Radiogenic isotope systematics of the Herefoss granite, South Norway: an indicator of Sveconorwegian (Grenvillian) crustal evolution in the Baltic Shield

,NCL”D,NC

ISOTOPE GEOSCIENCE

ELSEVIER

Chemical Geology

135 (1997) 139-158

Radiogenic isotope systematics of the Herefoss granite, South Norway: an indicator of Sveconorwegian ( Grenvillian) crustal evolution in the Baltic Shield Tom Andersen Laboratorium for Isotopgeologi, Mineralogisk-Geologisk Museum, San Gate I, N-0562 Oslo, Norway Received 26 October

1995; revised 7 June 1996; accepted 7 June 1996

Abstract A new Pb-Pb mineral isochron dates the crystallization of the Herefoss granite (South Norway) to 926 + 8 Ma, contirming earlier Rb-Sr evidence of a late Sveconorwegian (Grenvillian) age. Initial Sr and Nd isotopic compositions recalculated to es&926 Ma) I 40, e,,(926 Ma) = - 3.2 to - 0.8 indicate that the magma has originated from a mixture of two distinct source components: (1) A crustal component, which has resided in a moderately LILE-enriched reservoir (fat, = + 7 to + 8, 2381J/ *04Pb = 17-22) since 1.6-1.9 Ga. This crustal component is clearly distinct from the extremely LILE-enriched metasedimentary country rocks of the granite (fab > 21, 238U/ *@‘PI)> 24). (2): A mantle-derived component (eNd = +4.6 to +5.9, esr= -12 to -9, 238U/ 204Pb = 7.96) younger than 1.5 Ga. The ranges of initial Sr, Nd and Pb composition observed in the intrusion indicate that the magma was isotopically heterogeneous at the time of emplacement, and that the crustal contribution amounted to - 35-55%. By the end of the Sveconorwegian orogeny, the southwestern part of the Baltic Shield had acquired a compositionally layered structure, where an “uppermost” zone of the crust with extreme LILE enrichment was underlain by a heterogeneous layer consisting of rocks with “normal upper cmstal” degree of L1L.E enrichment and mafic intrusions younger than 1.5 Ga. There is no evidence of involvement of LILE-depleted “deep continental cmst”-source material in the petrogenesis of the Herefoss granite. Rejuvenation of the lower crust by injection of mantle-derived magmas (“underplating”) has been an important process in the evolution of the southwestern part of the Baltic Shield. The absence of a young mantle-derived component from u 1.12-Ga chamockitic intrusions in the Bamble sector and from the 989-Ma Grimstad granite suggests that this process may have been contemporaneous with late Sveconorwegian mafic magmatism in the Rogaland-Vest Agder sector further to the west. Keywords: Granites;

Baltic Shield; Continental

crust; Rb/Sr;

Sm/Nd;

1. Introduction The continental crust has developed by repeated erogenic and anorogenic events, in each of which new material has bee:n extracted from the mantle and added on to the continent, and material with a continental pre-history has been recycled and modified. 0009-2541/97/$17.00 Copyright PII SOOOS-2541(96)00095-2

Pb/Pb

Magmas formed by crustal anatexis will contain information on the average composition and history of the crust at depth at the time of formation, which is in turn reflected by the isotopic composition of strontium, neodymium and lead in granitic intrusions and their minerals. The Baltic Shield has a history of crustal accre-

0 1997 Elsevier Science B.V. All rights reserved.

T. Andersen/Chemical

140

Geology 135 (19971 139-158

(Sveconorwegian orogeny). It has been argued that the southwestern part of the Baltic Shield shows evidence of a somewhat irregular westwards younging trend, from a Svecofennian core-area in Centralsouth Sweden towards younger ages of crustal accretion in Southwest Sweden and South Norway, related to processes at successive destructive plate margins during the GTthian (or Kongsbergian) orogeny (Lindh, 1987; Ahall and Daly, 1989; Dahlgren et al., 1990a; De Haas et al., 1993). In contrast to this idea, Sveconorwegian elastic metasediments from southeastern Norway yield 1750-1900 Ma (i.e. Svecofen-

tion extending back into the Archaean (Gaal and Gorbatschev, 1987). The southwestern part of the shield, which is delimited to the east by the Protogine zone in South Sweden and to the NW by the Caledonian nappes (Fig. l>, is the youngest part of the shield in which a major crust-forming event has been shown to take place during the Svecofennian orogenx, at 1.9- 1.75 Ga (Gaal and Gorbatschev, 1987; Ahall and Daly, 1989; Gorbatschev and Bogdanova, 1993). Later tectonometamorphic episodes have been identified at 1.6- 1.5 Ga (Gothian or Kongsbergian orogeny) and at 1.25-0.9 Ga

a

Archaean Svecofennian m r%

Trans-Scandinavian Igneous belt

El

Caledonian and younger

C

Herefoss granite Medium- to coarse-grained (Herefoss pluton) Fine-grained

granite (Holtebu granite)

Porsgrunn-Kristiansand

-m

Faults Simplified from Elders (1963), wlh additional data from Annis (1974)

Fig. 1. a. Simplified geological map of the Herefoss granite, after Elders (1963). b. Simplified map of the geology of the Baltic Shield, with major geochronological c. Sketch map of the Bamble and SE parts of the Telemark sector.

provinces.

granite

shear zone

T. Andersen/Chemical

nian) neodymium model ages (Andersen et al., 19951, which indicates that little addition of juvenile material to this, supposedly younger, part of the Baltic Shield may have taken place in post-Svecofennian time. The Sveconorweg,ian event in South Norway is characterized by deformation, high-grade metamorphism and emplacement of mafic to granitic intrusions (e.g., Munz and Morvik, 1991; De Haas, 1992; Bingen et al., 1993; Kullerud and Dahlgren, 1993; Starmer, 1993; Andersen et al., 1994; Heaman and Smalley, 1994). From available isotopic data, the net contribution of new material from the mantle needs not have been large, except perhaps in the Telemark sector (Menuge, 1988; Dahlgren et al., 199Oa) and in the Rogaland area, where large mafic intrusions show initial Nd and Sr signatures of crustally contaminated mantle-derived magmas (Demaiffe et al., 1986; Menuge, 1988). Towards the end of the Sveconorwegian orogeny, a considerable number of post-tectonic granitic plutons were emplaced, these have been dated to 8501000 Ma by K-Ar, Rb-Sr and U-Pb methods (H. Neumann, 1960; Killeen and Heier, 1975; Skiold, 1976; Pedersen and M&e, 1990). Several of the plutons show comparatively low initial s7Sr/ 86Sr ratios (_ 0.705), which may indicate a significant contribution of relatively young mantle-derived material to the granitic magmas, or reflect an origin from a large-ion lithophile element (LILE) -depleted crustal source (L. Johansson and Kullerud, 1993). Few Pb and Nd isotope data have been published for these intrusions. The present study is an attempt to characterize the source of one of the late Sveconorwegian granites (the Herefoss pluton, Fig. 1) by integrating Sr, Nd and Pb isotope data, and thus to gain insight into the: composition and geochemical evolution of the unexposed continental crust in the southwestern part of the Baltic Shield.

2. Geological setting The Precambrian terrane of South Norway and adjoining parts of Southwest Sweden belongs to the Southwest Scandinavian Domain of the Baltic Shield (Gaal and Gorbatschev, 1987), and can be divided into five crustal sectors (Rogaland-Vest Agder,

Geology 135 (1997) 139-158

Bamble, Telemark, Kongsberg,

141

@@old-Akershus),

separated by shear zones or major brittle faults. The Bamble sector consists mainly of amphibolite- to granulite-facies supracrustal gneisses and quartzites, interlayered with amphibolites and penetrated by mafic intrusions, chamockites and late, post-tectonic granites. The rocks of the Bamble sector were, at some time during the Sveconorwegian erogenic cycle, thrust above continental rocks belonging to the Telemark sector along a shallow SE-dipping shearzone (the Porsgrunn-Kristiansand shear zone, Fig. 1). Later (in the Palaeozoic or more recently) this shear zone was re-activated as a major, brittle fault (“the great friction breccia”; Bugge, 19281, where the Bamble sector to the southeast has been downfaulted relative to the Telemark sector. The Herefoss granite (Fig. 1) is a nearly circular granitic body, emplaced into country rocks belonging to the Bamble and Telemark sectors. The intrusion can be divided in three sub-areas (Fig. 1): The largest of these, which consists of the southeastern part of the main intrusion, is separated from a smaller segment to the NW by the “friction breccia” fault, and has been displaced southwards and downwards (Elders, 1963). Both of these segments consist of medium- to coarse-grained granite. The third sub-area is an irregular, minor intrusion of fine-grained granite extending northwards from the main granitic body. This intrusion (the Holtebu granite) has a sharp, crosscutting contact towards the main Herefoss pluton, and differs from it in having more differentiated trace-element patterns (Annis, 1974). The contact relationships and internal structures of the Herefoss granite were studied in detail by Elders (1963). A weak negative gravity anomaly over the intrusion can be modelled by a plate-shaped granitic body, extending only l-2 km below the present surface, and with its deepest part in the southeast (Smithson, 1963). This probably represents the lower parts of a deeply eroded, diapiric intrusion. Southwest of the Porsgrunn-Kristiansand shear zone, the Herefoss granite intrudes into a series of amphibolite-facies metasedimentary gneisses, quartzites, amphibolites and granitic gneisses belonging to the Bamble sector. NW of the shear zone, the country rocks are banded gneisses and amphibolites. The external contacts of the pluton range from sharp to a zone of agmatitic migmatite; the intrusion mecha-

142

T. Andersen / Chemical Geology 135 (1997) 139-158

nism was forceful, deforming the foliation of the country rocks close to the contact to near conformity with the external margin of the granite; no thermal aureole is observed around the intrusion (Elders, 1963). The Herefoss granite has previously been dated to 909 + 29 Ma by a whole-rock Rb-Sr isochron (Killeen and Heier, 1975; recalculated to ha, = 1.42 . lo- ” yr- ‘1, and to 930-956 Ma by conventional K-Ar on mica (H. Neumann, 1960). Whole-rock major-element analyses were presented by Elders (1963), showing 64-75 wt% silica and minor amounts of normative wollastonite or corundum. The major-element compositions suggest an I-type affinity rather than S-type (White and Chappell, 1983), but the lack of trace-element data precludes a definite chemical classification. I-type character was also tentatively suggested for the Iddefjord granite in the 0stfold-Akershus sector by Pedersen and MHlae (1990). 3. The material studied The Herefoss intrusion consists of two-feldspar biotite and biotite-hornblende granite. Several distinct varieties can be recognized, from colour, grainsize and structure. A thorough petrographic description was given by Elders (1963). The present observations confirm his findings; a summary description of individual samples is given in Appendix A. The most abundant petrographic variety is a red, coarse-grained porphyritic granite with microcline phenocrysts in a groundmass of plagioclase, quartz, biotite (yellow to brown pleochroism) and Fe-Tioxides. Hornblende (olive-green to brown pleochroism) occurs in some samples only (Appendix A). Titanite and muscovite are minor minerals ( 2 l%), whereas apatite, zircon, fluorite, monazite and allanite occur as accessory minerals ( < 1%). Muscovite occurs both as a primary mineral (intergrown with biotite and as separate, interstitial crystals) and as a secondary alteration product (sericite after alkali feldspar, Appendix A). Some of the samples show a planar fabric defined by parallel orientation of microcline phenocrysts (106, 107). Elders (1963) showed that this foliation is in general parallel to the intrusive contact, and interpreted it as a magmatic flow foliation.

Fine- to medium-grained varieties, which are either red or pale grey occur with diffuse contacts to the coarse-grained granite. These are biotite and biotite-hornblende granites, resembling the coarsergrained varieties in terms of modal mineralogy. Samples 111 and 112 are from the fine-grained Holtebu granite (Fig. 1). The granite shows variable degrees of alteration, apparently uncorrelated with sampling locality or petrographic type. Alteration has affected feldspar, which has become turbid by sub-microscopic “dust” and locally contains recognizable sericite-flakes and needles of possible (clino)zoisite. Samples showing pronounced alteration of plagioclase also commonly contain minor to accessory amounts of anhedral, pistacitic epidote. In some samples (104A, 108, 1101, biotite is nearly totally replaced by chlorite, whereas others (104B, 111) show partial chloritization of biotite. In sample 111, the biotite is intergrown with chlorite; in this sample, the biotite is itself pleochroic in green (as opposed to brown), and contains opaque segregations along the cleavage traces. This may suggest that the alteration of biotite is a two-stage process, in which the green biotite is an intermediate product. In Appendix A, the state of alteration of feldspar minerals and biotite is indicated on a scale from 1 (fresh) to 3 (heavily altered). Sample 105 is a dark-grey, fine-grained quartzdioritic enclave in the granite, with the same mafic silicate and accessory minerals as the granite. This sample and the fine-grained grey granite from the main Herefoss pluton (101, 104B, 109) represent facies of the Herefoss granite interpreted by Elders (1963) as altered (“granitized”) xenoliths of the country-rock metasediments.

4. Radiogenic isotope geochemistry

4.1. Analytical methods Rb, Sr, Sm, Nd and Pb were separated from finely crushed and homogenized whole-rock powders by standard ion-exchange procedures. Sr, Nd and Pb isotopic ratios were determined by mass spectrometry, using a fully automated Finnigan MAT262 mass spectrometer in the Laboratory of Isotope Geology, Mineralogical-Geological Museum, Oslo. Nd was analysed on double rhenium filaments, Sr on single

T. Andersen/Chemical

tantalum filaments, both were loaded with phosphoric acid. Nd isotopic compositions are normalized to 146Nd/ 144Nd = 0.72.19. During the period the present analyses were made, the Johnson & Matthey batch No. S819093A Nd,O, gave 143Nd/144Nd = 0.511101 f0.000013 (2~). The NBS 987 Sr stan87Sr/87Sr = 0.710228 k 0.000050 dard yielded (2~). Pb was loaded on single rhenium filaments, using the phosphoric acid-silica gel method. Lead isotope analyses were corrected for mass fractionation off-line, using correction factors derived from multiple runs of the: NBS SRM 981 common lead standard, using the standard composition determined by Todt et al. (1984’1. The fractionation amounted to O.O95%/amu. Rb, Sr, Sm and Nd concentrations were determined by isotope dilution, using aliquots spiked in *‘Rb, 84Sr, ‘48Nd and 149Sm. A VG354 mass spectrometer was used for the isotope dilution analyses. One sample (10711 was selected for a lead mineral isochron study. Pure fractions of alkali feldspar, apatite and titanite were prepared by a combination of heavy-liquid and magnetic separation and handpicking. Plagioclase was not analysed, as it showed evidence of alteration (Appendix A). Zircon was excluded from the mineral isochron study because of the possibility of inhlerited components in disequilibrium with the magmatic minerals of the intrusion, but will be used for a future ion microprobe U-Pb study. Isochron calculations have been made using the ISOPLOT 2.57 software package (Ludwig, 1991), multi-stage lead modelling of alkali feldspar by a spreadsheet program written by M.J. Whitehouse (pers. con-n-nun.) and other modelling work by interactive spreadsheet programs written by the present author. Analytical data, calculated e-values and Nd model ages are given in Talble 1. 4.2. Lead mineral age The whole-rock and mineral separates of sample 107 span a range in “‘Pb/ ‘04Pb from 16.967 (alkali feldspar) to 92 (titanite), defining an isochron (Fig. 2a) with an age of 926 f 8 Ma (MSWD = 1.44). This age is equal within uncertainty to the 909 + 29Ma whole-rock Rb-Sr isochron age of Killeen and

Geology 135 (1997) 139-158

143

Heier (1975) and the age calculated from the present Rb-Sr data (below). 926 + 8 Ma is therefore regarded as the best estimate of the crystallization age of the Herefoss granite, and all time-dependent isotopic parameters have been calculated accordingly (Table 1). The Pb-Pb mineral isochron has a single-stage model p-value of 8.05 f 0.01, which is significantly lower than the model p of the source-terrane of Sveconorwegian metasediments (8.20 + 0.02, Andersen and Munz, 1995), but higher than the 7.907.95 range calculated for the mantle beneath this part of the Baltic Shield (Andersen et al., 1994). 4.3. Whole-rock

lead data

In the *07Pb/ *04Pb vs. *06Pb/ 204Pb diagram (Fig. 2a), the whole-rock samples scatter around the mineral isochron at a *06Pb/ *04Pb range from 17.249 to 20.803. The whole rocks alone yield a Pb-Pb isochron age of 1018 _+ 150 Ma (MSWD = 2.56). The somewhat higher MSWD-value suggests that the initial composition of Pb, although similar to that of the alkali feldspar of sample 107 (Table l>, was not completely homogeneous at the time of crystallization of the magma. The “unsupported” scatter of *“Pb/ *04Pb in the Herefoss granite is less than that observed in the N 1120-Ma Ubergsmoen intrusion in the Bamble sector (Andersen et al., 1994). The whole-rock samples show considerably more scatter in the *08Pb/ 204Pb ratio (Fig. 2b), but the composition of all samples can be fully explained by in-situ accumulation of radiogenic lead since 926 Ma in a system whose initial lead isotopic composition is given by the alkali feldspar of sample 107. The majority of samples can be accounted for by Th/U ratios between N 3 and N 9, whereas the two remaining samples (104A and 104B) require Th/U of 16 and 20, respectively. The range of in-situ Th/U ratios reflects varying abundance of Th-rich accessory minerals (monazite, allanite), which are particularly abundant in samples 104A and 104B. 4.4. Strontium The new whole-rock rubidium strontium data (Table 1) show considerable spread in “Rb/ 86Sr (from

15.555 15.545 15.468 15.589 15.486 15.464 15.494 15.502 15.547 15.511 15.722 15.702 15.718

15.445 16.904 20.697

18.483 18.397 17.535 19.194 17.594 17.249 17.618 17.626 18.736 17.866 20.803 20.347 20.696

16.967 37.613 92.004

101 102 103 104A 104B 105 106 107 108 109 110 111 112

107KP 107Ap 107Ttn

36.478 53.518 71.401

40.046 38.932 37.684 47.191 40.719 36.720 37.556 37.654 41.078 37.829 41.069 39.833 40.490

208Pb/ 204Pb

Nd

10.0 20.5 28.9 23.8 21.5 32.4 23.8 21.5 15.7 29.9 9.2 8.6 11.9

66.7 118.8 152.1 146.5 132.6 177.0 137.5 127.6 111.2 170.1 45.6 43.1 62.6

(ppm) (ppm)

Sm

0.09040 0.10497 0.11584 0.09830 0.09820 0.11070 0.10550 0.10242 0.08596 0.10682 0.12252 0.12217 0.1144

0.511951 0.511940 0.512086 0.511913 0.511894 0.512017 0.511964 0.511935 0.511883 0.511927 0.512069 0.512085 0.512098

6 25 84 6 7 6 8 8 5 6 5 5 11

+2a

141.2 345.2 499.6 207.6 247.1 694.7 611.0 386.9 73.6 667.6 168.1 67.8 93.2

(ppm)

(ppm)

270.4 196.1 110.9 194.6 166.4 77.7 114.1 145.6 214.9 99.5 223.8 361.4 335.1

Sr

Rb

5.6041 1.6533 0.6450 2.7316 1.9611 0.3251 0.5426 1.0947 8.5705 0.4329 3.8863 15.7844 10.5839

“Rb/ 86Sr

0.777521 0.727305 0.713220 0.737347 0.731613 0.709037 0.712328 0.719318 0.808389 0.710825 0.754467 0.897567 0.846564

“Sr/ 86Sr

20 64 16 20 18 16 16 14 200 18 20 20 40

+2a

1.34 1.54 1.48 1.48 1.51 1.51 1.51 1.51 1.38 1.58 1.62 1.58 1.44

DePaolo (1981)

fDM

1.41 1.62 1.58 1.56 1.59 1.60 1.60 1.59 1.45 1.67 1.72 1.69 1.53

DePaolo et al. (1991)

fDh4

-0.8 - 2.7 - 1.2 - 2.5 - 2.8 - 1.9 -2.3 - 2.5 - 1.6 -3.2 -2.3 - 1.9 -0.8

926 Ma

ENd

-3 26 16 -34 30 16 22 18 - 123 22 -8 -212 41

926 Ma

esr

The analytical uncertainty(reproducibility + fractionation) of the lead isotope data is 0.1% and the fractionation is - O.O95%/amu. Sample descriptions are given in Appendix A. The uncertainty figures given for Sr and Nd isotope ratios refer to the final digits of the respective ratios. The 2~ uncertainty in the e-values is +0.2 for Nd and f 1 for Sr.

“‘/Pb/ ‘04Pb

u)6Pb/ ‘04Pb

Sample

Table 1 Radiogenic isotope data and calculated parameters

T. Andersen/Chemical

N 0.6 to N 16; Fig. 3). Nine of the samples (101, 104A, 108 and 111 excluded) plot on a poorly fitted correlation line, with an age indication of 934 f 17 Ma (MSWD = 103) and an initial *‘Sr/ *%r ratio of 0.7046. This date is indistinguishable within uncertainty both from the 926 f 8Ma Pb-Pb mineral age of sample 107 and the 909 f 29-Ma Rb-Sr isochron age (initial ratio: 0..7051) reported by Killeen and Heier (1975). The four samples falling off the correlation line all plot significantly below the line; when recalculated to 926 Ma, they suggest unrealistically low initial *‘Sr/ 86Sr ratios ( < 0.7001, indicating that their Sr isotopic composition has been modified by

3!j

55

75

95

21

23

‘“Pb/m4Pb

19

145

Geology 135 (1997) 139-158 0.95

T

0.90 L

22 0 P

0.85 0.50 0.75 0.70 0

5

10

15

20

“Rb/“S r Fig. 3. Rb-Sr isochron diagram with the whole-rock scatter&on defined by nine of the present samples (934+ 17 Ma, initial “Sr/ 86Sr = 0.7046).

post-crystallization processes. In all of these samples, biotite is wholly or in part replaced by chlorite (Appendix A); 1‘t is thus feasible that radiogenic strontium can have been lost, due to alteration by fluids much later than the crystallization of the granite. Sample 101 was taken close to the external contact of the intrusion, the other three samples come from localities within or close to prominent fracture- or fault-zones (Fig. 1). Both the margins and the fracture/fault zones may have seen Permian or post-Permian fluid circulation, which may explain the secondary effects on the Rb-Sr system. However, other samples have escaped these effects, despite coming from similar settings, or from nearby localities (e.g., 109 and 110 compared to 108, 105 to 101 and 104A to 104B). This suggests that the secondary processes have only had local and not very penetrative effect. Also, the Pb and Nd isotopic systems in these samples are unaffected by the late processes. The Sr data on the four disturbed samples have not been used for petrogenetic modelling.

==Pb/=‘Pb Fig. 2. Pb isotope correlation in the Herefoss granite. The black circles are minerals in sample 107, the open circles are whole rocks. a. Uranogenic lead isotope correlations, with 926-Ma isochron. b. Thorogenic lead. Solid lines are 926-Ma reference isochrons for K,-values of 3 and 9. The broken line is a 1.9-Ga paleoisochron for K, = K? = 3.8 and p parameters derived from the multi-stage model discussed in the text and illustrated in Fig. 7, which reproduces the initial lead of sample 107. The ruled jield represents possible lead isotope compositions of protoliths with elevated Ks-values, characteristic of U-depleted “lower crust” (e.g., Taylor and McLennan, l!E35) at 926 Ma. See further discussion of Pb modelling in the text.

4.5. Neodymium The range of present-day 143Nd/ 144Nd is restricted, from 0.511883 to 0.512098. All samples are light rare-earth element (LREE) enriched, with 14’Sm/ 144Ndin the range 0.086-0.122, corresponding to fs,-values less than -0.56. Depleted-mantle model ages [DePaolo (1981) model] are in the range 1.34-1.62 Ga, with a frequency maximum in the range 1.5-1.6 Ga; these model ages are significantly younger than those obtained for metasedimentary

T. Andersen /Chemical Geology 135 (1997) 139-158

146

1.0

1.5

2.0

2.5

1, Pa) Fig. 4. Nd model ages [DePaolo’s (1981) depleted-mantle model]. Shaded = present data from the Herefoss granite; white = model ages of Bamble sector metasedimentary rocks (Andersen et al., 1995). The horizontal and vertical bars indicate the range and average, respectively, of Nd model ages of the Ubergsmoen chamockite (P. Hagelia and L. Kullerud, pers. commun.; Lindh and Persson, 1990).

rocks from South Norway (Fig. 4) suggesting that the sources of the Herefoss granite and the metasedimentary country rocks have different crustal histories. Recalculated to 926 Ma, the observed range of Nd isotopic composition corresponds to eNd from - 3.2 to -0.8. There is no correlation between initial neodymium isotope composition and either colour, grain size or geographic locality.

5. Discussion 5.1. Age of emplacement

-

regional correlation

The present-day lead and strontium data confirm the late Sveconorwegian age of the Herefoss granite. The best estimate of the emplacement age (926 + 8 Ma) overlaps within uncertainty with Rb-Sr ages on other post-tectonic Sveconorwegian granites from South Norway, such as 892-92%Ma granitic to charnockitic intrusions in the Rogaland sector (Pedersen, 1973; Pasteels et al., 1979; Wielens et al., 198 1;, and data of Pedersen and Falkum, 1975 in Verschure, 1985) and the younger members of the Setesdal igneous province (900-945 Ma; Pedersen, 1973; ,Pedersen and Konnerup-Madsen, 1994). The Herefoss granite shows marked petrographical similarities to ‘the adjacent Grimstad granite (Elders, 1963), but the two granites differ in terms of gravimetric and magnetic signature, which may be

due to different levels of exposure in the two intrusions (Smithson, 1963; Sindre, 1992). The Grimstad granite has been dated by whole-rock Rb-Sr to 946 + 66 Ma (Killeen and Heier, 1975) i.e. indistinguishable from the age of the Herefoss granite, and to 989 k 9 Ma by U-PI, on zircons and titanite (Kullerud and Machado, 1991). If the more precise U-Pb age is assumed to represent the emplacement of the Grimstad granite, this intrusion is significantly older than the Herefoss granite. 5.2. Grey vs. red granite The range of calculated initial Nd isotopic composition observed in the Herefoss granite indicates that the magma was somewhat heterogeneous at the time of crystallization. The present data do not indicate a systematic difference between the different colour or grain-size facies of the Herefoss granite. It is therefore very unlikely that the fine-grained and grey varieties represent local country-rock xenoliths which have been “granitized” as suggested by Elders (1963). If so, a trend towards higher ‘07Pb/ 204Pb and higher Nd model ages/lower ?? ,,-values should have been expected (cf. Andersen et al., 1995). The colour difference is most probably due to differences in pigmentation of the feldspar. The fine-grained varieties may be inclusions of early intrusive phases of the same magma, which have crystallized rapidly because of loss of their volatile contents. The quartz dioritic enclave studied here (sample 105) does not differ in isotopic composition from the granite itself, suggesting that it is a cognate inclusion, consisting of more primitive material which has crystallized from the same parent magma. 5.3. Sr and Nd data as source indicators The initial Sr and Nd isotopic compositions of the Herefoss granite provide constraints on the nature and composition of the source region of the magma. Different candidates for the source can be evaluated by comparison of the present data with data from other continental rocks from the Sveconorwegian province of the Baltic Shield and with general models for the Sr, Nd and Pb isotopic evolution of potential sources in the crust and mantle. In Figs. 5 and 6, time-corrected ?? sr- and +,-values are com-

T. Andersen/ 0

600 500

0.5

1

High' LILE crust 13,

o

;1

2

2.5

*1 ro2

\

\

1.5

Chemical Geology 135 (1997) 139-158

\

0

2t

.03 .a4

cu”

a5 *6

t, Ga Fig. 5. Sr evolution diagram for the southwestern part of the Baltic Shield. Black symbols indicate mafic rocks and anorthosites; open or shaded symbols granitoids and granitic gneisses. Legend: 1= Herefoss granite initial; 2 = Bamble, Kongsberg and Telemark sectors (except Telemark gneiss and Setesdal igneous province); 2b = Telemark gneiss; 3 = Rogaland sector and Setesda1 igneous province, Telemark sector: 4 = South and Central Sweden, including Protogine zone intrusions and rocks from the W. Bergslagen area as well as rocks from the 0stfold-Marstrand belt; 5 = Ostfold sector (Norway); 6 = depleted-mantle signature, Fen complex (539 Ma) and Oslo region (270 Ma). The definition of “depleted mantle”, “normal crust” and “high-LILE crust” are given in the text. Sources of data are given in Appendix B. fat, is the rubidium enrichmment factor relative to bulk Earth (e.g., Faure, 1986), defined by far, = [(“Rb/ 86Sr)~~,,,p,e

/(87Rb/86Sr)bulkEarthl~‘.

piled from published and unpublished data (see Appendix B for sources of data). fs, and fRb are enrichment factors relative to chondritic and bulkEarth reservoirs, respectively, as defined by Faure (1986). 5.3.1. Strontium Most crustally de.rived rocks from the Sveconorwegian province of South Norway and Southern Sweden west of the Protogine zone plot in two distinct regions in the esr vs. t diagram (Fig. 5). A majority of granitic intrusions and granitic gneisses has low to moderately high initial strontium ratios, corresponding to e,,(t)-values of 0 to N 150. The es,-values show a general increasing trend with younger age. Most of the points fall within a band delimited by growth curves with fRb = +7 to + 8, derived from a depleted-mantle source (fRb = - 0.45) at 1.7-1.9 Ga (field of “normal crust” in Fig. 5). Some Sveconorwegian granitic intrusions,

147

including rocks from the Rogaland igneous province and one of the data points from the 0stfold granite, fall distinctly below this zone in the diagram, suggesting a lower time-integrated rubidium enrichment or influence of a younger (Sveconorwegian) mantle component. A subordinate number of rocks plot far above this field, most of them within a field delimited by growth curves for fRb = 21, t,, = 1.7 and fRb = 3 1, = 1.9. This range of &,-values is based on data tDM on the South Telemark crustal composite of Andersen (1987) and a conservative estimate of the fRb of the protolith of elastic metasediments in the Bamble sector (Andersen et al., 1995). Mantle-derived rocks span a range from slightly above bulk-Earth values to the fRb = -0.45 depleted-mantle curve. The highest values come from Sveconorwegian mafic intrusions of the Rogaland province, which have been interpreted as crustally contaminated in both Sr and Nd (Demaiffe et al., 1986; Menuge, 1988). The initial 87Sr/ *‘Sr of the Herefoss granite falls below the “normal crust” range in Fig. 5, but within the range of contemporaneous granitic/charnockitic intrusions in the Rogaland-Vest Agder sector, and overlaps with the lowest initial Sr ratios observed in the Ostfold granite (Fig. 5). 5.3.2. Neodymium Fewer neodymium data are available on Precambrian felsic rocks from this region (Fig. 6): ,The 1.7-1.9-Ga crust field in Fig. 6 represents a crustal system equivalent to ~upracrustal rocks of the 0stfold-Marstrand belt (till1 and Daly, 1989) and the Bamble and Kongsberg sector metasediments (Munz et al., 1994; Andersen et al., 19951, having t,, = 1.7-1.9. The mantle model shown is the DePaolo (1981) model, which is chosen in preference to more recently proposed, linear models of global neodymium isotopic evolution (e.g., DePaolo et al.; 1991) because it accounts for a progressively increasing depletion of the upper mantle, and because it yields the youngest model ages for mid- to late Proterozoic rocks, and thus the most conservative estimates of crustal residence time. At 926 Ma, the Herefoss granite spans a range of eNd (- 3.2 to -0.8) which-is less negative than the range of Sweconorwegian metasediments and their

T. Andersen/Chemical

148

0.7

0.9

t 1.1

(W

1.3

1.5

1.7

1.9

8 6 4 B (0

2 0 -2 -4 -6 -8

Fig. 6. Nd evolution diagram for the southwestern part of the Baltic Shield. Legend: I = Herefoss granite, individual samples recalculated to 926 Ma; 2 = Ostfold-Marstrand belt, metasedimentary gneisses; 3 = Rogaland-Vest Agder Sector, chamockite, granite; 4 = Southwest Sweden, acid gneisses; 5 = South Norway, granulites; 6 = Bamble sector, Ubergsmoen charnockite; 7 = South Sweden, granulites; 8 = Grimstad granite. Sources of data are indicated in Appendix B. The shadedfieZd marked TG represents the evolution of Telemark gneisses, based on data of Menuge (1985). Mantle-derived components A and B are defined in Table 2. The arrows indicate mixing of mantle- and cmstally derived components when the Herefoss granite formed.

protoliths at this time ( I - 6). In contrast, the Grimstad granite shows lower eNd, approaching the 1.71.9-Ga crust (Fig. 6). The highest ?? ,,-values observed in the Herefoss granite overlap with coeval granitic rocks from the Rogaland-Vest Agder sector and from SW Sweden. The 1.12-1.15Ga chamockites and augen gneisses from the Bamble sector overlap with the Herefoss granite, but span a slightly larger range in ENd. 5.3.3. Restrictions on the source The present isotope data point to a source of the granitic magma within the continental crust, but with either a less Rb- and LREE-enriched composition than in other crustal rocks of the southwestern part of the Baltic Shield, or with a significant contribution of young, mantle-derived material. Anatexis of rocks equivalent to the surrounding metasediments would yield a magma with much higher eSr and lower eNd (and hence also older to,) than observed in the Herefoss granite. However, as local metasediments occur as xenoliths in the granite, a high-LILE metasedimentary end-member will have to be evaluated as a potential contaminant, to be mixed with melt derived from a less extreme source.

Geology 135 (1997) 139-158

Smithson (1963) suggested that tectonostratigraphically underlying rocks equivalent to the Telemark gneiss found north of the Porsgrunn-Kristiansand shear zone could be the source of anatectic magmas forming the Herefoss and Grimstad granites. The Telemark gneiss is a heterogeneous group of rocks, which is poorly constrained by Sr and Nd data, and whose origin is far from clear, as they may represent the basement on which the 1 S-l. 1-Ga Telemark supracrustals were deposited, or the metamorphic equivalents of the supracrustals (Ploquin, 1972; Sigmond, 1978; Starmer, 1993). Initial Sr ratios reported for Telemark gneisses by Kleppe (1980) span a large range, from the “normal crust” level to a maximum off the scale of Fig. 5. Only three Nd analyses of Telemark gneisses have been published (Menuge, 19851, these samples span a large field of evolution in Fig. 6, but all give model ages of < 1.5 Ga. From these data, the influence of Sveconorwegian mantle components in the Telemark ,,-valgneiss may be suspected. Only the highest ?? ues of the Herefoss granite fall within the Telemark gneiss range in Fig. 6. The poor correspondence with the Herefoss initial Sr and Nd composition suggests that the Telemark gneiss may be unlikely as source of the Herefoss granite; however, more data on the isotopic variation of these gneisses are needed to draw a firm conclusion. 5.4. Evidence from lead isotopes 5.4.1. Principles of multi-stage modelling of the U-Pb system Further constraints on the LILE characteristics and prehistory of the source of the Herefoss granitic magma can be obtained by reproducing the initial lead composition of the pluton by a multi-stage model of U-Pb evolution (e.g., Gale and Mussett, 1973; Zartman and Doe, 1981; Faure, 1986; Whitehouse, 1989). Lead isotope studies of Precambrian metasediments from the Bamble and Kong&erg sectors have shown that at least three consecutive stages of evolution are needed to account for the present-day lead isotope variations in this part of the Baltic Shield (Andersen et al., 1994; Andersen and Munz, 1995; Andersen et al., 1995; Andersen and Knudsen, 1996). This evolution can be modelled quantitatively, assuming that lead has resided in three subse-

T. Andersen/

Chemical Geology 135 (1997) 139-158

quent reservoirs since the formation of the Earth at t, = 4.57 Ga. Each reservoir is characterized by a constant 238U/204Pb ratio (pi, p2, p3, respectively). The pi’s are actually time-integrated parameters, which incorporate any internal U/Pb differentiation within the life-.time of the individual reservoir. Differentiation of the U/Pb ratio and isotopic homogenization of Pb has taken place during distinct events at t, and t,. In South Norway, the first stage ( pl) corresponds to evolution in a mantle reservoir prior to generation of the local continental crust, and the t, event can be: related to the generation of a regional continental protolith from the mantle. The t, event corresponds to the final isotopic homogenization, related to magmatic processes when the Herefoss granite formed. By modelling the composition of lead in potassium feldspar (which has k3 = 01, two of the three parameters p,, p2 and rr can be expressed as univariant functions of the third ( p3 = 0; to = 4.57 Ga; t, = 926 Ma, as given by the isochron slope), and illustrated in dia.grams with t, and p parameters as variables, as introduced by Whitehouse (1989).

149

range of 20-30 estimated for the metasediment source (Andersen and Munz, 1995; Andersen et al., 19951, and the range of the Ubergsmoen source ( p2 = 17-22 for tl = 1.6 Ga; Andersen et al., 1994), but still within the range of LILE-enriched rocks of the “upper continental crust”. Assuming younger t1 amounts to shifting the p.,-value towards slightly higher p2, as the p2 vs. t, curve has a negative slope (Fig. 7~). Mixing of material with /.L*2 17 and a younger,

a

5

1 0 8.00

7.90 5.4.2. Results The results of the multi-stage model calculation are shown in Fig. 7. For p3 = 0 and hence ~3//.~2 = 0, a pi-value of 7.96 can be estimated for t, = 1.6 Ga which is the maximum of the t,, range suggested by the present data (Fig. 7a). This is comparable to the CL, estimate of 7.95 obtained for the Ubergsmoen intrusion for t, = 1.6 Ga (Andersen et al., 1994). The best estimate of p2 based on t, = 1.6 Ga is 13 (Fig. 7b), which is distinctly lower than the

b

Pl

0.5

P3b2°.3 0.2 Ubergsmoen source 13 -

8 Fig. 7. Multi-stage evolution model for the alkali feldspar lead in sample 107, illustrated in ~-_IL and p-r diagrams (Whitehouse, 1989). The diagram are based on a three-stage model of lead evolution, as discussed in the text. a. First vs. final stage ~-values. The lines are drawn for t, = 1.91.6 Ga, at 0.1~Ga intervals. b: /.~s//.L~ vs. pLz diagram. The lines are drawn for f, = 1.9-1.6 Ga, at O.l-Ga intervals. ‘The alkali feldspar has j~s / w2 = 0. The shaded area represents the lower part of the pz range of the source of the - 1.12-Ga Ubergsmoen charnockite. c. y, vs. tt, for pI = 7.90, 7.95 and 8.0. The solid line C/L, = 7.95) represents the best. estimate of the average composition of the Herefoss source. Note that pL2 is dependent on the time of extraction of the crustal protolith from the mantle.

8.10

P2

* b

40 35 30

25 P2

18

20 15 10 I

8.0

7.9

_!&& ---_

150

T. Andersen/Chemical

mantle-derived component ( (u, = pz = 7.96) can explain the low p2-value of the Herefoss granite and the scatter of the whole-rock data in Fig. 2a. Due to reduced uranium concentration, LILE-depleted rocks of the lower continental crust will tend to evolve towards higher *‘*Pb/ *04Pb ratios at a given *06Pb/ *04Pb than will rocks of the LILE-enriched upper continental crust (Lambert and Heier, 1967; Zartman and Doe, 1981; Taylor and McLennan, 1985; Weis, 1986). For pr- and p2-values derived from Fig. 7 (i.e. p, = 7.96, pz = 13), the initial *“Pb/ *04Pb ratio of sample 107 can only be reproduced if K* = K, = 3.8, which is characteristic of LILE-enriched upper continental crust. More elevated K,-values, representative of Th/U ratios of LILE-depleted lower crust, yield significantly higher initial *08Pb/ *04Pb at 926 Ma than the observed composition, as illustrated by the ruled field in Fig. 2b. The Th/U ratio of the source of the Herefoss granite was thus within the range of the LILE-enriched upper continental crust (Taylor and McLennan, 19851, contradicting the presence of a significant “lower continental crust” component in the source region, as was suggested by L. Johansson and Kullerud ( 1993). 5.5. Mixing model The relative importance of mantle and crustally derived components in the Herefoss granite can be quantified by reproducing the initial Sr and Nd isotopic composition by binary mixing (Fig. 8; Table 2). Four different end-members have been chosen to represent the most likely reservoirs involved: A is juvenile, Sveconorwegian depleted-mantle-derived material. B is a mantle-derived component stored within the continental crust since 1.5 Ga. C is a crustal reservoir with high degree of LILE enrichment (fRb = 211, corresponding to the source of the Bamble metasediments (Andersen et al., 199.5) and D is a less LILE-enriched crustal component (fRr, = 81, corresponding to the source region of the Ubergsmoen chamockite and to the “normal crust” in Fig. 5. Since the amphibolites and mafic intrusions in the Bamble sector do not represent primitive mantle melt compositions (e.g., the Dale gabbro of De Haas et al., 1993, see Fig. 8; Andersen et al., 1995), average

Geology 135 (1997) 139-158 sBr(926Ma) 10.0

-50 ,

1

50 /

150

250

350

450

:

Fig. 8. Mixing model for the Herefoss granite, combining mantle (A, B) and crustal (C, D) components (Table 2). Binary mixing lines are tagged at 10% intervals. Data from the Dale gabbro are taken from De Haas et al. (1993). The shaded field at high es,-values and the triangles illustrate the range of Bamble metasediments (Andersen et al., 1995) recalculated to 926 Ma. One sample from the Herefoss granite falls off the diagram towards highly negative es,. Samples with ?? sr < 0 have altered biotite (Appendix A).

basaltic Sr (190 ppm) and Nd (15 ppm) concentrations have been used for the mantle-derived endmembers (Mason and Moore, 1982). The concentrations in the crustal end-members (25 ppm Nd, 50 ppm Sr) are averages of elastic metasediments from Bamble (Andersen et al., 1995). It should be noted that both of the crustal end-members (C and D) represent “upper continental crust” types of compositions (Taylor and McLennan, 1985). Mantle com-

Table 2 Input parameters ENd

Mantle-derived A B

c D

5.9 4.6

of binary mixing models %r

tDM

Sr

Nd

@em)

@em)

190 190

15 15

50 50

25 25

end-member: - 12 -9

Crustal end-member: -7.8 340 - 7.8 120

0.93

1.50

1.90 1.90

The mantle-derived end-members are based on the depleted-mantel model of DePaolo (1981) and average basaltic concentrations of Sr and Nd (Mason and Moore, 1982). The crustal end-members represent Bamble metasediments and their source (C) and a less LILE-enriched crustal source (D), with average Sm and Nd concentrations for Bamble metasedimentary gneisses (Andersen et al., 1995).

T. Andersen / Chemical Geology 135 (1997) 139-1.58

ponent B is assumed to have resided in a reservoir with a flat REE pattlern (i.e. fs, = 0) from 1.50 to 0.93 Ga, which is a simplifying, but not unrealistic assumption, given f,,,-values of -0.06 to -0.04 in some of the amphibolites analysed by Andersen et al. (1995). Regardless of which mantle-derived component is used, mixing with crustal component C fails to reproduce the range Iof eNd and esr observed in the undisturbed samples of the Herefoss granite, unless at unrealistically high Nd concentrations in the crustal component ( > 100 ppm Nd). Mixing of components D and A does, however, reproduce a majority of samples, at w 35-55% crustal contribution. If component B is used, 5--10% less of the crustal component would be neede’d (curve not shown in Fig. 8, to avoid clutter in the diagram). The ?? Nd of the samples which have had their strontium isotopic composition modified fall within the same range as the others (Table l), and thus isuggests similar amounts of the crustal and mantle-derived components; this cannot, of course, be backed up by Sr data. Local contamination with a metasediment-derived component would tend to pull samples off the mixing curve towards higher Q. A tendency in this direction can be seen in some of the samples, most notably in sample 112 from the Holtebu granite. Mixing between mantle and crustally derived

151

components may have taken place in situ, or within the source region in the underlying crust. As in-situ contamination of a mantle-derived magma would most likely involve surpracrustal gneisses with elevated fRb and U/W, which cannot have contributed significantly to the Herefoss granite, this process is unlikely, except at a local scale. The mixing process causing the characteristic radiogenic isotope signature of the Herefoss granite must therefore have taken place within the source region of the granitic magma. This could be brought about by partial melting of a heterogeneous crust, containing mafic rocks with a mantle-derived isotopic signature and felsic rocks with a longer crustal history, or by injection of mantle-derived magmas into a predominantly felsic crust immediately before partial melting (perhaps causing anatexis, e.g., Frost and Frost, 1987). The mixing model (Fig. 8) cannot distinguish between a Sveconorwegian or a N l-5-Ga mantlederived component. 1.5-Ga mantle-derived rocks would be temporally equivalent with the older Telemark volcanic rocks (Dahlgren et al., 1990a). During the Sveconorwegian orogeny, mafic magmatism has been recognized from the emplacement of early mafic intrusions at u 1.2-1.25 Ga (Dahlgren et al., 1990b; Munz and Morvik, 1991; De Haas et al., 1993) until the emplacement of the Rogaland anorthositic intrusions, which may be as young as 920-930 Ma

t DM=1.7-l .9 Ga f,,= 21-31 J&r 20 t DM=1.6-l .9 Ga f,,= 7-6 ,uu,= 17-22

.* .* . . . . . . .* . .

I.

.

.

.

.

. .

I.

. .

.I

. .

‘..

..

.d

I

I

Fig. 9. Cartoon of the continental crust in the southwestern part of the Baltic Shield at the end of the Sveconorwegian orogeny. The Herefoss granite (A), the Ubergsmoen chamockite (B) and a Sveconorwegian mafic intrusion (C) are illustrated, and their source regions indicated. See further discussion in the text.

152

T. Andersen/Chemical

(Duchesne et al., 1993). The observation that neither the N 1.12-Ga Ubergsmoen intrusion nor the 989-Ma Grimstad granite show a significant contribution of a young mantle component (except as a local contaminant in Ubergsmoen; Andersen et al., 1994) suggests that the mantle-derived component was most likely emplaced into the source region in tbe crust during the late stages of the Sveconorwegian orogeny. The presence of coeval mafic enclaves is a characteristic feature of granitic complexes where crustal anatexis result from emplacement of hot, mafic magmas into the crust (e.g., Platevoet and Bonin, 1991; Rossi and Cocherie, 1991; Bingen et al., 1993). Since such mafic enclaves are not found in the Herefoss granite (Elders, 19631, it is most likely that the mafic component was already established in the crust at the time of anatexis. 5.6. Geochemical

structure of the continental

crust

A simplified cross-section through tbe continental crust and underlying mantle at the end of the Sveconorwegian orogeny is given in Fig. 9. The uppermost part of the crust consisted of rocks with highly elevated Rb/Sr and U/Pb ratios, comprising Sveconorwegian metasediments, their yet unidentified source terrane(s) and granitic rocks formed by anatexis of this type of material (e.g., the N 1.4-Ga Tvedestrand granite; Field and R&eim, 1981). This crustal domain was penetrated by mafic intrusions (Sveconorwegian and possibly older; De Haas et al., 1993; T. Nijland and G.-J.L.M. de Haas, pers. commun.), chamockite/augen gneiss intrusions (such as the Ubergsmoen intrusion, Hagelia, 1989; Andersen et al., 1994) and post-erogenic intrusions (Herefoss, Grimstad). The structurally underlying “lower” crust consisted of two components: a moderately LILE-enriched crustal component with a crustal prehistory extending back to 1.6- 1.9 Ga, and a more recent, mantle-derived component. The crustal component served as a source of the Ubergsmoen and Grimstad granites, and a combination of both components gave rise to the Herefoss granite. Introduction of the mantle-derived component amounts to crustal rejuvenation by an <> process. The present data allow this process to be tentatively dated to the latest part of the Sveconorwegian orogeny, after

Geology 135 (1997) 139-158

emplacement of the Grimstad granite at 989 Ma. Possibly, “underplating” in the Bamble sector may be coeval with, and genetically related to late Sveconorwegian mafic-anorthositic magmatism in the Rogaland-Vest Agder sector. The LILE- and REE-depleted rocks of the Tromoy area (Clough and Field, 1980; Cameron, 1989a,b) are restricted to meta-igneous litbologies, and are associated with metasediments of the upper continental crust; low LILE concentrations may thus reflect other processes than regional, fluid induced depletion (T.-L. Knudsen, pers. commun.).

6. Conclusions The new Pb, Sr and Nd isotopic data on the Herefoss granite give new insight into the petrogenesis of this intrusion. The lead-lead isochron age of 926 f 8 Ma is based on unaltered rock-forming minerals of the granite, and is interpreted as the so far best estimate of the crystallization age of the intrusion. This age is within the range of earlier datings of the intrusion and of other late Sveconorwegian igneous rocks in South Norway, but is significantly more precise. This illustrates the power of this very simple isochron dating technique when applied to unmetamorphosed Precambrian granitic rocks. The Sr and Nd isotopic composition of the Herefoss granite at 926 Ma (esr < 41, eNd = - 3.2 to -0.8) is compatible with a mixed source of magma, containing mantle-derived and older crustal lithologies. This source was situated at depth in the continental crust, but cannot be related to a “depleted lower crust” in the sense of Zartman and Doe (1981) or Taylor and McLennan (1985), as the continental component was moderately enriched in LILE ( fRb= 7-8, 238U/204Pb = 17-22). This component was extracted from the mantle at or before 1.6 Ga. The mantle-derived component had the isotopic character of a normal depleted mantle, and was emplaced into the crust at 1.5 Ga or later, most probably during the later part of the Sveconorwegian period. The Herefoss granite shows little effect of contamination by its immediate country rocks, except perhaps on a local scale. The Sveconorwegian granitic intrusions of the

T. Andersen / Chemical Geology 135 (1997) 139-158

Bamble sector provide strong constraints on the composition and evolution of the unexposed parts of the Precambrian crust in the southwestern part of the Baltic Shield. By the end of the Sveconorwegian period, the continental crust of this part of the Baltic Shield had acquired a compositionally layered structure, in which the upper part of the crust (metasediments, their source-rocks and anatectic granites formed from them) is extremely enriched in LILE, whereas the underlying parts have the character of a more “normal” upper continental crust. Intrusions such as the Herefoss granite and the 1.12-Ga Ubergsmoen charnockite provide no evidence for the regional presence of a “LILE-depleted lower continental crust” at depth during the Sveconorwegian orogeny.

153

Acknowledgements The present study was financed by grants from Norges ForskningsrAd to the author and to the Laboratory of Isotope Geology at the Mineralogical-Geological Museum, Oslo. Gunnborg Bye-Fjeld, Tori1 Enger and Ame Stabel gave valuable analytical assistance. Special thanks are due to Lars Kullerud, for hospitality, assistance in the field and helpful discussions. Borghild Nilssen, Arthur Sylvester and numerous other friends and colleagues have provided comments and helpful suggestions. Martin Whitehouse kindly made his unpublished lead isotope modelling program available to the author. P.J. Patchett and D. Demaiffe are thanked for helpful and constructive reviews.

Appendix A. Petrographic characterisation, sample localities and alteration of the samples

Sample

Field characteristics

Colour in hand

Sub-area

UTM grid

Alteration

state a

E-W

N-S

alkali feldspar

plagioclase

biotite

101 102

fine-grained blotite granite coarse-grained, porphyritic biotite granite

grey red

eastern segment eastern segment

699 644

726 749

1 1

2 3

1 1

103

coarse-grained porphyritic biotite-hornblende granite

red

western segment

573

803

1

2

1

104a 104b 105

fine-grained biotite granite fine-grained granite dark, fine-grained quartz diorite w/biotite + bomblende

red grey dark grey

western segment western segment western segment

558 558 543

794 794 797

1 1 1

1 3 1

3 2 1

106

coarse-grab&, slightly foliated biotite-hornblende granite

red

western segment

558

793

1

3

1

107

coarse-grainesd, foliated biotite-hornblende granite

red

western segment

594

804

1

2

1

108

fine-grained, porphyritic biotite granite

red

eastern segment

658

783

2

3

3

109

medium-grained biotitehornblende granite

grey

eastern segment

660

789

1

1

1

110

biotite granite aplite in grey granite (109)

red

eastern segment

660

973

1

3

3

111 112 a Alteration (biotite) = commonly

fine-grained biotite granite red Holtebu granite 627 860 1 2 2 fine-grained biotite granite red Holtebu granite 648 844 1 2 1 state of alkali feldspar, plagioclase and biotite: 1 = fresh, or only slightly altered; 2 (feldspars) = partly turbid feldspar, partly replaced by chlorite; 3 (feldspars) = heavily altered (sericitized, saussuritized, minor amounts of pistacitic epidote present in sample), (biotite) = totally replaced by chlorite.

i? Andersen/Chemical

154

Geology 135 (1997) 139-158

Appendix B. Sources of radiogenic isotope data Sector

Locality/complex

Rock type

References

Rogaland

igneous complex

granite, charnockite,

Rogaland

igneous complex

mafic intrusions,

la) Strontium: Rogaland-Vest

Agder

Caledonian

Telemark

Bamble

nappes

reworked Precambrian granites and granitic gneiss acid volcanics post-tectonic granites

Telemark gneisses Telemark gneisses

granitic gneiss crustal composite

Fen complex Setesdal

initial of 539-Ma alkaline igneous rocks acid intrusions

Tvedestrand Ubergsmoen Levang Vestre Dale Grimstad

foliated granite charnockite granitic gneiss dome coronitic gabbro post-tectonic granite enderbite, granitic gneisses, granite, mafic intrusions

Iddefjord (Bihus) granite

post-tectonic

Central 0stfold

granite, granitic gneisses initial of _ 270-Ma alkaline igneous rocks

Oslo region

granite

Pedersen and Falkum (1975/, Versteeve (19751, Wilson et al. (1977), Pasteels et al. (1979). Wielens et al. (1981), Maijer et al. (19941, Zhou et al. (1995) Pedersen and Falkum (1975), DemaifSe et al. (1986), Menuge (1988) Andresen and Heier (19751, Berg (1977) Schnell and Falkum (1994) Kleppe (1980), Verschure et al. (1990) Priem et al. (1973/, Killeen and Heier (19751, Jacobsen and Heier (1978) Kleppe (19801, Andersen (1987), Andersen and Taylor (1988) Andersen and Sundvoll(1986). Andersen (1987) Pedersen (19731, Pedersen and Konnerup-Madsen ( 1994) Field and Riheim (1981) Hagelia (1989) O’Nions (1969) De Haas (1992) Kullerud and Machado (1991), L. Kullerud (pers. commun.) Jacobsen and Heier (1978) Killeen and Heier (1975), Skib’ld (1976), Pedersen and Miilge (1990) Skjemaa and Pedersen (1982) Sundvoll and Larsen (1990)

acid gneisses chamockite

A. Johansson et al. (1993) L. Johansson and Kullerud (1993)

Ostfold-Marstrand

acid and mafic metasupracrustals

.&ill and Daly (1989), .&Xl1

supracmstal Bergslagen, Vlrrnland

acid-intermediate

SW Sweden Varberg

S-central Sweden

anorthosite

Kristiansand Telemark supracrustals Telemark granites

Kong&erg 0stfold-Akershus

monzonite

belt Smiland,

Protogine zone

intrusions

acid and mtic intrusions, granitic gneisses

(19901, Ahill et al. (1990) Priem et al. (1970), L. Johansson and Johansson (19901, Valbracht (1991) L. Johansson and Johansson (1990)

T. Andersen/Chemical

Geology 135 (1997) 139-158

155

(b) Neodymium: Rogaland-Vest

Telemark

SW Sweden

Agder

Rogaland

igneous complex

Telemark supracrustals

augengneiss acid volcanics

Telemark gneiss Telemark gneisses

acid gneiss crustal composite

Fen complex

initial of 540-Ma alkaline igneous rocks

Demaiffe et al. (19861, Menuge (1988) Bingen et al. (1993) Menuge (1985) w/ age from Dahlgren et al. (199Oa) Menuge (1985) Andersen (19871, Andersen and Taylor (1988) Andersen and Sundvoll(1986). Andersen (1987)

metasedimentary

.&all and Daly (1989),

0stfold-Marstrand Protogine

zone

belt

acid-intermediate

gneisses intrusions

hhall et al. (19901. L. Johansson and Johansson

(1990)

A. Johansson et al. (1993) Bamble Ubergsmoen Lindh and Persson (19901, L. Kullerud and P. Hagelia (pers. commun.) Grimstad post-tectonic granite Kullerud and Machado (19911, L. Kullerud (pers. commun.) initial of w 270-Ma alkaline E.-R. Neumann et al. (19881, Oslo Region igneous rocks E.-R. Neumann et al. (19901, Sundvoll and Larsen (1993) References in italics: Initral ratios taken from the compilation of Verschure (1985). For a compilation of Nd data on anorthosites and mafic rocks, see Andersen and Sundvoll(l995). acid gneisses charnockite

References Ahall, K.I., 1990. An investigation of the Proterozoic Stenungsund granitoid intrusion, southwest Sweden; conflicting geochronological and field evidence. In: C.F. Gower, T. Rivers and B. Ryan, (Editors), Mid-Proterozoic Laurentia-Baltica. Geol. Assoc. Can., Spec. Pap., 38: 117-129. &all, K.I. and Daly, J.S., 1989. Age, tectonic setting and provenance of Bstfold-Marstrand Belt supracrustals: Westward crustal growth of the Baltic Shield at 1760 Ma. Precambrian Res., 45: 45-61. AhZll, K.-I., Daly, J.S. and Schdberg, H., 1990. Geochronological constraints on mid-F’roterozoic magmatism in the 0stfoldMarstrand Belt: implhcations for crustal evolution in Southwest Sweden. Geol. Assoc. Can., Spec. Pap., 38: 97-115. Andersen, T., 1987. Mantle and crustal components in a carbonatite complex, and the evolution of carbonatite magma: REE and isotopic evidence from the Fen complex, S.E. Norway. Chem. Geol. (Isot. Geosci. Sect.) 65: 147-166. Andersen, T. and Knudsen, T.-L., 1996. Precambrian crustal components in South Norway. Abstr., 22nd Nordic Geol. Winter Meet., Turku. p.12. Andersen, T. and Mum, LA., 1995. Radiogenic whole-rock lead in Precambrian metasedimentary gneisses from South Norway: Evidence for LILE mobility. Nor. Geol. Tidsskr., 75: 156-168. Andersen, T. and Sundvoll, B., 1986. Strontium and neodymium isotopic composition of an early tinguaite (nepheline mi-

crosyenite) in the Fen complex, S.E. Norway: Age and petrogenetic implications. Nor. Geol. Unders. Bull., 409: 29-34. Andersen, T. and Sundvoll, B., 1995. Neodymium isotope systematics of the mantle beneath the Baltic shield: Evidence for depleted mantle evolution since the Archaean. Lithos, 35: 235-243 Andersen, T. and Taylor, P.N., 1988. Lead isotope geochemistry of the Fen carbonatite complex, SE. Norway: Age and petrogenetic implications. Geochim. Cosmochim. Acta, 52: 209215. Andersen, T., Hagelia, P. and Whitehouse, M.J., 1994. Precambrian multi-stage crustal evolution in the Bamble sector of S. Norway: Pb isotopic evidence from a Sveconorwegian deepseated granitic intrusion. Chem. Geol. (Isot. Geosci. Sect.), 116: 327-343. Andersen, T., Maijer, C. and Verschure, R.H., 1995. Metamorphism, provenance ages and source characteristics of Precambrian elastic metasediments in the Bamble Sector, South Norway. Petrology, 3: 321-339. Andresen, A. and Heier, K.S., 1975. A Rb-Sr whole-rock isochron date on an igneous rock-body from the Stavanger area, south Norway. Geol. Rundsch., 64: 260-265. Annis, M.P., 1974. The highly differentiated Holtebu granite and its relation to the Herefoss pluton. Nor. Geol. Tidsskr., 54: 169-176. Berg, 0., 1977. En geokronologisk analyse av Prekambrisk basement i distriktet. Roldal-Haukelisaeter-Valdalen ved Rb-Sr whole-rock metoden, dets plass i den Sorvestnorske Prekam-

156

T. Andersen/

Chemical Geology 135 (1997) 139-158

briske Provinsen. Cand. Real. Thesis, Mineralogisk-Geologisk Museum, University of Oslo, Oslo, 125 pp. (unpublished). Bingen, B., Demaiffe, D., Hettogen, J., Weis, D. and Michot, J., 1993. K-rich talc-alkaline augen gneisses of Grenvillian age in SW Norway: Mingling of mantle-derived and crustal components. J. Geol., 101: 763-778. Bugge, A., 1928. En forkastning i det syd-norske grunnfjell. Nor. Geol. Unders., No. 130, 124 pp. Cameron, E.M., 1989a. Scouring of gold from the lower crust. Geology, 17: 26-29. Cameron, E.M., 1989b. Derivation of gold by oxidative metamorphism of a deep ductile shear zone; Part 2, evidence for the Bamble Belt, South Norway. J. Geochem. Explor., 31: 149169. Clough, P.W.L. and Field, D., 1980. Chemical variation in metabasites from a Proterozoic amphibolite-granulite transition zone, South Norway. Contrib. Mineral. Petrol., 73: 277-286. Dahlgren, S.H., Heaman, L. and Krogh, T.E., 1990a. Geological evolution and U-Pb geochronology of the Proterozoic central Telemark area, Norway (abstract). Geonytt, 17(l): 38. Dahlgren, S.H., Heaman, L. and Krogh, T.E., 1990b. U-PI, dating of coronitic metagabbros, hornblende-phlogopite-apatite pegmatites and albitites in the Proterozoic Bamble shear belt, South Norway. Abstr., 2nd Symp. on Baltic Shield, Lund, June 1990. De Haas, G.-J.L.M., 1992. Source, evolution and age of coronitic gabbros from the Arendal-Nelaug area, Bamble, Southeast Norway. Ph.D. Thesis, Utrecht University, Utrecht (published as Geol. Ultraiectina, No. 86, 129 pp.) De Haas, G-J.L.M., Verschure, R.H. and Maijer, C., 1993. Isotopic constraints on the timing of crustal accretion of the Bamble sector, Norway, as evidenced by coronitic gabbros. Precambrian Res., 64: 403-417. Demaiffe, D., Weis, D., Michot, J. and Duchesne, J.C., 1986. Isotopic constraints on the genesis of the Rogaland anorthositic suite (southwest Norway). Chem. Geol., 57: 167-179. DePaolo, D.J., 1981. Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic. Nature (London), 291: 193-196. DePaolo, D.J., Linn, A.M. and Schurbert, G., 1991. The continental crustal age distribution: Methods of determining mantle separation ages from Sm-Nd isotopic data and application to the Southwestern United States. J. Geophys. Res., 9O(B2): 2071-2088. Duchesne, J.C., Schlrer, U. and Wilma& E., 1993. A 10 Ma period of emplacement for Rogaland anorthosites, Norway: evidence from U-Pb ages. Terra Abstr., 5: 64 Elders, W.A., 1963. On the form and mode of emplacement of the Herefoss granite. Nor. Geol. Unders., 214: l-52. Faure, G., 1986. Principles of Isotope Geology. Wiley, New York, N.Y., 2nd ed., 589 pp. Field, D. and Riheim, A., 1981. Age relationships in the Proterozoic high-grade gneiss regions of Southern Norway. Precambrian Res., 14: 261-275. Frost, B.R. and Frost, C.D., 1987. CO,, melts and gram& metamorphism. Nature (London), 327: 503-506. GaPl, G. and Gorbatschev, R., 1987. An outline of the Precam-

brian evolution of the Baltic Shield. Precambrian Res., 35: 15-52. Gale, N.H. and Mussett, A.E., 1973. Episodic uranium-lead models and the interpretation of variations in the isotopic composition of lead in rocks. Rev. Geophys. Space Phys., 1 l(1): 37-86. Gorbatschev, R. and Bogdanova, S., 1993. Frontiers in the Baltic Shield. Precambrian Res., 64: 3-21. Hagelia, P, 1989. Structure, metamorphism and geochronology of the Skagerrak shear belt, as revealed by studies in the Hovdefjell-Ubergsmoen area, south Norway. Cand. Scient. Thesis, University of Oslo, Oslo, 236 pp. (unpublished). Heaman, L.M. and Smalley, P.C., 1994. A U-Pb study of the Morkheia Complex and associated gneisses, southern Norway: Implications for disturbed Rb-Sr systems and for the temporal evolution of Mesoproterozoic magmatism in Laurentia. Geochim. Cosmochim. Acta, 58: 1899-1911. Jacobsen, S.B. and Heier, K.S., 1978. Rb-Sr isotope systematics in metamorphic rocks, Kong&erg sector, south Norway. Lithos, 11: 257-276. Johansson, A,., Meier, M., Oberli, F. and Wikman, H., 1993. The early evolution of the Southwest Swedish Gneiss Province: geochronological and isotopic evidence from southernmost Sweden. Precambrian Res., 64: 361-388. Johansson, L. and Johansson, A., 1990. Isotope geochemistry and age relationships of matic intrusions along the Protogine Zone, southern Sweden. Precambrian Res., 48: 395-414. Johansson, L. and Kullerud, L., 1993. Late Sveconorwegian metamorphism and deformation in southwestern Sweden. Precambrian Res., 64: 347-360. Killeen, P.G. and Heier, K.S., 1975. Radioelement distribution and heat production in Precambrian granitic rocks, Southern Norway. Nor. Videnskaps-Akad., I Mat.-Naturv. Kl., Skr., Ny Ser. No. 35. Kleppe, A.V., 1980. Geologiske undersokelser fra sentrale deler av Telemark med hovedvekt pi geokronologi og kontaktrelasjonen mellom granittisk gneis og suprakrustalene. Cand. Real Thesis, University of Oslo, Oslo (unpublished). Kullerud, L. and Dahlgren, S.H., 1993. Sm-Nd geochronology of Sveconorwegian granulite-facies mineral assemblages in the Bamble Shear Belt, South Norway. Precambrian Res., 64: 389-402. Kullerud, L. and Machado, N., 1991. End of a controversy: U-Pb geochronological evidence for significant Grenvillian activity in the Bamble area, Norway. Terra Abstr., 3: 504. Lambert, LB. and Heier, K.S., 1967. The vertical distribution of uranium, thorium and potassium in the continental crust. Geochim. Cosmochim. Acta, 31: 377-390. Lindh, A., 1987. Westward growth of the Baltic Shield. Precambrian Res., 35: 53-70. Lindh, A. and Persson, P.O., 1990. Proterozoic granitoid rocks of the Baltic Shield - Trends of development. In: CF. Gower, T. Rivers and B. Ryan (Editors), Mid-Proterozoic LaurentiaBaltica. Geol. Assoc. Can., Spec. Pap., 38: 23-40 Ludwig, K.R., 1991. ISOPLOT a plotting and regression program for radiogenic-isotope data; version 2.53. U.S. Geol. Surv., Open-File Rep. 91-445, 39 pp.

T. Andersen/Chemical Maijer, C., Verschure, R.H. and Visser, D., 1994. Strontium isotope study of two supposed satellite massif of the Egersund anorthosite complex: the Sjelset igneous complex and the Undheim leuconorite. Nor. Geol. Tidsskr., 74: 58-69. Mason, B. and Moore, C.B., 1982. Principles of Geochemistry. Wiley, New York, N.Y., 4th ed., 344 pp. Menuge, J.F., 1985. Neodymium isotope evidence for the age and origin of the Proterozoic of Telemark, South Norway. In: A.C. Tobi and J.L.R. Touret (Editors): The Deep Proterozoic Crust in the North Atlantic Provinces. D. Reidel, Dordrecht, pp. 435-448. Menuge, J.F., 1988. The petrogenesis of massif anorthosites: a Nd and Sr isotopic investigation of the Proterozoic of Rogaland/Vest-Agder, SW Norway. Contrib. Mineral. Petrol., 98: 363-373. Munz, LA. and Morvik, R., 1991. Metagabbros in the Modum Complex, southern Norway: an important heat source for Sveconorwegian metamorphism. Precambrian Res., 52: 97113. Corrigendum: Precambrian Res., 53: 305. Munz, LA., Wayne, D. and Austrheim, H., 1994. Retrograde fluid infiltration in the high-grade Modum Complex, South Norway: evidence for age, source and REE mobility. Contrib. Mineral. Petrol., 116: 32-46. Neumann, E.-R., Tilton, G.R. and Tuen, E., 1988. Sr, Nd and Pb isotope geochemistry of the Oslo rift igneous province, southeast Norway. Geochim. Cosmochim. Acta, 52: 1997-2007. Neumann, E.-R., Sundvoll, B. and averlie, P.E., 1990. A mildly depleted upper mantle beneath southeast Norway: evidence from basalts in the Permo-Carboniferous Oslo Rift. Tectonophysics, 178: 89-107. Neumann, H., 1960. Apparent ages of Norwegian minerals and rocks. Nor. Geol. Tidsskr., 40: 173-191. O’Nions, R.K., 1969. Geochronology of the Bamble Sector of the Baltic Shield (Precambrian), South Norway. Ph.D. Thesis, University of Alberta, Edmonton, Alta., 172 pp. Pasteels, P., Demaiffe, D. and Michot, J., 1979. U- Pb and Rb-Sr geochronology of the eastern part of the south Rogaland igneous complex, sotnhern Norway. Lithos, 12: 199-208. Pedersen, S., 1973. Age determinations from the Iveland-Evje area, Aust Agder. Nor. Geol. Unders. Bull., 300: 33-39. Pedersen, S. and Falkum. T., 1975. Rb/Sr age determination of granitic rocks from the Precambrian of Agder, south Norway. Abstr., 4th Eur. Colloq. of Geochronology, Cosmochronology and Isotope Geology, Amsterdam. Pedersen, S. and Konnerup-Madsen, J., 1994. Geology of the Setesdal region, South Norway. Excursion guide to the SNF excursion August, 1994. Geol. Inst., University of Copenhagen, Copenhagen, 55 pp. Pedersen, S. and M&e, !j., 1990. The Iddefjord granite: Geology and age. Nor. Geol. Unders. Bull., 417: 55-64. Platevoet, B. and Bonin, B., 1991. Enclaves and mafic-felsic associations in the Permian alkaline province of Corsica, France: Physical and chemical interactions between coeval magmas. In: J. Didier and B. Barbarin (Editors), Granites and Their Enclaves. Elsevier, Amsterdam, pp. 191-204. Ploquin, A., 1972. Le granite acide d’Amli, Norvege meridonale: Transformation des laves acides du Tuddal (Formation

Geology 135 (1997) 139-158

157

inferieure des Series de roches supracrustales du Telemark). Sci. Terre, 17: 81-95. Priem, N.H.A., Verschure, R.H., Verdurmen, E.A.Th., Hebeda, E.H. and Boelrijk, N.A.I.M., 1970. Isotopic evidence on the age of the Trysil porphyries and granites in eastern Hedemark, Norway. Nor. Geol. Unders. Bull., 266: 263-276. Priem, N.H.A., Boelrijk, N.A.I.M., Hebeda, E.H., Verdurmen, E.A.Th. and Verschure, R.H., 1973. Rb-Sr investigations on Precambrian granites, granitic gneisses and acidic metavolcanics in central Telemark - Metamorphic resetting of Rb-Sr whole-rock systems. Nor. Geol Unders. Bull., 289: 37-53. Rossi, Ph. and Cocherie, A., 1991. Genesis of a Variscan batholith: Field, petrological and mineralogical evidence from the Corsica-Sardinia batholith. Tectonophysics, 195: 3 19-346. Schnell, U.A.S. and Falkum, T., 1994. Introduction to the geology of the Kristiansand area, south Norway. Excursion Guide, SNF (Sveconorwegian Friends), 7 pp. (unpublished). Sigmond, E.M.O., 1978. Beskrivelse til det bergrunnsgeologiske kartbladet Sauda 1:250 000. Nor. Geol. Unders., Bull. 341, 95 PP. Sindre, A., 1992. Regional tolkning av geofysiske data, karblad Arendal. Nor. Geol. Unders., Rapp. 92.213. Skiold, T., 1976. The interpretation of Rb-Sr and K-Ar ages of late Precambrian rocks in southwestern Sweden. Geol. Fiiren. Stockholm F&h., 98: 3-29. Skjernaa, L. and Pedersen, S., 1982. The effects of penetrative Sveconorwegian deformation on Rb-Sr isotope systems in the Romskog-Aurskog-Holand area, S.E. Norway. Precambrian Res., 17: 215-243. Smithson, S.B., 1963. Granite studies. I A gravity investigation of two Precambrian granites in S. Norway. Nor. Geol. Unders. Bull., 214: 53-140. Starmer, I.C., 1993. The Sveconorwegian Orogeny in southern Norway, relative to deep crustal structures and events in the North Atlantic Proterozoic Supercontinent. Nor. Geol. Tidsskr., 73: 109-132. Sundvoll, B. and Larsen, B.T., 1990. Rb-Sr systematics in the Oslo rift magmatic rocks. Nor. Geol. Unders. Bull., 418: 27-46. Sundvoll, B. and Larsen, B.T., 1993. Rb-Sr and Sm-Nd relationship in dyke and sill intrusions in the Oslo Rift and related areas. Nor. Geol. Unders. Bull., 425: 31-42. Taylor, S.R. and McLennan, SM., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312 pp. Todt, W., Cliff, R.A., Hanser, A. and Hofmann, A.W., 1984. z02Pb+20sPb double spike for lead isotopic analyses. Terra Cognita, 4: 209 (abstract). Valbracht, P.J., 1991. The origin of the continental crust of the Baltic Shield as seen through Nd and Sr isotopic variations in 1.89-1.85 Ga old rocks from western Bergslagen, Sweden. Ph.D. Thesis, University of Amsterdam, Amsterdam (published as GUA Pap. Geol., Ser. 1, No. 29, 222 pp.) Verschure, R.H., 1985. Geochronological framework for the lateProterozoic evolution of the Baltic Shield in south Scandinavia. In: A.C. Tobi and J.L.R. Touret (Editors), The Deep Proterozoic Crust in the North Atlantic Provinces. D. Reidel, Dordrecht, pp. 381-410.

158

T. Andersen/Chemical

Verschure, R.H., Maijer, C. and Andriessen, P.A.M., 1990. Isotopic age determinations in South Norway, II. The problem of errorchron ages from Telemark rhyolites. Nor. Geol. Unders. Bull., 418: 47-60. Versteeve, A., 1975. Isotope geochronology in the high-grade metamorphic Precambrian of southwestern Norway. Nor. Geol. Unders. Bull, 318: l-50. Weis, D., 1986. Genetic implications for Pb isotope geochemistry in the Rogaland anorthositic complex (southwestern Norway). Chem. Geol., 57: 181-199. White, A.J.R. and Chappell, B.W., 1983. Granitoid types and their distribution in the Lachlan Fold Belt, southeastern Australia. Geol. Sot. Am. Mem., 159: 21-34. Whitehouse, M.J., 1989. Pb-isotope evidence for U-Th-Pb behaviour in a prograde amphibolite to granulite facies transition from the Lewisian complex of north-west Scotland: Implications for Pb-Pb dating. Geochim. Cosmochim. Acta, 53: 717-724.

Geology 135 (1997) 139-158 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: l-30. Wilson, J.R., Pedersen, S., Berthelsen, C.R. and Jacobsen, B.M., 1977. New light on the Precambrian Holum granite, south Norway. Nor. Geol. Tidsskr., 57: 347-360. Zartman, R.E. and Doe, B.R., 1981. Plumbotectonics the model. Tectonophysics, 75: 135-162. Zhou, X.Q.. Bingen, B., Demaiffe, D., Lieigois, J.-P., Hertogen, J., Weis, D. and Michot, J., 1995. The 1160 Ma Hidderskog meta-chamockite: implications of this A-type pluton for the Sveconorwegian belt in Vest Agder (SW Norway). Lithos, 36: 51-66.