Contrasting magmatic arcs in the Palaeoproterozoic of the south-western Baltic Shield

Precambrian Research 92 (1998) 297–315

Contrasting magmatic arcs in the Palaeoproterozoic of the south-western Baltic Shield ˚ ha¨ll c Tim S. Brewer a,*, J. Stephen Daly b, Karl-Inge A a Department of Geology, University of Leicester, Leicester, LE1 7RH, UK b Department of Geology, University College Dublin, Belfield, Dublin 4, Ireland c Earth Sciences Centre, University of Go¨teborg, Box 460, SE-405 30 Go¨teborg, Sweden Received 14 January 1997; received in revised form 10 June 1998; accepted 10 June 1998

Abstract Following the Svecofennian arc accretionary growth and extensive granitoid magmatism in the Transscandinavian Igneous Belt (TIB), new crustal growth occurred west of the TIB to form the Gothian orogen. An early stage is ¨ tran Terrane. A second stage, forming an 140 km manifested by 1.69–1.65 Ga subduction-related magmatism in the A wide segment east of the Permian Oslo Rift, is recorded by three 1.66–1.59 Ga metamorphosed volcano-sedimentary ˚ ma˚l and Stora Le-Marstrand formations in the Idefjorden Terrane. The 1.66 Ga units, exposed in the Horred, A Horred Formation is dominated by felsic volcanics and has geochemical signatures indicative of formation in an ˚ ma˚l Formation have island arc setting. In contrast, the lithologically similar volcanic sequences in the 1.61 Ga A geochemical signatures consistent with a continental-margin setting. The 1.60–1.59 Ga Stora Le-Marstrand Formation is dominated by greywacke-type metasediments with subordinate metabasalts. These volcanics have markedly primitive trace element signatures and depleted Nd isotopic compositions, all consistent with their derivation in an oceanic island arc setting. The sediments document two provenances: a distal continental source and one with Nd isotopic compositions similar to the SLM volcanism. Many of the metasediments in the Stora Le-Marstrand formation have chemical signatures consistent with derivation from continental crust, suggesting that this volcanic arc developed in the vicinity of a continental massif, possibly in a setting similar to the Philippine Sea. Accretion of the Horred and Stora Le-Marstrand arc systems occurred prior to 1.61 and 1.59 Ga, respectively, and was followed by voluminous, ca 1.59 Ga calc-alkaline magmatism. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Palaeoproterozoic; Arc; Geochemistry; Gothian; Crustal growth

1. Introduction The Baltic/Fennoscandian Shield forms the major exposure of Precambrian crust in northern Europe, with Archaean units in the north-east and * Corresponding author. Fax: +44 116 2523918; e-mail: [email protected]

progressively younger Proterozoic terranes toward the south-west (Gaa´l and Gorbatschev, 1987; Gorbatschev and Bogdanova, 1993). During early Palaeoproterozoic time, the Archaean crust experienced several episodes of extension and rifting, which was terminated by the accretion of ca 1.90–1.85 Ga Svecofennian arc complexes (e.g. Patchett et al., 1987; Gorbatschev and Bogdanova,

0301-9268/98/$ – see front matter © 1998 Elsevier Science B.V. All rights reserved. PII S0 3 0 1 -9 2 6 8 ( 9 8 ) 0 0 07 9 - 5

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1993). A net result was the generation of a large volume of juvenile which now forms the Svecofennian Domain ( Fig. 1, inset). Following accretion, intermittent, 1.85–1.65 Ga granitoid magmatism occurred along the western protomargin of Baltica to form the Transscandinavian Igneous Belt (TIB; see below). Subsequent crustal growth was Gothian in age (ca 1.75–1.55 Ga) and resulted in an ca 175 km wide segment between the Permian Oslo Rift and TIB [Fig. 1; Gaa´l and ˚ ha¨ll and Gower (1997)]. Gorbatschev (1987); A West of the Oslo Rift in SW Norway, the early history is still poorly constrained, although generally attributed to ca 1.6 Ga Kongsbergian (Oftedahl, 1980; Starmer, 1991) or Gothian orogenesis (e.g. Gaa´l and Gorbatschev, 1987; Starmer, 1996). A distinction across the Oslo Rift between the Gothian and Kongsbergian orogens was sug˚ ha¨ll and Gower (1997) and appears gested by A important since pre-1.65 Ga gneisses in SW Norway (Ragnhildstveit et al., 1994) have been interpreted to represent a craton, or microconti˚ ha¨ll nent that docked onto Baltica at ca 1.58 Ga (A et al., 1997, 1998). This paper presents whole-rock geochemistry including rare-earth element (REE ) and Nd isotope data, from three distinct supracrustal units in the Idefjorden terrane of the Gothian orogen (Fig. 1). The results are used to determine the tectonic setting of the individual sequences and to constrain models for the Proterozoic evolution of the Baltic Shield. These results are also important in constraining the mechanism of crustal growth in the North Atlantic region. Unless otherwise stated, all subsequent ages discussed in this paper are U–Pb zircon ages. All of the rocks in this paper have been metamorphosed in greenschist or amphibolite facies. However, for simplicity rocks are generally referred to by their protolith names, that is, basalt rather than metabasalts etc.

2. Regional geology Gothian, and possibly the time-related Kongsbergian, crust characterize a 550 km wide area in the western part of the Baltic (Fennoscandian) Shield. The area is separated

from the Svecofennian Domain by mainly undeformed granitoid rocks of the 1.85–1.65 Ga TIB (Lindh and Gorbatschev, 1984; Andersson, 1991; Larson and Berglund, 1992; Wikstro¨m, 1996). Since most of the Gothian orogen was reworked during ca 1.15–0.9 Ga Sveconorwegian/ Grenvillian orogenesis, these areas are polymetamorphic and part of the Sveconorwegian Province (Berthelsen, 1980; Gorbatschev and Bogdanova, 1993). Recent work has further constrained the ca ˚ ha¨ll et al., 1.75–1.55 Ga Gothian evolution (A ˚ 1995; Connelly and Aha¨ll, 1996; Connelly et al., ˚ ha¨ll et al., 1996, 1997, 1998) and shown a 1996; A broadly coeval evolution with the Labradorian ˚ ha¨ll and Gower, history of eastern Laurentia (A 1997). The modified lithotectonic subdivision of ˚ ha¨ll and Gower (1997) for the region east of the A Oslo Rift is used here ( Fig. 1 inset), and the description below is also based on studies referred to therein. The Idefjorden terrane occupies the segment immediately east of the Oslo Rift and is underlain by diverse belts of 1.66–1.59 Ga supracrustal and intrusive rocks assembled by ca ¨ tran terrane is 1.58 Ga. To the southeast, the A dominated by older orthogneisses that represents a 1.7–1.65 Ga Gothian phase of crustal growth. To the northeast, the Klara¨lven terrane is dominated by 1.8–1.65 Ga gneissic granitoids, mostly regarded as deformed TIB rocks. The three lithotectonic terranes are delimited by N–S trending Sveconorwegian shear zones, some of which developed along structures related to Gothian amalgamation. One such shear zone is the prominent Mylonite Zone [Fig. 1; Berthelsen (1980); Park ˚ ha¨ll (1995); Page et al. (1996); et al. (1991); A Stephens et al. (1996)] that separates the ¨ tran and Klara¨lven terranes. Idefjorden from the A This lithotectonic subdivision, which largely follows the tectonic map of Stephens et al. (1994), is thus based on Gothian characteristics but is also important in a Sveconorwegian context. Gothian orogenic activity had ceased by 1.55 Ga ˚ ha¨ll, 1996) and there is no evi(Connelly and A dence for any substantial addition of crust thereafter (Lindh and Persson, 1990; Johansson et al., 1993). The subsequent cratonic stage was characterized by widespread mafic and granitoid pluton-

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299

Fig. 1. Generalized geology of SW Sweden, showing the distribution of Mid to Late Palaeoproterozoic volcanic units, modified after ˚ ha¨ll et al. (1995) and A ˚ ha¨ll and Gower (1997). 1, SLM formation (ca 1. 6 Ga); 2, granitoids (ca 1.59 Ga) with supracrustal rocks A ˚ ma˚l and Horred formations (undifferentiated ) shown in black. The cross-hatched line running from Go¨teborg to Lake Va¨nern of the A ˚ ma˚l Formation (1.61 Ga) and the Horred Formation (1.66 Ga); 3, A ¨ tran terrane rocks, mainly is the tentative boundary between the A granitoid gneisses; 4, Dal Group (ca 1.1 Ga); 5, Bohus granite (0.92 Ga). The inset map shows major tectonic units in SW Scandinavia. TIB, Transscandinavian Igneous Belt; stippling, Gothian orogen, variable reworked during Sveconorwegian orogeny. Idefjorden ˚ ), Klara¨lven terrane ( KI ), Kongsberg ( K ), Bamble (B), Telemark ( T ) and Rogaland-Vest Agder ¨ tran terrane (A terrane (Id ), A (RVA) sectors and the Western Gneiss Region ( WGR); PZ, Protogine Zone; MZ, Mylonite Zone; cross-hatching, phanerozoic rocks including the Oslo Rift.

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ism, forming discrete events between 1.51 and 1.20 Ga (Johansson et al., 1993 and references ˚ ha¨ll et al., 1997; A ˚ ha¨ll and Connelly, therein; A 1998). The Sveconorwegian Orogeny (1.15–0.9 Ga) variously reworked existing crust in the Gothian orogen, forming major N–S trending shear zones and the first fabric in the 1.51–1.20 Ga intrusions (Larson et al., 1990; Park et al., 1991; Wahlgren ˚ ha¨ll, 1995). The metamorphism et al., 1994; A mainly resulted in amphibolite-facies assemblages although widespread granulite rocks are known ¨ tran terrane (Johansson et al., from the southern A 1991). West of lake Va¨nern (Fig. 1), preserved greenschist parageneses records inhomogeneous Sveconorwegian overprinting in the Idefjorden terrane. Post-kinematic magmatism include 1.04–0.95 Ga pegmatites (Romer and Smeds, ˚ ha¨ll and 1996), the 0.96 Vinga porphyry (A Scho¨berg, 1996) and the 0.92 Ga Bohus granite ( Eliasson and Scho¨berg, 1991).

3. Volcanic units In the Idefjorden terrane, the earliest identified magmatic episodes are represented by three late Palaeoproterozoic volcanic sequences; the 1.66 Ga ˚ ma˚l and ca 1.60–1.59 Ga Stora Horred, 1.61 Ga A Le-Marstrand (SLM ) formations (see below). These are all intruded and separated by granitoids interpreted to form a single calc-alkaline unit, the ˚ ha¨ll et al. 1.59 Ga Go¨teborg Batholith [Fig. 1; A ˚ (1995); Connelly and Aha¨ll (1996)]. 3.1. Horred Formation In the Horred region, south of Go¨teborg, a sequence of banded metamorphosed supracrustal rocks are preserved as a large sheet within granitoids of the Go¨teborg Batholith (Fig. 1). Similar rocks are also found as smaller discontinuous sheets toward the north (i.e. east of Go¨teborg). These units have been grouped into the Horred formation and are interpreted as dominantly meta˚ ha¨ll et al., 1995). They range from volcanics (A mafic to felsic compositions, with a predominance of intermediate units. Most primary features have

been destroyed during amphibolite-facies metamorphism and deformation, and it is therefore difficult to identify the protoliths. However, the overall appearance is that of a volcanic sequence composed of lavas, pyroclastic deposits and sub˚ ha¨ll et al., 1995). volcanic rocks (A Metasedimentary units are scarce (<1%) and comprise well banded quartzo-feldspathic lithologies with possible slump structures and metre-wide laminated units, representing quartz-rich beds ˚ ha¨ll et al., 1995). and/or possibly cherts (A A 1643±29 Ma age for the Mjo¨sjo¨ dacite from the Horred Formation has been interpreted to date ˚ ha¨ll et al., 1995). Recently, this the volcanism (A age has been constrained to 1659+8/−6 Ma from another dacitic unit [unpublished data by ˚ ha¨ll (1996)]. J. Connelly, in Connelly and A ˚ ma˚l Formation 3.2. A ˚ ma˚l Formation is composed of The A greenschist- to amphibolite-facies volcanics and sediments, which outcrop to the west of lake Va¨nern ( Fig. 1). Similar rocks form a semi-continuous outcrop into Norway where they are termed Trysil porphyries (Priem et al., 1970). The volcanics are dominated by feldspar and/or quartzphyric rhyolites and dacites, which are intercalated with arkosic sandstones, subordinate conglomerates and pelitic units (Gorbatschev, 1977; Lundqvist and Skio¨ld, 1992). Mafic lithologies are present as minor lava flows, sills and dykes. A porphyritic dacite has yielded an age of 1614±7 Ma which has been interpreted as the age for this unit (Lundqvist and Skio¨ld, 1992). ˚ ma˚l ( Fig. 1), in the To¨sse area, To the south of A the supracrustals are particularly well-preserved, lacking any pervasive tectonic fabric and having only suffered low-grade greenschist-facies metamorphism. In this area, it is possible to identify primary volcanic structures, textures and mineral assemblages which demonstrate that the volcanic sequence is composed of welded ignimbritic tuffs, non-welded tuffs, flow-banded felsic lavas and subordinate amygdaloidal mafic lavas. ˚ ma˚l rocks are Outside of the To¨sse area, the A more intensely deformed, forming gneissic porphyries and quartz-mica schists, which often lack

T.S. Brewer et al. / Precambrian Research 92 (1998) 297–315

relict primary features. Furthermore, the field relationships of the amphibolites are obscured, such that it is unclear as to whether the minor mafic ˚ ma˚l volcabodies, sills and dykes are part of the A nism or some other magmatic event. Therefore, the identification of mafic lavas in the To¨sse area has proved critical, since it demonstrates a full ˚ ma˚l mafic to felsic compositional range for the A volcanism.

3.3. Stora Le-Marstrand (SLM) Formation The SLM Formation outcrops from islands south of Go¨teborg, along the western coast of Sweden and into SE Norway in the ¨ stfold–Marstrand Belt (Fig. 1), and has preO ˚ ha¨ll and Daly (1989). viously been described by A It is dominated by variably migmatized greywacketype sediments, with minor, but widespread, generally conformable mafic units. In areas of weak migmatization, the metasediments are composed of finely laminated units with occasional graded bedding and thicker psammatic or pelitic bands (1–3 m). Although the mafic units are amphibolitized and frequently migmatized, the least deformed rocks demonstrate a volcanic origin. In the Tjo¨rn–Orust area to the north of Go¨teborg ( Fig. 1), an especially well-preserved sequence, which is >400 m thick, includes well banded amphibolites, subordinate pillow lavas, mafic to intermediate dykes (garnet amphibolites) and felsic intercalations. The thin felsic bands (<1 m) are often laminated and are interpreted as volcanoclastic. The compositional banding, plagioclase phyric units and pillows are all relict primary features, whereas primary minerals have been recrystallized to amphibolite-facies assemblages. Recent constraints from ion probe dating of detrital zircons have evidenced a 1.60 Ga maxi˚ ha¨ll et al., mum age for the SLM deposition (A 1997, 1998), thus superseding the previous 1758±78 Ma Sm–Nd errorchron age derived from ˚ ha¨ll and Daly, 1989). A minimafic volcanics (A mum age is obtained from the 1.59 Ga Ro¨nna¨ng ˚ ha¨ll, 1996) which contains tonalite (Connelly and A ˚ ha¨ll, 1990). migmatized SLM xenoliths (A

301

4. Geochemistry Representative samples weighing 2–3 kg were collected for geochemical analysis from each of ˚ ma˚l: 58, the volcanic formations (Horred: 126, A SLM: 88 samples). However, owing to discontinuous outcrop patterns, complete lithostratigraphic sections were not available, and offset sampling within individual formations was used for the construction of near complete sequences. The samples used in the present study have been variably metamorphosed during the Gothian and Sveconorwegian orogenies so the possibility of element mobility must be considered. In metamorphosed volcanic sequences, large ion lithophile elements (LILE, i.e. Sr, Rb, K, Ba, Th and U ) have often been preferentially mobilized, whereas elements such as Zr, Y, Nb and the REEs are less mobile and frequently retain their primary geochemical signatures (e.g. Pearce and Cann, 1973; Brewer and Atkin, 1987). During sampling, obvious signs of alteration and/or veining were avoided. However, sparse ˚ ma˚l and occurrence of mafic units in both the A Horred Formations forced the use of some mafic samples that showed signs of minor alteration. In order to minimize the effects of alteration on the geochemistry, outliers were identified and removed using the following criteria: (1) relatively high loss on ignition values (LOI>3%, often as high as 9%), which correlate with high modal chlorite and calcite in the mafic compositions; these represent products of late stage retrogressive metamorphic effects; (2) relatively high and anomalous K O and Rb 2 values within individual rock groups (e.g. basalts), which often correlate with samples having significant (>5%) modal muscovite and/or biotite; (3) anomalously high CaO, Sr and LOI, which in the basaltic rocks correlate with high modal epidote and calcite. In the felsic rocks such chemistry correlates with epidote veined samples; and (4) in the felsic rocks some outliers have high SiO (>80%), representing silicified volcanics 2 (cf Brewer, 1985).

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However, in spite of sample screening, the more mobile LILE elements, such as K, Rb and Ba, still retained a greater dispersion than the high field strength elements (HFSE, i.e. Zr, Y ). This is a residual feature of metamorphic/alteration history and caution must therefore be exercised when using the LILE. In the following discussion, emphasis is placed on the more immobile elements. However, when the mobile elements display coherent inter-element relationships, they have also been used.

4.1. Analytical methods Individual samples were jaw-crushed and powdered. Six to eight grams of this powder was used to make press powder pellets with addition of a few millilitres of PVP–methycellulose binder. Glass fusion beads were made by combining ca 0.4 g of rock powder with ca 2.0 of Li–La tetraborate flux, using methods described by Harvey et al. (1973) and Brewer (1985). Major elements were determined on glass fusion beads and trace elements on pressed powder pellets by X-ray fluorescence spectrometry at Nottingham University, using the technique described by Harvey (1989). Representative analyses from each of the volcanic units are presented in Tables 1–3; a full list of analyses is available on request from the lead author. Following ion exchange separation, the REEs were determined by inductively coupled plasma emission spectrometry using the methods described in Harvey et al. (1996) and the data is presented in Table 4. Sm–Nd analyses were performed at University College Dublin following Menuge (1988) with minor modifications. 143Nd/144Nd ratios were determined on spiked samples and have been normalized to 146Nd/144Nd=0.7219. All Nd analyses are quoted relative to a 143Nd/144Nd ratio for the La Jolla standard of 0.511850. Quoted errors in 143Nd/144Nd are within run precision; reproducibility is ca 0.00002 or the within run precision, whichever is greater. Reproducibility of Sm and Nd concentrations is 2% and of 147Sm/144Nd ratios is 0.1%. Sm–Nd model ages (t ) were calculated DM relative to the model depleted mantle of DePaolo

(1981) using present-day CHUR ratios of 143Nd/144Nd=0.512638 and 147Sm/144Nd=0.1966.

4.2. Horred Formation The Horred Formation contains compositions ranging from basalt to rhyolite, with calc-alkaline affinities and dacites as the dominant rock type ( Fig. 2). Strong negative correlations of CaO, TiO and Fe O versus MgO suggest a genetic 2 2 3 relationship across the entire compositional range, with plagioclase and magnetite as major fractionating phases [Figs. 3(a) and 4(a)]. Magnetite fractionation was limited in the basalts, but inflections and subsequent negative correlations of MgO–TiO and MgO–Fe O from the basaltic 2 2 3 andesites to rhyolites demonstrate its importance as a fractionating phase in the more evolved compositions [Figs. 3(a) and 4(a)]. In the same compositions, an inflection in the MgO–P O 2 5 correlation indicates onset of apatite as a fractionating phase. Horred Formation basalts and basaltic andesites are moderately enriched in the LREE [[La/Lu]n 4–6.1, Fig. 5(a)], often have small but significant Eu anomalies and are depleted in HREE ([Gd/Lu]n 1.8–2.6). MORB normalized profiles are characterized by LILE and LREE enrichment, a negative Nb anomaly and depletion of the HFS elements [Fig. 6(a)]. This depletion of HFS elements is identical to that observed in modern island arc basalts (Pearce, 1983). Further support for an island arc setting comes from the Zr/Y ratios of the basalts, all of which are <3.0 ( Fig. 7). With increasing fractionation there is a general increase in LILE and HFSE abundances, although depletion of P O and TiO is consistent with 2 5 2 fractionation of magnetite and apatite in the more evolved lavas. Basalts and basaltic andesitic have positive e Nd (ca +2.1), whereas a dacite has a slightly lower value (+1.3; Fig. 8). The limited range and positive e values, combined with the similarity of Nd t model ages ( Table 5), suggest derivation from DM a similar source. Contamination by substantially older crust does not appear to have been an

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T.S. Brewer et al. / Precambrian Research 92 (1998) 297–315 Table 1 ˚ ma˚l and SLM volcanics: part 1 Representative whole rock geochemistry from the Horred, A Sample

H7

H9

H30

H20

H93

H62

H21

H25

H64

H96

H105

H106

H75

Formation Lithology SiO 2 TiO 2 Al O 2 3 Fe O 2 3 MnO MgO CaO Na 2 K 2 PO 2 5 LOI Total Cr Ni Co V Cu Pb Zn Rb Ba Sr Nb Zr Y Th U La Ce

Horred Basalt 47.36 1.31 16.98 12.18 0.18 6.30 10.52 O2.76 O0.87 0.15 1.08 99.69 30 2 38 264 28 11 72 21 225 489 3 60 23 1 1 17 17

Horred Basalt 47.71 0.71 18.07 10.48 0.20 6.38 11.09 3.12 0.87 0.24 0.98 99.85 128 32 35 212 1 12 88 13 372 550 1 63 18 1 1 26 13

Horred Basalt 49.03 0.75 17.99 11.48 0.21 5.37 10.12 2.98 0.93 0.44 0.48 99.78 25 14 35 228 85 7 100 19 512 1123 4 33 17 7 2 13 25

Horred B-And. 55.27 0.72 16.27 7.99 0.14 5.91 7.24 4.05 1.00 0.19 0.88 99.66 296 110 28 135 2 15 42 34 319 568 12 154 30 5 3 41 119

Horred B-And. 55.18 0.42 15.90 7.94 0.15 5.74 8.20 3.97 1.42 0.08 0.83 99.83 95 31 35 136 65 9 54 36 285 385 4 66 15 11 2 15 83

Horred And. 55.66 0.73 16.62 8.63 0.16 4.23 7.20 3.42 1.99 0.26 0.83 99.73 48 14 26 153 10 13 82 50 831 739 7 104 24 1 2 42 42

Horred And. 57.22 0.68 14.61 7.92 0.14 6.67 6.88 3.21 1.14 0.18 1.34 99.99 400 138 30 125 2 9 47 40 316 435 12 131 30 11 3 59 129

Horred And. 60.35 0.65 13.95 7.27 0.10 5.80 5.52 3.40 1.43 0.19 0.99 99.65 340 126 28 119 2 14 45 52 467 450 10 140 26 12 4 35 87

Horred And. 62.52 0.64 17.16 5.45 0.09 1.75 5.11 4.52 1.88 0.15 0.65 99.92 19 3 7 80 1 9 26 89 452 535 9 118 16 11 1 26 37

Horred Horred Horred Horred Dacite Dacite Dacite Rhyolite 63.01 64.43 68.62 73.03 0.65 0.69 0.40 0.24 15.49 16.33 15.00 13.64 5.96 4.17 3.61 2.04 0.13 0.09 0.06 0.02 2.12 1.42 1.46 0.56 4.93 4.13 3.97 1.43 3.86 3.58 3.44 2.92 2.67 3.79 2.46 5.44 0.14 0.18 0.08 0.05 0.91 0.83 0.55 0.38 99.87 99.64 99.65 99.75 46 24 45 25 9 7 6 3 19 14 11 8 92 54 55 23 54 8 5 8 17 18 15 24 58 37 28 8 109 107 95 223 673 1000 684 785 269 481 367 244 12 17 7 18 205 218 153 169 36 33 12 27 11 14 12 27 6 3 1 7 18 62 33 58 102 110 100 88

H52 Horred Rhyolite 77.15 0.24 11.71 1.05 0.00 0.18 0.60 1.92 6.82 0.02 0.13 99.82 12 5 2 16 6 29 3 149 1037 150 16 153 42 18 3 85 114

LOI is the loss on ignition value. Oxides in weight percent and trace elements in parts per million. Precision of major elements is better than 0.5%. For the trace elements precision is better than 3%. A full listing of standards is available from the lead author

important process in the genesis of the more evolved compositions. Thus, major and trace element data indicate an island arc setting within which the mantle source had been partly modified by subduction processes, as shown by the lower than depleted mantle value (Fig. 8) of DePaolo (1981). This source enrichment could have been produced by recycling of a sedimentary component having a Nd isotopic composition similar to the older Palaeoproterozoic crustal units in Baltica, such as the TIB or Svecofennian type (Huhma, 1986; Patchett and Kouvo, 1986; Patchett et al., 1987; Persson et al., 1995; Darbyshire et al., 1995).

˚ ma˚l Formation 4.3. A ˚ ma˚l Formation contains compositions The A ranging from basalt to rhyolite, having calcalkaline affinities, with dacitic compositions dominating (Fig. 2). Negative correlations of Al O , Fe O , MgO, 2 3 2 3 CaO, P O and TiO versus SiO [Fig. 4(b)] 2 5 2 2 are consistent with fractionation of plagioclase and ferromagnesium minerals from a parental basalt ( Wilson, 1989). The negative correlations between SiO –Fe O [Fig. 4(b)] and MgO–Fe O 2 2 3 2 3 [Fig. 3(b)] are also consistent with early and continued fractionation of magnetite. Inflection in the

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Table 2 ˚ ma˚l and SLM volcanics: part 2 Representative whole rock geochemistry from the Horred, A Sample

H70

D3

D105

D112

D111

D121

D29

D66

D69

D70

D54

D57

D76

D53

Formation Lithology SiO 2 TiO 2 Al O 2 3 Fe O 2 3 MnO MgO CaO Na 2 K 2 PO 2 5 LOI Total Cr Ni Co V Cu Pb Zn Rb Ba Sr Nb Zr Y Th U La Ce

Horred Rhyolite 70.69 0.24 15.47 2.30 0.03 0.51 2.50 O4.13 O3.30 0.07 0.65 99.89 28 2 7 26 2 19 23 85 887 361 6 172 9 9 13 27 18

˚ ma˚l A Basalt 49.89 1.07 15.24 10.30 0.14 7.18 8.12 2.50 3.03 0.36 2.04 99.87 180 96 41 209 83 16 127 145 529 641 4 97 19 5 1 29 47

˚ ma˚l A Basalt 51.93 0.73 16.10 10.23 0.21 5.88 8.81 1.45 1.03 0.17 3.41 99.95 54 25 39 183 31 14 94 38 324 400 4 110 20 4 <1 9 34

˚ ma˚l A Basalt 50.53 0.78 16.69 11.06 0.17 5.96 7.30 2.29 2.03 0.19 2.79 99.79 66 34 35 247 48 14 78 63 346 363 4 80 20 1 <1 4 56

˚ ma˚l A B-And. 53.06 0.70 16.79 9.86 0.19 3.87 8.09 2.88 2.21 0.21 1.93 99.79 57 37 35 175 129 18 72 63 503 501 5 80 17 2 <1 7 44

˚ ma˚l A B-And. 54.64 0.62 19.14 8.53 0.11 3.29 7.23 3.48 1.38 0.14 1.14 99.70 26 20 23 134 10 11 58 46 614 451 2 50 15 5 <1 11 37

˚ ma˚l A B-And. 55.97 0.51 18.57 8.05 0.19 3.48 5.48 3.59 2.18 0.21 1.55 99.78 19 13 22 96 3 13 96 78 512 394 3 59 20 2 3 13 30

˚ ma˚l A And. 58.92 0.71 16.40 7.17 0.12 3.43 5.53 3.18 2.25 0.24 1.65 99.60 58 40 23 103 12 9 78 86 464 439 10 184 37 2 2 5 48

˚ ma˚l A And. 62.02 0.68 15.93 5.76 0.10 2.74 4.87 3.05 2.53 0.20 1.89 99.77 41 26 19 81 15 15 59 92 514 429 8 135 13 4 <1 21 57

˚ ma˚l A And. 61.72 0.63 15.94 6.11 0.10 2.84 5.50 3.47 1.28 0.20 1.91 99.70 41 25 17 81 30 12 128 41 323 488 6 157 27 5 <1 25 80

˚ ma˚l A Dacite 63.05 0.62 16.19 5.42 0.04 2.89 2.48 4.59 2.59 0.15 1.74 99.76 35 12 7 74 4 5 30 99 409 229 9 198 19 13 <1 22 74

˚ ma˚l A Dacite 66.74 0.58 15.68 4.28 0.05 1.30 3.36 4.46 2.06 0.21 1.18 99.90 15 <2 5 57 8 4 18 56 648 302 5 233 18 6 1 21 64

˚ ma˚l A Dacite 67.50 0.42 15.11 4.11 0.06 1.70 3.78 3.23 2.53 0.12 1.13 99.69 31 17 7 49 4 11 36 88 929 395 6 133 13 7 <1 19 69

˚ ma˚l A Rhyolite 72.78 0.27 13.58 1.89 0.07 0.34 0.82 3.65 5.19 0.05 0.90 99.54 10 6 4 17 4 18 26 182 1014 113 16 215 34 14 2 51 82

For notes see Table 1.

Al O –MgO relationship indicates a change in the 2 3 fractionating assemblage, such that plagioclase extraction is more evident in the more evolved compositions. Onset of apatite fractionation in the more evolved rocks is indicated by an inflection in the MgO–P O correlation. Basalts and basaltic 2 5 andesites are moderately enriched in LREE ([La/Lu]n 4.9–5.1), often having small Eu anomalies and depleted HREE [ [Gd/Sm]n 1.6–1.8, Fig. 5(b)]. Both basalts and basaltic andesites are characterized by LILE/LREE enrichment, negative Nb anomalies and enrichment of HFS elements relative to MORB [Fig. 6(b)]. A slight trough in Ti [Fig. 6(b)] is consistent with fractionation of mag-

netite, and overall the patterns are extremely similar to present-day subduction related continental margin, calc-alkaline volcanics (Pearce, 1983). Support of a continental margin setting is provided by Zr/Y ratios for the basalts all being >3 [Fig. 7(a)], which is markedly different from Horred Formation basalts [Fig. 7(a)]. The andesites, dacites and rhyolites have similar MORB normalized profiles as the basalts, except for the greater degrees of enrichment and the pronounced troughs in TiO and P O ; reflecting extensive 2 2 5 fractionation of magnetite and apatite in these more evolved compositions. ˚ ma˚l volcanics have a narrow range of The A e values (+2.2 to +0.7; Fig. 8) similar to those Nd

305

T.S. Brewer et al. / Precambrian Research 92 (1998) 297–315 Table 3 ˚ ma˚l and SLM volcanics: part 3 Representative whole rock geochemistry from the Horred, A Sample

D103

D124

DS12 DS14 US3

GS23 DS13 GS12 GS221 GS216 GS16

GS19

GS20

GS71

Formation Lithology SiO 2 TiO 2 Al O 2 3 Fe O 2 3 MnO MgO CaO Na 2 K 2 PO 2 5 LOI Total Cr Ni Co V Cu Pb Zn Rb Ba Sr Nb Zr Y Th U La Ce

˚ ma˚l A Rhyolite 73.54 0.27 13.39 1.95 0.05 0.09 1.00 O3.80 O5.05 0.03 0.45 99.62 8 4 <3 11 3 17 14 161 983 142 16 234 35 14 5 33 88

˚ ma˚l A Rhyolite 75.48 0.22 12.58 1.44 0.06 0.19 0.87 3.28 5.11 0.02 0.40 99.65 7 2 3 8 5 25 16 148 816 127 15 180 32 17 5 46 108

SLM Basalt 46.80 1.77 13.63 16.27 0.23 6.10 11.96 1.12 0.48 0.13 1.29 99.78 94 45 51 394 98 8 148 15 57 211 4 103 39 4 <1 <4 48

SLM Basalt 47.49 1.30 14.28 15.26 0.22 6.63 10.79 2.73 0.25 0.07 0.90 99.92 128 81 71 369 82 2 120 3 140 92 2 63 27 5 <1 <4 12

SLM Bas-And. 55.65 1.89 11.94 17.97 0.30 2.05 7.12 2.22 0.52 0.24 -0.15 99.75 9 3 30 105 35 8 160 7 70 67 1 215 95 8 2 13 14

SLM And.esite 57.81 1.57 12.20 15.87 0.22 1.79 6.45 2.93 0.58 0.24 0.05 99.71 21 <2 24 75 17 6 157 6 53 71 3 247 93 4 <1 12 85

SLM And.esite 60.06 1.31 12.98 12.82 0.19 1.73 5.03 2.37 2.45 0.24 0.55 99.73 40 <2 21 77 13 14 110 99 396 143 11 185 48 11 2 27 62

SLM Basalt 46.87 1.87 13.06 16.68 0.26 6.35 11.35 2.00 0.52 0.15 0.50 99.61 87 52 51 385 101 4 122 7 16 160 5 102 40 2 <1 4 40

SLM Basalt 47.36 1.17 9.30 16.90 0.24 12.02 10.48 0.98 0.31 0.05 1.39 100.20 840 397 69 261 14 3 134 12 91 32 8 65 27 7 3 12 22

SLM Basalt 47.76 1.37 13.44 13.80 0.23 8.75 10.94 2.29 0.54 0.10 0.50 99.72 268 108 56 309 53 5 82 7 334 130 4 74 31 4 <1 <4 22

SLM Basalt 47.91 2.04 12.95 17.43 0.23 5.02 10.31 2.82 0.51 0.20 0.33 99.75 120 71 56 440 111 9 147 7 168 172 5 104 42 5 <1 12 <7

SLM Basalt 48.05 0.82 15.41 13.69 0.30 7.37 11.68 1.56 0.37 0.07 0.45 99.77 432 166 73 277 110 7 103 5 44 91 2 42 28 2 <1 <4 <7

SLM Basalt 48.08 0.88 14.35 13.22 0.22 8.36 12.19 1.61 0.35 0.07 0.50 99.83 316 131 53 253 66 5 81 4 98 76 2 47 24 1 <1 <4 22

SLM Bas-And. 54.53 1.84 11.51 20.43 0.30 2.00 7.84 1.08 0.09 0.37 -0.60 99.39 77 3 31 80 45 <1 204 3 22 46 2 211 92 6 <1 <1 12

For notes see Table 1.

of the Horred volcanics ( Table 5). The dacitic sample has an initial Nd isotopic composition similar to that of both the Horred rocks and the TIB (Fig. 8), whereas the basalts are slightly more depleted, although typical of magmas derived from a subduction modified mantle source. To conclude, the trace element and Nd isotopic ˚ ma˚l Formation develsignatures suggest that the A oped in a continental margin setting. In such a setting the basalts were derived from a subduction modified mantle source, whereas, the more evolved rocks probably represent a mix of this mantle component and melted juvenile Baltic crust (Huhma, 1986; Patchett and Kouvo, 1986;

Patchett et al., 1987; Persson et al., 1995; Darbyshire et al., 1995). 4.4. Stora-Le Marstrand Formation The SLM volcanics are dominated by sub-alkaline tholeiitic basalts which display marked Fe enrichment trends (Fig. 2). Garnet amphibolite sills and dykes, interpreted as cogenetic with the amphibolites, extend the compositional range to andesites (Fig. 2). With the exception of K O (mobile during meta2 morphism), the major elements display coherent correlations with MgO, all of which are consistent

306

SLM And. 4.89 16.57 3.05 17.69 6.77 1.78 8.77 9.63 5.70 5.86 1.05 SLM And. 3.79 13.36 2.82 16.77 7.68 2.22 11.00 14.16 10.45 11.63 2.32 SLM B-And. 2.98 11.73 2.44 16.30 6.88 2.45 11.84 14.66 9.35 9.60 1.92 SLM B-And. 3.00 11.71 2.56 16.44 6.99 2.43 11.67 13.24 8.84 9.40 1.88 SLM Basalt 1.56 4.35 0.88 5.75 2.28 0.76 3.57 4.46 2.89 2.59 0.42 SLM Basalt 3.12 10.00 1.85 11.08 3.87 1.09 5.76 7.29 4.77 4.51 0.68 ˚ ma˚l A B-And. 16.45 34.49 4.23 18.48 3.66 1.01 4.07 4.00 2.52 2.16 0.32 ˚ ma˚l A B-And. 14.56 29.71 3.74 14.55 3.26 0.85 3.76 3.53 2.18 1.74 0.28 ˚ ma˚l A Basalt 15.89 34.92 4.09 17.48 3.76 1.18 4.13 4.10 2.42 2.05 0.28 ˚ ma˚l A Basalt 13.12 27.13 3.31 16.10 3.90 0.92 3.47 3.30 1.85 1.47 0.22 Horred Basalt 12.73 30.97 3.77 17.81 4.28 1.32 4.89 4.66 2.75 2.05 0.28 Formation Lithology La Ce Pr Nd Sm Eu Gd Dy Er Yb Lu

Horred Basalt 14.23 30.22 3.77 17.91 3.59 1.16 3.87 3.47 2.12 1.69 0.24

Horred Basalt 19.26 46.87 6.12 28.00 7.50 1.90 7.80 7.09 3.68 3.00 0.37

Horred Basalt 13.19 27.62 3.60 15.20 3.41 1.20 3.53 3.07 1.73 1.30 0.20

Horred B-And. 29.34 62.84 8.02 34.33 6.43 1.60 5.78 5.65 3.73 3.65 0.56

Horred And. 30.00 61.41 7.23 28.78 5.11 1.05 5.20 4.73 3.00 2.77 0.39

˚ ma˚l A Basalt 23.61 49.33 6.00 23.65 4.59 1.22 4.36 3.64 2.03 1.53 0.27

D14 D31 D3 H21 H62 H30 H3 H9 H7 Sample

Table 4 ˚ ma˚l and SLM volcanics (all values in parts per million) REE concentrations form the Horred, A

D111

D105

GS12

GS23

GS18

GS16

GS20

GS53

T.S. Brewer et al. / Precambrian Research 92 (1998) 297–315

with an origin by fractional crystallization of a plagioclase+clinopyroxene±minor olivine assemblage from a parental magma [Fig. 3(c)]. The basaltic andesites and andesites are the most fractionated compositions. Inflections in MgO–TiO , 2 MgO–Fe O [Fig. 3(c)] and MgO–V trends 2 3 demonstrate magnetite as an important phase during fractionation in the more evolved compositions. Some of the metabasalts have high MgO, Ni and Cr values which indicate abundant olivine±spinel phenocrysts in the volcanic protoliths. Enrichment of P, Zr, Y and Rb with decreasing MgO illustrates the incompatible behaviour of these elements and the lack of any minor phase (e.g. apatite, zircon) during fractional crystallization even in the more evolved andesitic compositions. A paucity of magmatic zircon in analysed andesitic samples (J. Connelly, pers. comm., 1996) provides further support for the lack minor phase fractionation. All of the SLM basalts are characterized by a slight enrichment of LILE, a negative Nb anomaly and depleted HFSE concentrations [Fig. 6(c)], all of which are similar to the incompatible element signatures of basalts from primitive oceanic arcs (cf Pearce et al., 1995). REE profiles for the basalts, basaltic andesites and andesites are either LREE depleted [Fig. 5(c)] or flat profiles [basalts ˚ ha¨ll and Daly (1989)], which is typical of only, A low-K tholeiites and tholeiites from intra-oceanic arcs (Pearce et al., 1995). The basaltic-andesites and andesites have similar incompatible element profiles to the basalts [Fig. 5(c)], with the exception of a trough in TiO , which is consistent with 2 magnetite fractionation. A detailed Sm–Nd isotopic study of the SLM Formation suggested an age of ca 1.76 Ga based ˚ ha¨ll and Daly, on a whole-rock errorchron (A 1989). Given the new age estimate of ca ˚ ha¨ll et al., 1997, 1998), initial 1.60–1.59 Ga (A e values for the basalts have been calculated at Nd 1.6 Ga. These range from +5.9 to +3.0 ( Fig. 8) confirming the strongly depleted Nd isotopic char˚ ha¨ll and Daly, 1989). acter of these rocks (cf. A This coupled with the primitive trace element signatures (see above) are typical of magmas derived from a mantle source that suffered minor

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307

˚ ma˚l and (c) SLM formations. Discrimination fields SiO versus Fig. 2. Geochemical affinities of volcanics from the (a) Horred, (b) A 2 Na O+K O (Le Maitre, 1989) and AFM tholeiitic/calc alkaline (Irvine and Barager, 1971). $, Basalts; &, basaltic andesites; +, 2 2 andesites; ,, dacites; ×, rhyolites. Discrimination fields, BA, basaltic andesite; A, andesite; TB, trachybasalt; BTA, basaltic trachyandesite; TA, trachyandesite; PT, phonotephrite; TP, tephriphonolite.

subduction enrichment (affecting only LILE) and no crustal contamination. Genesis of the SLM volcanics in a primitive oceanic island arc setting, similar to the present-day South Sandwich arc ˚ ha¨ll and (Pearce et al., 1995) is supported (cf. A

Daly, 1989). Alternatively, the SLM basalts have geochemical affinities with back-arc basin basalts [Fig. 7(b)], which would also explain the limited subduction enrichment. At present it is not possible to effectively discriminate between these two hypo-

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T.S. Brewer et al. / Precambrian Research 92 (1998) 297–315

˚ ma˚l and (c) SLM formations. Symbols as in Fig. 2. Fig. 3. Selected major element variations from the (a) Horred, (b) A

these, due to the lack of geological data to the west of the present SLM outcrop, which is the supposed position of the complementary arc sequence.

A 400 m thick section of the SLM formation between Marstrand and Tjorn (Fig. 1) was sampled in detail. The section is dominated by mafic rocks, but ca 12% of felsic intercalations are

T.S. Brewer et al. / Precambrian Research 92 (1998) 297–315

309

˚ ma˚l formations. Symbols as in Fig. 2. Fig. 4. SiO versus Fe O and CaO for the (a) Horred and (b) A 2 2 3

present, as discrete units, generally well banded and 5–50 cm thick. In the field it is difficult to distinguish these felsic intercalations as representing either volcaniclastic or sedimentary protoliths. Compared to typical SLM metasedimentary lithologies, the felsic intercalations have similar bulk compositions but somewhatmore depleted Nd iso˚ ha¨ll and Daly (1989); Fig. 8]. topic signatures [A The e values of the felsic rocks (+0.6 to +1.8) Nd do not overlap with the SLM basalts but do tend towards them suggesting a common component, or mixing between older detrius and the volcanic

rock. One sedimentary sample not obviously associated with the basalts has an anomalously depleted e value (+1.6) showing that the metNd asediments also have a diverse, perhaps bimodal source. Overall the range of e values exhibited Nd by the sediments and felsic intercalations is more enriched than that of the SLM volcanics, and largely overlaps that of the older units in Baltica to the east, for example, the TIB and the Horred ˚ ma˚l Formations (Fig. 8), suggesting that and A much of the sedimentary detritus was derived from a juvenile continental terrain, possibly Baltica.

310

T.S. Brewer et al. / Precambrian Research 92 (1998) 297–315

Fig. 5. Chondrite normalized REE profiles for the (a) Horred, ˚ ma˚l and (c) SLM formations. Normalized to chondrite (b) A values of Nakamura (1974). $, Basalts; %, basaltic andesites; 6, andesites.

This type of detritus would not normally be associated with a primitive oceanic island arc. However, the presence of continental detritus does not contradict an oceanic setting. One explanation may be that the SLM oceanic arc developed near to a continental mass in a setting similar to some of the arc systems presently developing in the Philippine Sea. Alternatively it is well known that continental detritus may be transported for long distances (103 km) within trench systems, as in the present-day Indonesian arc (Hamilton, 1979). If, however, the SLM represented a back-arc environment, then the continental detrius could have been

Fig. 6. MORB normalized incompatible element diagrams for ˚ ma˚l and (c) SLM Formations. basalts from the (a) Horred, (b) A Normalized to MORB values from Pearce (1983).

shed from proto-Baltica, following the earlier accretion of the Horred arc system.

5. Arc accretion and crustal growth All three volcanic units discussed here ˚ ma˚l and SLM ), can be interpreted as (Horred, A juvenile 1.66–1.59 Ga crustal additions to Baltica, ˚ ma˚l rocks (felsic although at least some of the A compositions) may have been formed by reworking of pre-existing juvenile continental crust. The

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311

˚ ma˚l (A ˚) Fig. 8. Nd evolution diagram showing samples of the A and Horred (H ) formations at their time of formation: ), basalts/basaltic andesites; n, dacites. Ranges for the amphib˚ ha¨ll and Daly, oites and felsic rocks of the SLM Formation (A 1989) and the TIB [grey shading; Brewer and Darbyshire, unpublished data; Darbyshire et al. (1995)] are shown for comparison. DM, Depleted mantle (DePaolo, 1981).

Fig. 7. (a) Zr–Zr/Y discrimination diagram, showing the dis˚ ma˚l (%) basalts; tinction between the Horred ($) and A discrimination fields from Pearce (1983). (b) Zr–Zr/Y discrimination diagram for the SLM basalts, note the scatter between the fields for volcanic arc basalts ( VAB) and back-arc basin basalts (BABB); discrimination fields from Pearce and Norry (1979).

Horred and SLM arc systems are recognized as ˚ ma˚l formed in outboard settings, whereas the A volcanics formed in an continental margin setting. Archaean components are not indicated in these units, which is consistent with their separation from Archaean crust in the north-east by intervening belts of Svecofennian and TIB rocks (Fig. 1 inset). ˚ ha¨ll and Gower (1997), developAccording to A ment of the Gothian orogen between 1.69 and 1.65 Ga occurred in response to eastward-directed subduction. New crust associated with this stage of the Gothian occupies the region east of the ¨ tran Terrane (Fig. 1). Since Mylonite Zone in the A no older crustal units have been positively iden-

tified in the adjacent region to the west (Idefjorden terrane, Fig. 1), accretion of the 1.66 Ga Horred arc system is interpreted to have occurred onto a proto-margin of Baltica, which largely coincided ¨ tran with the present boundary between the A and Idefjorden terranes ( Fig. 1). Later, this zone was reactivated by the Sveconorwegian Orogeny to form the prominent Mylonite Zone (e.g. ˚ ha¨ll, 1995; Page et al., 1996; Berthelsen, 1980; A Stephens et al., 1996). The accretion of the Horred arc system is not well constrained temporally, but an early deformational event in this unit has been bracketed in the ˚ ha¨ll et al., 1995). 1.66–1.58 Ga interval (A Probably, this accretion caused deformation also ¨ tran gneisses in the east, and a 1.61 Ga in the A age for an aplitic dyke (Connelly et al., 1996) appears to provide a minimum age for such an event. Evidence for a second Gothian stage of subduction-related magmatism comes from the ca 1.59 Ga Go¨teborg Batholith which forms a major part ˚ ha¨ll et al. of the Idefjorden terrane [Fig. 1; A ˚ (1995) and Connelly and Aha¨ll (1996)]. This stage of calc-alkaline continental-margin magmatism ˚ ha¨ll, unpublished data) was accom(Brewer and A

312

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Table 5 ˚ ma˚l (samples prefixed D) formations Sm–Nd isotopic data from the Horred (samples prefixed H ) and A 147Sm/144Nd

143Nd/144Nd

t DM

e t Nd

˚ ha¨ll (1996)] Horred Formation [1659+8/−6 Ma, Connelly and A H9 b 1.66 3.93 17.29 H30 b 1.66 4.02 18.54 H62 ba 1.66 6.96 34.72 H105 d 1.66 7.29 41.50

0.1374 0.1311 0.1213 0.1062

0.512105±16 0.512034±14 0.511917±12 0.511720±18

1864 1852 1847 1867

2.21 2.16 1.96 1.33

˚ ma˚l Formation [1614±7 Ma, Lundqvist and Skio¨ld (1992)] A D3 b 1.61 5.08 27.17 D12 b 1.61 3.20 13.90 D118 d 1.61 6.23 32.15

0.1130 0.1391 0.1171

0.511810±10 0.512141±12 0.511832±12

1857 1833 1902

1.15 2.21 0.73

Sample

L

Age (Ga)

Sm

Nd

143Nd/144Nd ratios were measured on spiked samples and normalized to 146Nd/144Nd=0.7219. Within run (2s) errors are ca 0.1% for 147Sm/144 Nd and as listed for 143Nd/144Nd. Depleted mantle model ages (T ) were calculated using the model of DePaolo (1981). DM L, Lithologies; b, basalt; ba, basaltic andesite; d, dacite.

panied by 1.59–1.54 Ga rapakivi magmatism further east in Finland (Ra¨mo¨ et al., 1996 and references therein). Given the oceanic characteristics of the SLM volcanics, accretion of the SLM arc system must have occurred prior to 1.59 Ga, the age of intrusive granitoids dated by Connelly ˚ ha¨ll (1996). and A ˚ ma˚l Formations Previously, the Horred and A were interpreted to form a common calc-alkaline ˚ ha¨ll et al. (1995)]. Although belt [1.64–1.61 Ga; A lithologically and compositionally similar, the distinct geochemical signatures of their mafic components and the revised Horred age [1.66 Ga; ˚ ha¨ll (1996)] indicate reported by Connelly and A discrete volcanic episodes, in different tectonic settings. The continental-margin setting for ˚ ma˚l volcanism, and indistinguishable geothe A chemical trends for the ca 1.59 Ga Go¨teborg ˚ ha¨ll, unpublished data), Batholith (Brewer and A provides two possible scenarios: (1) they form a common ca 1.61–1.59 Ga continental-margin arc, representing at least 30 Ma of eastward subduction; or (2) they represent two distinct stages of eastward subduction. This distinction will require further geochronology, ˚ ma˚l area sugalthough field relationships in the A gest a common magmatic episode. It should also

be noted that granitoids included in the ca 1.59 Ga Go¨teborg Batholith have yielded ages coeval to ˚ ma˚l volcanics, although their 1616–1620 Ma the A ages, within errors, could not be separated from the better constrained 1.59 Ga granitoids, see ˚ ha¨ll et al. (1995) and additional data Table 2 in A ˚ ha¨ll (1996). A net result of the in Connelly and A 1.61–1.58 Ga Gothian stage was the development of a complex crustal collage, forming the Idefjorden terrane, where the original boundaries of the accreted units (the Horred and SLM arc systems) have been obscured by the voluminous emplacement of later granitoids, that is, the Go¨teborg Batholith, as well as subsequent orogenic deformation. The 1.80–1.65 Ga gneissic granitoids in the Klara¨lven terrane [Fig. 1; Welin and Ka¨hr (1980); Lindh et al. (1994); Persson et al. (1995)] have been interpreted to lack significant Gothian deformational fabric (Larson, 1996). Further south, in ¨ tran terrane ( Fig. 1), the 1.70–1.65 Ga veined the A and banded gneisses attest to severe Gothian deformation (Larson et al., 1986; Connelly et al., 1996; Cornell et al., 1996). This difference in Gothian deformation between the Klara¨lven rocks in the ¨ tran rocks in the south ( Fig. 1) north and the A may reflect the docking of the 1.66 Ga Horred formation in the south and absence of such

T.S. Brewer et al. / Precambrian Research 92 (1998) 297–315

Gothian events in the north. This difference also supports the geochemically based inference of ˚ ma˚l Formation as a continental-margin arc. the A

6. Conclusions

$

$

$

$

$

$

Metabasalts of the 1.66 Ga Horred, ˚ ma˚l and 1.60–1.59 Ga SLM forma1.61 Ga A tions have distinctive trace element geochemical signatures similar to those of modern calcalkaline island arcs, continental margin arcs and intra-oceanic tholeiitic arcs, respectively. ˚ ma˚l volcanics have slightly Horred and A depleted Nd isotopic signatures with narrow ranges of initial e values over the composiNd tional range from basalt to dacite. In comparison, initial e values of SLM basalts and Nd andesites have consistently more depleted values. The 1.66 Ga Horred and 1.60–1.59 Ga SLM formations originated as outboard arcs and were accreted prior to 1.61 and 1.59 Ga, respectively. The marked increase of Gothian deformation ¨ tran rocks compared to western in western A Klara¨lven rocks further north (Fig. 1) may reflect the docking of the Horred Formation against western Baltica then represented by the ¨ tran Terrane. A ˚ ma˚l Formation and ca Together the 1.61 Ga A 1.59 Ga Go¨teborg Batholith represent a second Gothian stage of subduction-related magmatism, which broadly out-lines the western edge of late Palaeoproterozoic Baltica. Whether they reflect two separate events or continuous eastward subduction, remains to be established. During this second episode the SLM arc system was accreted on to Baltica. The well-constrained HFSE signatures for each of the three investigated volcanic units in SW Sweden demonstrate the value of geochemistry in characterizing the tectonic settings of Proterozoic units that have suffered repeated amphibolite-facies deformation.

313

Acknowledgment This work has significantly benefited from the personal involvement and support of the late Lennart Samuelsson, Geological Survey of Sweden, Go¨teborg. The authors are also grateful to Michael Murphy, University College Dublin for technical assistance with Sm–Nd analyses. The study was financially supported in part by the Swedish Natural Research Council, Grant G-GU ˚ ) and the Geological Survey of 10286-300 to KIA ˚ ), by Sweden (Grant SGU 03-826/93 to KIA University College Dublin and by Nottingham University. This paper benefited from careful and ˚ . Johansson. detailed reviews by S. Bloomer and A

References ˚ ha¨ll, K.-I., 1990. An investigation of the Proterozoic A Stenungssund granitoid, SW Sweden: conflicting geochronological and field evidence. In: Gower, C.F., Rivers, T., Ryan, B. ( Eds.), Mid-Proterozoic Laurentica–Baltica. Geological Association of Canada Special Paper 38, Geological Association of Canada, pp. 117–129. ˚ ha¨ll, K-I., 1995. Crustal units and role of the Mylonite Zone A system in the Varberg–Horred region, SW Sweden. Geol. Fo¨ren. Stockholm Fo¨rh. 117, 185–198. ˚ ha¨ll, K-I., Connelly, J.N., 1998. Intermittent 1.53–1.13 Ga A anorogenic magmatism in SW Scandinavia; age constraints and correlations within Laurentia–Baltica. Precambrian Res. (in press). ˚ ha¨ll, K.-I., Daly, J.S., 1989. Age, tectonic setting and proveA ¨ stfold–Marstrand supracrustals: westward nance of the O growth of the Baltic Shield at 1760 Ma. Precambrian Res. 45, 45–61. ˚ ha¨ll, K-I., Gower, C.F., 1997. The Gothian and Labradorian A orogens; variations in accretionary tectonism along a late Paleoproterozoic Laurentia–Baltica margin. Geol. Fo¨reningens Stockholm Fo¨rhandl. 119, 181–191. ˚ ha¨ll, K.I., Scho¨berg, H., 1996. Age of the Vinga porphyry A and indications of 963 Ma block movements in the Swedish part of Sveconorwegian orogen. Geol. Fo¨reningens Stockholm Fo¨rhandl. 118, A6–A7. ˚ ha¨ll, K-I., Persson, P-O., Skio¨ld, T., 1995. Westward accretion A of the Baltic Shield: implications from the 1.6 Ga ˚ ma˚l–Horred Belt, SW Sweden. Precambrian Res. 70, A 235–251. ˚ ha¨ll, K-I., Brewer, T., Connelly, J.N., Larson, S.A ˚ ., 1996. A Temporal and spatial relationships between intra-cratonic magmatism and 1.70–1.55 Ga westward growth of the Baltic Shield. Geol. Fo¨reningens Stockholm Fo¨rhandl. 118, A5 ˚ ha¨ll, K-I., Samuelsson, L., Persson, P-O., 1997. A

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