Late Sveconorwegian metamorphism and deformation in southwestern Sweden

Precambrian Research, 64 (1993) 347-360

347

Elsevier Science Publishers B.V., Amsterdam

Late Sveconorwegian metamorphism and deformation in southwestern Sweden Leif Johansson a,*, Lars Kullerud b'** alnstitute of Geology, University of Lund, S6lvegatan 13, S-223 62 Lund, Sweden bMineralogical-Geological Museum, Sars gate 1, N-0562 Oslo 5, Norway (Received July 10, 1991; revised version accepted November 3, 1992 )

ABSTRACT The Precambrian of southwestern Sweden is divided into eastern and western crustal segments separated by a major tectonic boundary, the Mylonite Zone. The southern part of the eastern segment is characterized by numerous occurrences of mafic granulites and charnockites, the western segment comprising greenschist to amphibolite facies gneisses of supracrustal and plutonic origin. Sm-Nd mineral ages of the granulite facies metamorphism in the eastern segment range from ~ 920 to ~ 880 Ma. Garnet + pyroxene + feldspar + whole-rock Sm-Nd isochrons of the Varberg charnockite and the Tr~is16vsliige mafic granulite yield ages of 893 _+5 Ma and 881 _ 4 Ma, respectively. This late Sveconorwegian (late Grenvillian ) granulite facies metamorphism in southern Sweden appears to be confined to the eastern segment, while the last high-grade metamorphic event in the western segment is contemporaneous with the ~ l 100 Ma old granulite facies metamorphism in the Bamhle area of SE Norway. At least three phases of regionally important deformation postdate the late Sveconorwegian metamorphism in southwestern Sweden. Deformation related to movements within the Mylonite Zone was contemporaneous with or later than the granulite facies metamorphism in the Varberg area, and took place mainly under retrograde metamorphic conditions leading to almost complete replacement of garnet and pyroxene by amphibole, biotite and plagioclase. Extensional tectonics were presumably important during the uplift of the granulite terrain east of the Mylonite Zone. Peak P-T conditions during the late Sveconorwegian metamorphic event were considerably higher than those required for the melting of granitoid rocks. The Sm-Nd ages of granulite facies parageneses within the eastern segment are approximately the same as those of numerous late Sveconorwegian granite intrusions in southwestern Scandinavia, suggesting a genetic relationship between high-grade metamorphism at depth and the emplacement of granites at higher crustal levels. The Nd model ages of the Varherg charnockite agree reasonably well with the Nd model ages of 1700-1600 Ma old granites within the same crustal domain, while the model age of the mafic granulite (originally a mafic dyke) corresponds roughly to the model ages of ~ 930 Ma old mafic dykes in SE Sweden.

1. Introduction Southwestern Scandinavia (Fig. 1 ) is a tectonically complex area comprising several crustal segments separated by shear zones. The region has been correlated with the Grenvillian Province in Canada (e.g. Gower, 1985; Gower et al., 1990). In Sweden, the Southwest *Corresponding author. **Present address: GRID-Arendal, Longum Park, N-4800 Arendal, Norway.

Swedish Gneiss Region (Fig. 1 ) is divided into eastern and western crustal segments separated by a tectonic boundary, the Mylonite Zone. It has long been known that there are considerable differences between the eastern and western crustal segments as regards magmatism (e.g. Lindh, 1987), resetting of isotopic ages, and the distribution of supracrustal rocks. The lack of age data particularly from the eastern segment has made it difficult to reconstruct the tectonometamorphic evolution.

0301-9268/93/$06.00 © 1993 Elsevier Science Publishers B.V. All fights reserved. SSD10301-9268 ( 93 ) EO073-L

348

L. JOHANSSON AND L. KULLERUD

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Dal Group (DG) AlmeshkraGroup (AG) # Majorshear zones #

Fig. 1. Map showingthe main Precambrianregionsin the southwestern part of the Baltic Shield, the Caledonides, the Oslo Palaeo rift and the Phanerozoic cover. M Z = Mylonite Zone; PZ=Protogine Zone; DBT=Dalsland Boundary Thrust; GZ=GSta ~lv Zone; B=BoHts; BE=Bergen area; H= Hallands~s; J= JSnkSping; U= Ullared. However, new S m - N d ages and P-Testimates from the Bamble area (Fig. 1 ) of Norway (Kullerud and Dahlgren, 1993) and the eastern segment of the Southwest Swedish Gneiss Region (Johansson et al., 1991) have now made it possible to at least partly reconstruct the Sveconorwegian tectonic and thermal evolution. S m - N d dating has proved to be particularly useful for dating high-grade, garnetbearing parageneses. Studies by Jacobsen and Wasserburg ( 1979 ), Mearns ( 1986 ), Cohen et al. (1988), Jagoutz (1988) and Johansson et al. (1991 ) have clearly shown the usefulness of this method. The relationship between Sveconorwegian high-grade metamorphism in the rocks of the eastern segment and the deformation in the Mylonite Zone can be studied in the Varberg

area (Fig. 2) which is also well suited for the investigation of the tectonometamorphic relationships between the eastern and western parts of the Southwest Swedish Gneiss Region. Isotopic dating of high-grade metamorphic mineral assemblages in rocks from the Varberg area provides age estimates of the metamorphism and constrains the age of regionally important deformation events that took place before or after the metamorphism. The age of the highgrade metamorphism and formation of charnockites can also form the basis for a discussion of the genesis of the Sveconorwegian granites in southwestern Scandinavia. This paper presents new Sm-Nd age data on the latest high-grade metamorphism which led to the formation of mafic granulites and orthopyroxene + clinopyroxene + garnet + perthiticfeldspar assemblages in granitic rocks (i.e. charnockites) in southwestern Sweden. Finally, the age and the significance of the subsequent deformation and retrogression of the charnockite and other high-grade rocks are discussed in a regional context. 2. R e g i o n a l g e o l o g y

The main crustal provinces of southern Sweden and Norway are shown in Fig. 1. The Precambrian of southern Norway is divided into four segments. Sveconorwegian charnockites and associated granulite facies rocks are common in the Rogaland and Bamble-Kongsberg segments, these regions thus being of particular importance when comparing Sveconorwegian tectonometamorphic evolution in southwestern Sweden and southern Norway. For details of the geology of the Telemark and Kongsberg segments the reader is referred to Brewer and Field ( 1985 ) and Starmer ( 1990 ). The Rogaland segment comprises amphibolite and granulite facies gneisses with large intrusions of Neoproterozoic charnockites, monzonorites and anorthosites (Demaiffe and Michot, 1985 ). Age relationships and the general geology of the Bamble area are discussed

LATE SVECONORWEGIAN METAMORPHISM AND DEFORMATION IN SOUTHWESTERN SWEDEN

by Kullerud and Dahlgren (1993) and by Starmer ( 1991 ). The Precambrian of southern Sweden can be divided into three crustal segments separated by two large deformation belts: the Protogine Zone in the east and the Mylonite Zone in the west. These deformation belts are systems of branching and anastomosing shear zones rather than well-defined single mylonite zones. The western segment (Fig. 1 ) of the Southwest Swedish Gneiss Region comprises ~ 1760-1600 Ma old ortho- and para-gneisses (/~da~ill et al., 1990) into which at least three generations of granites have been intruded ~ 1550, 1250 and 900 Ma ago. Mesoproterozoic thrusting and thrusting in the earlier part of the Sveconorwegian orogeny have been recognized within the western segment (Park et al., 1991 ). The youngest supracrustal rock unit in the western segment is the Dal Group (Fig. 1 ) which comprises low-grade metasediments that were deposited between 1075 Ma and 1030 Ma ago (Rb-Sr ages, recalculated from Ski~51d, 1976) atop a 1250-1200 Ma old granitic basement. Between the Mylonite and Protogine Zones is the eastern segment of the Southwest Swedish Gneiss Region (Fig. 1 ), the northern part of which is dominated by generally strongly metamorphosed and deformed granitic gneisses ranging from ~ 1800 Ma to ~ 1600 Ma in age (Lindh, 1987). The deformation history of the eastern segment has been outlined by Larson et al. ( 1986 ). As a result of the latest intense deformation and high-grade metamorphism there is little clear evidence of earlier metamorphic events. The southern part of the eastern segment is characterized by numerous minor occurrences of granulite facies rocks and is known as the Southwestern Granulite Province (SGP, Fig. 1; Johansson et al., 1991 ). It is unclear how far north these granulite facies rocks extend. The largest coherent complex of charnockite and granulites occurs in the Varberg area (Figs. 2 and 3 ).

349

t

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~6

Fig. 2. Geological map of the Varberg-Bua region, modified after Talbot and Heeroma ( 1989 ), who largely based their map on maps by Hubbard (1975), Constable and Hubbard (1981), and unpublished maps by J.L. Constable and by L. Samuelsson. Legend: 1= Bua gneisses; 2=granite (the so-called Torpa granite) and granitic gneisses partly formed from retrogressed charnockites; 3 = pyroxene-bearing granitoid rocks; 4 = granitic gneisses and migmatites; 5=Varberg charnockite sensu stricto; 6 = mafic granulites and garnet amphibolites. The three generations of major shear zones are shown and numbered B1 (oldest) to B3 (youngest) according to the terminology of Talbot and Heeroma (1989). B2 zones are considered to be branches of the Mylonite Zone (cf. Fig. 1).

The Varberg area was first described and mapped by Svedmark (1893) and was later considered in greater detail by Quensel ( 1951 ), Hubbard (1975), and Talbot and Heeroma (1989). Hubbard ( 1978, 1989), and Hubbard and Whitely (1979) discussed the geochemistry of the charnockite and associated rocks while the structural development of the area was treated by Hubbard (1975), and Talbot and Heeroma (1989). Hubbard (1975) divided the rocks in the Varberg area into three major complexes, the Varberg Series, the Bua Series and the Char-

3 50

L. JOHANSSONAND L. KULLERUD

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4

5' Fig. 3. Simplified geological map of the Tr~isl6vsl~ige area based on Quensel ( 19 51 ). / = granitic gneiss interspersed with chamockite; 2 = i n t e r m e d i a t e to felsi¢ charnockite and hybrid rocks; 3 = marie granulites; 4 = undifferentiated gneisses.

nockite-Granite Association (CGA). The Varberg Series comprises gneissic granitic rocks interlayered with a banded sequence of felsic and mafic granulites. Similar gneisses are found in scattered occurrences farther south along the coast and probably underlie part of the Phanerozoic cover in southernmost Sweden (Fig. 1 ). The CGA contains charnockites and granites with a plutonic appearance. Based on geochemical data, Hubbard ( 1988, 1989) emphasized a close genetic relationship between the charnockites and granites of the Torpa type (Fig. 2). In his genetic interpretation, the Torpa granite represents a mobile melt fraction that was extracted from the site of melting leaving behind a residual of depleted charnockites. The relative amounts of charnockite and granite are difficult to estimate since deformed granites and retrograded charnockites are virtually indistinguishable. Within the area shown as granite on the maps there are also some occurrences of charnockite. The high-grade rocks of the Varberg Series

and the CGA are overlain tectonically by rocks of a lower metamorphic grade (Fig. 2 ) named the "Bua Series" by Hubbard ( 1975 ). According to Hubbard, the Bua Series is largely made up of psammitic and pelitic rocks, quartz-feldspar gneisses and other rocks of supracrustal origin. L. Samuelsson (unpubl. map 1988, pers. commun., 1991 ), however, found only minor occurrences of gneisses of supracrustal origin in the Bua area which he sees as dominated by severely deformed migmatized granitoid orthogneisses. In intensely deformed zones, these gneisses locally develop a layer-like compositional banding that has been generated by metamorphism and deformation. Since there are few, if any, rocks of indisputable supracrustal origin in the Bua area, we prefer the term "Bua gneisses" to "Bua Series". Mafic dykes and rounded or irregularly shaped mafic bodies are abundant in all the three major rock complexes. Some mafic dykes cut across deformational structures in the granitic country rocks indicating pre-dyke deformational events. The primary igneous minerals of the mafic rocks, that intruded the CGA and the Varberg Series, have been completely replaced by garnetiferous amphibolite to granulite facies mineral assemblages. The mafic rocks within the Bua gneisses, in contrast, are hornblende- and plagioclase-dominated rocks generally lacking garnet and pyroxene. The youngest rocks in the area are deformed and/or partially recrystallised granitic pegmatites that were intruded into the charnockitic rocks.

3. Previous geochronological studies Few modern radiometric datings are available from the SGP, and only one of these is from the Varberg area. Magnusson (1960) published some K-Ar ages as between 900 and 1000 Ma which he considered dated the resetting of the K-Ar system during the Sveconorwegian metamorphism. Uranium minerals from pegmatites gave U -

LATE SVECONORWEGIAN METAMORPHISM AND DEFORMATION IN SOUTHWESTERN SWEDEN

Pb intrusion ages of ~910 Ma (Welin and Blomqvist, 1964). Rb-Sr dating of the Varberg charnockite (Welin and Gorbatschev, 1978 ) yielded a poorly defined whole-rock isochron with an age of 1420 + 25 Ma which the authors suggested could be interpreted as the age of the "intrusion of the charnockite". They pointed out that there must have been an additional high-grade event that is evidenced by granulite facies mineralogies found in mafic dykes cutting across the Varberg charnockite, and emphasized that nothing was known about the age difference between the two granulite facies events. Hubbard (1975), on the other hand, considered that the formation of the charnockite and the granulite facies metamorphism was part of a single evolutionary process (fig. 6 of Hubbard, 1975 ) which he named "Hallandian". Hubbard's "Hallandian" also comprised the emplacement of granites and abundant intrusions of mafic rocks. An attempt to date the zircons of the Varberg charnockite by the U - P b multigrain method was made by Welin et al. (1982). Because of substantial scatter no age was obtained. One possible reason is that the charnockite may contain zircons of more than a single origin and age, an assumption which is supported by the occurrence of numerous partially assimilated granitoid xenoliths in the charnockite. Multistage zircon growth and lead loss could be other factors contributing to the complexity of the data. The authors showed, however, that in an episodic lead-loss model at least three out of the eleven zircon fractions were aligned along a discordia line with an upper intercept at 1420 Ma ( = t h e Rb-Sr age) and a lower intercept at 950 Ma ( = t h e approximate age of Sveconorwegian reworking). Johansson et al. ( 1991 ) published two SmNd ages for the granulite facies mineral assemblages in mafic granulites from the SGP (Fig. 1 ). The ages of 907 + 12 Ma (Hallandshs) and 916 + 11 Ma (Ullared) clearly demonstrate the importance of late Sveconorwegian high-grade metamorphism in both the northern and

35 1

southern parts of the SGP. Similar ages have been obtained for amphibolite and granulite facies rocks at three other localities in the southwesternmost part of the SGP (A. Johansson, pers. commun., 1991 ). All these S m - N d ages are surprisingly low when compared with the Rb-Sr age of the Varberg charnockite and with ages suggested for metamorphic and deformational events in southwestern Sweden ( ~ 1090 Ma, Daly et al., 1983; ~ 1130 Ma and ~ 1100 Ma, Talbot and Heeroma, 1989). The S m - N d ages are also much lower than the age obtained for the granulite facies metamorphism in the Bamble area, southernmost Norway ( ~ 1100 Ma, Kullerud and Dahlgren, 1993).

4. Sampling and petrography Samples with granulite facies mineral assemblages were taken from two localities in the Varberg area. The sample of Varberg charnockite is from a quarry a few hundred metres southeast of Varberg castle; this quarry is the type locality of the Varberg charnockite. A sample from this locality was also included in the Rb-Sr dating of Welin and Gorbatschev (1978). The charnockite is greenish-brown, medium-grained equigranular and consists of quartz, orthoclase, plagioclase, clinopyroxene, orthopyroxene, garnet, and minor amounts of hornblende and biotite, zircon and apatite being the main accessory minerals. The sample is almost free of secondary alterations. The other sample, a mafic granulite from Tr/islrvsl~ige, ~ 7 km south of Varberg (Fig. 3 ), was taken from one of the many elongated mafic intrusions within the granulite facies gneisses in that area. Most of these intrusions feature well-preserved primary igneous layering of alternating light plagioclase-rich layers and darker layers with higher contents of hornblende and pyroxenes. The layers vary in thickness from less than 1 cm to ~ 1 m. No igneous minerals have been preserved. The metamorphic mineral assemblage in the sample

352 consists almost exclusively of plagioclase, pyroxene, garnet, quartz and opaques. The rock is fine grained and equigranular with a wellequilibrated granoblastic texture. It is noteworthy that the sampled mafic intrusion lacks pre-metamorphic deformational structures and mineral textures, which together with the preserved igneous layering suggests that the dykes were undeformed and unmetamorphosed at the onset of the high-grade metamorphism.

5. Analytical procedure

The mineral separates used for isotopic analysis were purified by hand-picking under a binocular microscope. Between 70 and 300 mg of cleaned separates were crushed and dissolved. Sm and Nd were separated by standard wet-chemical methods (Mearns, 1986). The isotopic analyses for spiked and unspiked samples were performed using a fully automatic VG 354 five-collector mass spectrometer and a Finnigan 262 automatic, nine-collector instrument. The 147Sm/144Ndratio is usually assumed to have a precision of 0.5% (2o; Mearns, 1986 ). Mearns ( 1986 ) also used the precision obtained on the mass spectrometer as the best estimate of the precision of the ~43Nd/144Nd ratio. The long-term ( ~ 1 year) variation in the 143Nd/144Nd ratio for a standard is approximately 0.000040 (2a), which is considerably greater than the typical mass-spectrometer precision for the 143Nd/ ~44Nd ratio of our data (0.000002 to 0.000014, 2tr). All the ~43Nd/ 144Nd ratios were analysed automatically and in a single batch. Thus the expected precision is somewhere between the long-term precision and the mass-spectrometer precision. To test the precision values used in the calculations we re-analysed three separates (Varberg: cpx, fsp and whole rock), the differences obtained being 0.46, 0.18, and 0.07%, respectively, verifying that the 0.5% (2a) estimate we used is a realistic error estimate of the 147Sm/144Ndra-

L. JOHANSSON AND L. KULLERUD

tio. The error of the 143Nd/144Ndratio varies between 0.000035 (clinopyroxene) and 0.000014 (feldspar). We consider 0.000020 to be the best estimate of the 2tr error for analyses performed in one batch. The MSWD close to unity obtained using this value verify the error estimates. Isochrons were calculated according to York (1969) with a decay constant of 6.54.10-12. The isotopic ratios have been normalized to 146Nd/la4Nd=0.7219. The shortterm variation (five analyses, three days) of the Johnson and Matthew 321 Nd standard (batch no. $819093A) was 143Nd/144Nd =0.511140-+20 (2tr). The standard samples were analysed during the same time period as the Varberg and Tr/isl~ivsl~ige samples.

6. Results and interpretation

The analytical data are presented in Table 1. The mineral analyses of the Varberg charnockite sample define an isochron with an age of 893_+5 Ma (Fig. 4). The MSWD is 0.7. The initial 143Nd/144Nd ratio is 0.511140 _+0.000020 corresponding to eNd(893) = -- 6.5 and eNd(0) = -- 15.5. The mafic granulite sample gave a well-defined age of 881 + 4 Ma with an MSWD of 0.7 (Fig. 4). The initial 143Nd/li4Nd ratio is 0.511500_+0.000020 with ENd(881)=--0.2 and ENd(0) = -- 5.0. The S m - N d isotopic system is suitable for dating well-equilibrated granulite facies rocks. Cohen et al. ( 1988 ) demonstrated that even in slowly cooled high-temperature rocks, garnet grains can be expected to give ages very close to the time of mineral growth. These ages are interpreted as high-temperature blocking ages of the S m - N d system at or near the peak metamorphic conditions. This interpretation is consistent with the high blocking temperature of REE volume diffusion in silicates (Sneeringer et al., 1984).

353

LATE SVECONORWEGIAN METAMORPHISM AND DEFORMATION IN SOUTHWESTERN SWEDEN

TABLE 1 Sm-Nd data

Sample

Sm

Nd

(ppm)

(ppm)

147Sm/144Nda

143Nd/144Nd

--- S E b

Varberg CPX CPX II GT PL PLII WR WR II

23.21 23.19 9.89 2.16 2.16 9.26 9.26

58.30 57.90 12.70 13.80 13.80 47.80 47.80

0.2424 0.2435 0.4731 0.0947 0.0949 0.1179 0.1179

0.512554 0.512664 0.513919 0.511699 0.511685 0.511842 0.511807

0.000004 0.000006 0.000005 0.000005 0.000001 0.000004 0.000007

Tr~sl6vsl~ge CPX GT PL WR

11.69 7.30 0.68 8.36

39.30 7.58 5.70 33.10

0.1808 0.5857 0.0726 0.1536

0.512556 0.514882 0.511915 0.512384

0.000003 0.000003 0.000005 0.000005

aThe estimated analytical uncertainty is 0.5% (see text for explanation) bSE = standard error

0,514(}(}

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VARBERG CHARNOCKITE

~ ~ f f T

0.51300

0.51200 ~,~pWR 0.51100 0.00 0.5)500

2

.

, 0.10

.

, 0.20

Age: 893 + 5.3 Ma lr: 0.51114 ± 0.00002 MSWD: 0.7 • , , . 0.30 0.40 0,50 147Sm/144Nd

TR,~SLOVSLAGE MAFIC GRANULITE

/

GT

0.51400 0.51300 0.51200

0.51100 0.00

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Age: 881 ± 4.3 Ma It: 0.51150+0.00002 MSWD: 0.7 - , - , , • 0.30 0.40 0.50 0.60 147Sm / 144Nd

Fig. 4. S m - N d isochrons of the Varberg charnockite and the Tr~isl6vsl~ige mafic granulite. GT=garnet; CPX= clinopyroxene; PL = plagioclase; FSP= K-feldspar and plagioclase; WR = whole-rock; M S W D = mean squared weighted deviate (s); Ir = initial 143Nd / 144Nd ratio.

7. Discussion

7.1. Formation of the charnockites and granites

The ages obtained agree closely with the ages of similar granulite facies mineral assemblages in mafic rocks at Ullared and Hallands~s (Fig. 1, Johansson et al., 1991 ). Regardless of what the protoliths may have been, the granulite facies parageneses now present in the Varberg charnockite, the Tr~isl6vsl~ige mafic granulite and elsewhere in SGP appear to have been formed during a metamorphic event ~ 900 Ma ago. The significance of the 1420 _+25 Ma Rb-Sr isochron of Welin et al. (1982) remains unclear. It may represent the intrusion age of the granitoid protolith of the charnockite, in which case it approximately corresponds in age to the anorogenic Karlshamn and Vhnga granites (Aberg et al., 1985a, b ) within and just west of the Blekinge region (Fig. 1 ). The Rb-Sr age is somewhat lower than the poorly defined U - P b zircon age ( 1 4 5 2 _ 4-i-349 7 Ma) obtained for the t3rkelljunga charnockite in the southernmost part of the SGP (Johansson, 1991 ). Another possibility is that the Rb-Sr isochron is a mixing-line and that the age is geologically meaningless. Since no concentration data have been

354

L. J O H A N S S O N A N D L. K U L L E R U D

published, it is not possible to test the mixing hypothesis. The negative ~Nd( 893 ) of the Varberg charnockite clearly shows that its protolith had a relatively long crustal prehistory. Because of the low 1475m/144Nd ratio the 143Nd/144Nd versus time plot has a gentle slope (Fig. 5 ) and the time difference between the model ages TciatJR (1605 Ma) and TDM (1940 Ma; DePaolo, 1981 ) is large. Thus the model ages indicate that the rock may have resided within the crust for a period of 200 to 500 Ma prior to its possible intrusion as a granitoid ~ 1400 Ma ago. Isotopic studies by Mearns et al. (1986), Patchett et al. (1987) and Claesson (1987) indicate that the lithospheric mantle underlying central and southern Scandinavia had a moderately depleted character with significantly lower end values than those predicted by the depleted mantle model of DePaolo ( 1981 ). TDMis therefore probably much higher than the age of protolith separation from the mantle. However, both model ages suggest an Palaeoproterozoic origin for the protolith of the Var-

04) 4

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...........

........

-4 -8 -12 -16 500

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1500

TIME (Ma)

Fig. 5. Diagram showing the similarity in evolution of the 143Nd/1"Nd ratio with time for the Tr~isl~vsl~ige mafic granulite (black line in field A) and the ~ 930 Ma old Blekinge-Dalarna dolerite dykes (stippled field A; Nddata from Johansson and Johansson, 1990). A comparison between the Varberg charnockite (black line in field B) and three 1680 to 1610 Ma old granites (stippled field B; Nd-data from P.-O. Persson pers. commun., 1991 ) of the eastern segment of the Southwest Swedish Gneiss Region is also shown. Field C denotes the approximate age interval of late Sveconorwegian high-grade metamorphism in the SGP as based on seven Sm-Nd wholerock + mineral isochrons.

berg charnockite. In terms of 147Sm/144Nd and 143Nd/la4Nd ratios, the Varberg charnockite closely resembles (Fig. 5 ) granitoids occurring in the northern part of the crustal segment between the Protogine and the Mylonite Zones (Persson, 1986 ). These granitoids have yielded intrusion ages of between 1680 and 1610 Ma (Persson, 1986; P.-O. Persson, pers. commun., 1991). The model ages of the mafic granulite at Tr~isl/Svsl~ige (TcHuR=898, TDM = 1700 Ma) are considerably lower than the model ages of the charnockite sample. The 147Sm/la4Nd and 143Nd/144Nd ratios (Fig. 5) are close to those of essentially unmetamorphosed mafic dykes east of the Protogine Zone (Johansson and Johansson, 1990). These ~930 Ma old dykes constitute part of the major Blekinge-Dalarna dyke swarm that trends roughly parallel to the Protogine Zone from Blekinge in the south to the Caledonide Front in the north (Fig. 1 ). Mafic intrusions are common in the SGP. Hubbard (1988) suggested that the Varberg granulite metamorphism had mainly been caused by heat transferred to the crust by these intrusions. Mafic intrusions could also introduce CO2 and thereby reduce the partial water pressure which facilitates the formation of granulites. If, as may be suggested by the SmNd model ages, the mafic intrusions are largely of Sveconorwegian age, their appearance on the scene could well be related to the occurrence of granulite facies metamorphism ~ 900 Ma ago. There is at present no evidence that the Varberg charnockite crystallized as a charnockite during the "Hallandian orogenic event" ~ 1450 Ma ago, as proposed by Hubbard ( 1975 ) and Talbot and Heeroma (1989). Note that, the late Sveconorwegian mineral ages obtained for the charnockite and the mafic granulite in the Varberg area correspond to the Sm-Nd mineral ages of granulite facies parageneses in samples from Hallands~ts and UUared, and three other widely spaced localities in the SGP. This suggests that the parageneses defining the Varberg charnockite and the granulites were all

LATESVECONORWEGIANMETAMORPHISMAND DEFORMATIONIN SOUTHWESTERNSWEDEN

formed during a period of widespread granulite facies metamorphism and anatexis in southwestern Sweden. The estimated P-Tconditions during the peak of metamorphism range from 705 °C and 8.1 kbar in the south at Hallandsgs to 770°C and 10.5 kbar in the north at Ullared (Johansson et al., 1991 ). Similar values can be expected for the rocks in the Varberg region (C. M611er, pers. commun., 1990). These pressures correspond to depths of 30-35 km. At present, our knowledge of the P - T - t evolution of the SGP is limited but structural and metamorphic results suggest that the SGP was depressed to great depths during the Sveconorwegian orogeny. The widespread occurrences of coronitic dolerites support this conclusion. In these rocks the igneous olivines, plagioclases and clinopyroxenesare replaced by a metamorphic mineral assemblage consisting of garnet, ortho- and clino-pyroxene, and plagioclase indicating an adjustment to increasing pressures (Johansson, 1992). Structural observations from the western segment (Park et al., 1991 ) suggest that the piling of nappes in the west may have led to a depression of the eastern segment of the Southwest Swedish Gneiss Region.

355

The later part of the Sveconorwegian orogeny 950-880 Ma ago is characterized by extensive magmatic activity with intrusions of charnockites, norites and mangerites in the Rogaland region of southern Norway (Demaiffe and Michot, 1985). Coeval granulite facies metamorphism has been recorded from the allochthonous rocks of the Bergen Arcs (Fig. l, cf. Cohen et al., 1988) just north of Rogaland. This was also a period of granite emplacement in the western segment of the Southwest Swedish Gneiss Region and in the Bamble, Rogaland and Telemark areas of southern Norway (Table 2 ). All granites listed in Table 2 have initial 8 7 S r / S 6 S r ratios of between 0.705 and 0.711 which suggests that they originated from a somewhat LIL-depleted lower crust. A genetic relationship between the high-grade metamorphism and the formation of the granitoid melts is therefore highly probable and should be investigated further. The view that there had been an active orogenic environment along the southwestern margin of the Baltic Shield during the late Sveconorwegian is in striking contrast with some previous suggestions that this was a period of declining activity, low-grade metamorphism, uplift, and

TABLE 2 Rb-Sr whole-rock ages of late Sveconorwegian granitoids in southwestern Scandinavia Rock

Crustal segment

Age (Ma)

Ref.

Bjerkreim-Sokndal qz.-monzonite Seterdalen granite Lyngdal granite Kleivan granite Herefoss granite Bessefjell granite Fl/t granite Hidra charnockitic dikes Vradal granite Bohus-Iddel~ord granite Bohus-Iddefjord granite Bohus-Iddel]ord granite

Rogaland-Vest Agder Western segment of SSGR Rogaland-Vest Agder Rogaland-Vest Agder Bamble-Telemark Telemark Telemark Rogaland-Vest Agder Telemark Western segment of SSGR Western segment of SSGR Western segment of SSGR

928 + 50 925 + 56 912 _+38 910 + 07 909 _+26 904 + 16 894 _+28 892 + 25 888 + 46 918 _+07 890 _+35 881 _+35

1 2 3 4 5 5 5 6 3 7 8 5

SSGR = Southwest Swedish Gneiss Region; crustal segments refer to Fig. 1. References: 1 = Wielens et al., 1981,2 = Skjernaa and Pedersen, 1982; 3=Pederscn and Falkum, 1975; 4=Petersen, 1977, 5=Killeen and Heier, 1975; 6=Pasteels et al., 1979; 7 =Pedersen and Maal~e, 1990; 8 =Ski61d, 1976.

356

cooling (e.g. Talbot and Heeroma, 1989; Park et al., 1991 ). One of the late Sveconorwegian granites close to the Varberg area is the Bohus-Iddel]ord granite which has yielded Rb-Sr wholerock intrusion ages of 881_+35 Ma (Killeen and Heier, 1975 ), 890 _+35 Ma (Ski61d, 1976 ) and 918 _+7 Ma (Pedersen and Maaloe, 1990 ), and U / P b ages of 919 _-25 Ma for monazite and 922 _+5 Ma for xenotime (Eliasson and Sch6berg, 1991 ). The Sm-Nd ages, the high-pressure metamorphism and the fact that temperatures in the SGP were high enough to cause anatexis (Johansson et al., 1991 ) suggest that the SGP represents an exposed part of late Sveconorwegian deep crustal levels from which high-level granite intrusions can have originated. The western segment of the Southwest Swedish Gneiss Region was at a much higher crustal level than its eastern segment during most of the late Sveconorwegian. This is also supported by the biotite cooling ages (Jarl, 1992 ) discussed below and by the presence of the low-grade Dal Group ( 1075-1030 Ma old, SkiNd, 1976), which escaped the high-grade metamorphism of the eastern segment. 7.2. Deformation in relation to metamorphism There are numerous observations (Park et al., 1991) of southeast- and east-directed thrusting along the Mylonite Zone, the Grta )klv Zone and the Dalsland Boundary Thrust (Fig. 1 ) within the western segment of the Southwest Swedish Gneiss Region. The age of this thrusting is poorly defined. Movements along these zones are probably not restricted to a single well-defined period. It is more likely that the rocks in these zones have been repeatedly deformed. Park et al. (1991 ) place the thrusting around 1100 Ma. However, it is obviously younger than the diagenesis of the Dal Group (1075-1030 Ma; Ski/Jld, 1976) and older than the undeformed lamprophyre dykes (,,-900 Ma) that cut across the northern part of the Mylonite Zone (Wahlgren, 1979). Ap-

L. JOHANSSON AND L. KULLERUD

proximately 920 Ma old titanites in the southernmost part of the Mylonite Zone have been deformed; this indicates late Sveconorwegian movements along that zone (Johansson and Johansson, 1993). The southeast-directed thrusting, that resulted in a thickening of the crust and depression of the rocks of the eastern segment, probably predated the granulite facies metamorphism. All rock units in the Varberg area, including the late pegmatites, have undergone deformation during the late Sveconorwegian. The brittle deformation in the area may be considerably younger and unrelated to the Sveconorwegian events. Deformation postdating the metamorphic peak is characterized by retrogression and hydration of granulite facies mineral assemblages to parageneses that are stable under amphibolite facies or greenschist facies conditions. The Sm-Nd ages obtained imply that extensive retrogression and deformation must also have taken place in the Mylonite and the Protogine Zones ~ 900 Ma ago. Hubbard ( 1975, fig. 6) showed that the main phases of deformation in the Varberg area were contemporaneous with the high-grade metamorphism. Talbot and Heeroma (1989) supported these findings and mapped three generations (BI to B3) of major deformation zones in the area (Fig. 2). However, in their model the first main deformation (B 1 ) took place during the Hallandian orogeny ( ~ 1400 Ma; Hubbard, 1975) to be followed by a second phase (B2, ~ 1130 Ma) and a third phase (B3, ~ 1100 Ma). The presumed ages of these deformation events are based on the Rb-Sr isochron of the Varberg charnockite (B 1; Welin and Gorbatschev, 1978 ) and for the B2 and the B3 phases on poorly substantiated correlations with tectonometamorphic events in the crustal region to the west of the Mylonite Zone. K-Ar cooling ages of biotites (i.e. passage through ~ 250°C) in rocks to the west of the Mylonite Zone are between 1100 and 1000 Ma (Jarl, 1992 ), whereas the K-Ar ages ofbiotites

LATE SVECONORWEGIAN METAMORPHISM AND DEFORMATION IN SOUTHWESTERN SWEDEN

from the northern part of the area between the Protogine and the Mylonite Zones are between 975 and 900 Ma. No K - A r cooling ages ofbiotires from the SGP are available. From the radiometric datings of Skitild ( 1976 ), Zeck and Wallin (1980), and Jarl (1992), and from the presence of the low-grade metasediments of the Dal Group and the absence of late Sveconorwegian granulites in the western segment, it is obvious that the region to the west of the Mylonite Zone had undergone a middle to late Sveconorwegian metamorphic evolution characterized by lower metamorphic grades and earlier cooling than in the eastern segment. The ages of metamorphism recorded in the western segment of the Southwest Swedish Gneiss Region ( ~ 1090 Ma, Daly et al., 1983) suggest a closer time relationship with the middle Sveconorwegian high-grade event in the Bamble region ( ,-, 1100 Ma) than with the late Sveconorwegian metamorphism in the east. According to Talbot and Heeroma (1989) the rocks in the Mylonite Zone, which separates the Bua gneisses from the CGA (Fig. 2), were deformed during the B2 and B3 phases. Massive mafic and felsic granulites were retrogressed and transformed into amphibolite facies gneisses. Remnants of these granulites occur as lenses in the intensely sheared amphibolite facies rocks of the Mylonite Zone. All available radiometric data on granulite facies mineral assemblages in the SGP show that the high-grade metamorphism is late Sveconorwegian. Consequently, the B2 and B3 deformation took place essentially after the late Sveconorwegian metamorphic peak. Similar relationships between age, deformation and metamorphism have been observed in many other places within the SGP and are thus characteristic of the SGP. The rate of uplift of the late Sveconorwegian granulite terrain in southwestern Sweden is not known. However, as in many other high-grade areas (e.g. Coney and Harms, 1984; England and Thompson, 1984; Platt, 1986) the uplift may well have been rapid, being enhanced by

357

tectonic unroofing including extensional tectonics. The possibility of late or even post-Sveconorwegian extensional tectonics in the western segment and along the Mylonite and Protogine Zones should be investigated. Park et al. (1991 ) mentioned briefly that there is some evidence of west-directed extensional movements within the Mylonite Zone, the Gtita )~lv Zone and the Dalsland Boundary Thrust (Fig. 1 ) in the western segment. They suggested that these movements may relate to the collapse of an over-thickened nappe pile that was built up during an earlier compressional phase. The age of the extensional deformation is not known, but this must obviously be one of the youngest deformation phases in the region, which suggests a relationship with deformation accompanying the latest Sveconorwegian metamorphism and uplift. In their reinterpretation of the geology of the Protogine Zone, Andreasson and Rodhe (1990) emphasized the importance of extensional tectonics during the uplift of the granulite terrain west of the Protogine Zone. Much more information on metamorphism, structures and ages (particularly from shear zones) in southwestern Sweden is needed before the relationship between the crustal segments and the role of the major shear zones during the uplift of the SGP can be fully understood. The late Sveconorwegian metamorphism reached granulite facies conditions and probably caused partial melting and migmatization. It was accompanied or followed by at least three phases of compressional or extensional deformation. These facts together with the apparent differences in age of deformation, metamorphism and cooling make it necessary to exercise caution when correlating preto mid-Sveconorwegian deformation phases across the Mylonite and Protogine Zones. 8. Conclusions

( 1 ) The mineral assemblage (pyroxene, garnet and feldspar) defining the Varberg char-

3 58

nockite yielded a mineral + whole-rock Sm-Nd age of 893+ 5 Ma, and the granulite facies mineral assemblage in a mafic granulite within the charnockite an S m - N d mineral+wholerock age of 881 + 4 Ma. (2) Nd model ages suggest that the Varberg charnockite was formed from Palaeoproterozoic protoliths that had a crustal residence time of 200-500 Ma before its intrusion as a granitoid ~ 1400 Ma ago (cf. Rb-Sr whole-rock age). (3) The late Sveconorwegian granulite facies metamorphism affected the entire southern part of the crustal segment between the Mylonite and Protogine Zones and was not restricted to the Varberg region. (4) The eastern segment of the Southwest Swedish Gneiss Region was at a deeper crustal level than the western segment ~ 900 Ma ago. (5) At least three phases of regionally important deformation are coeval with or younger than the late Sveconorwegian granulite facies metamorphism. (6) Extensional tectonics were presumably an important factor during the uplift of the granulite terrain. (7) The final juxtaposition of the eastern and western segments of the Southwest Swedish Gneiss Region took place in the late Sveconorwegian. (8) Late Sveconorwegian high-grade metamorphism at depth and the approximately contemporaneous emplacement of crustally derived granites at upper crustal levels may have been related genetically. Acknowledgements This study was funded by a grant from the Swedish Natural Sciences Research Council (NFR G-GU 8869-304). The use of the analytical facilities at the Mineralogical Geological Museum in Oslo is gratefully acknowledged. Tom Andersen (Oslo), Stephen Daly (Dublin) and Roland Gorbatschev (Lund), and an anonymous referee read an early ver-

L. JOHANSSONANDL. KULLERUD

sion the manuscript. Their suggestions improved the manuscript considerably. Sincere thanks to Margaret Greenwood-Petersson for her improvement of the English. References Aberg, G., Kornf~ilt, K.-A. and Nord, A.G., 1985a. The V~inga granite south Sweden--a complex granitic intrusion. Geol. F6ren. Stockholm F6rh., 107:153-159. /l,berg, G., Kornfiilt, K.-A. and Nord, A.G., 1985b. Further radiometric dating of the Karlshamn granite, south Sweden. Geol. F6ren. Stockholm F6rh., 107: 197-202. Ahiill, K.I., Daly, J.S. and Sch6berg, H., 1990. Geochronological constraints on Mid-Proterozoic magmatism in the t~stfold-Marsstrand belt; implications for crustal evolution in SW Sweden. In: C.F. Gowers, T. Rivers and B. Ryan (Editors), Mid-Proterozoic Geology of the Southern Margin of Proto Laurentia-Baltica. Geol. Assoc. Can., Spec. Pap., 38:97-116. Andreasson, P.G. and Rodhe, A., 1990. Geology of the Protogine Zone south of Lake V~ittern, southern Sweden--a reinterpretation. Geol. F6ren. Stockholm F6rh., 112: 107-125. Brewer, T.S. and Field, D., 1985. Tectonic environment and age relationships of the Telemark supracrustals, southern Norway. In: A.C. Tobi and J.L.R. Touret (Editors), The Deep Proterozoic Crust in the North Atlantic Provinces. D. Reidel Publ. Co., Dordrecht, pp. 291-307. Claesson, S., 1987. Nd isotope data on 1.9-1.2 Ga-old basic rocks and metasediments from the Bothnian basin, central Sweden. Precambrian Res., 35:115-126. Cohen, A.S., O'Nions, R.K., Siegenthaler, R. and Griffin, W.L., 1988. Chronology of the pressure-temperature history recorded by a granulite terrain. Contrib. Mineral. Petrol., 98:303-311. Coney, P.J. and Harms, T.A., 1984. Cordilleran metamorphic core complexes. Cenozoic extensional relics of Mesozoic compression. Geology, 12:550-554. Constable, J.L. and Hubbard, F.H., 1981. U, Th, and K distribution in a differentiated charnockite-granite intrusion and associated rocks from SW Sweden. Mineral. Mag., 44: 409-415. Daly, J.S., Park, R.G. and Cliff, R.A., 1983. Rb-Sr isotopic equilibrium during Sveconorwegian (=Grenville) deformation and metamorphism of the Orust dykes, SW Sweden. Lithos, 16:307-318. Demaiffe, D. and Michot, J., 1985. Isotope geochronology of the Proterozoic crustal segments of southern Norway. A review. In: A.C. Tobi and J.L.R. Touret (Editors), The Deep Proterozoic Crust in the North Atlantic Provinces. D. Reidel Publ. Co., Dordrecht, pp. 411-433. DePaolo, D.J., 1981. Neodymium isotopes in the Colo-

LATE SVECONORWEGIAN METAMORPHISM AND DEFORMATION IN SOUTHWESTERN SWEDEN

rado Front Range and crust-mantle evolution in the Proterozoic. Nature, 291: 193-196. Eliasson, T. and SchSberg, H., 1991. U-Pb dating of the post-kinematic Sveconorwegian (Grenvillian) Bohus granite, SW Sweden: evidence of restitic zircon. Precambrian Res., 51: 337-350. England, P.C. and Thompson, A.B., 1984. Pressure-temperature-time paths of regional metamorphism, I. Heat transfer during the evolution of regions of thickened continental crust. J. Petrol., 25: 894-928. Gower, C.F., 1985. Correlations between the Grenville Province and the Sveconorwegian orogenic belt. In: A.C. Tobi and J.L.R. Touret (Editors), The deep Proterozoic Crust in the North Atlantic Provinces. D. Reidel Publ. Co., Dordrecht, pp. 247-257. Gower, C.F., Ryan, A.B. and Rivers, T., 1990. Mid-Proterozoic Laurentia,vBaltica: an overview of its geological evolution and a summary of the contributions made by this volume. In: C.F. Gower, T. Rivers and A.B. Ryan (Editors), Mid-Proterozoic Laurentia-Baltica. Geol. Assoc. Can., Spec. Pap., 38: 1-22. Hubbard, F.H., 1975. The Precambrian crystalline complex of south-western Sweden. The geology petrogenetic development of the Varberg Region. Geol. F6ren. Stockholm FSrh., 97: 223-236. Hubbard, F.H., 1978. Geochemistry of the Varberg granite gneisses. Geol. F6ren. Stockholm F6rh., 100:3138. Hubbard, F.H., 1988. Basic intrusion, charnockite-rapakivi granite plutonism and crustal depletion, SW Sweden. Rend. Soc. Ital. Mineral. Petrol., 43-2: 543554. Hubbard, F.H., 1989. The geochemistry of Proterozoic lower-crustal depletion in southwest Sweden. Lithos, 23: 101-113. Hubbard, F.H. and Whitely, J.E., 1979. REE in charnockite and associated rocks, southwest Sweden. Lithos, 12: 1-11. Jacobsen, S.B. and Wasserburg, G.J., 1979. Nd and Sr isotopic study of the Bay of Island ophiolite complex and the evolution of the source of mid-ocean ridge basalts. J. Geophys. Res., 84: 7429-7445. Jagoutz, E., 1988. Nd and Sr systematics in an eclogite xenolith from Tanzania. Evidence for frozen mineral equilibria in the continental lithosphere. Geochim. Cosmochim. Acta, 52: 1285-1293. Jarl, L.-G., 1992. New isotope data from the Protogine Zone and southwestern Sweden. Geol. F6ren. Stockholm F6rh., 114: 349. Johansson,/~., 1991. The early evolution of the Southwest Swedish Gneiss Province: geochronological and isotopic evidence from southernmost Sweden (abstr.). IGCP 275 Task Group Meeting, The Westward Accretion of the Baltic Shield. GSteborgs Universitet, Geologiska Institutionen, Publ. B 356. Johansson, L., 1993. The late Sveconorwegian metamor-

359

phic discontinuity across the Protogine Zone. Geol. F(Sren. Stockholm F6rh., 114: 350-353. Johansson, L. and Johansson, A., 1990. Isotope geochemistry and age relationships of mafic intrusions along the Protogine Zone, southern Sweden. Precambrian Res., 48: 395-414. Johansson, L., and Johansson, A., 1993. U-Pb age oftitanite in the Mylonite Zone, southwestern Sweden. Geol. F6ren. Stockholm F6rh., 115: 1-7. Johansson, L., Lindh, A. and M611er, L., 1991. The late Sveconorwegian (Grenville) high-pressure granulite metamorphism in southwest Sweden. J. Metamorph. Geol., 9: 283-292. Killeen, P.G. and Heier, K.S., 1975. Radioelement distribution and heat production in Precambrian granitic rocks, southern Norway. Nor. Vidensk.-Akad., 1. Mat. Nat. K1. Skr. Ny Ser., 35, 32 pp. Kullerud, L. and Dahlgren S.H., 1993. Sm-Nd geochronology of Sveconorwegian granulite facies mineral assemblages in the Bamble Shear Belt, South Norway. In: R. Gorbatschev (Editor), The Baltic Shield. Precambrian Res., 64:389-402 (this volume). Larson, S.A., Stigh, J. and Tullborg, E.-L., 1986. The deformation history of the eastern part of the southwest Swedish gneiss belt. Precambrian Res., 31: 237-258. Lindh, A., 1987. Westward growth of the Baltic Shield. Precambrian Res., 35:53-7 I. Magnusson, N.H., 1960. Age determinations of Swedish Precambrian rocks. Geol. F6ren. Stockholm F6rh., 82: 407-432. Mearns, E.W., 1986. Sm-Nd ages for Norwegian garnet peridotites. Lithos, 19: 269-278. Mearns, E.W., Andersen, T., M6rk, M.B.E. and Morvik, R., 1986. 143Nd/144Nd evolution in depleted Baltoscandian mantle. Terra Cognita, 6: 247. Park, R.G., Ah~ill, K.-I. and Boland, M.P., 1991. The Sveconorwegian shear-zone network of SW Sweden in relation to mid-Proterozoic plate movements. Precambrian Res., 49: 245-260. Pasteels, P., Demaiffe, D. and Michot, J., 1979. U-Pb and Rb-Sr geochronology of the eastern part of the Rogaland igneous complex, southern Norway. Lithos, 12: 199-208. Patchett, P.J., Todt, W. and Gorbatschev, R., 1987. Origin of continental crust of 1.9-1.7 Ga age: Nd isotopes in the Svecofennian orogenic terrains of Sweden. Precambrian Res., 35: 145-160. Pedersen, S. and Falkum, T., 1975. Rb-Sr isochrons for the granitic plutons around Farsund, southern Norway. Chem. Geol., 15: 97-101. Pedersen, S. and Maaloe, S., 1990. The Idde0ord granite, geology and age. Nor. Geol. Unders., 417: 56-84. Persson, P.-O., 1986. A Geochronological Study of Proterozoic Granitoids in the Gneiss Complex of Southwestern Sweden. PhD-thesis, Lund University, 105 pp. (unpublished).

360 Petersen, J.S., 1977. The migmatite complex near Lyngdal, southern Norway and related granulite metamorphism. Nor. Geol. Tidsskr., 57: 65-83. Platt, J.P., 1986. Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks. Geol. Soc. Am. Bull., 97: 1037-1053. Quensel, P., 1951. The charnockite series of the Varberg district on the south-western coast of Sweden. KVA. Ark. Mineral. Geol., 1: 227-332. Ski/Sld, T., 1976. The interpretation of the Rb-Sr and K Ar ages of late Precambrian rocks in the south-western Sweden. Geol. F6ren. Stockholm F6rh., 98: 3-29. Skjernaa, L. and Pedersen, S., 1982. The effects of penetrative Sveconorwegian deformation on Rb-Sr isotope systems in the Ramskog-Aurskog-Holand area, SE Norway. Precambrian Res., 17:215-243. Sneeringer, M., Hart, S.R. and Shimuzu, N., 1984. Strontium and samarium diffusion in diopside. Geochim. Cosmochim. Acta, 48: 1589-1608. Starmer, I.C., 1990. Mid-Proterozoic evolution of the Kongsberg--Bamble belt and adjacent areas. In: C.F. Gower, T. Rivers and A.B. Ryan (Editors), Mid-Proterozoic Laurentia-Baltica. Geol. Assoc. Can., Spec. Pap., 38: 279-306. Starmer, I.C., 1991. The Proterozoic evolution of the Bamble Sector shear belt, Southern Norway: correlations across southern Scandinavia and the Grenvillian controversy. Precambrian Res., 49: 107-139. Svedmark, E., 1893. Beskrifning till kartbladet Varberg. Sver. Geol. Unders., Set. Ab, 13, 82 pp.

L. JOHANSSON AND L. KULLERUD

Talbot, C.J. and Heeroma, P., 1989. Cover/basement relationships in the SW Swedish gneisses near Varberg. Geol. F/Sren. Stockholm F6rh., 111:105-119. Wahlgren, C.-H., 1979. A cratonic ultrabasite in southwestern Sweden. Geol. F6ren. Stockholm F6rh., 101: 125-129. Welin, E. and Blomqvist, G., 1964. Age measurements on radioactive minerals from Sweden. Geol. F6ren. Stockholm F6rh., 86: 33-50. Welin, E. and Gorbatschev, R., 1978. The Rb-Sr age of the Varberg Charnockite, Sweden. Geol. F6ren. Stockholm F6rh., 100: 225-227. Welin, E., Gorbatschev, R. and K~hr, A.-M., 1982. Zircon dating of polymetamorphic rocks in southwestern Sweden. Sver. Geol. Unders., Ser. C, 797: 1-34. Wielens, J.B.W., Andriessen, P.A.M., Boelrijk, N.A.I.M., Hebeda, E.H., Verdurmen, E.A.Th. and Verschure, R.H., 1981. Isotope geochronology in the high-grade metamorphic Precambrian of southwestern Norway: new data and reinterpretations. Nor. Geol. Unders. Bull., 359: 1-30. York, D., 1969. Least square fitting of a straight line with correlated errors. Earth Planet. Sci. Lett., 5: 320-324. Zeck, H.P. and Wallin, B., 1980. A 122060 m.y. Rb-Sr isochron age representing a Taylor-convection caused recrystallisation event in a granitic rock suite. Contrib. Mineral. Petrol., 74: 45-53.