Rb-Sr isotope systematics in metamorphic rocks, Kongsberg sector, south Norway

STEIN B. JACOBSEN & KNUT S. HEIER

LITHOS Jacobsen, S. B. & Heier, K. S. 1978: Rb-Sr isotope systematics in metamorphic rocks, Kongsberg sector, south Norway. Lithos 11, 257-276. Oslo. ISSN 0024-4937.

Rb-Sr isotope data are presented for gneisses, migmatite neosome material and granitic and gabbroic intrusive rocks from the southern part of the Kongsberg sector, south Norway. The maximum age of the crust in this area appears to be ,-. !.6 AE. Two metamorphic episodes at ,-, 1.5-1.6 AE and at ~ 1.1-1.2 AE are recognized. Initial a~Sr/SeSr ratios for the granitic rocks give evidence for reworking of sialie crust and indicate that approximately 1.6 AE old crust repeatedly acted as a source for granitic magmas for a timespan of ~0.5 AE.

,Stein B. Jacobsen, Mineralogisk-Geologisk Museum, Sars Gale I, Oslo 5, Norway. Present address: The Lunatic Asylum of the Charles Arms Laboratory, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, U.S.A. Knut S. Heier, Mineralogisk-Geologisk Museum, Sars Gate 1, Oslo 5, Norway. Present address: Norges Geoiogiske Undersokelse, Leiv Eiriksons vei 39, 7000 Trondheim, Norway.

Initial 87Sr/S~Sr ratios favor a growth of the continents through time by differentiation from mantle raaterial (Hurley et al. 1962; Wasserburg 1966; Moorbath 1975) but do not rule out the possibility that many granitic igneous rocks have formed by the remelting of older sialic crust. On the basis of petrochemical and structural data, Engel et al. (1974) have suggested a profound episodisity in crustal evolution and global tectonics. This suggests that the processes involved in the formation of continental crust may have varied through time. To assess the importance of juvenile additions from the mantle as opposed to remeiting of older crustal rocks in the formation of quartzo- feldspathic igneous rocks we must determine, for an orogenic period in any area, the relative contributions of the two processes. This may in principle be done by Rb-Sr isotope systematies.

The Precambrian of south Norway is a classic region for the study of regional metamorphism and the formation of granitic rocks. The purpose of this paper is to use Rb-Sr isotope data to discuss the importance of remelting of crustal rocks versus juvenile additions in the formation of quartzo- feldspathic rocks in the Kongsberg sector of the Precambrian of south Norway. Crustal evolution in the Kongsberg sector will also be compared with that of adjacent areas. We have sampled and analyzed various types of granitic rocks, gabbroic rocks and their surrounding gneisses occurring in the southern part of the Kongsberg sector. The granitic rocks range from neosome material in

17 - Lithos4/78

high-grade gneisses and small anatectic bodies to granites of batholithic dimensions.

The data in this paper indicate that in south Norway crustal material derived from the mantle ,~1.6 AE ago repeatedly acted as source for granitic magmas during the subsequent -,-0.5 AE, i.e. from ~ 1.6 AE to -,, 1. I AE.

Geology The Baltic Shield has, on the basis of K-Ar mineral dates, been subdivided by Kratz et al. (1968) into three zones: a Saamo-Karelian zone (3.6-1.9 AE), a Svecofennian zone (2.3-1.6 AE) and a Sveco- norwegian zone (1.2-0.9 AE). The Precambrian of south Norway belongs tc, the Sveconorwegian zone. Rb-Sr whole rock studies (O'Nions & Baadsgaard 1971; O'Nions & Heier 1972) give evidence for the existence of rocks with ages up to ~ 1.7 AE in this zone.

West of the Oslo Rift (Fig. 1) Precambrian supracrustal rocks occur in the Telemark area (the Telemark suite) and the Kongsberg (the Kongsberg series) and Bamble (the Bamble series) sectors. The Telemark suite (Dons 1960, 1972) is mainly built up of well-preserved continental volcanic rocks and shallow water sedimentary rocks. The metamorphic grade ranges from greenschist to upper amphibolite facies. Bugge (1936) considered the Kongsberg sector to have many lithologica! and petrological similarities with the Bamble sector

258 S. B. Jacobsen & K. S, Heier LITHOS 11 (1978)

Fig. 1. Geological sketch map of the Kongsberg area, south Norway. Sample localities are indicated. A = Quartzo-feldspathic gneisses, amphibolites and dioritic gneisses. B = Mica schists and quartzites. C = Granitic gneisses. D = Telemark supracrustals, i.e. interlayered acid and basic !avas and quartzites. E = Hyperites. F = Medium- to coarse- grained gneissose granite (Helgevannet granite}. G = Medium- to fine-grained gneissose granite (Meheia granite in the south, Rollag granite in the north}. H = Cambro-silurian sediments. I = Permian igneous rock~. UTM coordinates for the sample locations indicated on fhe map are given in Tables l to 5.

further south, from which it is separated by the Permian Oslo Rift. The Kongsberg and Bamble series are composed of slieeply dipping gneisses and amphibolites with gabbroic and granitic intrusions (Morton et al. 1970; Starmer 1972; Jacobsen 1975). The metamorphic grade is mainly upper amphib- olite facies and intermediate pressure granulite facies. The latter is mostly recognized in the southern Bamble sector. 1"here is abundant evidence of migmatization in the gneisses. Small synkine- matic granites occur as elongate bodies (Bugge 1936; Touret 1968) and as gneiss domes (O'Nions & Baadsg~ard 1971).

The Telemark suite is welded together with the gneisses in the Kongsberg and Bamble series by a series of synkinematic granites, augengneisses and

migmatites (often called the Telemark granite gneiss) (Barth & Dons 1960; Barth & Reitan 19'63; Barth 1966; Smithson 1965; Mitchell 1967; Touter 1968; Martins 1969; Cramez 1970; Dons 1972). Their composition commonly approximates to 'the minimum-melting composition in the granite sys- tem. The Telemark granite gneiss makes up ~ 33~ of the surface area of the Precambrian of south Norway. The remainder is mainly made ~p of mixed gneisses (~48)~) and well-preserved supracrustai rocks (~ 12~). Large blocks (up to 20x l0 km) of supracrustal rocks are 'floating' within these granitic gneisses and several intrusive phases can be recognized ;n them. These granitic gneisses have been assumed to represent a remobil~zed older basement to the supracrustals. According to

LITHOS 11 (1978)

Table 1. Mean values and ranges of K, Rb and Sr concentrations.

Rb-Sr isotopes, Kongsberg 259

Rock type N" Rb(ppm) b Sr(ppm) b K(wt.~o) b Rb/Sr c K/Rb d

I. Quartzo-feldspathic gneisses Quartz-plagioclase gneiss, all samples Quartz-plagioclase gneiss, Loc. no. 3 Quartz-plagioclase gneiss, Loc. no. 4 Pink granodioritic gneiss, Loc. no. 4 Enderbitic granolite, Loc. no. 5 Charnoenderbitic granolite, Loc. no. 6

II. Granitic neosomes Loc. no. 7

Loc. no. 8 lII. Mica schist, Loc. no. 9 IV. Medium-grained red granitic gneiss

Kongsberg Vatnaas

V. Gneissic granites Helgevannet Meheia

24 18(2.2-47) 180(31-241) 0.85(0.11-2.00) 0.100(0.01-0.43) 470(179-756) 5 10(2.2-16.7) 133(31-241) 0.42(0.11-0.89) 0.075(0.01-0.43) 420(179-756) 4 34(22.3--4'7) 142(115-167) 1.23(0.94-2.00) 0.239(0.194--0.286) 362(284-426)

17 86(55-97) 140(134-248) 2.88(2.31-3.06) 0.614(0.35-0.70) 335(304-420) 9 15(8.0-21.7) 225(193-250) 0.61(0.42-0.95) 0.067(0.032-0.108) 407(286-588) 7 40(14.1-48) 211(196-231) 2.00(1.57-2.19) 0.190(0.061-0.241) 500(396-1:!3)

7 116(79-153) 45(22-84) 4.06(3.28-4.74) 2.58(1.06-7.05) 350(285-473) 5 89(63-103) 70(35-90) 4.72(3.58-6.11) 1.27(1.01-1.80) 530(424-630)

10 41(28-57) 50(21-95) 1.89(!.0i-2.43) 0.82(0.33-2.26) ~61(367-565)

17 85(60-95) 8~(49-111) 4.45(3.60-5.69) 0.97(0.74-1.95) 520(324-738) 6 73(32-94) 88(24-152) . 3.53(2.10-4.44) 0.83(0.21-3.83) 484(370-656)

24 153(69-268) 66(33--117) 3.91(3.04--4.56) 2.32(0.962-7.26) 256(132--441) 25 206(116-265) 44(16-88) 4.31(2.75-4.62) 4.68(1.44-15.7) 209(172-360)

(a) N = the number of samples analyzed. (b) The range of measured values are given in parentheses. (c) Calculated by dividing mean values of Rb with mean values of Sr, the range of measured values are given in parentheses. (d) Calculated by dividing mean values of K with mean values of Rb, the range of measured values are given in parentheses.

Mitchell (1967) the Telemark granite gneiss in some areas consists o f inhomogeneous gneisses in which partially granitized metamorphic rocks occur as remnants of the pre-existing terrain which was subjected to the granitization. In south Norway an Archean basement for the supracrustal rocks has not so far been recognized. Such a basement may however exist at a deeper level in the crust.

A large number of diapir like, postkinematic granites (~0.90-0.96 AE, Killeen & Hcier 1975) occur throughout the Precambrian of south Nor- way. Bart:h & Reitan (1963) and Smithson (1965) have suggested that these formed by remobilization of the Telemark granite gneiss.

The geology and petrology of the southern part of the Kongsberg sector have been presented by C. Bugge (1917), A. Bugge (1928, 1936, 1937), J. A. W. Bugge (1943), Kayode (1974) and .~acobsen (1975). O'Nions & Heier (1972) carried out a recon- naissance geochronologic study in this area. Kayode (1974) discussed the geochemistry of the granitic rocks around Kongsberg. These studies indicate that the sequence of events in the southern part of the Kongsberg sector may be summarized as follows (Jacobsen 1975): (a) Deposition of sodic volcanics, with major and

trace element characteristics similar to the present day island-arc tholeiite series, and some sedimentary rocks.

(b) ln t rwion of calc-alkaline gabbroic to dioritic rocks; deformation, isoclinal folding, mig-

matization and metamorphism (M 1) produced a series of quartzo-feldspathic gneisses inter- layered with amphibolites (from the voleanics) and dioritic gneiss (from the gabbroic to dioritic intrusions). The metamorphic grade varied from amphibolite to granulite facies. Intrusion of small bodies of potassic granite.

(c) Intrusion of gabbros, and dolerites (olivine- tholeiites).

(d) Intrusion of potassic metaluminous to subal- uminous batholithic granites (Meheia and Helgevannet granites; they belong to the Tele- mark granite gneiss), deformation and meta- morphism (M2) up to amphibolite facies.

Sampling and analytical methods Samples used in this study were selected on the basis of field, petrographical and petrochemical studies by Kayode (1974) and Jacobsen (1975). They were collected from visibly homogeneous outcrops; in general the quartzo-feldspathic gnelsses and the mica schists are less homogeneous than the other rocks in the area. Samples of 5 to 20 kg were used from the quartzo-feldspathic gneisses. Most mica schist samples were ~ 10 kg; one (52), however, weighed ~ 30 I~g and four ~3 kg (51, 55, 56, 57). The samples used for all other rock types were ~ 3 kg. The samples were first crushed to a fine gravel, after which an approximately 400 gram representative fraction was split from this gravel and crushed to a fine powder. All sample locations are shown in Fig. 1 and the Universal Transverse Mercator (UTM) grid coordinates are given in the tables.

Rb/Sr ratios and Rb and Sr concentrations were deter- mined by XRF methods. The USGS standard G2 has

260 S. B. Jacovsen & K. S. Heier LtTHOS 11 (1978)

476.3 ppm Sr and 169.3 ppm Rb by isotope dilution (Pankhurst & O'Nions 1973) and all XRF results were measured relative to these values. Potassium concentrations were determined by both y-ray spectrometry and XRF. Sr was separated from ~0.5 g samples by cation exchange ~echuiques, and isotopically analyzed on the VG-Micromass 30 mass spectrometers in Oxford and Oslo (Mineralogisk- Geologisk Museum). Several samples were analyzed in both laboratories and the results were to within ~0.003%. The a~Sr/SbSr obtained for the NBS standard SRM-987 was 0.71037+2 and 0.70815+2 for the Elmer & Amend standard. This is 0.00023 higher than the NBS certified value for SRM-987. The methods used are described by O'Nions & Pankhurst (1973) and Pankhurst & O'Njons (1973). Ana- lytical errors for the individual samples are given in the tables. All errors quoted in this paper are two sigma errors. Regression lines were calculated using the York 2 model (York 1969; Brooks et al. 1972) with uncorrelated errors.

Rb-Sr systematics Rb-Sr isotope data may give information about the time of crystallization (igneou.~ or metamorphic) and the pre- crystallization history of metamorphic rocks or of magmatic rocks and their sources. For total rocks with the same initial Sr isotopic coml~,osition (at time T) that were closed to gain or loss of Rb, Sr or radiogenic strontium (=a~Sr*) since time T, the isotopi,: compositions measured today form a linear array on a Rb-Sr evolution diagram. Because Rb, Sr and S~Sr* may migrate on a whole rock scale during metamorphism (Lanphere e t a [ . 1964, Wasserburg et al. 1964) most metamorphic rocks do not behave as simple systems. The data points may thus not define a precise line and then neither age nor initial STSr/SbSr (=1) are well defined.

in general there may be a tendency toward local redistribu- tion of Rb and Sr due to mineralogical control during low to intermediate grade metamorphism, but gross changes throughout a thick sequence of metamorphic rocks are extremely unlikely (Kragh & Davis 1973). Rb-Sr isochron ages of whole rock samples may record premetamorphic ages, especially in orlhogneisses (Lanphere et al. 1964). Large homogeneou~ samples representative of the minera- logical mode. or sample composites may help to minimize the metamorphic effect. The highest grade metamorphic rocks (i.e. granulite facies) often show a strong depletion of Rb relative to potassium (Heier 1964; Lambert & Heier 1968; Heier & Thoresen 1971) so K/Rb ratios may give a clue to migrations of Rb. Such depletions have not been observed for greenscl~ist and amphibolite grade meta- morphism.

The fractionation history for a metamorphic rock may be described by a two-stage model in which it is assumed that the rock crystallized at time r with initial 87Sr/S6Sr=l(z). Fractionation of Rb, 5r or a~Sr* may then have occurred during a later metamorphic event at Tu. Further, let the model age of the rock relative to I(Q be T,. Then the ratio G between the enrichraent factor for Rb/Sr and for radio- genic Sr at T u is given by

.f* e *r* - e ~'''u - 7", - T~

where

f, (8~Rb/a6Sr)~

The subscripts refer to stages 1 and 2, and so (STRb/S~Sr)2 = (STRb/S6Sr) . . . . . The fractional loss of radiogenic Sr, produced since time x, at time Tu is ( i - f * ) where f , = s ~ Sr* (TM) 2# 7 Sr* (TM) ~.

DePaolo & Wasserburg (1976a, b) have shown that throughout the Earth's history most continental igneous rocks have been derived from a mantle reservoir (called CHUR) with a chondritic SmlNd ratio. They also showed from a negative correlation between initial Sr and initial Nd that the STSr/S~Sr in this reservoir today is 0.7045. From this they calculated that the e 'gb/S°Sr ratio of this reservoir is 0.0836 and inferred that it is representative of the bulk earth today. The strontium isotope evolution of this reservoir (UR) through time is given by l u g ( T ) = = l u j ( O ) - ( a T R b / a 6 S r ) ° ~ (e ~ r - 1) where l w ( O )=0 .7045 and (aTRb/a6Sr)O =0.0836.

The concept of model age is very useful for discussing disturbed systems. Usually they are calculated relative to B A B I ( B A B l = 0 . 6 9 8 9 8 , Papanastassiou & Wasserburg 1969); TaAB! model ages give an upper limit to the last time the rock was an open system for Rb and Sr (Wasserburg & Papanastassiou 1976). If 7aA~!=4.6 AE (~ the age of the Earth) it suggests that the rock forming processes have n¢.t changed the Rb/Sr ratio in the material involved. If TsArs>4.6 AE then the time-integrated effect is to reduce the Rb/Sr ratio; conversely, if TRial<4.6 AE, the time- integrated effect is to increase the Rb/Sr ratio of the material involved. Thus TB~a~ is a measure of the time-integrated fractionation history of a rock after ~4.6 AE. Most initial Sr values for terrestrial rocks are, however, considerably enriched in a?sr relative to BABI , so the TBa~ ages will be close to the last time the rock was an open system only if this was accompanied by very large enrichments of Rb relative to St. In many cases it may therefore be useful to estimate the lowest plausible initial ratio on samples with very low Rb/Sr ratios and calculate a model age relative to this initial ratio or calculate the model age T~,'~ ra[ative to the bulk earth evolution line:

1 [- . (B~Sr/a6Sr) . . . . - l w ( O )

~--- L "[- . . . . . . I s [ -A In l (87Rb/s6Sr) . . . . _(S~Rb/8~Sr)Ok.]

These model ages may further constrain the upper limit for the last time of disturbance of the Rb-Sr system in a rock.

Discussion of analytical data Quartzo-feldspathic gneisses and related rocks

Quartzo-feldspathic gneisses are the most abundant rocks and together with their interlayered amphi- bolites and mica schists they represent the oldest part of the gneiss complex. The petrography of the samples is summarized in the Appendix. A nine- point total rock isochron was obtained on homo- geneous samples of enderbitic granolite from locality 5 (Table 2, Fig. 2) giving an age T= --1.58_0.05 AE and an initial a7Sr/a~Sr, I= =0.70236-T-14. Ar shown on the insert in Fig. 2, all of the nine data points are within error of the

LITHOS 11 (1978) Rb-Sr isotopes, Kongsberg 261

best fit line. This result will be used as a reference line in the tbllowing discussion.

These samples have higher K/Rb ratios than the 'Main Trend' of Shaw (1968) for rocks of the upper continental crust. However, they plot within the field of volcanic rocks from modern island-arcs of Jakes & White (1970) in a K-Rb diagram. There is thus no clearcut evidence for the Rb-depletion that has been observed in many other g~anulite facies rocks (Heier 1973). The large range of measured KfRb ratios (286 to 588) for a single outcrop gives evidence for redistribution of Rb between total rocks during metamorphism, sug- gesting that the isochron most likely dates the granulite facies metamorphism.

Samples from eight other localities were analysed:

(a) To get an indication of whether age and initial Sr reflect igneous or mete~morphic vahes. Samples were therefore selected from localities with different average Rb/Sr ratios. The mean values and ranges for K, Rb and Sr concentra- tions are given in Table 1.

(b) To see how homogeneous the quartzo-feld- spathic gneisses and related rocks are with respect to age ~nd initial Sr. Locations were chosen to cover a large area (24 km x 6 kin, Fig. 1) and the most abundant rock types.

The results (Tables 3 and 4) are described below. Five granulite facies samples from locality 6 are within error of the T= 1.58 AE reference isochron (Fig. 3). A best-fit line through these data points yields T= 1.55+_0.04 AE and I=0.70256~22. One sample (27) of charnoenderbitic granolit¢ has K/Rb= i 120 and K=1.57 wt.% which evidently must imply Rb-depletion during metamorphism. Still this data point is within error of the best-fit line,

I I i I I |

0.710 [- ENDERBITIC GRANOLITE /

0.708 T = 1.58 + - O.05/E £~'19

. ~ 0 23

/ 24 _ 0.1 0.2 0. 0.704 / +4 ~T'=0.05IE' T '

0. 702 - 2

87Rb186Sr

0. 700 I ~ i i i i ,_ 0.1 0.2 0.3

Fig, 2. Rb-Sr evolution diagram for the enderbitzc granolite from locality 5. The insert shows the deviations ~ (in parts in !0'*) of the measured S~Sr/S%r from the best-fit line. I A E = IAEON = 109 year~.

which suggest,,, that the timespan between M 1 and the igneous crystallization was small. The age and initial Sr for locations 5 and 6 are within error of each other. This is important since the average Rb/Sr ratios fo~" the two outcrops are, quite different.

Table 2. Analytical results for eaderbitic granolite samples from Loc. no. 5?

Sampleno. Rb(ppm) b Sr(ppm) c K(ppm) d K/Rb S~Rb/S6Sr STSr/S6gr

! 7 21.7 224 7600 350 0.28 ! + 6 0.70863 + 6 18 9.1 193 4900 538 0 , 1 3 6 + 6 0.7053~+6 19 19.9 240 5700 286 0 . 2 4 0 + 6 0.7076~, +4 20 17.9 196 6900 385 0 . 2 6 4 + 6 0.70840+6 21 11.2 233 4900 438 0 . 1 3 9 + 6 0.70543+6 22 25.5 236 9500 373 0.313 + 6 0.70927 +_4 23 9.3 217 4200 452 0.124 + 6 0,70503 + 8 24 8.0 250 4700 588 0.093 + 6 0.70447 + 4 25 16.6 234 6100 367 0 . 2 0 5 + 6 0.70694+_4

" The Universal Transverse Mercator (UTM) grid coordinates for this locali':, is 32VNM 462,301. These UTM gr[d lines are available on standard 1:5~100 topographic maps of this area. Uncertainties: b 0.5 ppm, = 4 ppm, a 100 ppm.

262 S. B. Jacobsen & K. S. Heier LITHOS 11 (1978)

Table 3. Analytical results for quartzo-feldspathic gneisses and amphibolites from various localities.

Sample Loc. UT M- no. no. coordinates" Rb(ppm) b Sr(ppm) ~ K(wt.%) d K/Rb S~Rb/S'SSr STSr/S6Sr T,(AE)' G r

i. Quartz-plagioelase gneiss 1 I 326,129 19.6 124 0.75 383 0.458 +9 0.713844.12 1.78 0.706 2 2 306,150 16.8 137 0.77 458 0.354+7 0.71077+6 1.69 0.813 3 3 329,151 15.6 175 0.28 179 0.258 4. 4 0.7,0708 :]: 6 1.30 2.40 4 3 329,151 2.2 241 0.11 500 0.026+3 0.7,03024-16 1.80 0.686 5 3 329,151 16.7 79 0.89 533 0.614+ 11 0.7191 4.4 1.94 0.571 6 3 329,151 4.1 141 0.31 756 0.084+2 0.7069+4 3.79 0.178 7 3 329,151 13.5 31 0.52 385 1.264-2 0.73134.3 1.63 0.906 8 4 350,142 47 163 2.00 426 0.831 4. 17 0.720344-8 1.54 1.09 9 4 350,142 22.3 115 0.95 426 0.5634-12 0.71478+8 1.57 1.02

10 4 350,142 37 167 !.05 284 0.641 + 13 0.715574-12 1.47 1.30 Ii 4 350,142 29.6 123 0.94 318 0.6984-16 0.717374-4 1.53 1.12

II. Pink granodioritic gneiss 12 4 350,142 55 135 2.31 420 1.18+2 0.72892+6 1.60 0.96 13 4 350,142 88 243 2.96 336 1.02 + I 0.72458 -I- 8 1.55 1.07 14 4 350,142 81 137 2.61 322 1.72+2 0.74225+6 1.65 0.873 15 4 350,142 76 139 2.53 333 i.59+2 0.73960+6 !.67 0.842 16 4 350,142 97 141 3.11 321 2.01 +2 0.7462+4 1.55 1.07

!I!. Enderbitic granolite 26 6 443,335 7.0 279 0.25 357 0.0725:6 0.70410+4 1.72 0.774

IV. Cha rnoenderbitic grano~ite 27 6 443,335 14.1 231 1.57 1113 0.176+6 0.70654.2 1.67 0.842 28 6 443,335 47 196 2.13 453 0.6985:12 0.71763+6 !.56 1.0~ 29 6 443,335 39 205 2.19 562 0.550+ 12 0.71~!40 + 8 1.56 1.04 30 6 443,335 48 209 2.09 435 0.663+ 12 0.71"4+ 3 1.61 0.941

V. Aml:~hibolite 31 4 350,142 23.5 253 0.48 204 0.269+5 0.706944- l0 1.21 4.36 32 ', 350,142 22.0 227 0.46 209 0.281 5:6 0.707555:14 1.32 z i g 33 4 350,142 29.5 250 0.42 1:42 0.3405:7 0.70959+14 1.51 1.17 34 4 350,142 44 31- 0.115 193 0.4025:8 0.71015+ 10 1.38 1.71 35 4 350,142 18.0 232 0.~17 253 0.231 5:6 0.70683+8 1.38 1.71

" The UTM grid coordinates are available on standard 1:50000 topographic maps of this are~t. All locations have grid zone designation 32V and 100 km square identification of ?qM. b ~ relative uncertainty for Rb and Sr is 2 to 4%. d relative uncertainty for K is I to 2?/o. " T, is the model age relative to an initial 8~Sr/a~:~r ratio of 0.70236.

2 = 9.0139AE- t r G = ( I . 5 8 - . I . I ) / ( T , - I . I ) .

The maximum dilference ir~ initial ratios between these two localities is dl<~ 0.00042 and A(a~Rb/a6Sr) = 0.36, so if initially they had exactly the same I and were rehomogenized locaUy during M 1 then the timespan between the igneous crystallization and M 1 is d T ~ - d l /U.ASTRb/S"Sr)~O.09 AE.

Total rock da~a fcom amphibolite facies localities (Loc. nos. l, Z 3, 4) in the quartzo-feldspathic gneisses are shown relative to the enderbitic grano- lite isochron in Fig. 3. All the amphibolite facies data together form a reasonable li~ear array that ¢orrespo~ds to a slope of ~ 1.6 AE and I~0.7025. However~ many of the points deviate significantly from the F= 1.58 AE reference isochron. Since M 1

and the time of igneous crystallization are essenti~dly indistinguishable the data may be interpreted with the two-stage model described earlier. It is well established that M2 in south Norway is at ~ 1.1 AE (O'Nions et al. 1969; O~Nions & Baadsgaard 1971). We use z = 1.58 AE for the original age, so it follows that G = (1.58 - 1. I)/(T, - 1.1) where 7", is the model age relative to the initial Sr for the enderbitic granolite (I=0.70236). Note that the following discussion would be essentially the same if instead of T, we used the TSo~ model ages since the/-value for the enderbitic granolite only deviates ~ 7 parts in 104 from the bulk earth evolution line. The devia- tions of all the amphibollite facies samples from the

LITHOS II (1978) Rb-Sr isotopes, Kongsberg 263

1,58 AE reference isochron are shown in Fig. 3 and T, and G are given in Table 3.

Some of the scatter of the data points may be explained in terms of mineralogy or chemistry of the samples. Samples 12 to ]6 show slight chloriti- zation of biotite. Epidotes often show large enrich- ments ofSTSr * (Wasserburg & Steiger 1967) and this may well explain why samples 5 and 6, which contain retrograde epidote, plot to the left of the reference isochron. Sample 3 plots to the right of the reference isochron and has a model age T, = 1.30 AE and an urmsually low K/Rb ratio of 179, indicating Rb gain during M2. The Rb/Sr ratios are uniform and low (,,~0.1) in the quartz-plagioclase gneisses which make up ~ 90% of the quartzo-feldspathic gneisses. The remaining 10% is essentially made up of pink granodioritic gneiss with a higher mean Rb/Sr ratio (~0.61). The K/Rb ratios show roughly the same range for both amphibolite facies and granulite fa~:ies samples from this are~. Sample 4 has a very low Rb/Sr ratio of 0.009 which is a ratio typical of mid-ocean ridge tholeiites (Hart 1971) or oceanic plagiogranite (Coleman & Peterman 1975). It has K/Rb=500 and Tsam=10.4+l .7 AE which re- quires either that this sample suffered Rb-depletion during the 1.58 AE event or that its source was depleted in Rb prior to this time.

We also an, lyzed samples from an 8 m thick ~4,.mphibolite layer situated between the granodioritic gneiss and the quartz-plagioclase gneiss in locality 4. The samples were collected in a profile from the con- tact between the amphibolite and the granodioritic gneiss towards the interior of the arnphibolite. The distances from the contact t:or samples 34, 33, 31, 32 and 35 are 0.5 m, 1 m, 2 m, 3 m, and 4 m, respectively.

From Table 3 it is clear that tTr~e Rb (19-44 ppm) and Sr (227-313 ppm) concentrations and the S7Rb/SeSr(0.23-0.40) and K/Rb (142-253) ratios change systematically from the core to the margin of the amphibolite. The very low K/Rb ratios in 33 and 34 are good indications that the margin of the amphibolite behaved as a sink for Rb during meta- morphism apparently because biotite amphibolite was produced by metamorphic reactions between granodioritic gneiss and amphibolite. A similar amphibolite was analyzed from another locality where the adjacent quartzo-feldspathic gneisses had much lower Rb-content (~ 10 ppm). This gave K/Rb = 800 giving further evidence for the addition of Rb during metamorphism. However, it cannot be determined from the concentration data alone whether these migrations occurred during M 1, M2, or both.

0.76[

0.75 [

• . ' " ' , ' , | , 1 I ' " ' ' 1

QUARTZO-FELDSPATHIC GNEISSES AND AMPHIBOLITES

+4

*2

I o

-2 0.74 I'i

-4

0.0

0.73 %

AT - 0.05/E

. . . . 1.0 l I i i I 1 ~

0. 72

31 3

Y° 1 0 28 ~F LOC. ~L). l 0 ) [] 4 Im 6

• 4

AMPH BOL HI: ;ACIES ~N[ i5S£S

GRANULIE FACIES GhEISSES AMPHI ;JOLITES

87Rb/86Sr o.7o

0.0 1.0 20

Fig. 3. Rb-Sr evolution diagram for quartzo-fcidspathic gneisses and their interlayered amphibolites. A referellce line with T= 1.58AE and 1=0.70236 corresponding to tl,e enderbitic granolite isochron in Fig. 2 is shown and the insert shows the deviations (in parts in 103) of the ~easurcd S?Sr/StSr from this line.

The isotopic data for the amphibolite samples are shown in Fig. 3 relative to the T= !.58 AE reference isochron. Only one point (33) is essentially within error of it ~.nd all points plot to the right, as would be expected if the samples gained Rb during metamorphism. If, prior to M2, all samples had the same Sr isotope composition, then we would have an isochron giving the time of Rb-addition. This is evidently not the case, so the samples must have a more complex history. The calculated G- values (Table 3) from core to margin are 1.71, 2.18, 4.36, 1.17 and 1.71. If we assume no preferential loss of S:Sr* then G =fsb-S, and increases, as expected, from the core towards the margin in the first three samples; but 1he G-value suddenly decreases in the two samples r, earest to the margin (33 and 34). This could be due ro gain of STSr* in these two samples. All mc,:Iel ages T, are lower than 1.58 AE; sample 31 has the lowest model age T, = 1.21 AE, which in- dicates that th,~ migrations must have occurred more

264 S. B. Jacobsen & K. S. Heier LITHOS It (1978)

1.20

1.10

1.00

0.90

0. 80

0.70

I I I

GRANITIC NEOSOME5 FROM THE QUARTZO- FELDSPATHIC GNEISSES

OLOC. NO. 7 A LOC. NO. 8

38

I

3(

T = 1 .58~

39 45

47

+12

+8

+4

go

-8

87Rb/865 r -12

0 10

. . . . 3 5 r"--

_.--___._t~_____..L 10 2O

| _ 1

2O

t

Fig. 4. Rb-Sr evolution diagram for granitic neosomes from the quarti, o-feldspathic gneisses (Loc. 7 & 8, Fig. I), The insert shows the deviations ~ tin parts in 102) of the measured HTRb/S~Sr from the 1.58AE reference line from Fig. 2.

recently than this time, i.e, most likely at M2. The isotopic data thus also indicate that the amphibolite has been a sink for Rb and that migrations of Rb occurred on a scale of ~ 5 m. The adjacent grano- dioritic gneiss is the m~,st likely source for Rb and this may be the reason that two of the data points from the granodioritic gneiss plot to the left of the 1.58 AE reference isochron.

From sample 35 we get an estimate of the lower limit for the initial Sr for the amphibolite layer I >f 0.7017 since it has probably gained Rb. An upper limit can be arrived at by calculating its STSr/S6Sr value at 1.1 AE, which is 0.7033. It is thus not possible to decide whether the amphibolite had a a~Sr/a~Sr ratio that was different from that in the adjacent quartzo-feldspathic gncisses al t.58 AE on the basis of these data.

Fine grained granitic dykes and veins occur in the quartzo-feldspathic gneisses and are interpreted as

migmatite neosome material (i.e. partial melts). These migmatitic rocks occur in a zone (mainly within granodioritic gneisses) or: both sides of L~igen around the city of Kongsberg. In some places agmatitic rocks have developed with a neosome of fine grained red granitic gneiss.

Twelve samples of neosomes from two localities (7 and 8) have been analyzed. All have much higher Rb/3r "atios than the quartzo-feldspathic gneisses. One sample of the paleosome at locality 7 has Rb/Sr=0.576, while a sample of paleosome from locality 8 has Rb/Sr = 0.541. The enrichment factor (Rb/Sr)~jco,omJ(Rb/Sr)v=l=~¢,= shows a variation from 1.8 to 12 in locality 7 while in locality 8 it shows a range from 1.9 to 3.3 and demonstrates that significant Rb/Sr fractionation occurred at the time of melting. Fig. 4 presents a Rb-Sr evolution dia- gram for all 12 analysed samples. Five points plot within error of the 1.58 AE reference isochron. The model ages all lie in the range 1.44 AE to 1.62 AE except for one point that givcs T,= 1.72 AE. Considering the large enrichment factor for Rb/Sr in the neosomes relative to the paleosome we conclude from these data that the partial melting event occurred at ,,, 1.5 to ~ 1.6 AE.

Small sulfide-rich zones in gneisses and schists occur throughout the investigated area. Just west of Kongsberg there are two major zones (up to 800 m wide) with mainly sulfide-rich schists and gneisses paralleling the north-south regional strike of the gneiss sequence. These zones are termed 'fahlbands' in the local nomenclature (Gammon 1966). We selected samples from metasedimentary, sulfide- poor mica schists within one of these zones (Loe. 9). In this zone r~he dolerite dykes are thoroughly altered to amphibolites and often to strongly foliated hornblende-andesine-ilmenite rocks. This indicates that the post-dolerite deformation had much more pelwasive effects here than in adjacent quartzo-feldspathic gneisses where relict igneous textures frequently occur in the dolerites.

It is evident that the data do not define a precise line in the Rb-Sr evolution diagram (Fig. 5). The schists occur xs a zone in the quartzo-feldspathic gneisses and raust be considered to be of the same age as these. :$ample 56 has the lowest model age T . = 1.21 +0.02 AE. Assuming that the initial Sr is well defineci this is a strict upper limit to the last time the Rb-S: system in 1Ehis outcrop was disturbed. This disturbance most likely reflects the 1.1 AE metamorphism (M2). We also infer that this event was responsible for the pervasive deformation in this schist zone. It is chmr from the way the data scatter in Fi~-;. 5 tha~ this :metamorphic event did not

LITHOS 11 (1978)

Table 4. Analytical results for granitic neosomes and mica schists.

Rb-Sr isotopes, Kongsberg 265

Sampleno. gb(ppm) b Sr~ppm) c K(wt.~/o) d K/Rb S T R b / S S S r STSr/S6Sr T.(AE)" G f

I. Granitic neosomes. Loe. no. 7. UTM'=356 ,167 36 153 22 4.74 310 21.3+0.2 1.16657___ 16 1.55 1.07 37 150 24 4.45 297 19.2 5:0.4 1.10370-t- 10 i .49 1.23 38 136 25 4.47 329 16.1 + 0.2 1.04190 + 8 i. 50 1.20 39 115 28 3.28 285 12.15:0.1 0.96839 5:8 !.56 1.04 40 79 61 3.28 415 3.77 + 4 0.78662 5: I 0 1.59 0.979 41 93 74 4.40 473 3.69 5:4 0.78662 5:10 1.62 0.923 42 89 84 3.79 426 3.09 5:6 0.77696 5:24 1.72 0.774

I1. Granitic neosomes. Loc. no. 8. UTM'--349,158 43 63 35 3.58 568 5.26+ 12 0.81811 + l 1.57 t.02 44 94 57 4.72 502 4.83 + 9 0.80175 +_. 10 i .47 1.30 45 97 80 6.11 630 3.56 5:7 0.77960_+ 6 1.54 1.09 46 103 90 4.37 424 3.33 + 7 0.76979 + 6 1.44 1.41 47 88 87 4.81 547 2.94 + 6 0.76294-t- 20 i .47 1.40

III. Mica schists. Loc. no. 9. U T M ' = 326,145 48 31 95 1.42 458 0.962 __. 17 0.72780 ___ 10 1.88 0.615 49 28 75 1.58 565 1.I 1 + 2 0.72970+ 10 1.75 {.I.738 50 36 58 1.80 500 1.80 + 3 0.74750 + 10 ! .78 0.706 51 57 68 2.43 426 2.41 _+ 2 0.74480_ 30 1.26 3.00 52 45 26 1.65 367 5.02 + 11 0.79050 5:50 1.25 3.20 53 51 27 2.38 467 5.55 _+ I l 0.80240+ t0 1.29 2.53 54 44 21 2.18 495 6.05 + 11 0.80940+ 10 1.26 3.00 55 41 19 2.10 512 6 .40+6 0.81180+20 1.22 4.00 56 57 25 2.35 412 6.61 + 6 0.81420+90 1.21 4.36 57 21 87 !.01 481 0.712+7 0.71720+80 1.48 1.26

For explanation see Table 3.

lead to a complete Sr isotope homogenization be- 0.85 tween total rocks within this outcrop. All the samples are bounded by two reference lines with T= 1.17 AE and T= 1.05 AE. In two cases, a pair of samples with both a high and a low Rb/Sr ratio were collected within one meter of each other (50- 53 and 49-54). The 50-53 tie line gives T= 0.80 =1.05+0.04 AE and I----0.721 T-1. The 49-54 tie line corresponds to an age of T= 1.15 + 0.04 AE and I=0.7112-T-6. The ages derived from these two tie lines are significantly different, demonstrating that complete Sr isotope homogenization did not even 0.75 occur at the scale of I meter during the 1. l AE event in this locality.

It is also evident from the data that this outcrop showed a range of STSr/S~Sr ratios of at least ~ 0.706 to ~0.721 at T= 1.1 AE. This is consistent with the average ~ 7Rb/86Sr ratio for the outcrop of 2.4 (Table 0. 70 1) which gives an average 87Sr/S6Sr of 0.718 at 1.1 AE assuming that the average S~Rbff6Sr was the same prior to the M2 metamorphism. Since there are samples that plot both to the left and right of the T= 1.58 ?,E reference isochron, it is not

[ 1 !

MICASCHIST

o~

I I I

Y / 51

T " 1.58/E

T ~- 1.05/E 52

T = l . 17/E

J57 87Rb/86S r I I I I I l

0 2,0 4.0 6.0

Fig. 5. Rb-Sr evolution diagram lbr mica schist samples from locality 9 (Fig. I). Several reference lines are shown. The T = 1.58AE reference line corresponds to the enderbitic granolite isoc hron.

266 S. B. Jacobsen & K. S. Heier LITtlOS it (1978)

0.710

0.708

0.706

0.704

0.702

I I t I I I DIOR ITIC GNEISS

1 52-+o o5, j

+4 0 , 0;1 , O.j2

I 0 0.1 0.2

Fig. 6. Rb-Sr evolution diagram for the dioritic gneisses. The insert shows the deviations .~ (in parts in 10') of the measured SrSr/a6Sr from the best-fit line.

necessary to invoke any net gain or loss of Rb and/or Sr isotopes from the total outcrop sampled. The M2 metamorphism merely resulted in a redistribution of Rb and/or Sr isotopes between total rocks with- out complete Sr isotope homogenization.

Despite the scat',er ~f the data points about the T= ! .58 AE reference isochror~ for both the quartzo- feldspathic gneisses and the granitic neosomes, these rocks have similar age and initial Sr ratio over wide areas. There seems to be little justification to suggest any real age differences between the various localities of quartzo-feldspathic gneisses investi- gated in this study. The scatter about the T= !.58 AE reference isochron within a single outcrop may be just as great as on a regional scale. This is most likely due to migrations of Rb and Sr isotopes during M 2 without accompanying Sr isotope homogeniza- tion on a total rock scale.

The average Rb/Sr ratios for the individual out- crops of the quartzo-feldspathic gneisses show quite a large range (0.067-0.614). Thus, if local homogenization of Sr isotopes occt~rred on an out- crop scale during M l we should expect a large range of initial Sr values unless the timespan between the

igneous crystallization of the rocks and the meta- morphism were short (~< 0.1 AE). Since the initial Sr value appears to be the same for all localities, we conclude that the result obtained on locality 5 of T=1.58-1-0.05 AE and I=0.70236-T-14 is within error of the igneous values, although redistribution of Rb and/or Sr isotopes may have occurred at M 1.

The result for the granitic neosomes also shows that the timespan between the deposition and metamorphism and migmatization of the protoliths of the quartzo-feldspathic gneisses was ~<0.1 AE. Interlayered mica schists, however, have been affected more by M2 than the quartzo-feldspathic gneisses. Although they were evidently not homo- genized with respect to Sr isotopes during this event the data for the mica schists strongly suggest a metamorphic event at ~ 1.1 AE (M2) which is consistent with ocher evidence. The M2 meta- morphism did not disturb the Rb-Sr system in the granulite facies rocks significantly since all data points are within error of the T= 1.58 AE reference isochron. However, all of the amphibolite facies localities studied show evidence of migration of Rb and/or Sr isotopes. This is probably due to the difference in the amount of H20 available in the amphibolite relative to the granulite facies rocks durint, M2.

Gneisses with an age of ~ 1.6 AE are known from several other places in the Precambrian of south Norway. A Rb-Sr whole rock isochron on the Levang Gneiss Dome in the Bamble sector gives T= 1.(:i2+0.08 AE and I=0.701-T- 1 (O'Nions & Baadsgaard 1971). U-Pb data on zircons from granitic gneisses in south Rogaland and Vest Agder give a discordia with an upper intersection at 1.49 AE 0:'asteels & Michot 1975); however, the data points fall along a curved line and fit a model with 1.6 AE as a primary age of the zircc~ns and episodic loss at ,,, 1.1 AE and 0.0 AE. Rb-Sr data on acid metavolcanics of the Rjukan group :in the Telemark area do not form any precise linear arrays (Priem et al. 1973) but give model ages T ~ in the ran[:e -,, 1.0 AE to 1.6 AE. The ~ 1.6 AE e Jent tiros seems to be represented in most parts of the Precambrian of south Norway.

Basic intrusive rocks

Gabbroic to dioritic intrusives that have been meta- morphosed to dioritic (hornblende+andesirLe) gneisses occur frequently within the investigat,:xl area. Similar rocks also oo;ur within the granui~tf~ facies area as pyroclase granolite. Chemically these intrusives are calc-alkaline (Jacobsen 1975). Sb;

LITHOS II (1978)

Table 5. Analytical re:mRs for basic intrusive rocks.

Rb-Sr isotopes, Kongsberg 267

Sample Loc. UTM- no. no. coordinates" Rb(ppm) b Sr(ppm) ~ K(wt.~/o) d K/Rb aTRb/aSSr a~Sr/arSr

I. Dioritic gneiss 4001 13 306,156 5.4 523 0.19 352 0.030i + 18 0.70314__.6 4092 13 306,156 6.0 521 0.22 367 0.0330 + 18 0.70323 +_ 4 4003 6 443,335 5.7 406 0.32 561 9.04 i + 6 0.70322 + 6 4004 6 443,335 6.8 35 ! 0.57 838 0.056 + 6 0.70370 +_ 6 4005 8 348,161 30 534 1.03 343 9.16.5-+_ 3 0.70588 + 6 4006 8 348,161 26 384 0.81 312 0.194_+4 0.70667+ 12

II. Doicritcs 2005 6 443,335 7.0 220 0.27 386 0.092 _+ 2 0.70423 + 6 2225 20 397,099 15. I 387 0.40 265 0. ! 13 + 2 0.70374_+ 8

Ill. Gabbros 2070 19 430,240 8.8 230 0.33 375 0.1 ! l +_ 2 0.70411 _+_6 2409 14 305,378 i 3.9 252 O.a9 374 O. 150 +_ 3 0.70537 _+ 8 2606 ! 5 285,335 16.0 275 0.57 360 0.169 + 3 0.70526 + 20 2608 16 306,315 14.2 331 0.35 250 0. i 24 + 2 0.70472 + 8 2609 16 306,315 19.2 325 0.43 220 0.171 +2 0.70528 -+,~ 2610 17 287,289 3.8 216 0.16 420 0.051 _+4 0.70324___6 2611 ! 8 283,275 20.8 254 0.52 250 0.237 + 4 0.70635 _+ 4

For explanation see Table 3.

samples have been analyzed from widely separated localities (Fig. 6). Since they all appear to be of intrusive origin and have similar geochemical characteristics (which are different from other basic rocks in the area) the samples are considered cogenetic. The data define a precise linear array in the Rb-Sr evolution diagram (Fig. 6) and the best-fit line gives T=1.52+0.05 AE and I=0.70249T-6. The samples 4001 and 4002 are plagioclase-rich (70% Anso-60) gabbroic cumulates where most of the pyroxenes have been replaced by hornb|ende but the igneous texture is still partly preserved. They have model ages TrAm of 9.3+0.7 AE and 8.7+0.6 AE, respectively, and this is consistent with the interpretation of these samples as plagioclase cumulates. 4003 and 4004 from the granulite facies aree: are also gabbroie samples with partly preserved igneous textures but mostly recrystallized to grano- blastic aggregate~s of -~ 15% orthopyroxene, ~ 10% clinopyroxene and ~ 60% plagioclase (An3s). Sub- solidus reactions also produc.~i garnet-clinopy- roxene-quartz sympleetites at orthopyroxene-pla- gioclase grain boundarie~ reflecting cooling in a dry environment deep in the crust (Griffin & Heier 1973). These samples also have model ages TB~t (7.1+!.2 and 5.8_+0.8 AE respec- tively) higher than the age of the Earth, but it is not clear from the textures that these samples are plagioclase cumulates, so this is probably due to Rb-depletion during the granulite facies metamor-

phism. Finally 4C~)5 and 4006 have completely metamorphic fabrics with ,-,50% plagioclase (An3o), ~ 50% hornblende and minor amounts of biotite, sphene and ore. These saraples are from a gabbroic to dioritic intrusion which shows igneous textures in some places, especially in its plagioclase- rich portions. These gabbroic to dioritic intrusives seem, on structural grounds, ~o have intruded during M 1. Thus even if M ! caused migrations of Rb and Sr in these rocks this would not have affected the isochron, which is therefore interpreted to give the time of intrusion. The good fit of these data to a line shows that the effect of the M2 meta- morphism on these samples must be insignificant.

Hyporite is used by Scandinavian geologists as a name for basic plutonic rocks in the Kongsberg and Bamble sectors and for similar rocks in south- eastern Norway and southwestern Sweden (Barth & Reitan 1963). Hyperites occur as dykes and intrusive bodies throughout the investigated area and have been termed the Vinor diabase by Bugge (1917). They are composed of rocks which include gabbro, olivine gabbro, hypersthene gabbro (norite), and amphibolite derived from these rocks. The large bodies of gabbro consist of a core ofcoronite gabbro and a margin metamorphosed to amphibolite. Be- tween these two extreme ~ypes, all alteration stages seem to occur.

The hyperites in many places cut across the early regional foliation and gneiss banding in the are;: but

268 S. B. Jacobsen & K. S. Heier LITHOS 11 (1978)

0.706

O. I04

0. 702

I I i ' l " ~ I / I •

o GABBRO, VINOREN , , / m GABBRO, JUNGEREN : .~ ,1~ .= . - ,

_ A DOLER ITE / ~',xT 1.t'O-0.05~ 2409 / I = 0. 70240 7,. 12

0 ,~"2609

2608 / ~06 ~ ' / 0 0.1 0.2 0.3

2005/x / i . , i i 2o70 +8 (>

87Rb1865 r ~ i i I !

I I _ _ _ _ 1 l I ,

0 O. 10 O. 20 O. 30

Fig. 7. Rb-Sr evolution diagram for Hyperites (Loc. 14 to 20, Fig. i). The insert shows deviations

(in parts in 104) of the measured STSr/S6Sr from the best-fit line.

~hey have also to, a large degree been deformed and metamorphosed ~o amphibolite subsequent to their intrusion. They thus appear to have intruded between two major episodes of metamorphism and deformation and are believed to mark tl~e begin- nings of the ~1.1 AE metamorphism (Starmer 1972). They intrude the quartzo-feldspathic gneisses, the granitic gneisses and the dioritic gneisses and are mostly older than the Helgevannet granite (Kayode 1974; Jacobsen 1975). From data presented later in this paper these relations show that they intruded mainly between ,,-1.20 and --- 1.37 AE. Some of the dykes appear to post-date the Helgevannet granite but all seem to be older than the Meheia granite (Kayode 1974) which will be shown to be ,~ 1.07 AE old.

Six samples from non-metamorphic interior parts of the large gabbro at Vinoren (Fig. 1) have been analyzed. Four of them define a good linear array (Fig. 7) with a slope corresponding to T= 1.20-1- 0.05 AE and an intercept I=0.70240-T-12. Two other points deviate significantly from the best-fit line. This gabbro is in many places net-veined with granitic material which probably formed by partial melting of the granodioritic gneisses surrounding the gabbro at the time of its intrusion. The two samples that deviate from the best-fit line lie on a m~xing line between the Rb and Sr isotopic composi- tion of the granodioritic gneiss at !.2 AE and an isotopic composition for the magma of STSr/a6Sr = =0.70240 and STRb/S6Sr =0,093 +0.010 at 1.2 AE. Only a few percent contamination with this gtano-

dioritic material is enough to explain the deviation of these two points from the best-fit line to the re- maining four points. The evidence for contamina- tion makes the age determined on the four colinear points uncertain. All six samples ofthe Vinor gabbro have ortho- to meso-cumulate textures (cumulus phases: ~-,60% plagioclase (An so-6o), ~15% oli- vine, intercumulus phases: ~ 10% clinopyroxene, ~5% orthopyroxene, ~5% opaques and ,-, 1-3% biotite) with thin (<0.5 mm) coronas as the only evidence of subsolidus reactions. This shows that diffusion of ions subsequent to the time of intnJsion was very limited. Total rocks have probably there- fore been closed systems since the time of intrusion and thus the obsdrved variation in S7Sr/SeSr at 1.2 AE is most likely due to contamination. One sample (2610) hits a model age TB,tst = 5.8 4:0.6 AE, which is greater than the age of the Earth, and is probably a plagioclase-enriched Cumulate.

Two samples (2005 and 2225) wit's strongly zoned plagioclase, olivine and pyroxenes and a subophitic texture were selected from the interior parts of doledte dykes. Assuming that they have an age similar to the Vinor gabbro (T= 1.20+0.05 AE). they then have initial Sr values of I=0.70268T-16 and I = 0.70184 -T-19 respectively. A sample (2070) ofa gabbro from Jungeren (Fig. 1) with a subophitic texture and strongly zoned plagioclase and pyrox- enes has I=0.70224~ 17 assuming T= 1 20_+0.05 AE. Thi,~ sample and the two dolerite samples all have 8~l;,b/S6Sr ratios of -,,0.1, wMch is probably close to the ratio in the melt they crystallized from.

LITHOS 11 (1978) Rb-Sr isotopes, Kongsberg 269

The Vinor gabbro which exhibits cumulate textures, shows a much larger range of S~Rb/86Sr ratios from ~0.05 to ~0.24. Although the precise age of these intrusive rocks is not known the initial Sr values are very well determined because of the low Rb/Sr ratios in these rocks. From arguments presented above the age is within the range 1.07 to 1.37 AE. This gives a range of/-values from 0.7018 to 0.7027 for the hyperit~.

Granitic gneisses and granites

The granitic gr~eisses at Kongsberg and Vatnaas are intrusive into the quartzo-feldspathic gueisses and the interlayered amphibolites. They are red and medium grained and always show complete meta- morphic textures with ~ 30% quartz, ~ 30% micro- cline, ~30% p.1ag~odase, ~5% amphibole and minor amounts of biotite, garnet and ore. The average Rb, Sr and K concentrations and Rb/Sr and K/Rb ratios are fairly similar in both of the analyzed granitic gneisses {Table 1). The mean K/Rb ratio in both (~ 500) is fairly similar to that observed in the surrounding quartzo-feldspathic gneisses. It is distinctly higher than the main trend of Shaw (1968) for upper crustal materials and this suggests that these rocks have been dedved by partial melting of a Rb-depleted source or have lost Rb during meta- morphism.

Nine samples from the body ofgranitic gneiss just NE of the town of Kongsberg lie on a straight line with a best-fit slope corresponding to an age of T= !.56+0.04 AE and I=0.7014-T- 18. Six samples from the body of granitic gneiss at Vatnaas have also been analyzed (Fig. 8, Table 6). Five of the data points fit perfectly to a line with. a slope corresponding to an age of T = ! . 3 7 + +_ 0.02 AE and an intercept ! = 0.70643 -T- 48. Sample 103 plots significantly to the let~: of this best-fit line, probably due to loss of Rb or gain of STSr* during the M2 metamorphism. The age and initial Sr is clearly significantly different from that obtained for the body just NE of Kongsberg.

The quartzo-feldspathic gneisses are intruded in the west by the Helgevannet and Meheia granites (Fig. 1). Both granites are gaeissic but relict igneous textures occur. The Meheia granite is a light pink medium-grained rock with taint or distinct foliation and intrudes the Helgevannet granite. It has ~ 35% quartz, -,, 30% K-feldspar, -,, 30% plagioclase and minor amounts of biotite. The Helgevannet granite is ~. pink or gr~y, medium- to coarse-grained rock, usually with distinct gneissic texture and ~30% qt~artz, ~ 30% K-feldspar, ~ 30% plagioclase and

. G R A N I T I C GNEISSES ,'~ KONGSBERG

1.00 ~, • VATNAAS / .

,.~ T = 1.56 +- O.04/E o ~ I = 0.7014~ 18

i 0.78 106

0.90 - 10 0.7 -1.37-'002R

4 2 3 15 J~./,/ 0 2 4 6

1~4 ~ 0 0.80 -4

lo2 { ii 87R#s, -8 0. 70 ~-

0 5 l0 1~

Fig. 8, Rb-Sr evolution diagram for the granitic gneisses at Kongsberg (Loc. 10, Fig. I )and Vataaas (Loc. I1 & 12, Fig. l). The insert shows the deviations ~ (in parts in 10 2) of the measured S~gb/"6Sr from the best-fit line.

~-5% biotite. The granites are intensly sheared in many places and are locally transformed 1o myl- oblastites. These granites have higher mean Rb concentrations and Rb/Sr ratios and lower K/Rb ratios (close to Shaw's 'Main Trend') than other rocks in this area (TaMe 1). This is most likely due to these rocks having suffered extensive fractional crystallization involving plagioclase.

Five samples of the Helgevannet granite from Jondalen define a perfect linear array; a best-fit line gives T = 1.20+0.02 AE and I=0.708-T-2 (Fig. 9) The samples from Jondalen were taken ~800 m apart along a 4 km profile through the western part of this granite. The Rb and Sr concentrations and the Rb/Sr ratio change systematically along this profile. The s ~Rb/S6Sr ratio is ~ 4.6 near the contact to the Telemark supracrusta]s in the west and in- creases gradually to ,,, 17 7 in the core ofthe granite. This variation is most likely due to fractional crystallization from the margin towards the core of this body. Only mixing of Sr isotopes on the scale of the whole body could have reset the S~Sr/S6Sr ratio to a single value during a metamorphic event. As this seems to be unlikely (Krogh & Davis .1973) we

270 S. B. Jacobsen & K. S. Heier

Table 6. Analytical results tbr granitic gneisses and gneissic granites.

LITHOS 11 (1978)

Sample Loc. UTM- no. no. coordinates" Rb(ppm) b Sr(ppm) ~ K(wt.%) e K/Rb B~Rb/s6sr STSr/86Sr

I. Medium-grained red 101 12 102 II 103 I1 104 12 105 11 106 12 107 10 108 10 109 10 110 10 i l l l0 112 10 113 l0 114 10 115 10

granitic gneiss 381 431 32 362 437 63 362 437 75 381 431 83 368 434 94 387 426 92 367 194 60 367 194 96 367 194 75 367 194 75 367,194 76 367,194 80 367,194 89 367,194 91 367,194 95

152 2.10 656 0.610+-10 0.71800+-20 147 2.65 421 1.25+_ 1 0.73076+_20 88 4.44 592 2 . 4 7 + _ 3 0.75790+_20 84 4.30 518 2 . 8 5 + _ 3 0.76075+_20 32 4.28 455 8.70+_ 12 0.87475+_20 24 3.40 370 ll.4+_0.1 0.92544+_20 77 4.43 738 2.26+_4 0.7527+_20

II1 3.60 375 2.51 +_5 0.75526+_ 14 83 4.14 552 2.58+_5 0.75740+_ 10 85 4.12 549 2.58+_5 0.75798+_ 12 84 4.07 536 2.62+_5 0.75813 +_ 12 83 4.19 524 2.82+_6 0.76412+_ 10 82 4.11 462 3.18+_6 0.77028 +_ 16 51 4.42 486 5.23+_ 10 0.81411 +_58 49 4.59 483 5.69 ± 11 0.82647 ±86

II. Coarse to medium-grained gneis~icgranite 1250 21 199,212 183 117 1252 21 217,201 191 85 1253 21 227,195 235 47 1254 21 234,192 249 46 1256 21 242,190 260 44

I11. Medium to fine-grained gneissic granite 1002 22 281,119 178 53 1004 22 282,120 209 51 1005 22 278,122 116 67 1006 22 279,123 221 33 1008 22 278,12~ 206 40 1009 22 278,125 242 23

4.24 232 4.55 +6 0.78490+ 10 3.90 204 6.61 + 12 0.81860+_ 12 3.97 169 14.8 +0.3 0.96031 +- 12 3.81 153 15.9 +- 0.3 0.97785 +- 14 4.15 160 17.7 +- 0.4 1.00236 +_ 16

4.00 225 9.87 +_ 12 0.8638 +_ 4 4.18 200 1 2 . 0 + _ 0 . 1 0.8934___5 4.18 360 5.02 +_ 5 0.7897 +_ 6 4.49 203 20. I +_ 0.2 1.0157 +_ 7 4.42 215 1 5 . 4 _ _ . 0 . 5 0.9474__.2 4.50 186 32.4+-0.9 !.1912+-B

For explanation see Table 3.

interpret the resuk ~.s giving the age and initial Sr of the igneous crystallization of this granite.

Si~ samples of the Meheia granite spaced over an area of ~ 1.5 ~ 0.5 km give an isochron (Fig. 9) corresponding to T= 1.07 + 0.01 AE and I= 0.715 -T- ~2. The 87Rb/S~Sr ratio within a single outcrop varies only by a few percent while the total variation within the sampled area is from ~ 5.0 to ~ 32, and on a more region al scale the variation is from ~ 4 to ~ 48. Thus, the isochron obtained for the Meheia granite also probably reflects the age and initial Sr of the original magma. Because both the Helge- vannet and the l~4eheia granites are cross-cutting batholiths and have a metamorphic overprint it is concluded that the Helgevannet body intruded early or slightly prior to, and the Meheia body later during the M2 metamorphic event at ~ ~.1 AE.

Discussion of the variation of initial Sr isotope ratios with time The initial STSr/S6Sr ratios for the various rocks are plotted in Fig. 10 and the data are summarized in Table 7. The post-crystallization s 7Sr/S6Sr evolution for most rock types is indicated by the arrows on Fig. 10. The slopes of these lines are given by the average S TRb/S6Sr ratio of the respective rock types. How well defined these are may be judged from Table 1. The/-values given in Table 7 and other referenced /-values have been standardized to 0.71014 for the NBS standard SRM-987. Initial Sr for a rock which crystallized at time Tis also giwm in Table 7 as the fractional deviation in parts in 104. from the STSr/S6Sr in the bulk earth at time 7". This quantity is termed e Sr = ((lsocK(7)/Iua(7))- 1) × 104 and is plotted versus time in Fig. 11. Assuming that the earth formed with S~Sr/SOSr-= BABI at T0 =L}.6 AE we can calculate the STRb/S6Sr of the source (Table 7) for a single stage evolution up to the time

LITHOS 11 (1978) Rb-Sr is6topes, Kongsberg 271

1.2 MEHEIA & HELGEVANNET GR AN I TES

0 Meheia [] Helgevannet

1.1

1.0

0.9

0.8

0.7

O. 718

0.716

0.714

- 0.0ZE \ I = 0 . 7 0 8 ~ 0 . 7 1 7

T = 1.07-+ 0.01~ I - 0. 715 T- 0. 002. 0.110

[006

1253 O. 108 0 lO 20 30

+4 ' " ' ' O. ?06 1004 +2

0 -2 .L~ 0. 704

• - 4 I -4

+ 4 f .--a , ' t 0.702 r loo5 +2 j.,

40.,00 87Rb/86Sr -4

l~.O 20.0 30.0

Fig. 9. Rb-Sr evolution diagram for the Helgevannet and Meheia granites (Loc. 21 & 22, Fig. I). The insert shows the deviations ~ (in parts in 102) of the measured STRb/S6Sr from the best-fit lines.

of crystallization of the rock (:/;,) from (87 Rb/8 SOsouRc = (l(T.0 - B,4 m)/(e o For a more complex evolution this is still the time- averaged 87Rb/SeSr for the source material.

It is at present not clear when the heterogeneities in the mantle originated but two well-studied Archean rocks indicate that 87',~r/S6Sr evolution in the mantle was close to the bulk earth evolution line early in the Earth's history. These rocks are: (1) A basaltic komatiite from the Barberton area (Jahn & Shih 1974) and (2) clinopyroxenes from mafic to ultramafic rocks in the Albiti belt, Superior Provino:, Canada (Hart & Brooks 1977)o It is thus possible that all mantle sources for magmatic rocks had the bulk earth Rb/Sr ratio early in the Pre- cambrian and became fractionated later tt, gl~ e the large range in initial Sr values observed in young volcanic rocks. Such a fractionation event is con- strained by the initial ratio of a mantle derived rock and its time of crystallization according to the equation ( F - I)A T= ( IaocK( Tx) - Ius( T~) )/ 2(S T Rb/

! I I I !

fl

~' II MEHEI A GR AN I TE

| I I I

HELGEVANNET~ GRANITE~

GV

~ G G V

GGK.., T Age (/E)

I I I I I I

0.8 1.0 1.2 1.4

QFG

I i

1.6 1.8

Fig. 10. initial STSr/S%r versus age diagram for the rocLs in the Kongsberg area. The slopes of the arrows are given by the average Rb/Sr ratios of the various rock types ('fable 7). The range of different evolution lines for the various petrographic types of the quartzo-feldspathic gneis- ses are also indicated. QFG =quartzo-feldspathic gneisses. DG = dioi~tic gneisses. GGK = granitic gneiss, Kongsberg. GGV=granitic gneiss, Vatnaas. GV=gabbro, Vinoren. For all rocks we show 2~r error ellipses whose size and shape depend on the isochron parameters and the S~Rb/S6Sr of the centroid of the isochron. The point taken to represent the quartzo-feldspathic gneisses in thi~ diagram is the result obtained on the enderbitic granolites "tom locality 5.

S6Sr)[,a where F=(Rb/Sr)sovece/(Rb/Sr)ur. and Tx + A Tis the time the source wa.~ fractionated from the bulk earth evolution line. This information is also listed in Table 7.

Finally we give the fractionation factor for eL rock relative to its source, f--(Rb/Sr)eccK/(Rb/Sr).~owc~ (Table 7) which is calculated by dividing the STRb/S6Sr in column two of this table with tha~ of column five. The f-value for a rock will clearly depend on fractionation during partial melting of the source, fractional crystallization of the magma and possible redistribution of Rb and Sr during metamorphism. With the exception of the gabbro sample fro~l Jungeren and the two dolerite we use average values of S7Rb/s6Sr fo: a large number of

272 S. B. Jacobsen & K. S. Heier

Table 7. Rb-Sr evolutionary parameters.

LITHOS 11 (1978)

Rock type Age(AE) (8"tRb/~6Sl')~ocK l~ocx es, (a'Rb/SSSr)sovac~ (F-I)A T(AE) f

Quartzo-feldspathicgneisses 1.58+0.05 0.70213-T-14 - 7 . 3 + ! . 2 0.072+2 -0 .44+0.07 Enderbitic granolite 0.19 2.6 Quartz-plagioclase gneiss 0.29 4.0 Charnoenderbitic grz~nolit¢ 0.69 9.6 Pink granodioritic g~,eiss 1.78 25

Dioritic gneiss 1.52_+0.05 0.085 0.70226T-6 -6 .5+0 .4 0.0734+2 -0.39-+0.03 1.2 Hyperites

Gabbro, Vinoren 1.20+0.05 0.154 0.70217-T-12-13.1+0.8 0.065-+15 -0.81-t-0.06 2.4 Gabbro, Jungeren 1.22+0.15 0 . 1 1 1 0.70198-T-33 -15 .6+2.2 0.061___4 -0.94-+0.13 1.8 Dolerite 2005 1.22_+0.15 0.092 0.70242:1:29 -9 .3+1 .6 0.070+3 -0 .56+0.10 1.3 Dolerite 2225 1.22+0.15 0.113 0.70157T-36-21.2+2.5 0.053+5 -1.29_+0.16 2.1

Granitic rocks Graniticgneiss, Kongsberg 1.56+0.04 2.8 0.7012-T-18 -20.9_+25.0 0.05+4 - 1 . 3 ± 1 . 5 56 Graniticgneiss~Vatnaas 1.37+0.02 2.4 0.70620-T-48 +47.1+6.5 0.15±1 +2.8-+0.4 16 Helgevannet gra~i;.~. 1.20+0.02 6.8 0.708T-2 +70+__28 0.18-+4 +4 .2± !.7 37 Meheia granite 1.07+0.01 13.8 0.715-T-2 +167+28 0.31+4 +10.1_+1.7 44

Mean values from Tables I and 5. b All initial Sr isotope values have been standardized to the NBS certified value of 0.71014 for SRM-987.

samples to calculate the fractionation factorf. This should tend to minimize the effect of fractional crystallization and of redistribution of Rb and Sr during metamorphism, such that the f-value given in Table 7 should ~rnainly reflect fractionation of a magma relative to its source.

The quartzo-feldspathic gneisses, the dioritic gneiss, and the hyperites all have. esr<0 which implies that they have been derived from mantle reservoirs with time-average Rb/Sr ratios lower than that for the bulk earth. The quartzo-feldspathic gneisses and their interlayered amphibolites have high Na/K ratio,,; and follow a tholeiitic trend in the Na20 + K 2 0 - FeO - MgO diagram, and show depletion in Rb, Zr, Nb, LREE, Th and U (Jacobsen 1975).

The e, sr = for these rocks of - 7.3 requires that the source had a Rb/Sr ratio different from that in the bulk earth for at least 0.44 AE. The fractionation factorfvaries from -,- 2.6 to 25 and shows that these rocks were enriched in Rb relative to Sr at 1.58 AE. The result for the dioritic gneiss is within error of that for the quartzo-feldspathic gneisses except for the average f-value of 1.2 which indicates that the magma from which these rocks crystallized was essentially not fractionated relative to its source; bu~ the large range in Rb/Sr ratios of the individual samples (Table 5) shows clearly that these are both enriched and depleted relative to their source. The -,, 1.2 AE old hyperites show es~ values in the range - 9 to - 21 and this implies that their sources had a Rb/Sr ratio lower than the bulk earth ratio tbr at least -,-0.5 to 1.3 AE.

Geologic relationships in the Precambrian of south Norway (Barth & Reitan 1963) give us a clue to tl:.~e structure of the crust in this region. The regional variations in lithologies and metamorphism have generally been explained in terms of a vertically layered crust, where subsequent erosion has caused deeper levels to be exposed towards the coast (Weg- mann 1960; Barth & Dons 1960; Barth & Reitan 1963; Smithson 1965). Geophysical and geechemical studies show that the lower continental crust in shield areas most likely consists of medium to high pressure granulite facies rocks (Heier 1974). Low to intermediate pressure granulite facies rocks occur in ~the Bamble and Kongsberg sectors, which have been uplifted relative to the Telemark granite gneiss along late-Precambrian cataclastic zones (Touret 196~), and in the Rogaland area, together with anorthosites and mangerites, and apparently form the transition to the deep crust. The lower part of the upper crust is made up of the amphibolite facies supracrustal rocks and migmatites in the same areas and the Telema~.'k granite gneiss. These rocks again underlie the generally more marie supracrustal rocks exposed in the central Telemark area. These upper crustal rocks are frequently penetrated by the shallow level postkinematic granites while these are absent in areas with granulite facies rocks. The crust is relatively heterogeneous at all exposed levels and complicated isotope systemat~cs may be expected if granites formed by remelting 0,fthis crustal m~.terial.

The initial STSr/S6Sr data for the synkincmatic granitic rocks are shown in Fig. 10 and "Fig. ! 1. The ~s~ values vary from ~ 0 tc ~ 170. A straight line

LITHOS 11 0978) Rb-Sr isotopes, Kongsberg 273

evolution curve can be drawn throv.gh the four data points, which indicates that these rocks could all have been derived from the same reservoir at diffe- rent times. The slope of this line corresponds to 87Rb/86Sr ratio o f ~ 1.5 for this reservoir. The average 87Rb/StSr ratios of all these granitic rocks are higher than this, which is consistent with enrich- ment of Rb relative to Sr during formation of the magmas. The trend in initial Sr versus time for these granitic rocks intersects the mantle growth curves at ~ 1.5-1.6 AE, which may be the time of formation of the source reservoir for these rocks. The high 87Rb/86Sr strongly suggests that the source was a part of the continental crust.

There is thus no direct evidence for crust signifi- cantly older than ,,-1.6 AE at depth in the in- vestigated area. However, if the trend in initial ratio is only due to chance then we cannot rule out that these granites formed by remelting of crust much older than !.6 AE, except for the granitic gneiss at Kongsberg. If, for example, the source of the Meheia granite had the average upper crustal 87Rb/86Sr ratio of 0.73 (Hurley et al. 1962), then F= 8.7, and from the ( F - I),4 T value in Table 7 a reservoir with this ratio would intersect the bulk-earth evolution line at ~2.4 AE. This then could be interpreted as the time of formation of the crustal source for these rocks. Since the inter.~tion of the trend in initial Sr ratios for these granites with the bulk earth evolu- tion line corresponds closely to the age of the oldest known rocks in the area it suggests that those rocks were the source of the granites. An S~Rb/86Sr ratio of 1.5 is high compared to that of most of the rock types within the quartzo-feldspathic gneisses (Table 7) and fits best with the data for the pink grano- dioritic gneiss. This suggests that the most granitic components have been preferentially melted out. If this is the case, then the Sr isotopes in these rocks could not have equilibrated with the surrounding rocks having lower BTRb]86Sr ratios. Metasedi- mentary gneiss sequences, which are generally richer in potassium feldspar and mica, are known at Modum (Jzsang 1966) further north in the Kongs- berg sector. Such rocks may also be possible sources for these granites.

The data for the granitic rocks suggest that ,,, 1.6 AE old crust in south Norway repeatedly acted as a so arce for granitic magmas over a timespan of ~0.5 AE. Although we have investigated only a small portion of this area we infer by comparison with other geologic data (e.g. Barth & Reitan 1963) that this is representative of large areas of the Precambrian of south Norway. The Telemark graniite gneiss covers an area 18,000 km 2 (Barth &

18 - Lthos 4]78

+180

+150

+120

+90

+60

+30

-3C

q ! OUARTZO-

\ FELDSPAT~IC GNEISSE5 I~slc

F ~ J L 0 iNTRUSiVE

N [Jii\

I I I I I I I s - ' ; ~ ; . ' ~ 2 I

1.0 1.2 1.4 1.6 T(/E)

I I I ! I

~ POSTKINEMATIC GRANITES

~ SYNKINEMATIC GRANITES

Fig. 11. Fractional deviations in parts in 104 of initial S~Sr/StSr from the bulk earth evolution line of DcPaolo & Wasserburg 0976). The sources of the data for the post- kinematic granites are given in the text.

Reitan 1963) and thus makes up a sigmficant pro- portion of the crust in this area.

The large areas of granitic rocks in the Pre- cambrian of south Norway are highly silicic (70- 75% SiO2) and potassic (4-5% KzO), and are not associated with the more basic members of tile calc- alkaline series. In our opinion they would l:e diffi- cult to explain as anything else than products of large-scale rcmelting of continental crust. Other cases where it can be argued from Sr isotope data that granites have been produced by melting of significantly older crust are also known (Fullagar & Odom 1973; Brewer & Lippholt 1974, Vitrac & Allegre 1975; Davies & Allsopp 1976) and th,s indicates that large scale remelting of coatinental crust is not only restricted to the PrecambLian of south Norway.

The continental cru~ has apparently grown through time mainly by additions from (1) island arc volcanism (.lakes & White 1971), (2) vok:anism and plutonism in continental magmatic arcs along continental margins, or (3) continental volcano.- plutonic arcs in intra-continental orogem¢ chains. Which mechanism ha~ been the most important

274 S. B. Jacobsen & K. S. Heier

Appendix. Petrography of the quartzo-feldspathie gneisses and related rocks.

LITHOS i I (1978)

Locality no. and size of sampled ' a rea Rock type Sample no. Mineralogy

1. 2. 3. 20 mx40 m

4a. 5mxl0m 4b. i0mxt0m

20 m E of 4a 4c. 4mx2m

between 4a & 4b

5. 20 m x 20 m 6. 20 m x 50 m

7. 5mx5m 8. 5mx5m 9. 20mxl0m

Quartz-plagioclase gneiss Quartz-plagioclase gneiss Quartz-plagioclase gneiss

Pink granodioritic gneiss Quartz-plagioclase gneiss

I 2 3 4 5 6 7

12 to 16 8to l l

Amphibolite 32 ~.nd 35

Bi-amphibolite 31, 33 and 34 Enderbitic granolite 17 to 25 Charnoenderbitic granolite 27 to 30 Enderbitic granolite 26 Granitic neosome 36 to ~t2 Granitic neosome 43 to 47 Mica schist 52 to 56

57 48 to 50 51

40%Q, 50%Pl(Anto), 10%Hb 40%Q, 50%Pl(Aato), 10%Hb 35%Q, 55%Pl(An3o), 10%Hb 40%Q, 60%Pl(Anlo) 30%Q, 55%Pl(Anas), 10%Bi, 5%Ep 40%Q, 40%Pl(An3o), 10%Hb, 10%Ep 40%Q, 50%Pl(Anlo), 10%Ga 30%Q, 45%Pl(An3o), 20%Ksp, 5%Bi 25%Q, 35%Pl(An35), 10%Bi, 10%Hb, 20%Ga

45%PI(An3s), 50%Hb, 5%Ore

45%PI(An35), 50%Hb, 5%Bi, 2%Ore 30%Q, 50%Pl(An4a), 5%Ga, 10%Opx, 3%Hb, 2%Bi 20%Q, 15%Ksp, 45%Pl(An3a), 5%Cpx, 1%Opt. 5%Bi, 10%Hb 40%Q, 50%Pl(An4o), 5%Opx, 5%Ga 30%Q, 35%Ksp, 30%Pl(Anlo), 3%Ga, Bi or Mu 30%Q, 35%Ksp, 30%Pl(Anto ), 3%Ga, Bi or Mu 50%Q, 10%Pl(An3o), 20%Mu, 10%Bi, 10%Ga, 3%St 50%Q, 30%P1, 5%Mu, 3%Bi, 10%Ga, 3%St 50%Q, 25%PI, 15%Mu, 5%Bi, 10%Ga, 3%St 35ToQ. 25%P1, 10%Mu, 20%Bi, 10%Ga, 3%St

Abbreviations: PI = plagioclase, Q,-- quartz, Ga = garneL Opx = orthopyroxene, Cpx = clinopyroxene, Ksp = K-feldspar, Bi = biotite, Hb = hornblende, Ore = iron ore minerals, Ep = epidote, Mu = muscovite, St = staurolite.

through time is at present ,difficult to tell. However, to be able to explain the concentrations of K, Rb, Ba and REE in average upper crustal rocks Jakes & White (1971) have to invoke later remelting on a large scale of the island-arc crust. The data presented here indicate that such large-scale remelting may occur up to at least 0.5 AE after the initial formation of crust in this particular area.

Data for the post-kinematic granites in south Norway are also plotted in Fig. 11. Geographically these are divided into two groups. For the western group ~,,ge and Sr isotope data exist for the Grimstad, Herefoss, Vr~dal, Bessefjell and Fyresdal granites (Sy!vester 1964; Venugopal 1970; Brueckner 1972; Priem et al. i 973; Killeen & Heier 1975). These data give the following range in initial Sr and age for this g~:oup: / '=0.90 to 0.97 AE and , s , = - 6 to +51. T~e eastern group consists of the Heddal, A~dal, lddefiord and Bohus granites. Data from Killeen & Hcier (1975) and Ski61d (1976) give T=0.;~7 to 0.94 AE and es,= +79 to +93.

Smithson (1965) suggested that the post-kine- matic granites were diapirs from the underlying Telemark granitic gneisses and migmatites. This is clearly wrong in light of Sr isotope data, since at the time of crystallizatign of these granites the Telemark

granitic gneisses would have evolved to S7Sr/S6Sr = =0.720 to 0.734 and the source of the Telemark granitic gneisses would have evolved to ~0.717. The post-kinematic granites could however have formed from a mixture of mantle-derived and crustal-derived material. Some of the granites in the western group are within error of the bulk earth evolution line and could represent granites derived by differentiati,m from mantle-derived magmas without involving mixing with older crust. If we assume that these granites were derived by melting 1.6 AE crustal rc,cks then for the western group this source m~terial would have to have STRb/S6Sr ratios in the range ~ 0.08 to 0.5, similar to the range in granu!ite facies rocks and quartz-pl~gioclase gneisses. For the eastern group the source material would have an STRb/a6Sr ratio of ~0.78, similar to the average for K-feldspar-bearing granulite facies rocks (i.e. charnoeuderbitic granolite).

In conclusion, the lower to middle part of the crust in the Precambrian of south Norway is made up of high grade supracrustal rocks, incl~di~ag rocks of island-arc affinity that were separated from their mantle sources ,~ 1 6 AE ago. These rocI:s were re- melted to form the Telemark granitic gnei,,~ses and migmatites of the lower part of the upper crust;

LITHOS I 1 (1978) R b - S r isotopes, Kongsberg 275

mainly in the time interval ~ 1.1 to 1.2 AE during the so-called Sveconorwegian orogeny. This orog- eny was mainly a reworking of older !ithologies; the only identified juvenile additions during and slightly prior to this time are the hyperites. The upper part of the crust is made up of continental volcanics and shallow-water sediments that predate this er~geny. At ~ !.0 AE the lower crust was intruded by magmas that differentiated to form the anorthosites and mangerites now exposed in the granulite facies gneisses of the Egersund area (Pasteels & Michot 1975). The upper crust was penetrated by post- kinematic, diapiric granites of ~0 .9 -0 .96 AE age that cannot have formed from the same source as the synkinematic granites. However, older crustal rocks evidently also played a role in the genesis of most of these rocks.

Acknowledgements. - About half of the Sr isotope determina- tions were made in the Department of Geology and Mineralogy, University of Oxford. We thank N. H. Gale end S. Moorbath of the'Age and Isotope Group' at the University of Oxford for generously giving us the use of their laboratory facilities and for assisting us in the work, and E. A. Vincent for the use of general facilities in the Department of Geology and Mineralogy. We also wish to thank B. Jensen for her assistance with the XRF analyses and P. N. Taylor for assistance with the mass spectrometry and helpful di~us- sions while the work was carried out in Oxford and Oslo. We are grateful to D. J. DePaolo, A. J. Gancarz, W. L. Griffin, M. T. McCulloch, J. Vizgirda and G. J. Wasserburg for their comments on the manuscript. Publication No. 126 m the Norwegian Geotraverse project.

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Accepted for publication December 1977 Printed October 1978