). metamorphic Ceol., 1991, 9, 283-292

Late Sveconorwegian (Grenville) high-pressure granulite facies metamorphism in southwest Sweden L. JOHANSSON, A. LINDH AND C. MOLLER Department of Mineralogy and Petrology, Institute of Geology, Solvegatan 13, S-223 62 Lund, Sweden

ABSTRACT Mafic granulite, garnet amphibolite and charnockite occur in the southwest Swedish part of the Baltic Shield. This part is generally considered to be the continuation of the Grenville collisional belt in Canada. The area with granulite facies rocks, the Southwest Swedish Granulite Region (SGR), is considerably larger than previously thought. The SGR is bounded to the east and west by two major tectonic zones. The first quantitative age data and P-T determinations for the high-grade metamorphism in the SGR are presented.

Conventional geothermobarometry was applied to mafic granulites from five localities. The estimated P-T conditions for the peak of metamorphism range from 705" C and 8.1 kbar at HallandsHs in the south, to 770" C and 10.5 kbar at Ullared in the north (medium- to high-P granulite facies conditions). Sm-Nd geochronology on minerals from the mafic granulites at HallandsHs and Ullared give late Sveconorwegian (Grenville) ages of 907 f 12 and 916 f 11 Ma for the high-grade metamorphism, which is considerably younger than previously thought.

Our results stress the hitherto underestimated importance of the late Sveconorwegian high-grade metamorphism in the southwestern part of the Baltic Shield.

Key words: age determination; Baltic Shield; charnockite; granulite; P-T estimate.

INTRODUCTION

The existence of scattered charnockite and mafic granulites in southwestern Sweden has been known for many years. Newly discovered occurrences extend considerably the high-grade region to become one of the largest known in Scandinavia. No P-T estimates or age determinations of the high-grade metamorphism in this region have been published previously. This paper reports the first quantitative P-T estimates and two Sm-Nd mineral isochrons on mafic granulites.

CRUSTAL REGIONS AND BOUNDARIES

The Swedish part of the SW Baltic Shield is made up of three major crustal regions which are separated by two tectonic zones, the Mylonite (MZ) and the Protogine (PZ) Zones (Fig. 1). East of the PZ there are two subregions, the Svecofennian region (c. 1900-1770 Ma) and the Transscandinavian Igneous Belt (c. 1800-1650 Ma). The area between the PZ and MZ is made up of granitoid gneisses and high-grade metamorphic rocks of unknown ages. This paper concerns the high-grade, southern part of the area between the MZ and PZ, here called the Southwest Swedish Granulite Region (SGR). To the west of the MZ both orthogneisses and rocks of supracrustal origin are found. The oldest rocks have intrusion ages of 1770-1650Ma. These are intruded by at least three generations of granites at c. 1550, 1250 and 900 Ma. For a

more detailed review of the geology of the Baltic Shield the reader is referred to Gail & Gorbatschev (1987).

The MZ and PZ represent important breaks in metamorphic grade and structural development. Mafic rocks to the east of the PZ contain chlorite, epidote and actinolite-bearing assemblages characteristic of low-grade conditions. Within the PZ, amphibolite facies hornblende- bearing assemblages with zoisite, biotite and almandine- rich garnet dominate the mafic rocks, and to the west of the PZ granulite facies pyroxene-bearing assemblages appear (Johansson, 1991). In the west, the MZ (Fig. 1) delimits the high-grade metamorphic SGR abruptly. Immediately beneath the N- to NW-dipping MZ, mafic rocks still retain their high-grade pyroxene- and garnet- bearing assemblages, whereas the mafic rocks exposed within the strongly deformed overlying migmatite-gneisses (the Bua Series; Hubbard, 1975) are devoid of both pyroxene and garnet. One exception is a small garnet-bearing mafic lens found at the very base of the Bua series.

The metamorphic breaks across the MZ and PZ north of Lake Vanern are also recognized in K-Ar cooling ages of biotite (Jarl, 1988). West of the MZ ages between 1100 and 1000Ma are obtained, and east of the PZ the ages scatter in the interval 1600-1000Ma. The biotite ages in the area between the MZ and PZ are younger, 975-900 Ma, which shows that this region cooled later than the neighbouring ones.

The geological significance of the PZ is far from clear.

283

284 L. JOHANSSON, A. LINDH & c . MOLLER

Fig. 1. Geological map of south Sweden showing the major provinces. Key: ( 1 ) Svecofennian Province; (2) Transscandinavian Igneous Belt; ( 3 ) Blekinge Province; (4) western part of the Southwest Swedish Gneiss Region; (5) eastern part of the Southwest Swedish Gneiss Region; the southern third, south of the line through B and U is the Southwest Swedish Granulite Region (SGR); (6) the southern part of the Protogine Zone; (7) Phanerozoic cover; (8) undifferentiated mafic rocks including gabbros, amphibolites, dolerite dyke swarms, etc. Tectonic boundaries: MZ = Mylonite Zone, and PZ = Protogine Zone. Towns: B = Borls; U = Ulricehamn; V = Varberg; F = Falkenberg. The localities investigated are shown as numbered circles: (1 ) Hallandsls; (2) Ullared; ( 3 ) Tylosand; (4) Grotvik and (5) Klnna.

The PZ is a tectonic zone with essentially vertical movements (Patchett et al., 1978). During the last few years the PZ has been interpreted as a major strike-slip zone (Talbot, 1986; Henkel & Ericsson, 1987), as a collision suture (Burke et al., 1977) or as a failed arm of an intracontinental rift (Andreasson et al., 1987). The PZ has been correlated with the Grenville Front in Canada either as its direct continuation (Gower & Owen, 1984) or as the opposite front of a two-sided Grenville orogenic belt.

GEOLOGICAL SETTING OF THE SGR The gneiss region between the PZ and MZ, in contrast to the neighbouring regions, is characterized by numerous Occurrences of high-grade rocks, from northern Skine in the south to at least Ulricehamn in the north (Fig. 1). With the exception of regional mapping by the Swedish Geological Survey (Caldenius et al., 1966; Mohren & Larsson, 1968) and structural studies in the Ulricehamn region (Fig. 1) by Larson et al. (1986), previous work has

concentrated on the charnockite area at Varberg. Most previous investigations concern the structural build-up of the charnockite complex (Quensel, 1950; Hubbard, 1975; Talbot & Heeroma, 1989). Hubbard (1978) and Hubbard & Whitely (1979) in addition discussed the geochemistry of the charnockite complex. Welin & Gorbatschev (1978) reported a Rb-Sr whole-rock isochron of 1420 f 25 Ma for the Varberg charnockite complex. The age was interpreted as an intrusion age. Efforts to date the intrusion with U-Pb techniques on zircon failed since no discordia line was obtained. The interpretation of the Rb-Sr age must therefore be considered as tentative.

The granulite facies metamorphism in southwestern Sweden, the associated deformation, and the emplacement of charnockites and mafic intrusions have been termed ‘the Hallandian event’ (Hubbard, 1975). The time period 1500-1400 Ma has been assigned to this ‘Hallandian quasi-orogeny’ (GaAI & Gorbatschev, 1987, and references therein) based mainly on the Rb-Sr age determination of the Varberg charnockite. The influence of the Grenvillian

LATE SVECONORWECIAN METAMORPHISM IN SWEDEN 285

or Sveconorwegian orogeny, 1250-900 Ma ago, has hitherto been considered to be minor in the SGR (cf. GaAl & Gorbatschev, 1987; Talbot & Heeroma, 1989).

The SGR (Fig. 1) is made up of variably deformed and migmatized granitic to intermediate gneisses and numerous metamorphosed large mafic bodies. Isotopic age deter- minations for these rocks are as yet not available. The granitoid rocks are commonly assumed to have ages similar to the granitoids around and north of Lake Vanern, i.e. to have a crustal history beginning about 1800- 1700 Ma ago.

Several small bodies of charnockite occur throughout the SGR, especially in the well-exposed coastal areas in the west. The largest of them, the Varberg charnockite, has compositions which range from quartz-diorite to granodiorite (Quensel, 1950; Hubbard, 1975). The rocks are dark brownish or greenish in colour due to the dark feldspars. The medium-grained charnockite is locally rich in large orthoclase megacrysts up to 5cm in size, but pyroxene-bearing, fine-grained, granulitic gneisses are also found. Mafic minerals are fine-grained ortho- and clinopyroxene, garnet, hornblende and biotite. These minerals form aggregates which define a foliation or gneissosity. The contact relationships to the surrounding gneisses are complex. At many localities the Varberg charnockites grade smoothly into the gneisses, and hybrid rocks are common. At other places further south along the coast there are metre-wide charnockite zones in amphibol- ite facies gneisses; a contact relationship similar to those described from southern India (Janardhan et al., 1982).

Numerous folded layers and lenses of garnet- amphibolite are scattered within the gneisses and charnockites of the SGR. The garnet amphibolites vary in size, reaching up to several kilometres in length and up to 1 km in thickness. They are generally medium grained and have a compositional layering with a subparallel gneissosity and thin felsic veins. Many amphibolites contain megacrysts of ortho- or clinopyroxene. In the coastal westernmost parts of the region, medium-grained mafic granulites of metagabbroic appearance are common. Garnet, pyroxene, amphibole and plagioclase are the major minerals. Some of these mafic rocks are layered (which in at least some rocks is clearly a relict primary structure) with decimetre- to metre-thick layers of alternating mafic and plagioclase-rich compositions. Thin dykes of very fine-grained mafic granulites or garnet amphibolites are common. Such dykes cross-cut older migmatite veins and the rock units described above, which indicates a polymetamorphic evolution of the SGR.

Aluminium-silicate-bearing rocks are rare and hitherto only found at two localities within the SGR. At Ullared (Fig. 1, locality 2) kyanite is abundant in a garnet- pyroxenite, and south of Falkenberg (Fig. 1) a sillimanite-rich, garnet-bearing quartzo-feldspathic gneiss is found. Sillimanite has been reported from the Bua Series gneisses (Hubbard, 1975), which overlie the SGR in the northwest.

In the coastal parts of the SGR the high-grade, upper amphibolite to granulite facies metamorphism overprints

intrusive contacts between mafic dykes and deformed granitic gneisses (Quensel, 1950; Caldenius et al., 1966; Mohren & Larsson, 1968). However, there are also numerous large-scale shear zones (such as the MZ) that deform and retrograde the granulite facies rocks and thereby post-date the high-grade metamorphism.

PETROLOGY

Since a large part of the area is made up of metamorphosed granitic rocks with mineral parageneses not very suitable for P-T determinations, we have chosen to work with five metamorphosed mafic rocks.

Mineral analyses were performed with standard microprobe techniques using natural and synthetic mineral standards. The samples from Tylosand, Grotvik and Kbnna were analysed with a Kevex energy-dispersive system (EDS) at the Institute of Geology, Lund, Sweden and the sample from Kbnna also with a Cameca wavelength-dispersive system (WDS) at the Institute of Geology, Uppsala, Sweden. No systematic differences could be found using the two different probes. The analyses from the Hallandsis and Ullared samples were made with a Jeol WDS equipment at the Department of Geology, Rensselaer Polytechnic Institute, Troy, New York, USA.

Representative mineral compositions in mafic granulites at Hallandsbs, Ullared, Tylosand, Grotvik and Kinna (Fig. 1) are given in Table 1. Based on the charge balance calculation scheme of Neumann (1976), the Fe3+ contents in garnets and pyroxenes were found to be very small or negligible except in the samples from Hallandsbs and Ullared. Calculation of Fe3+ in the clinopyroxenes from these localities, after normalization to four cations, commonly results in values of Fe3+/Fe2+ varying up to c. 0.5. The estimation of Fe3+ content in pyroxenes is strongly dependent on the quality of the analyses of all other elements, particularly in SOz , and it is therefore difficult to assess a 'true' Fe3' content. A probable bias towards too high Fe3+/Fe2+ ratios is a well-known and a major problem in geothermometry based on Fe-Mg exchange reactions (cf. Carswell et al., 1985). All temperatures and pressures presented in Tables 2 & 3 and in Fig. 5 are therefore calculated with all iron as Fe2+.

Sample descriptions

Hallan&&

The southernmost sample is taken at Hallandsls (625535/132315; all coordinates are given in the Swedish national grid), in a single outcrop of mafic granulite surrounded by reddish granitoid gneisses. This rock may serve as an example of the mineral assemblage, texture and compositions of a common type of fine-grained mafic granulite in the area. The rock consists of less than 0.5-mm-sized, more or less granoblastic or rounded grains of ortho- and clinopyroxene, garnet, hornblende, plagio- clase (in places anti-perthitic), biotite, opaques (ilmenite

L . IOHANSSON, A . LINDH & c. MOLLER

-

0.0-

0.6 -

Table 1. Representative microprobe analyses of minerals in the investigated rocks from the SGR.

HallandsHs Ullared Tylijsand Grotvik KBnna

Grt Cpx Opx PI Hbl Grt Cpx PI Grt Cpx Opx PI Grt Cpx PI Grt Cpx PI

RIM GARNET CORE

Fe/Fe+Mg I : : : : . .

Almandine -

SiO,

TiOz

FeO MnO CaO Na,O

Total

Si A1 Ti

AI?O3

MgO

KZO

3+ Mn Ca Na K

Total

38.76 51.69 51.01 57.04 41.61 39.03 51.19 58.32 38.21 52.62 51.97 21.74 2.02 1.42 27.60 11.73 21.96 3.88 26.47 22.07 2.26 1.10 - 0.21 0.12 - 2.39 - 0.48 - - - - 5.56 13.01 19.19 - 10.14 6.32 12.80 - 4.63 12.11 17.71

27.81 9.58 26.77 0.66 16.26 25.31 8.76 0.17 27.54 10.88 28.31 1 . 1 1 0.10 0.32 - 0.10 0.48 0.06 - 1.18 - 0.43 6.93 23.14 0.44 9.83 11.88 8.55 21.95 8.98 6.86 22.04 0.58 - 0.48 - 6.02 1.48 - 0.77 6.62 - - -

0.41 1.76 - - 0.04 - - - - - _

57.16 37.87 52.35 61.02 39.07 27.37 20.70 1.27 23.23 21.56

2.14 9.2 - 5.46 - 31.26 16.08 - 28.21 - 1.95 0.38 - 0.86 7.80 6.93 21.25 4.46 6.93

0.30 - - 0.73 -

_ - _ - - -

8.07 - - 8.58 -

53.68 2.15

12.79 9.4

22.08

-

-

- -

60.08 24.89 - - - - 6.35 8.18 0.56

101.91 100.23 99.27 101.56 97.35 101.65 99.89 100.60 100.49 99.91 100.10 100.70 100.85 100.53 98.02 102.09 100.10 100.56

2.98 1.97

0.64 1.79 0.07 0.57

-

- -

1.94 1.96 2.53 6.31 2.98 1.91 0.09 0.06 1.45 2.10 1.98 0.17 0.01 0.00 - 0.27 - 0.01 0.73 1.10 - 2.29 0.72 0.71 0.30 0.86 0.02 2.06 1.62 0.27 0.00 0.01 - 0.01 0.03 0.00 0.93 0.02 , 0.47 1.93 0.70 0.88 0.04 - 0.52 0.43 - 0.06

0.02 0.34 - - - -

2.60 1.39 - - 0.01

0.43 0.57 0.00

-

2.98 1.97 2.03 0.10

0.54 0.68 1.80 0.34 0.08 - 0.57 0.89

- -

- - - -

1.99 0.05

1.01 0.91 0.01 0.02

-

- -

2.55 3.01 1.44 1.94

0.00 - 0.25 - 2.01 - 0.13 0.37 0.59 0.70 - 0.02 -

-

2.00 0.06 0.00 0.52 0.51 0.01 0.87 - -

2.76 3.00 1.24 1.95

- 0.63 1.81

- 0.06 0.22 0.57 0.75 - 0.03 -

_ - -

I .99 0.09

0.71 0.29

0.88

-

-

- -

2.68 1.31 - - - - 0.30 0.71 0.03

8.02 4.04 4.01 5.01 15.74 8.03 4.01 5.00 8.00 3.9X 3.99 5.08 7.93 3.97 5.00 8.02 3.96 5.03

No. o f 0 12 6 6 8 23 12 6 8 12 6 6 8 12 6 8 12 6 8 0.01 - - - - - -

Fe * 74 29 44 - 47 69 28 - 77 33 47 - 89 49 - Fe:,,, 73 20 43

Fe' = FeZ+/(FeZ+ + Mg) ratio calculated with all Fe as Fez+; Fe:,,, = marks the same ratio but corrected for the calculated amount of Fe3+.

0.06 - - 74 29 -

Fe'+ 0.09 0.12 0.03 - - 0.09 0.11 -

- - 89 - - 74 - _ - 77 - - 68 19 -

Fez+ = the total Fe contents calculated as Fez+; Fe3+ = the amount of ferric Fe resulting from recalculation according to Neumann (1976);

and pyrrhotite) and small amounts of quartz (Fig. 2). The texture indicates that the rock is well equilibrated.

Most minerals are virtually unzoned, like the garnet shown in Fig. 3. An exception is plagioclase which shows an increase in anorthite content from An,, to AndR towards the rim (Fig. 3). The rim composition of plagioclase is assumed to be in equilibrium with the other unzoned minerals; this is consistent with the occurrence of plagioclase of this composition as tiny inclusions in unzoned garnet. The zoning in plagioclase is probably a

Fig. 2. Back-scattered electron image of the texture of the Hallandsis mafic granulite, with garnet (GRT), clinopyroxene (CPX), orthopyroxene (OPX), hornblende (HBL), plagioclase (PL) and quartz (QZ). The composition of the garnet in the centre of the picture is shown in Fig. 3. The profile (black dotted line) is taken from the Cpx-PI contact towards the centre of the garnet. The composition of the plagioclase is shown in Fig. 3. This profile (white dotted line) is taken from the Opx towards the Opx-PI-Grt contact. Scale bar = 0.5 mm.

relict growth zonation, since the dithsivity of components in plagioclase is slow at metamorphic temperatures.

Pyrope

Grossular

Spessarline

0

PLAGIOCLASE o'6 { RIM RIM

Anorthile

0.4

0 0 1 0 2 mrn

Fig. 3. Compositional zoning profiles in garnet and plagioclase from the HallandsHs sample.

LATE S V E C O N O R W E G I A N METAMORPHISM IN SWEDEN 287

Ullured

The Ullared Complex (634075/131240) is a layered complex, c. 1 x 2 km in size, surrounded by strongly deformed, amphibolite facies, granitoid gneisses. It is made up of various mafic to intermediate rocks, individual layers ranging between ten and several tens of metres in thickness. The complex includes Al-rich bulk compositions such as kyanite-bearing garnet-pyroxenites.

The sample used for thermobarometry and geochronol- ogy is made up of up to 6-mm garnet porphyroblasts set in a finer-grained matrix of clinopyroxene, plagioclase, quartz, opaques (mainly secondary ilmenite) and minor hornblende (Fig. 4). Clinopyroxene, quartz, rutile, apatite and titanite occur as inclusions in the garnets. The P-T estimate for this locality is based on rim-rim composi- tions. The garnets are slightly zoned with increasing pyrope content and Mg/(Mg + Fe) ratio, and decreasing grossular content from core (Alm,,Py,,Gro,,Sp,) to rim (Alm,,Py,,Gro,,Sp,). Intracrystalline diffusion is expected to have modified a remnant growth zoning at the temperature conditions recorded here. A first-hand interpretation of the compositional change, based on the expected partitioning of Ca, Mg, Fe and Mn between garnet, clinopyroxene and plagioclase, suggests increasing temperature and decreasing pressure conditions from core to rim.

The existence of kyanite in aluminous garnet- pyroxenites within the same complex is a good pressure indicator for the peak P-T conditions of the Ullared complex. However, it is also possible that the kyanite is metastably persistent from earlier, higher pressure conditions. The kyanite-bearing rocks at Ullared show extensive symplectite reaction textures which most likely were formed during uplift. Kyanite is partly to completely replaced by an intergrowth of sapphirine + corundum + plagioclase. Clinopyroxene is replaced by a clinopyroxene + plagioclase intergrowth and an outer rim of orthopyroxene + plagioclase symplectite. The latter

Fig. 4. Photomicrograph from the Ullared sample. Garnet (GRT) with inclusions of quartz (white) and rutile (black and grey). Matrix of clinopyroxene (CPX), plagioclase (PL) and minor hornblende (HBL), quartz, and ilrnenite (black). Scale bar = 1 rnm.

symplectites are likely replacements of more omphacite- rich clinopyroxene precursors which have been stable at still higher pressures.

Tylosand and Grotvik

Two metamorphosed mafic dykes, situated c. 700 m apart, have been sampled for P-T determinations. They occur in a rock complex made up of siliceous inhomogeneous gneisses and small amphibolitic schlieren and bands. The mafic granulite at Tylosand (628400/131350) consists of plagioclase, clino- and orthopyroxene, garnet, biotite, hornblende, and ilmenite as major phases. It is fairly fine grained with occasional megacrysts of clinopyroxene. The mineral assemblage at Grotvik (628360/131410) is dominated by hornblende, plagioclase and garnet; clinopyroxene and ilmenite occur in lesser amounts. Both rocks are granoblastic with no trace of a relict igneous texture. The bulk and mineral chemical compositions of the two dykes differ, with significantly lower Mg/(Mg+ Fe) ratios in the mafic minerals at Grotvik. No mineral zonation has been recorded.

Kdnna

The sampling points at Klnna (629655/138200 and 629455/138265) are situated much closer to the PZ than the Tylosand locality (Fig. 1). Two different dykes have been sampled. They are both situated within in- homogeneous gneisses. One of the dykes is medium- to coarse grained and consists primarily of hornblende, plagioclase and up to centimetre-sized garnet porphyro- blasts. Biotite occurs in minor amounts, and rutile and ilmenite are found as inclusions in garnets. The other rock is more fine- and even grained with garnet; clinopyroxene, plagioclase and biotite as the major phases.

P-T ESTIMATES

P-T estimates obtained from geobarometers and geother- mometers are summarized in Tables 2 & 3. A computer program 'GEOTHERMOBAROMETRY' coded by M. Kohn, Rensselaer Polytechnic Institute, Troy, NY, was used for the calculations. Since all of these rocks contain the assemblage garnet-clinopyroxene-plagioclase-quartz, calibrations based on this assemblage are used for comparative P - T estimates (Table 2 , Fig. 5). The calibrations applied include garnet-clinopyroxene thermo- meters by Ellis & Green (1979) and Pattison & Newton (1989) combined with the barometers by Moecher et al. (1988) and Newton & Perkins (1982). It must be emphasized that the quoted small uncertainties in the P-T estimates given in Tables 2 & 3 and Fig. 5 represent the range of our relatively small P - T database.

The temperature estimates for the different localities are relatively uniform (Table 2, Fig. 5). Using the Ellis & Green (1979) calibration, the mafic granulites at the Hallandsls and Tylosand localities record temperatures of c. 705" C, while the Grotvik and Klnna localities indicate

288 L . JOHANSSON, A . L I N D H & c . MOLLER

Table 2. Pressure (kbar) and temperature ("C) estimates based on garnet-clinopyroxene thermometers of Ellis & Green, (1979) (EG) and Pattison & Newton (1989) (PN) combined with three different barometer calibrations of the assemblage garnet-clinopyroxene- plagioclase-quartz: the Mg-endrnember (MEA-Mg) and Fe-endmember (MEA-Fe) of Moecher et al. (1988) and the Mg-endmember (NP-cDx) of Newton & Perkins (1982).

HallandsHs Ullared Tylhand Grotvik KHnna

Calihrations T n P n T n P n T n P n T n P n T n P n

EG/MEA-Mg 705f20 5 8.1 f0.3 4 770f35 8 10.5f0.5 4 705f30 15 10.5f0.5 9 660f25 8 10.2f0.3 7 670f25 8 10.0f0.3 10 PN/MEA-Mg 605f25 5 7.6f0.3 4 635f 40 8 9.5 f0.4 4 635 f40 15 9.5 f0.4 9 480f 50 8 8.8 f0.3 7 585 f 25 8 9.2f0.3 10 EG/MEA-Fe 695f20 5 4.6f0.3 4 765 f 35 8 9.0f0.5 4 765 f35 15 9.0f0.5 9 6 4 0 f 2 5 8 2.9 f0.3 7 665 f 2 5 8 8.0f0.3 10

565 f 2 5 8 6.3 f0.3 10 EG/NP-cpx 700f20 5 6.5f0.2 4 765f 35 8 9.1 f0.4 5 765f35 15 9.1 f0.4 9 655f25 8 7.2f0.4 7 (%Sf25 8 8.4f0.3 10 PN/NP-cpx 595f25 5 6.0f0.2 4 630f 40 8 7.8f0.3 5 630f40 15 7.8f0.3 9 470f50 8 5.7f0.4 7 575 f 2 5 8 7.6f0.3 10

PN/MEA-Fe 580f25 5 3.1 f0.3 4 620f40 8 6.4f0.4 4 620f40 15 6.4f0.4 9 - -

~~

Note that the quoted uncertainties in pressures and temperatures only reflect the range of our limited database: n = numher of mineral data sets on which the P - T estimate is based

slightly lower temperatures around 665" C. The highest temperature recorded, c. 770"C, is from the Ullared kyanite-bearing garnet-pyroxenite. If the Ullared estimate had been corrected for Fe3+ based on charge balance calculations (Neumann, 1976), the temperature would be lowered by c. 70" C.

Recently, Pattison & Newton (1989) and Green el al. (1989) advocated that the Ellis & Green (1979) geothermometer gives very good results a t mantle conditions (30 kbar, 1150-1200" C) but gives increasing overestimates at crustal conditions. Green et al. (1989)

Table 3. Temperature ("C) and pressure (kbar) estimates for the Hallandsis mafic granulite using calibrations other than those presented in Table 2.

TIP calibrations T n P n

SB/MEA-Mg-cpx 715f 10 4 8.1 f0.3 4

GP/MEA-Mg-cpx 700f20 4 8.1 f 0 . 3 4 H/MEA-Mg-cpx W f 1 0 4 7.9f0 .3 4

EG/MEA-Fe-opx 710f25 5 9 . 6 f 0 . 2 4 PN/MEA-Fe-opx 610f25 5 8 . 2 f 0 . 2 4 EG/PC-Mg 705f25 5 7 .2f0 .2 4 PN/PC-Mg 590f25 5 6 .1f0 .2 4 EG/PC-Fe 705f25 5 7 .2f0 .2 4 PN/PC-Fe 590f25 5 5 .2f0 .2 4 EG/BWB-Fe 710f25 5 9 .6f0 .2 4 PN/BWB-Fe 610 f 25 5 8 . 2 f 0 . 2 4 EG/NP-opx 710f20 5 7 .5f0 .2 4 PN/NP-opx 605 f 25 5 7.0f0 .2 4 EG/KS-Mg 705f20 5 7.7f0.2 4 PN/KS-Mg 605f25 5 7 .7f0 .2 4 EG/KS-Fe 705f20 5 7.8f0 .2 4 PN/KS-Fe 600f25 5 7.2f0 .2 4

Thermometers based on garnet-orthopyroxene: Sen & Bhattacharaya (1984) (SB); Harley (1984) (H); hornblende- garnet: Graham & Powell (1984) (GP); garnet-clinopyroxene: Ellis & Green (1979) (EG); Pattison & Newton (1989) (PN).

Barometers based on garnet-clinopyroxene-plagioclase- quartz: Moecher er al. (1988) (MEA Mg-endmember); garnet- orthopyroxene-plagioclase-quartz: Moecher et af. (1988) (MEA Fe-endmember); Perkins & Chipera (1985) (PC Mg-endmember, PC Fe-endmember); Bohlen er al. (1983) (BWB Fe-endmember); Newton & Perkins (1982) (NP-orthopyroxene); garnet- hornblende: Kohn & Spear (1990) (KS Mg-endmember, Fe-endmember).

Note that the quoted uncertainties in pressures and temperatures only reflect the range of our limited database: n =number of mineral data sets on which the P-T estimate is based.

suggested an overestimate of c. 145°C at P-T conditions similar to those presented here. Application of the calibration by Pattison & Newton (1989) t o the present data set, however, gives unrealistically low temperatures of 480-635°C. With the available data , we have not been able to detect rimwards compositional gradients in garnets and clinopyroxenes indicating late diffusional Fe-Mg exchange (cf. Fig. 3). Thus, there is no reason to assume that the Fe-Mg distributions represent very late cooling, and therefore we consider the temperatures obtained with the Ellis & Green (1979) thermometer to be more realistic than those obtained with the Pattison & Newton (1989) thermometer.

Pressure estimates obtained are also uniform: between 10 and 10.5 kbar with the clinopyroxene Mg-endmember of Moecher et al. (1988), with the exception of Hallandsls which records lower pressures around 8.1 kbar. Applica- tion of the clinopyroxene Fe-endmember calibration by the same authors gave significantly lower pressures, which in some cases clearly are unrealistic (Table 2). T h e Newton & Perkins (1982) clinopyroxene Mg-calibration gives consis- tently c. 1.5 kbar lower pressures than the Mg-endmember by Moecher el al. (1988) (Table 2).

D

0 100 200 300 400 500 600 700 800 7- ("C)

Fig. 5. P-T diagram with the 'best' P-T estimates from the SGR. The temperature estimates are based on the Pattison & Newton (1989; black boxes) and Ellis & Green (1979; shaded boxes) garnet-clinopyroxene calibrations. Pressure estimates are based on the Mg-endmember garnet-clinopyroxene-plagioclase-quartz calibration of Moecher el al. (1988). See text for further information. The Al-silicate stability fields are after Holdaway (1971).

LATE SVECONORWEGIAN METAMORPHISM IN SWEDEN 289

143Nd 144~d

0.5 1 400 -

0.5 1300-

0.5 I 200 -

0 . 5 1 100,

Analytical data of hornblende and orthopyroxene in the mafic granulite from Hallandsls (locality 1) have permitted application of thermobarometers based also on other equilibria. The calibrations used include the garnet- orthopyroxene thermometer by Harley (1984), Sen & Bhattacharaya (1984) and the garnet-orthopyroxene- plagioclase-quartz barometers by Newton & Perkins (1982), Bohlen et al. (1983), Perkins & Chipera (1985) and Moecher et al. (1988). The results are listed in Table 3. Amphibole-garnet thermometry (Graham & Powell, 1984) and amphibole-garnet-plagioclase-quartz baro- metry (Kohn & Spear, 1990) have also been applied to the Hallandsls mafic granulite (Table 3). Thermometers which give very similar results (700-715" C) are those by Ellis & Green (1979, Grt-Cpx), Sen & Bhattacharaya (1984, Grt-Opx) and Graham & Powell (1984, Grt-Hbl). Barometers which are in mutual agreement are the clinopyroxene Mg-endmember by Moecher et al. (1988, Grt-Cpx-PI-Qtz) and the calibration by Kohn & Spear (1990, Grt-Hbl-PI-Qtz) which give 7.7-8.1 kbar com- bined with the Ellis & Green temperatures, or the orthopyroxene Fe-endmember calibrations by Bohlen et al. (1983) and Moecher et al. (1988) which both give c. 9.6 kbar. It should be noted that the thermobarometers involving hornblende are empirically calibrated against other calibrations, for example the one by Ellis & Green (1979).

As discussed above, the temperature estimates presently preferred by us are the ones obtained with the Ellis & Green (1979) thermometer. For all granulites, it can be questioned whether the temperatures recorded are peak or closure temperatures. As mentioned above, in the sample from Hallandsfs the minerals analysed show flat zoning profiles and the samples from Grotvik, Tylosand and Kfnna have essentially unzoned minerals. There is thus no reason to believe that we have recorded extreme cooling. However, for a rock in which the minerals have flat zoning profiles, it cannot be excluded that the rock has experienced higher temperatures at earlier stages in the metamorphic cycle. At the Ullared locality, the large garnets show zoning patterns which indicate that the rim P-T conditions were pre-dated by lower temperatures and higher pressures. Thus, with the data available we consider the rim P-T data presented for the Ullared sample likely to represent a true peak P-T point of a clockwise P-T loop.

ULLAREII GKANULI'I'E

916k I I Ma

1.r. = 0.511394 MSWD = 2.8

.

AGE DATING

Two mafic granulites at Ullared and Hallandsls were sampled for Sm-Nd mineral isotope analysis. Both samples are described in the previous section (localities 1

The isotope analyses were performed at the Mineralogical-Geological Museum in Oslo. Details of the analytical procedures for Sm-Nd analysis used in Oslo are described by Mearns (1986).

Clinopyroxene, garnet, magnetite and plagioclase were separated using conventional magnetic separation and

& 2).

Table 4. Sm-Nd data for the Hallandsls (H) and Ullared (U) samples.

Sm Nd I4'Sm/ I4'Nd/ f 2 o (ppm) (ppm) '"Nd '"Nd

U-WR u-PL u-CPX U-GRT H-WR H-FSP H-MT H-GRT

8.31 2.55 9.61 4.83 4.45 0.58 0.92 2.58

39.43 16.19 27.91 6.23

17.16 4.63 3.85 2.78

0.128361 0.512180 0.095832 0.511964 0.209656 * 0.5 12635 0.472036 0.514237 0.157819 0.512327 0.076036 0.511848 0.146376 0.512251 0.566709 0.5 14769

o.oooo1o O.ooOo36 o.oooo1o o.oooo1o o.oooo1o 0.000004 0.000006 o.oooo1o

heavy-liquid techniques. The mineral concentrates were further purified by hand-picking under the microscope. Between 200 and 300 mg of very pure mineral concentrate was used for the analysis. Sm and Nd contents were determined by isotope dilution. The blank was 0.03 ng Sm and 0.3 ng Nd. Sm and Nd were loaded as phosphates on triple filaments and analysed on a VG 354 multi-collector mass spectrometer. The Nd compositions have been normalized to lMNd/l"Nd = 0.7219. Twelve analyses of the La Jolla Nd standard analysed during the relevant time periods gave 0.511864 f 13 (2a). The decay constant 6.54 x lo-'* for Sm has been used in the calculations. Regression calculations of the isochrons followed York (1969). The analytical data are given in Table 4.

The Ullared and Hallandsfs isotope data define isochrons of 916 f 11 Ma and 907 f 12 Ma respectively (Fig. 6). Cohen et al. (1988) demonstrated that even in

144Nd 143Nd 144Nd

0.51400

0.5 1300

0 .5 1200

0.5 I 100

907k12 Ma MSWD = 2.2 1.r. = 0.5 1 139 1

. . 00 0.10 0.20 0.30 0.40 ~ 147Srn

144Nd

Fig. 6 . Sm-Nd isochron diagrams for the Hallandsik and Ullared samples. Mineral abbreviations as i n Fig. 2, MT = magnetite, FSP = erthitic feldspar and WR = whole rock. 1 .r . = initial '43Nd/LNd ratio.

290 L . J O H A N S S O N , A . LINDH I% C . M O L L E R

slowly cooled granulite terranes, garnet grains are expected to yield Sm-Nd chronologies very close to the time of the mineral growth. Sneeringer el al. (1984), in a study of Sr and Sm volume diffusion in clinopyroxenes, concluded that high blocking temperatures indicate that the Sm-Nd isotope system records emplacement events and are generally not disturbed unless a fluid enters the system or melting takes place.

Both ages are therefore interpreted as high-temperature blocking ages of the Sm-Nd system at or near peak metamorphic conditions.

DISCUSSION

The present results are the first quantitative P-T estimates and age determinations of high-grade metamorphism in southwestern Sweden. These give temperatures of 660-770" C and pressures of 7.5-10.5 kbar. Using an average density of 2.7 g cm-3 for the overburden, the pressures recorded correspond to a crustal depth of 28-39 km. The calculated mean geothermal gradient at the time of peak metamorphism is 18-28" C km-', which is a normal to slightly high value in the range of continental gradients (Chapman, 1986). The age of the high-grade metamorphism as recorded by Sm-Nd mineral ages in two mafic granulites in different parts of the SGR is much younger than previously thought: c. 900Ma, which corresponds to the late Sveconorwegian (late Grenville).

The present crustal thickness below the SGR is in the range 35-40 km (EUGENO-S Working Group, 1988), thickening towards the northeast. The present crustal thickness added to the estimated depth of the peak metamorphism suggest a considerable crustal thickening during the late Sveconorwegian metamorphic event, which supports Sveconorwegian collisional processes in the southwest Baltic Shield.

Field relations support a polymetamorphic evolution for the SGR. Migmatitic structures and veins in gneisses and mafic rocks are cut by mafic granulite facies metamor- phosed dykes, and thus provide good evidence that relatively high-grade conditions have prevailed also at some time before the late Sveconorwegian. There are no time constraints for this migmatization, however, and the age difference may well be small.

Previous investigators have considered an older age for the high-grade metamorphism more plausible. Welin & Gorbatschev (1978) and Gorbatschev & Welin (1980) interpreted the Rb-Sr whole-rock isochron of 1420 Ma obtained from the Varberg charnockite to date the time of intrusion and implicitly also to approximate the age of the high-grade metamorphic event. Data have, however, also been presented that indirectly support the idea of a late Sveconorwegian metamorphic disturbance, but the condi- tions of this overprint were not quantified. Welin et af. (1982) presented a model for the episodic lead loss in zircons in the Varberg charnockite based on an inferred metamorphic event around 950 Ma. In areas further north, and areas west of the MZ, a late Sveconorwegian metamorphism has been suggested as the cause for the

disturbance of the Rb-Sr isotope system (Welin e l a/., 1980) and for the complete re-equilibration of the K-Ar system (Magnusson, 1960; Jarl, 1988).

The MZ and PZ tectonic zones must have played important roles during the uplift of the SGR, as evidenced by the metamorphic breaks across them. Just north of Varberg, the MZ separates the high-grade metamorphic SGR from the overlying rocks of the Bua series (Hubbard, 1975). Along the MZ, the charnockites and rnafic granulites are deformed and retrograded. The abrupt break in metamorphic grade across the M Z at Varberg suggests that the rocks of the overlying Bua series were thrust onto the charnockite complex at a time when the SGR already had been partially uplifted and cooled. If, as suggested by the presented Sm-Nd isotope results, the high-grade metamorphism is late Sveconorwegian, the thrusting of the Bua series should be even younger. There are no radiometric data concerning the age of the metamorphism in the Bua series. Therefore, there is no evidence that the metamorphism that affected the Bua series is contemporaneous with the late Sveconorwegian granulite metamorphism recorded in the underlying rocks.

Imbricate structures and separate thrust units of gneisses and supracrustal rocks have been described from the gneiss area west of the MZ, northwest of Lake Vanern (Gorbatschev, 1988). Rb-Sr dating (Wahlgren et af., 1981) indicates that this thrusting took place during the late Sveconorwegian, around 960 Ma.

The occurrence of rocks of similarly high grade in the Grenville province in North America and in the Bamble-Arendal and Rogaland areas in south Norway is of crucial importance for the understanding of the Grenvillian-Sveconorwegian orogeny and the correlation of crustal segments in the North American and Baltic Shields. The granulite facies metamorphism in the Bamble-Arendal area has been dated at 1150-1100 Ma, using the Sm-Nd method on high-grade mineral assemblages (L. Kullerud, pers. comm. 1989). Estimates of P-T for the deepest levels of the Grenville Orogen in Canada are 700-800°C and 10-11 kbar (Gower & Edmer, 1988). The timing of this metamorphism is uncertain; no Sm-Nd mineral isochrons or other isotope determinations which date the high-grade assemblages have been published. The preferred model suggests a high-grade event at approximately 1425 Ma and sporadic but lower-grade overprinting during the Grenvillian orogeny. These opinions are very similar to those previously expressed for the SW Baltic Shield.

ACKNOWLEDGEMENTS

This work was financially supported by the Swedish Natural Science Research Council (NFR). The Sm-Nd datings were carried out at the Mineralogical-Geological Museum in Oslo. Special thanks go to T. Enger, E. Mearns and A. Stabel for skilled technical advice and to T. Miyazu for solving technical problems during probe work in Lund. Sincere thanks go to F. Spear for providing the access to the microprobe equipment and other facilities at

LATE SVECONORWECIAN METAMORPHISM IN SWEDEN 291

Rensselaer Polytechnic Institute, Troy, New York, and to M. Kohn for providing us with a version of his GEOTHERMOBAKOMETRY program.

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Received 12 November 1989; revision accepted 24 May 1990