoning inzircons an example from the Spirit Mountain batholith Nevada MineralogicalMagazine 70 517ndash543

Ludwig KR 2002 SQUID 102 a users manual Berkeley Geochronology Center SpecialPublication 2 17

Ludwig KR 2003 ISOPLOTEx version 3 a geochronological toolkit for MicrosoftExcel Berkeley Geochronology Center Special Publication 4 71

Ludwig KR Mundil R 2002 Extracting reliable UndashPb ages and errors from complexpopulations of zircons from Phanerozoic tuffs Geochimica et Cosmochimica Acta66 463

Martiacutenez Catalaacuten JR Arenas R Abati J Saacutenchez Martiacutenez S Diacuteaz Garciacutea FFernaacutendez Suaacuterez J Gonzaacutelez Cuadra P Castintildeeiras P Goacutemez Barreiro J DiacuteezMontes A Gonzaacutelez Clavijo E Rubio Pascual FJ Andonaegui P Jeffries TEAlcock JE Diacuteez Fernaacutendez R Loacutepez Carmona A 2009 A rootless suture and the


lost roots of a mountain chain the Variscan belt of NW Iberia Comptes RendusGeoscience 341 114ndash126

Martiacutenez Catalaacuten JR Arenas R Diacuteaz Garciacutea F Gonzaacutelez Cuadra P Goacutemez- BarreiroJ Abati J Castintildeeiras P Fernaacutendez-Suaacuterez J Saacutenchez Martiacutenez S AndonaeguiP Gonzaacutelez Clavijo E Diacuteez Montes A Rubio Pascual FJ Valle Aguado B 2007Space and time in the tectonic evolution of the northwestern Iberian Massifimplications for the Variscan belt In Hatcher Jr RD Carlson MP McBride JHMartiacutenez Catalaacuten JR (Eds) 4-D Framework of Continental Crust GeologicalSociety of America Memoir 200 pp 403ndash423

Martiacutenez Catalaacuten JR Diacuteaz Garciacutea F Arenas R Abati J Castintildeeiras P Gonzaacutelez CuadraP Goacutemez Barreiro J Rubio Pascual F 2002 Thrust and detachment systems in theOacuterdenes Complex (northwestern Spain) implications for the VariscanndashAppala-chian geodynamics In Martiacutenez Catalaacuten JR Hatcher Jr RD Arenas R DiacuteazGarciacutea F (Eds) VariscanndashAppalachianDynamics the building of the Late PaleozoicBasement Geological Society of America Special Paper 364 pp 163ndash182

Mattinson CG Wooden JL Zhang JX Bird DK 2009 Paragneiss zircongeochronology and trace element geochemistry North Qaidam HPUHP terranewestern China Journal of Asian Earth Sciences 35 298ndash309

McClelland WC Power SE Gilotti JA Mazdab FK Wopenka B 2006 UndashPbSHRIMP geochronology and trace-element geochemistry of coesite-bearingzircons North-East Greenland Caledonides In Hacker BR McClelland WCLiou JG (Eds) Ultrahigh-pressure metamorphism Deep continental subductionGeological Society of America Boulder Geological Society of America Special Paper403 pp 23ndash43

Moreno T GibbonsW Prichard HM Lunar R 2001 Platiniferous chromitite and thetectonic setting of ultramafic rocks in Cabo Ortegal NW Spain Journal of theGeological Society 158 601ndash614

Murphy JB Gutieacuterrez Alonso G Nance RD Fernaacutendez-Suaacuterez J Keppie JDQuesada C Strachan RA Dostal J 2006 Origin of the Rheic Ocean rifting along aNeoproterozoic suture Geology 34 325ndash328

Murphy JB Keppie JD Nance RD Dostal J 2010 Comparative evolution of theIapetus and Rheic Oceans a North America perspective Gondwana Research 17482ndash499 doi101016jgr200908009

Nance RD Gutieacuterrez-Alonso G Keppie JD Linnemann U Murphy JB Quesada CStrachan RA Woodcock NH 2010 Evolution of the Rheic Ocean GondwanaResearch in press doi101016jgr200908001

Ordontildeez Casado B Gebauer D Schaumlfer HJ Gil Ibarguchi JI Peucat JJ 2001 A singleDevonian subduction event for the HPHT metamorphism of the Cabo Ortegalcomplex within the Iberian Massif Tectonophysics 332 359ndash385

Peucat JJ Bernard-Griffiths J Gil Ibarguchi JI Dallmeyer RD Menot RP Cornichet JPonce Iglesias de Leoacuten M 1990 Geochemical and geochronological cross section ofthe deep Variscan crust the Cabo Ortegal high-pressure nappe (northwestern Spain)Tectonophysics 177 263ndash292

Puelles P Aacutebalos B Gil Ibarguchi JI 2005 Metamorphic evolution and thermobaricstructure of the subduction-related Bacariza high-pressure granulite formation(Cabo Ortegal Complex NW Spain) Lithos 84 125ndash149

Ribeiro A Munha J Dias R Mateus A Pereira E Ribeiro L Fonseca P Araujo AOliveira T Romao J Chamine H Coke C Pedro J 2007 Geodynamic evolutionof the SW Europe Variscides Tectonics 26 1ndash24

Roger F Matte Ph 2005 Early Variscan HP metamorphism in the western IberianAllochthon A 390 MaUndashPb age for the Braganccedila eclogite (NWPortugal) InternationalJournal of Earth Sciences 94 173ndash179

Rubatto D 2002 Zircon trace element geochemistry partitioning with garnet and thelink between UndashPb ages and metamorphism Chemical Geology 184 123ndash138

Rubatto D Gebauer D 2000 Use of cathodoluminescence for UndashPb zircon dating byion microprobe some examples from the Western Alps In Pagel M Barbin VBlanc P Ohnenstetter D (Eds) Cathodoluminescence in Geosciences SpringerBerlin pp 373ndash400

Saacutenchez Martiacutenez S Arenas R Diacuteaz Garciacutea S Martiacutenez Catalaacuten JR Goacutemez BarreiroJ Pearce J 2007 The Careoacuten Ophiolite NW Spain supra- subduction zone settingfor the youngest Rheic Ocean floor Geology 35 53ndash56

Santos Zalduegui JF SchaumlrerU Gil Ibarguchi JI Girardeau J 1996 Origin and evolutionof thePaleozoicCaboOrtegal ultramaficndashmafic complex (NWSpain)UndashPb RbndashSr andPbndashPb isotope data Chemical Geology 129 281ndash304

Santos Zalduegui JF Schaumlrer U Gil Ibarguchi JI Girardeau J 2002 Genesis ofpyroxenite-rich peritotite at Cabo Ortegal (NW Spain) geochemical and PbndashSrndashNdisotope data Journal of Petrology 43 17ndash43

Schaumlfer HG Gebauer D Gil Ibarguchi JI Peucat JJ 1993 Ion-microprobe UndashPbzircon dating on the HPHt Cabo Ortegal Complex (Galicia NW Spain) preliminaryresults Terra Abstracts 5 (4) 22

Stacey JS Kramers JD 1975 Approximation of terrestrial lead isotope evolution by atwo-stage model Earth and Planetary Science Letters 26 207ndash221

van Calsteren PWC Boelrijk NAIM Hebeda EH Priem HNA Den Tex EVerdurmen EAT Verschure RH 1979 Isotopic dating of older elements (includingthe Cabo Ortegal maficndashultramafic complex) in the Hercynian orogen of NW Spainmanifestations of a presumed early Palaeozoic mantle-plume Chemical Geology 2435ndash56

Vavra G Gebauer D Schmid R Compston W 1996 Multiple zircon growth andrecrystallization during polyphase Late Carboniferous to Triassic metamorphism ingranulites of the Ivrea Zone (Southern Alps) an ion microprobe (SHRIMP) studyContributions to Mineralogy and Petrology 122 337ndash358

von Raumer JF Stampfli GM 2008 The birth of the Rheic OceanndashEarly Palaeozoicsubsidence patterns and subsequent tectonic plate scenarios Tectonophysics 4619ndash20

Whitehouse MJ Platt JP 2003 Dating high-grade metamorphism-constraints fromrare-earth elements in zircon and garnet Contributions to Mineralogy and Petrology145 61ndash74

Williams IS 1997 UndashThndashPb geochronology by ion microprobe not just ages buthistories Economic Geology 7 1ndash35

Wooden JL Mazdab FK Barth AP Miller CF Lowery LE 2006 Temperatures (Ti)and compositional characteristics of zircon early observations using high massresolution on the USGS-Stanford SHRIMP-RG Third SHRIMP workshop RottnestIsland Australia Program and abstracts 64ndash65

Yui TF Okamoto K Usuki T Lan CY Liou JG 2009 TriassicndashJurassic accretionsubduction in Taiwan region along the SE margin of South China evidence fromzircon SHRIMP dating International Geology Review 51 304ndash328


abraded by transport) or metamorphic zircons from those grains thatare grown during magmatism

In the present paper we use this new approach (REE-assisted zircondating) to interpret the age distribution obtained in zircons from the ASilva granodiorite Additionally the well-preserved intrinsic contactsand the general weak deformation in the A Silva granodiorite make it agood tectonic marker that enables us to better understand therelationships among its intrusion and the development of the regionalfoliation the generalized top-to-the-NNW shearing and the partialmeltingprocesses operating in themetasedimentary country rocks Thisleads us to the recognition of a transitory stage of vertical decouplingbetween the upper and intermediate levels and the lower levels of thearc during its tectonic evolution

We also first show through field mapping structural analysis andUndashPb zircon dating that the A Silva granodiorite is older thanpreviously thought and it is related to a CambrianndashOrdovician arcdeveloped in the northern margin of Gondwana (Goacutemez Barreiroet al 2007) This arc drifted away from the continent giving rise tothe Rheic Ocean (Bozkurt et al 2008 von Raumer and Stampfli 2008Nance in press)

Finally we consider the general implications of our study for thecharacterization of the type timing and kinematics of the tectonicprocesses that contributed to the exhumation of the orogenic rootgenerated in the Gondwana continental margin during CambrianndashOrdovician times

2 Geological background

Preserved as megaklippen in the core of open synforms theallochthonous complexes occupy the highest structural position in theVariscan belt of the NW Iberian massif (see Martiacutenez Catalaacuten et al2009 and references therein) These complexes include a number ofallochthonous slices that appear well represented in the largest theOacuterdenes Complex (Fig 1) These slices have been grouped into threemain units named from bottom to top the basal units the ophioliticunits (representing the Variscan suture) and the upper units Thebasal and ophiolitic units record a westward oceanic subduction (DiacuteazGarciacutea et al 1999a Saacutenchez Martiacutenez et al 2007 Goacutemez Barreiroet al 2010) that evolved to continental subduction of the Gondwanancontinental margin during Devonian to Early Carboniferous times(Arenas et al 1995 Martiacutenez Catalaacuten et al 2007 2009)

The upper unit is tectonically emplaced above the ophiolitic unitand it presents increasing petrological evidence suggesting that it wasgenerated in an arc environment of the Gondwana continental margin(eg Abati et al 1999 2003 Fernaacutendez Suaacuterez et al 2003Andonaegui et al 2002 Puelles et al 2005) This arc was separatedfrom Gondwana during the Early Ordovician starting to drift towardsthe north at the same time of the opening of the Rheic Ocean (GoacutemezBarreiro et al 2007 Martiacutenez Catalaacuten et al 2009) During the EarlyDevonian the upper unit was emplaced over the Gondwanacontinental margin at the top of the Variscan pile (eg Diacuteaz Garciacuteaet al 1999a) This main tectonic process allows us to directly examinedifferent levels of an arc-derived terrane that would originally haverepresented an arc crust nearly 55 km thick according to paleopres-sure estimates (eg Gil Ibarguchi et al 1999 Arenas and MartiacutenezCatalaacuten 2002 Puelles et al 2005) This crustal ensemble is nowstrongly reduced to approximately 12 km in thickness and it can befurther divided into three additional units with contrasting tectono-metamorphic evolution exhumed from different depths and sepa-rated by extensional detachments that from bottom to top are(Martiacutenez Catalaacuten et al 2002) the high-pressure and high-temperature unit (HPndashHT with remnants of the lower crust andupper mantle) the intermediate-pressure unit with intermediate tohigh-temperature (IPndashIHT a section of the middle crust) and theintermediate-pressure unit with low-temperature (IPndashLT represent-ing the upper crust)

The HPndashHT upper units consist of high-pressure mafic to felsicgranulites eclogites and high-pressure gneisses with closely relatedultramafic massifs Most of the mafic rocks are metagabbros oftholeiitic composition Their geochemical signature has been com-pared to MORB (Gil Ibarguchi et al 1990) and geochemical studies onthe ultramafic rocks from the Cabo Ortegal outcrops are consistentwith the hypothesis that these rocks were generated in an arc setting(Moreno et al 2001 Santos Zalduegui et al 2002)

Fabric studies of the main foliation (eg Engels 1972 Aacutebalos1997) demonstrate that eclogite and granulite rocks underwent aregional intense ductile deformation during the highest pressureconditions followed by widespread development of an amphibolitefacies foliation related to their continued exhumation Later duringthe Variscan Orogeny progress isoclinal folds and thrusts underepizonal conditions (eg Martiacutenez Catalaacuten et al 2002 GoacutemezBarreiro et al 2007) reworked the whole ensemble

The age of the HPndashHT metamorphism in the allochthonouscomplexes is a matter of intense debate which will be summarized asfollows A Precambrian age was proposed based only on the tectoniccomplexity (eg Ribeiro et al 2007) On the basis of UndashPb zircon datasome authors have proposed a Late CambrianndashEarly Ordovician age (ca480ndash495 Ma Peucat et al 1990 Fernaacutendez Suaacuterez et al 2002 2007)whereas others favor a roughly Middle Devonian age (ca 380ndash390 MaSchafer et al 1993 Santos Zalduegui et al 1996 Ordontildeez Casado et al2001 Roger and Matte 2005)

The IPndashIHT (intermediate-pressure intermediate- to high-tem-perature) upper unit consists of a metasedimentary sequence locallynamed the O Pino unit which is dominated by staurolitendashkyaniteschists and sillimanitendashbiotite paragneisses (Abati et al 2003Castintildeeiras 2005 Goacutemez Barreiro 2007) Late Cambrian metaplu-tonic massifs such us the Monte Castelo gabbro and the Corredoirasorthogneiss occur near the base of the unit together with a number ofdifferently sized gabbroic and granodioritic plutonic bodies TheMonte Castelo gabbro cropping out to the west of the OacuterdenesComplex is the largest and is a massive two-pyroxene gabbro with atholeiitic character very similar to those of island arc tholeiites(Andonaegui et al 2002) Under the microscope textures vary fromgranular to intergranular and ophitic The presence of olivine and thecommon ophitic textures indicate a relatively shallow emplacement

The Corredoiras orthogneiss is a large granodioritic massif withminor tonalitic and mafic rocks that crops out in the eastern part ofthe Oacuterdenes Complex It exhibits a porphyritic texture with alkalifeldspar and plagioclase as porphyroclasts in a coarse-grained matrixThe Corredoiras orthogneiss and the related lithologies define a high-K calc-alkaline series and tectonic discrimination diagrams based onimmobile elements suggest a volcanic-arc setting (Andonaegui et al2007)

The Corredoiras orthogneiss has been affected by intense top-to-the-north shearing developing hectometre-thick shear zones withinthe body and in the basal part that brought it into contact with theunderlying HPndashHT Sobrado unit (Gonzaacutelez Cuadra 2007) Thestructure responsible for its final emplacement above the ophioliticunit is also a shear zone developed under greenschist facies conditionsand located in its eastern part where top-to-the-SE kinematic criteriahave been obtained (Diacuteaz Garciacutea et al 1999b)

Ages belonging to these two main plutonic bodies (ID-TIMS UndashPbin zircon 499plusmn2 Ma for theMonte Castelo gabbro and 500plusmn2 Ma forthe Corredoiras orthogneiss Abati et al 1999) confirm thatvoluminous tholeiitic and calc-alkaline magmatism had a peak ataround 500 Ma and was shortly followed by the development ofshear zones In the basal section of the Monte Castelo gabbro thisshearing was developed under granulite facies conditions andrecorded a pressure increase of 2ndash4 kb (Abati et al 2003) Theshearing was first indirectly dated using UndashPb in monazites from thehost high-grade metasediments that yielded an age between 493 and498 Ma (Abati et al 1999) More recently an age of 483plusmn4 Ma was




































43 UndashPb results







5 Discussion





Common 206Pb()b

U(ppm)

Th(ppm)








Table 1 (continued)


Common 206Pb()b

U(ppm)

Th(ppm)


























4 483 58 13184 0135 na 111 492 009 64 372 703 1285 227 4055 012 200 002 ndash ndash



6 351 45 14076 0009 na 071 261 011 38 214 362 573 99 2393 013 149 003 ndash ndash


72 598 56 12791 0710 na 161 443 022 58 329 454 605 94 3474 009 105 004 ndash ndash












161 759 37 14474 0008 na 038 229 006 42 320 534 840 132 3613 005 201 002 ndash ndash


17 687 74 12758 0013 na 098 500 011 69 372 601 937 154 3829 011 136 002 ndash ndash

18 295 47 12853 0793 na 180 386 020 50 256 371 523 86 2441 016 105 004 ndash ndash

19 614 37 13680 0052 na 050 262 009 40 248 356 512 81 2518 006 127 003 ndash ndash

20 433 26 13644 0004 na 028 184 006 29 199 312 443 70 2109 006 155 002 ndash ndash

21 333 60 11940 0008 na 149 617 018 82 441 756 1200 204 4461 018 146 002 ndash ndash

A Silva-21 243 41 13179 0440 230 150 388 018 46 210 301 446 73 1922 017 98 004 059 1062 337 53 12363 0019 213 287 1000 043 109 376 314 290 39 2925 016 27 004 021 1593 244 63 11640 0545 302 234 750 025 79 331 533 830 142 3282 026 106 003 040 814 654 83 13553 0104 184 135 593 022 83 501 896 1442 238 5356 013 174 003 031 3555 HcPb 962 79 14884 15400 5098 2830 2608 359 132 625 828 1266 200 5195 008 96 019 195 196 377 50 12637 0016 133 094 406 012 55 291 476 746 124 2906 013 135 002 033 2847 388 28 13021 0422 136 054 253 009 40 282 505 778 124 3128 007 196 003 054 2868 334 20 13293 0009 057 031 161 005 26 167 242 340 53 1747 006 132 003 035 5919 358 39 12570 0108 117 096 461 014 59 288 396 541 86 2764 011 91 003 025 30510 1536 55 17130 0639 248 128 393 021 53 387 586 911 140 4041 004 172 004 063 62011 383 30 13051 0003 082 062 294 010 42 224 303 403 63 2295 008 97 003 028 46412 323 66 12128 0019 174 155 585 012 65 304 455 675 111 2920 020 104 002 030 18613 177 46 11651 0019 219 227 639 031 64 282 495 784 136 2929 026 122 005 034 8114 880 46 14078 0009 079 057 327 009 51 348 493 650 90 3477 005 129 002 024 111015 320 53 11946 0443 290 144 408 021 50 270 433 653 107 2660 016 130 005 071 11116 331 43 12204 0035 137 130 571 021 72 374 587 882 147 3687 013 123 003 024 24117 298 24 12689 0008 065 040 234 008 35 196 290 421 69 1963 008 119 003 028 45718 273 41 11899 0010 135 120 464 016 61 319 572 930 161 3270 015 152 003 029 20219 343 34 12370 0008 101 084 420 014 61 342 548 810 132 3575 010 132 003 024 33920 551 44 13079 0006 087 054 300 009 46 301 443 595 89 3065 008 128 002 029 63021 222 68 10810 0030 264 269 846 021 89 380 651 1038 180 3788 031 117 002 031 84



Table 2 (continued)



A Silva-222 544 50 12980 1594 284 112 289 009 42 270 409 572 89 2915 009 137 002 098 19123 577 63 11859 0161 163 195 573 017 71 372 554 814 132 3812 011 115 002 028 35424 772 95 13821 1622 568 304 658 049 70 401 631 933 145 3927 012 133 007 086 13625 319 25 13196 0008 086 078 355 017 44 165 150 167 25 1292 008 38 004 024 369261 603 29 13645 0007 063 024 172 005 29 233 364 485 69 2554 005 165 002 037 949262 576 91 12694 0047 240 322 1177 023 145 818 1508 2462 401 8796 016 170 002 020 24027 908 45 14994 0109 100 045 278 008 45 348 536 740 107 3651 005 166 002 036 91028 357 39 12656 0014 113 098 463 015 64 373 622 947 156 3776 011 148 003 024 31529 306 99 11662 0028 280 227 657 025 72 351 622 977 166 3576 032 137 004 043 10930 372 52 11778 0015 147 130 484 015 63 334 565 879 147 3463 014 139 003 030 25231 654 49 13086 0476 185 099 408 015 58 372 518 683 101 3633 008 118 003 045 35432 239 37 11444 0025 142 110 432 014 56 299 536 855 146 3086 016 152 003 033 16833 189 26 12360 0012 114 071 294 012 40 240 470 807 142 2698 014 203 003 039 16634 393 37 12408 0005 102 084 410 010 56 347 606 1014 171 3575 009 183 002 025 38635 453 53 12177 0010 162 130 574 020 77 393 584 826 135 3902 012 107 003 028 280



























6 Conclusions






Acknowledgements




References





















































































































43 UndashPb results







5 Discussion





Common 206Pb()b

U(ppm)

Th(ppm)








Table 1 (continued)


Common 206Pb()b

U(ppm)

Th(ppm)


























4 483 58 13184 0135 na 111 492 009 64 372 703 1285 227 4055 012 200 002 ndash ndash



6 351 45 14076 0009 na 071 261 011 38 214 362 573 99 2393 013 149 003 ndash ndash


72 598 56 12791 0710 na 161 443 022 58 329 454 605 94 3474 009 105 004 ndash ndash












161 759 37 14474 0008 na 038 229 006 42 320 534 840 132 3613 005 201 002 ndash ndash


17 687 74 12758 0013 na 098 500 011 69 372 601 937 154 3829 011 136 002 ndash ndash

18 295 47 12853 0793 na 180 386 020 50 256 371 523 86 2441 016 105 004 ndash ndash

19 614 37 13680 0052 na 050 262 009 40 248 356 512 81 2518 006 127 003 ndash ndash

20 433 26 13644 0004 na 028 184 006 29 199 312 443 70 2109 006 155 002 ndash ndash

21 333 60 11940 0008 na 149 617 018 82 441 756 1200 204 4461 018 146 002 ndash ndash

A Silva-21 243 41 13179 0440 230 150 388 018 46 210 301 446 73 1922 017 98 004 059 1062 337 53 12363 0019 213 287 1000 043 109 376 314 290 39 2925 016 27 004 021 1593 244 63 11640 0545 302 234 750 025 79 331 533 830 142 3282 026 106 003 040 814 654 83 13553 0104 184 135 593 022 83 501 896 1442 238 5356 013 174 003 031 3555 HcPb 962 79 14884 15400 5098 2830 2608 359 132 625 828 1266 200 5195 008 96 019 195 196 377 50 12637 0016 133 094 406 012 55 291 476 746 124 2906 013 135 002 033 2847 388 28 13021 0422 136 054 253 009 40 282 505 778 124 3128 007 196 003 054 2868 334 20 13293 0009 057 031 161 005 26 167 242 340 53 1747 006 132 003 035 5919 358 39 12570 0108 117 096 461 014 59 288 396 541 86 2764 011 91 003 025 30510 1536 55 17130 0639 248 128 393 021 53 387 586 911 140 4041 004 172 004 063 62011 383 30 13051 0003 082 062 294 010 42 224 303 403 63 2295 008 97 003 028 46412 323 66 12128 0019 174 155 585 012 65 304 455 675 111 2920 020 104 002 030 18613 177 46 11651 0019 219 227 639 031 64 282 495 784 136 2929 026 122 005 034 8114 880 46 14078 0009 079 057 327 009 51 348 493 650 90 3477 005 129 002 024 111015 320 53 11946 0443 290 144 408 021 50 270 433 653 107 2660 016 130 005 071 11116 331 43 12204 0035 137 130 571 021 72 374 587 882 147 3687 013 123 003 024 24117 298 24 12689 0008 065 040 234 008 35 196 290 421 69 1963 008 119 003 028 45718 273 41 11899 0010 135 120 464 016 61 319 572 930 161 3270 015 152 003 029 20219 343 34 12370 0008 101 084 420 014 61 342 548 810 132 3575 010 132 003 024 33920 551 44 13079 0006 087 054 300 009 46 301 443 595 89 3065 008 128 002 029 63021 222 68 10810 0030 264 269 846 021 89 380 651 1038 180 3788 031 117 002 031 84



Table 2 (continued)



A Silva-222 544 50 12980 1594 284 112 289 009 42 270 409 572 89 2915 009 137 002 098 19123 577 63 11859 0161 163 195 573 017 71 372 554 814 132 3812 011 115 002 028 35424 772 95 13821 1622 568 304 658 049 70 401 631 933 145 3927 012 133 007 086 13625 319 25 13196 0008 086 078 355 017 44 165 150 167 25 1292 008 38 004 024 369261 603 29 13645 0007 063 024 172 005 29 233 364 485 69 2554 005 165 002 037 949262 576 91 12694 0047 240 322 1177 023 145 818 1508 2462 401 8796 016 170 002 020 24027 908 45 14994 0109 100 045 278 008 45 348 536 740 107 3651 005 166 002 036 91028 357 39 12656 0014 113 098 463 015 64 373 622 947 156 3776 011 148 003 024 31529 306 99 11662 0028 280 227 657 025 72 351 622 977 166 3576 032 137 004 043 10930 372 52 11778 0015 147 130 484 015 63 334 565 879 147 3463 014 139 003 030 25231 654 49 13086 0476 185 099 408 015 58 372 518 683 101 3633 008 118 003 045 35432 239 37 11444 0025 142 110 432 014 56 299 536 855 146 3086 016 152 003 033 16833 189 26 12360 0012 114 071 294 012 40 240 470 807 142 2698 014 203 003 039 16634 393 37 12408 0005 102 084 410 010 56 347 606 1014 171 3575 009 183 002 025 38635 453 53 12177 0010 162 130 574 020 77 393 584 826 135 3902 012 107 003 028 280



























6 Conclusions






Acknowledgements




References




















































































Common 206Pb()b

U(ppm)

Th(ppm)








Table 1 (continued)


Common 206Pb()b

U(ppm)

Th(ppm)


























4 483 58 13184 0135 na 111 492 009 64 372 703 1285 227 4055 012 200 002 ndash ndash



6 351 45 14076 0009 na 071 261 011 38 214 362 573 99 2393 013 149 003 ndash ndash


72 598 56 12791 0710 na 161 443 022 58 329 454 605 94 3474 009 105 004 ndash ndash












161 759 37 14474 0008 na 038 229 006 42 320 534 840 132 3613 005 201 002 ndash ndash


17 687 74 12758 0013 na 098 500 011 69 372 601 937 154 3829 011 136 002 ndash ndash

18 295 47 12853 0793 na 180 386 020 50 256 371 523 86 2441 016 105 004 ndash ndash

19 614 37 13680 0052 na 050 262 009 40 248 356 512 81 2518 006 127 003 ndash ndash

20 433 26 13644 0004 na 028 184 006 29 199 312 443 70 2109 006 155 002 ndash ndash

21 333 60 11940 0008 na 149 617 018 82 441 756 1200 204 4461 018 146 002 ndash ndash

A Silva-21 243 41 13179 0440 230 150 388 018 46 210 301 446 73 1922 017 98 004 059 1062 337 53 12363 0019 213 287 1000 043 109 376 314 290 39 2925 016 27 004 021 1593 244 63 11640 0545 302 234 750 025 79 331 533 830 142 3282 026 106 003 040 814 654 83 13553 0104 184 135 593 022 83 501 896 1442 238 5356 013 174 003 031 3555 HcPb 962 79 14884 15400 5098 2830 2608 359 132 625 828 1266 200 5195 008 96 019 195 196 377 50 12637 0016 133 094 406 012 55 291 476 746 124 2906 013 135 002 033 2847 388 28 13021 0422 136 054 253 009 40 282 505 778 124 3128 007 196 003 054 2868 334 20 13293 0009 057 031 161 005 26 167 242 340 53 1747 006 132 003 035 5919 358 39 12570 0108 117 096 461 014 59 288 396 541 86 2764 011 91 003 025 30510 1536 55 17130 0639 248 128 393 021 53 387 586 911 140 4041 004 172 004 063 62011 383 30 13051 0003 082 062 294 010 42 224 303 403 63 2295 008 97 003 028 46412 323 66 12128 0019 174 155 585 012 65 304 455 675 111 2920 020 104 002 030 18613 177 46 11651 0019 219 227 639 031 64 282 495 784 136 2929 026 122 005 034 8114 880 46 14078 0009 079 057 327 009 51 348 493 650 90 3477 005 129 002 024 111015 320 53 11946 0443 290 144 408 021 50 270 433 653 107 2660 016 130 005 071 11116 331 43 12204 0035 137 130 571 021 72 374 587 882 147 3687 013 123 003 024 24117 298 24 12689 0008 065 040 234 008 35 196 290 421 69 1963 008 119 003 028 45718 273 41 11899 0010 135 120 464 016 61 319 572 930 161 3270 015 152 003 029 20219 343 34 12370 0008 101 084 420 014 61 342 548 810 132 3575 010 132 003 024 33920 551 44 13079 0006 087 054 300 009 46 301 443 595 89 3065 008 128 002 029 63021 222 68 10810 0030 264 269 846 021 89 380 651 1038 180 3788 031 117 002 031 84



Table 2 (continued)



A Silva-222 544 50 12980 1594 284 112 289 009 42 270 409 572 89 2915 009 137 002 098 19123 577 63 11859 0161 163 195 573 017 71 372 554 814 132 3812 011 115 002 028 35424 772 95 13821 1622 568 304 658 049 70 401 631 933 145 3927 012 133 007 086 13625 319 25 13196 0008 086 078 355 017 44 165 150 167 25 1292 008 38 004 024 369261 603 29 13645 0007 063 024 172 005 29 233 364 485 69 2554 005 165 002 037 949262 576 91 12694 0047 240 322 1177 023 145 818 1508 2462 401 8796 016 170 002 020 24027 908 45 14994 0109 100 045 278 008 45 348 536 740 107 3651 005 166 002 036 91028 357 39 12656 0014 113 098 463 015 64 373 622 947 156 3776 011 148 003 024 31529 306 99 11662 0028 280 227 657 025 72 351 622 977 166 3576 032 137 004 043 10930 372 52 11778 0015 147 130 484 015 63 334 565 879 147 3463 014 139 003 030 25231 654 49 13086 0476 185 099 408 015 58 372 518 683 101 3633 008 118 003 045 35432 239 37 11444 0025 142 110 432 014 56 299 536 855 146 3086 016 152 003 033 16833 189 26 12360 0012 114 071 294 012 40 240 470 807 142 2698 014 203 003 039 16634 393 37 12408 0005 102 084 410 010 56 347 606 1014 171 3575 009 183 002 025 38635 453 53 12177 0010 162 130 574 020 77 393 584 826 135 3902 012 107 003 028 280



























6 Conclusions






Acknowledgements




References


















































































Table 1 (continued)


Common 206Pb()b

U(ppm)

Th(ppm)


























4 483 58 13184 0135 na 111 492 009 64 372 703 1285 227 4055 012 200 002 ndash ndash



6 351 45 14076 0009 na 071 261 011 38 214 362 573 99 2393 013 149 003 ndash ndash


72 598 56 12791 0710 na 161 443 022 58 329 454 605 94 3474 009 105 004 ndash ndash












161 759 37 14474 0008 na 038 229 006 42 320 534 840 132 3613 005 201 002 ndash ndash


17 687 74 12758 0013 na 098 500 011 69 372 601 937 154 3829 011 136 002 ndash ndash

18 295 47 12853 0793 na 180 386 020 50 256 371 523 86 2441 016 105 004 ndash ndash

19 614 37 13680 0052 na 050 262 009 40 248 356 512 81 2518 006 127 003 ndash ndash

20 433 26 13644 0004 na 028 184 006 29 199 312 443 70 2109 006 155 002 ndash ndash

21 333 60 11940 0008 na 149 617 018 82 441 756 1200 204 4461 018 146 002 ndash ndash

A Silva-21 243 41 13179 0440 230 150 388 018 46 210 301 446 73 1922 017 98 004 059 1062 337 53 12363 0019 213 287 1000 043 109 376 314 290 39 2925 016 27 004 021 1593 244 63 11640 0545 302 234 750 025 79 331 533 830 142 3282 026 106 003 040 814 654 83 13553 0104 184 135 593 022 83 501 896 1442 238 5356 013 174 003 031 3555 HcPb 962 79 14884 15400 5098 2830 2608 359 132 625 828 1266 200 5195 008 96 019 195 196 377 50 12637 0016 133 094 406 012 55 291 476 746 124 2906 013 135 002 033 2847 388 28 13021 0422 136 054 253 009 40 282 505 778 124 3128 007 196 003 054 2868 334 20 13293 0009 057 031 161 005 26 167 242 340 53 1747 006 132 003 035 5919 358 39 12570 0108 117 096 461 014 59 288 396 541 86 2764 011 91 003 025 30510 1536 55 17130 0639 248 128 393 021 53 387 586 911 140 4041 004 172 004 063 62011 383 30 13051 0003 082 062 294 010 42 224 303 403 63 2295 008 97 003 028 46412 323 66 12128 0019 174 155 585 012 65 304 455 675 111 2920 020 104 002 030 18613 177 46 11651 0019 219 227 639 031 64 282 495 784 136 2929 026 122 005 034 8114 880 46 14078 0009 079 057 327 009 51 348 493 650 90 3477 005 129 002 024 111015 320 53 11946 0443 290 144 408 021 50 270 433 653 107 2660 016 130 005 071 11116 331 43 12204 0035 137 130 571 021 72 374 587 882 147 3687 013 123 003 024 24117 298 24 12689 0008 065 040 234 008 35 196 290 421 69 1963 008 119 003 028 45718 273 41 11899 0010 135 120 464 016 61 319 572 930 161 3270 015 152 003 029 20219 343 34 12370 0008 101 084 420 014 61 342 548 810 132 3575 010 132 003 024 33920 551 44 13079 0006 087 054 300 009 46 301 443 595 89 3065 008 128 002 029 63021 222 68 10810 0030 264 269 846 021 89 380 651 1038 180 3788 031 117 002 031 84



Table 2 (continued)



A Silva-222 544 50 12980 1594 284 112 289 009 42 270 409 572 89 2915 009 137 002 098 19123 577 63 11859 0161 163 195 573 017 71 372 554 814 132 3812 011 115 002 028 35424 772 95 13821 1622 568 304 658 049 70 401 631 933 145 3927 012 133 007 086 13625 319 25 13196 0008 086 078 355 017 44 165 150 167 25 1292 008 38 004 024 369261 603 29 13645 0007 063 024 172 005 29 233 364 485 69 2554 005 165 002 037 949262 576 91 12694 0047 240 322 1177 023 145 818 1508 2462 401 8796 016 170 002 020 24027 908 45 14994 0109 100 045 278 008 45 348 536 740 107 3651 005 166 002 036 91028 357 39 12656 0014 113 098 463 015 64 373 622 947 156 3776 011 148 003 024 31529 306 99 11662 0028 280 227 657 025 72 351 622 977 166 3576 032 137 004 043 10930 372 52 11778 0015 147 130 484 015 63 334 565 879 147 3463 014 139 003 030 25231 654 49 13086 0476 185 099 408 015 58 372 518 683 101 3633 008 118 003 045 35432 239 37 11444 0025 142 110 432 014 56 299 536 855 146 3086 016 152 003 033 16833 189 26 12360 0012 114 071 294 012 40 240 470 807 142 2698 014 203 003 039 16634 393 37 12408 0005 102 084 410 010 56 347 606 1014 171 3575 009 183 002 025 38635 453 53 12177 0010 162 130 574 020 77 393 584 826 135 3902 012 107 003 028 280



























6 Conclusions






Acknowledgements




References




























































































4 483 58 13184 0135 na 111 492 009 64 372 703 1285 227 4055 012 200 002 ndash ndash



6 351 45 14076 0009 na 071 261 011 38 214 362 573 99 2393 013 149 003 ndash ndash


72 598 56 12791 0710 na 161 443 022 58 329 454 605 94 3474 009 105 004 ndash ndash












161 759 37 14474 0008 na 038 229 006 42 320 534 840 132 3613 005 201 002 ndash ndash


17 687 74 12758 0013 na 098 500 011 69 372 601 937 154 3829 011 136 002 ndash ndash

18 295 47 12853 0793 na 180 386 020 50 256 371 523 86 2441 016 105 004 ndash ndash

19 614 37 13680 0052 na 050 262 009 40 248 356 512 81 2518 006 127 003 ndash ndash

20 433 26 13644 0004 na 028 184 006 29 199 312 443 70 2109 006 155 002 ndash ndash

21 333 60 11940 0008 na 149 617 018 82 441 756 1200 204 4461 018 146 002 ndash ndash

A Silva-21 243 41 13179 0440 230 150 388 018 46 210 301 446 73 1922 017 98 004 059 1062 337 53 12363 0019 213 287 1000 043 109 376 314 290 39 2925 016 27 004 021 1593 244 63 11640 0545 302 234 750 025 79 331 533 830 142 3282 026 106 003 040 814 654 83 13553 0104 184 135 593 022 83 501 896 1442 238 5356 013 174 003 031 3555 HcPb 962 79 14884 15400 5098 2830 2608 359 132 625 828 1266 200 5195 008 96 019 195 196 377 50 12637 0016 133 094 406 012 55 291 476 746 124 2906 013 135 002 033 2847 388 28 13021 0422 136 054 253 009 40 282 505 778 124 3128 007 196 003 054 2868 334 20 13293 0009 057 031 161 005 26 167 242 340 53 1747 006 132 003 035 5919 358 39 12570 0108 117 096 461 014 59 288 396 541 86 2764 011 91 003 025 30510 1536 55 17130 0639 248 128 393 021 53 387 586 911 140 4041 004 172 004 063 62011 383 30 13051 0003 082 062 294 010 42 224 303 403 63 2295 008 97 003 028 46412 323 66 12128 0019 174 155 585 012 65 304 455 675 111 2920 020 104 002 030 18613 177 46 11651 0019 219 227 639 031 64 282 495 784 136 2929 026 122 005 034 8114 880 46 14078 0009 079 057 327 009 51 348 493 650 90 3477 005 129 002 024 111015 320 53 11946 0443 290 144 408 021 50 270 433 653 107 2660 016 130 005 071 11116 331 43 12204 0035 137 130 571 021 72 374 587 882 147 3687 013 123 003 024 24117 298 24 12689 0008 065 040 234 008 35 196 290 421 69 1963 008 119 003 028 45718 273 41 11899 0010 135 120 464 016 61 319 572 930 161 3270 015 152 003 029 20219 343 34 12370 0008 101 084 420 014 61 342 548 810 132 3575 010 132 003 024 33920 551 44 13079 0006 087 054 300 009 46 301 443 595 89 3065 008 128 002 029 63021 222 68 10810 0030 264 269 846 021 89 380 651 1038 180 3788 031 117 002 031 84



Table 2 (continued)



A Silva-222 544 50 12980 1594 284 112 289 009 42 270 409 572 89 2915 009 137 002 098 19123 577 63 11859 0161 163 195 573 017 71 372 554 814 132 3812 011 115 002 028 35424 772 95 13821 1622 568 304 658 049 70 401 631 933 145 3927 012 133 007 086 13625 319 25 13196 0008 086 078 355 017 44 165 150 167 25 1292 008 38 004 024 369261 603 29 13645 0007 063 024 172 005 29 233 364 485 69 2554 005 165 002 037 949262 576 91 12694 0047 240 322 1177 023 145 818 1508 2462 401 8796 016 170 002 020 24027 908 45 14994 0109 100 045 278 008 45 348 536 740 107 3651 005 166 002 036 91028 357 39 12656 0014 113 098 463 015 64 373 622 947 156 3776 011 148 003 024 31529 306 99 11662 0028 280 227 657 025 72 351 622 977 166 3576 032 137 004 043 10930 372 52 11778 0015 147 130 484 015 63 334 565 879 147 3463 014 139 003 030 25231 654 49 13086 0476 185 099 408 015 58 372 518 683 101 3633 008 118 003 045 35432 239 37 11444 0025 142 110 432 014 56 299 536 855 146 3086 016 152 003 033 16833 189 26 12360 0012 114 071 294 012 40 240 470 807 142 2698 014 203 003 039 16634 393 37 12408 0005 102 084 410 010 56 347 606 1014 171 3575 009 183 002 025 38635 453 53 12177 0010 162 130 574 020 77 393 584 826 135 3902 012 107 003 028 280



























6 Conclusions






Acknowledgements




References


















































































Table 2 (continued)



A Silva-222 544 50 12980 1594 284 112 289 009 42 270 409 572 89 2915 009 137 002 098 19123 577 63 11859 0161 163 195 573 017 71 372 554 814 132 3812 011 115 002 028 35424 772 95 13821 1622 568 304 658 049 70 401 631 933 145 3927 012 133 007 086 13625 319 25 13196 0008 086 078 355 017 44 165 150 167 25 1292 008 38 004 024 369261 603 29 13645 0007 063 024 172 005 29 233 364 485 69 2554 005 165 002 037 949262 576 91 12694 0047 240 322 1177 023 145 818 1508 2462 401 8796 016 170 002 020 24027 908 45 14994 0109 100 045 278 008 45 348 536 740 107 3651 005 166 002 036 91028 357 39 12656 0014 113 098 463 015 64 373 622 947 156 3776 011 148 003 024 31529 306 99 11662 0028 280 227 657 025 72 351 622 977 166 3576 032 137 004 043 10930 372 52 11778 0015 147 130 484 015 63 334 565 879 147 3463 014 139 003 030 25231 654 49 13086 0476 185 099 408 015 58 372 518 683 101 3633 008 118 003 045 35432 239 37 11444 0025 142 110 432 014 56 299 536 855 146 3086 016 152 003 033 16833 189 26 12360 0012 114 071 294 012 40 240 470 807 142 2698 014 203 003 039 16634 393 37 12408 0005 102 084 410 010 56 347 606 1014 171 3575 009 183 002 025 38635 453 53 12177 0010 162 130 574 020 77 393 584 826 135 3902 012 107 003 028 280



























6 Conclusions






Acknowledgements




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6 Conclusions






Acknowledgements




References


















































































REE-assisted UPb zircon age (SHRIMP) of an anatectic granodiorite: Constraints on the evolution of...

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