RbSr total rock isotope studies on Precambrian charnockitic gneisses from South Norway: evidence for isochron resetting during a low-grade metamorphic-deformational event

Earth and Planetary Science Letters, 45 (1979) 3 2 - 4 4 © Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m - Printed in The Netherlands

32

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Rb-Sr TOTAL ROCK ISOTOPE STUDIES ON PRECAMBRIAN CHARNOCKITIC GNEISSES FROM SOUTH NORWAY EVIDENCE FOR ISOCHRON RESETTING DURING A LOW-GRADE METAMORPHIC-DEFORMATIONAL EVENT DENNIS FIELD

Department of Geology, UntverslO, of Nottingham, Nottmgham {fingland) and ARNE RAHEIM

Mmeraloglsk Oeologtsk Museum, Sirs Gate 1, Oslo (Norway)

Received March 9, 1979 Revised version received June 1, 1979

The quartzo-feldspathlc charnockltlC orthognelsses within the Bamble sector of the so-called SveconorwegIan (1 2 - 0 9 b y ) zone are highly fractlonated in K and Rb such that they comprise two chemically contrasting zones one highly K, Rb-deflclent and the other with values of the same order as tipper crustal hthologles Eight series of samples, each collected from single outcrops, have yielded Rb-Sr total rock apparent ages in two distinct groups, at - 1 5 4 0 and - 1 0 6 0 nl y Outcrops in both the K-deficient and normal-K suites have produced examples ol each age The older age relates to the high-grade charnocklte event, and the younger to a superimposed low-grade event which occurred at the same time as the intrusion of u n d e f o r m e d granite sheets and pegmatlte dikes, one of the granites has yielded an lsochron age of 1063 -+ 20 m y The low-grade event Involved only the partial alteratlon of o r t h o p y r o x e n e s to chlorite and/or serpentine, coupled with some corrosion of biotite, the alterations were initiated along narrow, irregularly spaced, cracks and it was their development which faclhtated open system behavlour of the total rock isotopic systems at some locahtles The d~gree o! rehomogenlsatlon IS a function of the intensity ol the secondary alterations Confirmation of resetting at - 1 0 6 0 m y is given by four mineral + host rock lsochrons all yielding ages within error of the age for the intrusive granite, tx~o of these are from outcrops where the rocks retain the older ~ 1 5 4 0 - m y age The secondary total rock isotopic homogenxsatlon cannot be explained adequately by Rb mobility or by simple rnlxmg with a fluid having ItS own initial 87Sr/86Sr composition The primary mineralogy m a y have determined whethei indlvldtlal locahtles and/or samples sulfered net increases or net decreases in 87Sr/86Sr An i m p o r t a n t implication of the results is that in this, or any slmdar geological situatmn, there would be a very real posslbihty of drawing erroneous conclusions from regionally-collected samples, particularly if the full significance of the later, relatively minor P-T event r c m m n e d undetected and/or the scale of isotopic (re-)homogemsatlon, were u n k n o w n It is only because of the m e t h o d s adopted that it can be stated that there is no isotopic evidence for a high grade Sveconorwegian (Grenvflllan) event in this part of ; o u t h e r n Norway

1 Introduction

S v e c o n o r w e g l a n (1 2 - - 0 9 m y ) z o n e o f s o u t h e r n Norway

This paper reports on the Rb-Sr isotope systematlcs m a high-grade quartzo-feldspathlc

charnockltlc gneiss

sequence from the Bamble sector of the so-called

These hlgh-Fe (rapaklvl-type) acid-Inter-

m e d i a t e o r t h o g n e l s s e s ( m e a n S102 ~ 6 8 % ) a r e h i g h l y f r a c t l o n e d in K a n d R b s u c h t h a t t h e y c o m p r i s e t w o chemically contrastmg zones - one highly deficient

33 in K and Rb with average values of K20 ~ 0 5%, Rb ~ 8 ppm, Rb/Sr - 0 05, K/Rb ~ 1300 (n = 78) [1 ] and the other with more "normal" upper crustal values (K20 ~ 4 0%, Rb ~ 150 ppm, Rb/Sr ~ 1 2 and K/Rb ~ 290 (n = 40, Field, unpubhshed data) This extreme fractlonatlon has previously been ascribed to a regional metasomatasm related to granullte facies metamorphism [2], but subsequent modelling of the K, Rb (and REE, Ba, St) data has shown that the LIL element dlStrlbuuons are consistent with, and better explained by, an essentially primary fractlonatmn involving the separatmn of cumulus (K-deficient) phases from magma which was emplaced directly under high-grade conditions, leaving a residual hquld from which the normal-K charnockltes crystalhsed [3] The present study was mmated in an attempt to estabhsh the Rb-Sr isotope systematlcs relating to the high-grade event and its associated K-Rb fractionatlon We began by analysing samples collected on a regional scale for the earlier geochemical programme The results were inconclusive, mainly because of scatter on the Rb-Sr evolution diagrams Both the K-deficient and normal-K groups were similarly affected and no systematic differences could be Identified At that stage we were unable to identify with any certainty either the causes or the timing of the apparent disturbance The latter could not be explained by the imposition of any later high-grade event, because the only identifiable secondary effects are related to a relatively minor, low-grade deformation In the light of these data our investigation continued with the added objectives of (a) testing for possible isotopic homogenlsatmn on a smaller scale and (b) identifying the mechanism responsible for the apparent disturbance We recollected suites from both groups of gnelsses from individual small outcrops, at the same time carefully monitoring the nature and intensity of secondary mineralogical alterations

2 Geological setting and

petrography

Using K-Ar mineral data, the Baltic Shield was subdivided by Kratz et al [4] into three zones Saamo-Karehan (3 6 - 1 9 b y ), Svecofennlan (2 3 1 6 b y ) and Sveconorweglan (1 2 - 0 9 b y ) According to this scheme, the Bamble and Kongsberg

sectors of southern Norway (Fig 1) are placed within the Sveconorweglan zone, which has long been correlated with the Grenville Province of North America [5,6] In a later K-Ar study, O'Nlons et al [7] placed the thermal maximum of the Sveconorweglan event at ca 1100 m y , referring to this as the "mare (high-grade) metamorphic episode", corresponding to the "Sveconorweglan Regeneration Period" of Magnusson [8] Subsequent Rb-Sr total rock data from Bamble [9] and Kongsberg [10j both confirmed the ca 1 1b y event and, although inconclusive, also provided the first radlomemc evidence for an earlier Svecofennlan metamorphic (and possibly gneiss-forming) event at ~1 7 - 1 5 b y This earher event has recently been confirmed In Kongsberg, where a nine-point lsochron on "enderbmc granohtes" has yielded 1 58 -+ 005by wlthlR =070236-+14[11] The Bamble and Kongsberg sectors have long been recogmsed to share slmflarmes m their petrological and structural evoluuon [12,13], and there have been

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Fig 1 GeoloDcal setting and samphng localities The Bamble, Kongsberg, Rogaland and Telemark regions comprise the socalled Sveconorweglanzone of southern Norway

34 two recent attempts towards estabhshlng a detailed correlation of their tectonic hlstones [ 11,14] Both areas consist largely of steeply dipping, intermediateacid gnelsses associated with supracrustals and both gabbrolc (hyperite) and granitic intrusions Cor&erlte is a common constituent of the supracrustals, and sdhmanlte ts the stable alummo-slhcate Although dominated by upper amphlbolate facies assemblages, intermediate-pressure granuhtes are developed in both sectors The largest outcrop of granuhtes occurs in the environs of Arendal, within Bamble (Fig 1), and it is the charnockItlC orthognmsses from this region which are the subject of the present study For ease of reference, a zonal scheme ( A - D ) has been adopted across the amphlbohte-granuhte facies transition zone (Fig 1) Zone A is amphibohte facies, and zones B, C and D are all in granullte facies, as dehneated by a well-defined orthopyroxene isograd m metabasites [2] The degree of dehydration gradually increases from this lsograd, across strike, towards the Skagerrak coast In zone B, the quartzo-feldspathlc gnelsses exhibit a totally hydrous (biotite + hornblende) mineralogy, whilst in zones C and D they are charnockltlc, containing orthopyroxene (+-hornblende, biotite, chnopyroxene) Through zone C these, too, become progressively less hydrous, with biotite and hornblende often completely absent m zone D Where pyroxene(s) occur together with hornblende -+ biotite, there IS no evidence, textural or otherwise, to suggest that the

coexisting anhydrous and hydrous phases are in disequilibrium - they developed during the same highgrade event The only ldentxfiable secondary effects are spatially and temporally related to a later series of subvertical, Intrusive and undeformed, granite sheets (~<30 m) and associated pegmatate dikes (~<2 m) Imme&ately adjacent to these, the typical dark green colouratlon is removed to give pink (zone C) or grey (zone D) gnelsses We refer to these as decoloured gnelsses The extent of the decolouratlon varies between ca 1 and 15 m There is no new penetrative fabric developed, only minor, irregularly-spaced, cracks (~<1 ram), now infilled with chlorite In these rocks, orthopyroxenes are pseudomorphed by mixtures dominated by serpentine, chlorite or actinohtac amphibole, primary amphibole is sometmaes altered to chlorite and/or actmohte, occasionally with carbonate, and there is some secondary biotite growth The secondary alterations w~than the charnockltes from outside the decoloured zones are more hmlted both in nature and extent there xs only pamal, and often variable, alteration of orthopyroxene to chlorite and/ or serpentine (no amphibole), primary biotite IS usually corroded along grain edges The feldspars have remained largely unaffected except for incipient turbidity within plagxoclases adjacent to the cracks m the more altered samples (Fig 2) These photomicrographs (Fig 2) demonstrate (a) the causal relation-

Fig 2 The causal relationship between chlorite-refilled cracks and secondary alteration of orthopyroxene (a) One of the leastaltered samples (locahty 12) - recipient alteration only where the crack crosses orthopyroxene (sample 12 17, X20) (b) A more intensively altered sample (locality 10), yet with relatively fresh orthopyroxene (bottom right) persisting within 2 mm of the widest crack (sample 10 12, × 12) The larger cracks are interconnected by a myrxad of fine velnlets and there is local, lnclpxent alteration of feldspar to an unidentified, free-grained, turbid product

35 ship between the secondary alterations and the development of the cracks and (b) the differences an intensity of the alterations between adjacent outcrops, one of which contains a late~ Intrusive granate sheet

3 Samphng and analytical techniques Eight seraes of charnockltac rocks have been collected from fresh road-cuts Each seraes represents collectaon at a separate location, and these are shown in Fag 1 together wath their identifying numbers At all but one of the sates, the samples were collected over a dastance m the range 10 50 m, the exception is site 11, where the dastance as 130 m In all cases the samples weaghed approxamately 2 kg, of whxch half was prepared for chemical and isotopic analysas The selectaon of samplang localltaes was governed by the attempt to obtain suates of charnockatac rock whach were the most hkely to reveal Rb-Sr isotope systematlcs relating to the high-grade P-T event and the assocaated K-Rb fractaonataon Sates 10, 11 and 12 are within the K-deficient zone (D) and the remainder from the "normal-K" zone (C) Only those outcrops were used where apparently fresh material could be collected In pamcular, heavily joanted and sheared zones were avoided At those localltaes where the antruslve granite shects/pegmatate dakes outcropped, care was taken not to collect an, or adjacent to, the decoloured zones One of the antrusave granite sheets was sampled at localaty 10 (seraes 10B) The rocks were crushed m a steel-jaw crusher and finely ground an an agate Tema )roll For those samples with Rb > 30 ppm, Rb/Sr rataos were determaned darectly by a precase X-ray fluorescence method [15] For these samples, unspaked measurements of 8~Sr/ 86Sr were made Rubldaum, Sr contents of the minerals and rocks wath Rb < 30 ppm were determined by 1sotope dilution using a mbxed 87Rb/84Sr spike Varmble mass discrimination an 87Sr/86Sr was corrected by normahslng 88Sr/86Sr to 8 3752 Mass spectrometry was performed on a Mlcromass MS30 using procedures slmalar to those described elsewhere [15,16] The 87Rb decay constant used is 1 42 X 10 -11 yr -1, and the data have been regressed by the technique of York [17] In assagnang errors to the data points the coefficient of variation is taken as 1% for 87Rb/86Sr, the errors for the aVsr/a6Sr measure-

ments are given in Table 1 Age and intercept errors are quoted at the 2o confidence level

4 Results The Rb, Sr data are given in Table 1 and the total rock lsochron diagrams illustrated an Figs 3 (the charnocklte samples) and 4A (the gramte sheet) Data are presented for all samples analysed asotoplcally The following age calculations have been made Senes 2 These samples were collected from wathm 40 m, at the southern end o f a 100-m road cut None was taken from the northern zone because of a large number of late shears, sample 2 1 is from w~thln 1 m of the most southern shear At the southern end of the collectaon sate there is a 5-m zone of patchy, mcomplete, decolourataon effects, sample 2 15 as the closest, at 1 m dastance Orthopyroxene alterataon effects are relatively manor, but variable even within single thin sectaons The 14 poants define a lanear array with a best-fit regression yielding 1443 + 104 m y , with I R = 0 70956 + 0 00578 The mean square of wexghted deviates (MSWD) as 8 71, because this value is hagher than the ~2 5 upper limit for an acceptable lsochron relataonshap [ 18] the scatter about the regression slope cannot be assagned to experimental error alone and there as some geological disturbance present If, for geologxcal reasons, samples 2 1, 2 15 (see above) and 2 4 (near joint surfaces, slaghtly weathered) are excluded, the values become 1509 + 79 m y wath I R = 0 70539 + 0 00456 and MSWD = 3 60 The mineral (K-feldspar, biotite, plagaoclase) + rock data for sample 2 18 give 1043 + 48 m y with an MSWD of 5 95 Isotopic eqmhbrmm was probably not attaaned between the rock and its constatuent minerals The manerals alone gave an age of 1047 +- 20 m y (MSWD = 0 54) Series 3 This outcrop as 21 m long and the samples all come from within the northern 16 m Apart from 3 2 and 3 3, they are all from wathm a 6-m zone This outcrop contaans no evidence for the pegmatate antruslon/decolouratlon event and there as only recipIent mineral alterataon The rocks gave an asochron age of 1535 + 123 m y , with I R = 0 70349 + 0 00928 (MSWD = 1 41)

36 TABLE 1 Rock and mineral analytacal data Sample 1 2 3 4 8 10 ll 12 13 14 15 17 18 18 PL a 18 BI a 18 KF a 19

Rb b

Sr b

87Rb/86Sr c

87Sr/86Sr d

142 3 158 5 146 6 133 3 143 6 130 6 1730 1544 138 2 169 2 173 6 1585 173 2 122 70 891 04 389 66 191 3

120 6 114 0 144 9 135 1 130 6 120 0 1005 764 104 4 112 8 104 2 117 1 105 7 107 42 9 91 217 91 98 6

3 4386 4 0629 2 9396 2 8948 3 2039 3 1665 50320 59189 3 8699 4 3792 4 8402 39467 4 7846 3 3313 420 8809 5 2300 5 6880

0 78217 -+ 0 00004 0 79482_+ 0 00022 0 76795-+ 0 00014 0 77290_+ 0 00012 0 77521 -+ 0 00016 0 77570 ± 0 00012 081111+- 0 0 0 0 1 4 083499± 000018 0 78987 _+ 0 00018 0 80048-+ 0 00020 0 8 0 5 0 7 -+ 0 0 0 0 1 8 0 7 8 8 3 6 -+ 0 0 0 0 2 0 0 80991 -+ 0 00008 0 79079-+ 0 00014 7 02845 -+ 0 00140 0 81987 -+ 0 00004 0 82828-+ 0 00008

5 4465 5 5349 4 9692 4 5960 46512 5 1910 6 0452 5 5277

0 82381 -+ 0 00006 0 82669-+ 0 00014 0 81218-+ 0 00010 0 80355 -+ 0 00006 080750-+ 0 0 0 0 1 2 0 81800-+ 0 00018 0 83457 -+ 0 00020 0 82629-+ 0 00014

Series 2

2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2

Series 3

32 3 3 35 36 37 3 8 3 11 3 12

182 6 184 4 190 8 184 4 1869 181 0 211 1 193 9

98 1 97 4 112 4 116 7 1180 102 l 102 5 102 6

Series 4A

4 4 4 4 4 4 4 4

2 4 4 PL a 4 BI a 4 KF a 5 7 8

149 176 9 642 455 182 180 194

100 103 68 19 202 110 100 104

Series 6

6 6 6 6 6 6 6

1 6 8 10 17 18 19

79 9 85 6 83 9 86 4 931 102 1 949

Series 8

8 8 8 8 8 8 8 8

2 3 8 10 11 12 15 18

67 64 76 69 75 62 78 76

0 7 19 00 96 5 7 0

9 0 1 5 7 0 7 8

1 0 29 79 41 1 9 l

4 5 0 109 6 4 5 5

3407 0076 3902 1011 5928 8391 2334 4425

0 0 0 2 0 0 0 0

79268-+ 0 80146 -+ 0 73443-+ 0 36800 -+ 0 82656 -+ 0 79956 +- 0 80673 -+ 0 80928 -+ 0

00026 00026 00018 00014 00014 00016 00008 00006

167 4 160 0 155 5 169 4 1564 160 7 161 8

1 3836 1 5510 1 5644 1 4777 1 7260 1 8421 1 7010

0 72899+- 0 00020 0 73142 -+ 0 00020 0 73178-+ 0 00016 0 73022-+ 0 00020 0 7 3 3 7 6 -+ 0 0 0 0 1 8 0 73582-+ 0 00010 073346 + - 000012

197 196 206 200 213 197 212 198

0 9992 0 9485 1 0701 1 0019 1 0261 0 9120 1 0757 1 1241

0 0 0 0 0 0 0 0

6 4 8 8 7 0 0 0

72261 -+ 0 72141 -+ 0 72427 -+ 0 72269-+ 0 72297 -+ 0 72134-+ 0 72502-+ 0 72501 -+ 0

00020 00006 00018 00022 00020 00010 00018 00020

37 TABLE 1 (continued)

Series 10A

Sample

Rb b

Sr b

10 2 10 6 107 10 7 PL a 10 7 BI a 10 7 KF a

12 32 21 5 474 83

20 3 50 01 87 70

183 112 145 204 9 251

41 6 21 26 36 13 9 10

5 16 44 07 7 96 16 97

10 10 10 10 10 10 10 10

9 11 12 13 15 17 23 24

Series 11

11 1 11 6 11 12 11 22 l l 24 1125 11 30

Series 12

12 12 12 12 12 12 12 12 12 12 12

Series 10B

10 31 10 32 1033 10 34 10 36 10 38 10 40 1041

a b c d

1 2 3 8 11 12 17 19 19 PL a 19 BI a 21

87Rb/86Sr c

87Sr/86Sr d

39 6 84 43 03 86

0 1925 0 8278 04278 0 0710 197 0548 0 9627

0 71179-+ 0 00010 0 72402 _+0 00006 0 7 1 3 0 6 -+ 0 0 0 0 1 6 0 70831 -+ 0 00016 3 58494 -+ 0 00088 0 72185 -+ 0 00020

148 139 70 72 67 71 70 152

2 06 99 65 9 00 84 10

0 8123 0 1281 0 8739 1 0361 l 5773 0 5695 0 3305 0 2088

0 0 0 0 0 0 0 0

4 47 1 73 2 68 7 86 17 62 12 16 4 39

142 115 119 129 112 135 207

79 95 43 43 01 07 97

0 0904 0 0431 0 0649 0 1758 0 4554 02605 0 0610

0 70517-+ 0 00008 0 70662-+ 0 00016 0 70613 -+ 0 00010 0 70840-+ 0 00018 0 71423 ± 0 00014 0711025 000020 0 70687 -+ 0 00020

10 97 4 37 5 54 2 01 10 11 0 83 1 36 17 17 6 12 669 60 6 75

127 206 224 189 161 205 202 66 109 9 180

92 12 18 55 66 61 12 15 27 79 49

76 8 46 5 417 103 0 131 7 107 9 146 0 121 5

82 4 69 7 733 79 0 84 4 88 2 89 2 88 8

0 0 0 0 0 0 0 0 0 277 0

2482 0613 0715 0306 1808 0116 0195 7519 1621 8722 10811

2 7066 1 9262 16616 3 7833 4 5242 3 5611 4 7698 3 9734

0 0 0 0 0 0 0 0 0 4 0

71845 -+ 0 70956-+ 0 72040-+ 0 72327 + 0 73242-+ 0 71729-+ 0 71087-+ 0 70972-+ 0

70878 -+ 0 70473 -+ 0 70536-+ 0 70402-+ 0 70720 ± 0 70384 -+ 0 70411 -+ 0 72126 -+ 0 71224 ± 0 84320-+ 0 70607 -+ 0

00016 00010 00010 00008 00012 00014 00014 00008

00012 00016 00016 00014 00006 00014 00020 00036 00006 00448 00020

0 75693 -+ 0 00012 0 74478 ± 0 00016 0 7 4 1 1 0 -+ 0 0 0 0 1 6 0 77414 -+ 0 00016 0 78426 -+ 0 00024 0 76988-+ 0 00014 0 78807 -+ 0 00030 0 77561 -+ 0 00012

PL = plagloclase, BI = biotite, KF = K-feldspar Rocks with Rb < 30 ppm, and all minerals, by ISotope dilution, others by XRF Precise to ± 1% Preclslons quoted as 20 standard error of mean

Series 4A T h e s e s a m p l e s are f r o m a n e w 1 0 - m c u t

n o u n c e d T h e 5 p o i n t s y i e l d a g o o d - f i t age o f 1075 +

o f w h i c h a p p r o x i m a t e l y h a l f is c o m p o s e d o f d e c o l o u r -

122 m y ( M S W D = 1 10) A m i n e r a l ( K - f e l d s p a r ,

ed r o c k s a d j a c e n t to a d i s c o r d a n t 3 0 - c m - w l d e p e g m a -

b i o t i t e , plagloclase) + t o t a l r o c k r e g r e s s i o n for

tlte dike T h e s e c o n d a r y m i n e r a l a l t e r a t i o n s are pro-

s a m p l e 4 4 g w e s 1 0 3 6 + 20 m y , t h e M S W D o f

38

SERIES

SERIES

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SERIES IOA

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57 R b/56 Sr Fig 3 Total rock Rb-Sr nsochron diagrams for eight series of charnockltlC gnelsses

2 69 is close to the limit of reasonable fit When the minerals are regressed together with all the charnockmc rocks from this locality, and 8-point asochron age of 1035 + 9 m y , with I R = 0 72862 and MSWD = 1 19 is obtained

Senes 6 Locality 6 is a 60-m road-cut, adjacent to a zone of extensive decolouratlon The secondary mineral alterations are pronounced The samples give a good-fit age of 1023 -+ 90 m y , w i t h I R = 0 70870 -+ 0 00206 (MSWD = 0 40)

39 Senes 8 Locality 8 1s a 40-m (N-S) road-cut At the extreme northern end, there IS a 2-m decoloured zone adjacent to a 1-m pegmatlte dike The eight samples were collected from a 30-m zone at the southern end, all exhibit the secondary mineral alterations Because the samples fall within a narrow range of STRb/s6Sr, and there IS accompanying scatter, no best-fit line has been applied to the points Series 10.4 The eleven samples were taken from within 45 m of each other There IS continuous outcrop into a 5-m-wide decoloured zone caused by a 25-m-wide granite sheet Samples from this form Series 10B (see later) The intensity of the secondary alteration effects varies from sample to sample One example is illustrated In Fig 2 Regression gives 1162 -+ 214 m y , but with a bad-fit (MSWD = 250) If sample 10 6 is excluded, an apparent age of 1065 -+ 191 m y is obtained, but with only marginally better fit (MSWD = 193) When the minerals (K-feldspar, plagloclase, biotite) of sample 10 7 are included, the data yield 1125 + 150 m y with 1 R = 0 70720 + 0 00096 (MSWD = 198) Regression of only the minerals and their host rock gives 1034 + 59 m y , but an MSWD of 12 74 indicates that isotopic equilibrium was not fully attained between the rock and three of ItS constituent minerals The minerals alone give a value of 1041 +- 28 m y (MSWD = 2 70) Sertes 11 These samples were collected over the greatest distance of all localities (130 m), and there are both granite sheets and pegmatlte dikes, with accompanying decolouratlon, within the outcrop Some samples have suffered extensive secondary alteration, whilst others have remained relatively unaltered Because of scatter, no regression data are presented Senes 12 This outcrop (40 m) IS the only one besides locality 3 to show no evidence for the granite sheet/ pegmatlte dlke/decolouratlon event, and there is only Incipient alteratxon of the pyroxenes (Fag 2) The nine samples define an apparent age of 1537 + 118 m y with I R = 0 70344-+ 0 00026 An MSWD of 12 13 indicates that there is some geological disturbance present The minerals (plagaoclase, biotite) of sample 12 19 (K-feldspar-free) give, together with their host rock, a good-fat age of 1044 + 19 m y (MSWD = 1 13)

SERIES lOB

/

-7-

SERIES .083

2

3

~2

- c so

87Sf/ 86Sr

/f

/

87Sr/ 86Sr

/ A

~ ~-

~

B

oo,

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}/ IR

071572 I

2

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Fig 4 Total rock Rb-Sr isochron diagrams for (A) an mtruslve gramte sheet at locahty 10, (B) Series 2, 3 and ] 2 combined

Senes 10B The samples from the intrusive undeformed granite sheet at locahty 10 give a good-fit lsochron age of 1063 -+ 20 m y with I R = 0 71572 _+ 0 00082 (MSWD = 0 68)

5 Age interpretations The Interpretation of the data must take into account the following (1) The total rock apparent ages all fall within two distinct groups at ~1 4 5 - 1 55 b y (Series 2, 3, 12) and ~1 0 - 1 1 b y (Series 4, 6, 10) Outcrops In both the K-deficient and normal-K states have produced examples of each age (2) Mineral lsochrons for both total rock age groups are within error of each other at ~ 1 0 4 0 m y (3) The charnockatlC rocks have suffered only two detectable metamorphic events - one high-grade (the charnockite mineralogy) and the second low-grade The latter event Involved the passage of fluids which faclhtated the formation of hydrous minerals (4) The older apparent ages were obtained where the formation of the secondary hydrous minerals and the corrosion of biotite are at a minimum At all of the other localities, the secondary effects are much more pronounced (5) The observed regional differences in mineralogy, K, Rb, e t c , can be explained in terms of an essentially

40 primary fractlonatlon related to the emplacement and crystalhsatlon of rapaklvl-type magmas directly under high-grade condmons The lsochron age of the intrusive granite at locality l 0 is 1063 -+ 20 m y At this locahty, charnockItes from outside the decoloured zone show a considerable variation in intensity of the secondary effects, and these samples define a linear array corresponding to 1162 +- 214 m y , the exclusion of one sample gwes 1065 -+ 191 m y In the light of the older ages obtained from the least altered salnples (Series 2, 3, 12), the most reasonable interpretation of the Series 10A data is that an earlier ISotopic system was subjected to a secondary disturbance which occurred at the same time as the granite intrusion The mineral + total rock age of 1034 -+ 59 m y (minerals alone-- 1041 -+ 28 m y ) for sample 10 7 provides additional evidence for isotopic disturbance at this time The importance of the ~1060-m y event is emphaslsed by (l) The total rock ages obtained at localmes 4 (1075 + 122 m y , including minerals from sample 4 4 = 1035 + 9 m y ) a n d 6 ( 1 0 2 3 - + 9 0 m y ) At each of these locahtles the degree of secondary alteratlon is both pronounced and uniform between samples (2) Mineral + total rock isochrons of 1044 + 19 m y a n d 1 0 4 3 -+ 48 m y for samplesl2 1 9 a n d 2 18 which belong to two of the groups which are relatively unaltered and which have yielded the older (~1 4 5 - 1 55 b y ) ages It IS evident that total rock ages from those localrues which have suffered the more intensive secondary alteration, and all mineral ages, fall within error of the 1063 -+ 20 m y age for the intrusive gramte sheet We interpret these data as meaning that (a) there has been resetting of the total rock systems within some localities, (b) the open system behavIour was facilitated by the development of cracks during a relatively minor, low-grade metamorphic and defortuitional event (of [19,20]), and the passage of flmds occurred at the same time as the intrusion of the granite sheets and related pegmatlte dikes, (c) the degree of rehomogenlsatlon of the isotopic systems is related to the intensity of the secondary alteration (cf [211) Because only two metamorphic episodes are recognlsed, and the H1060-m y (Sveconorweglan) age

relates to the later low-grade event, we interpret the older (~1 4 5 - 1 55 b y ) age as relating to the highgrade event which involved the formation of the charnocklte mineralogy Our interpretation that the high-grade event should not be correlated with the younger (Grenvllllan) age is supported by a recently acquired 9-point total rock lsochron of 1397 -+ 57 m y (MSWD = 1 12) (Field and Rl]helm, unpublished data) on massive, locally derived and locally crosscutting homogeneous granite pods which represent the last recogmsable event involved in the high-grade metamorphism, on geological grounds these formed earlier than the ~1060-m y granite sheets/pegmatlte dikes In view of our later comments, this 1397-m y apparent age should only be regarded as a minimum, and not necessarily be equated to the time of melt formation Nevertheless, the data are Important m that they place a definite constraint on the timing of the high-grade event (x e pre-1397 -+ 57 m y ), and effectively preclude any alternative hypothesis involving an early intrusive/gneiss forming event at ~1540 m y , followed by selectwe hlgh-grade reworkmgat~1060my (cf [9]) Unfortunately, the error hmlts on the three plots revolving Series 2, 3 and 12 do not allow a precise determmatmn of the timing of the charnockJte event on an individual locahty basis However, the two best preserved (3, 12) give very similar values for both age and 87Sr/86Sr initial ratio, and the combined data for these two groups give 1536 + 26 m y with I R = 0 70345 -+ 0 00014 Even then the scatter cannot be totally assigned to experimental error (MSWD = 6 22), but we regard this as the best available estimate for the high-grade event (Fig 4B) The fact that representatives of both the K-deftcient (series 12 - low Rb/Sr) and normal-K (series 3 - high Rb/Sr) varieties have yielded a common age and 87Sr/a6Sr initial ratio IS entirely consistent with the other evidence [ 1 - 3 ] that the fractlonatxon in K, Rb, etc, was directly related to the granuhte metamorphism The remaining question concerns the timing of intrusion of the gneiss precursors whether this occurred more or less synchronously with the metamorphism, or significantly earlier An averaged 87Sr/S6Sr growth line has been constructed for the combined charnockIte suite, this was based on the aTSr/S6Sr initial ratio of 0 70345 and an average 87Rb/S6Sr value of 2 5 (Fig 4B) Follow-

41 lng the arguments developed in a series of papers by Moorbath (e g [22,23]), extrapolation of this line back to a hypothetical linear upper-mantle growth curve extending from 0 699 to 0 703 (Rb/Sr ~ 0 02) yields a maximum age limit of ~50 m y for the gneiss precursors, and precludes any significant crustal history prior to ~1 6 b y ago This interpretation assumes closed system behavlour of Rb and Sr w~thln the charnocklte body during the gneiss formlng episode There could have been a longer crustal history if the metamorphism revolved a significant increase an Rb/Sr, e g by an influx of Rb [ 2 4 - 2 6 ] but, although the average 87Rb/a6Sr ratio of 2 5 is high, the proposition of an overall large scale metasomatic introduction of Rb (or removal o f St) is untenable In the present case because of the close, genetic association between a highly K, Rb-deficlent zone and a normal-K, Rb zone within the same body The isotopic data are in accord with the evidence that the fractlonatlon was essentially primary, involving the emplacement of intermediate magma directly under the high-grade conditions They are also consistent with Jacobsen and Heier's [11 ] conclusion that the Precambrlan of southern Norway includes significant volumes of rocks which separated from the mantle at ~1 6 b y ago

6 Discussion Having established the age of the gneiss-forming event as ~1540 m y and the superimposed low-grade event as ~ I 0 6 0 m y , examination of the combined data (Fig 5) reveals several features which have a bearing on the problems associated with secondary homogenlsatlon mechanisms All of the reset and disturbed samples with 87Rb/ 86Sr ratios > c 0 8 (Series 4, 6, 8, in part 10) plot excluswely below the primary 1536-m y reference lsochron, whilst those with lower ratios (Series 11, In part I0) plot exclusively above Locahtles 4, 6, 8 and 11 cannot have behaved as closed systems with respect to Rb and/or Sr during the 1060-m y event At localities 4, 6 and 8 there must have been either a net gain in Rb or net reduction In 87Sr/86 Sr (or combination) The nature of the retrogression reactions suggests that Rb mobility was not an important factor, no new biotite was generated (e g at the

expense of orthopyroxene), which it might have been had the penetrating solutions been K,Rb-rlch This textural evidence is consistent with the results of an independent investigation of the chemical changes associated with the pegmatlte intrusions [27], which showed that (variable) changes in Rb and Sr were confined to within 30 cm of the intrusive contacts and were, therefore, restricted to the tuner parts of the decoloured zones, at a distance of 1 m from the contacts, only increases in H20 and C1 could be detected Both lines of evidence suggest that the open system behaviour more probably Involved modification of the 87Sr/86Sr ratios, and that this occurred by a subtle Interaction between the fluids and the rocks they permeated Recent work on hydrothermally altered basalts suggests that the 87Sr/a6Sr ratio in the fluid itself may have been important mixing can cause an enhancement when the initial ratio in the fluid is higher than that in the rocks [28,29], and a reduction when lower [21] Apphcatlon of a mixing model to the altered charnockltes requires that the initial ratio in the fluid was ~0 709, the value that rocks with 87Rb/86Sr ~ 0 8 would have attained by 1060 m y ago There is no supporting evidence for this as the initial composition of the fluid In fact, if the fluid did have ItS own 8VSr/a6Sr composition, this would logically have more nearly approximated to 0 716, the initial 8 VSr/86 Sr ratio of the lntruswe granite at locality 10 In this case, simple mixing would have raised the 87Sr/86Sr ratios in all rocks with 87Rb/86Sr < c a 1 9 (corresponding to 87Sr/86Sr = 0 716 at 1060 m y ), whereas those with 87Rb/86Sr In the range 0 8 - 1 9 actually show a net reduction Consequently, if the fluid did initially have a 878r/86St composition ~ 0 716, there must have been some other control involved in determining whether there was to be an increase or a decrease in 87Sr/86Sr within any particular sample or locahty The same conclusion would be reqmred if another posslblhty is considered, namely that the fluid might not have Initially possessed a significant 87Sr/86Sr at all The necessary chemical control may have been provided by the rocks themselves [30] If so, it was related to the Rb/Sr ratios and therefore to the preexisting mineralogy The possible role is best illustrated by examination of the Series 10A rocks, the only state in which both posmve and negative changes In 878r/

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Fig 5 Total rock Rb-Sr lsochron diagrams tot tile reset total rock series and minerals for sample 4 4, showing their posmons relative to the 1536-m y primary reference lsochron Diagram B is an enlargement of the lower part of A TR4 = total rock, PL4 = plagloclase, BI4 = bxotlte and KF 4 = K-feldspar for sample 4 4 Mineral lsochrons for samples 2 18, 10 7 and 12 19 are omitted for clarity

868r have been recognlsed C o m p a r e d w i t h the Series 6, 8 and 11 samples, these rocks cover a relatively large range in 87Rb/S 6Sr which, because t h e y contain only m i n o r a m o u n t s o f K-feldspar, is due almost entirely to variations in biotite c o n t e n t As at the other collection sites, the blotltes have b e e n c o r r o d e d at their edges by the percolating fluids, and Sr leached

by this reaction would have been relatively enriched in 87Sr, tending to Increase the 87Sr/86Sr ratio In the fluid This fluid Sr w o u l d t h e n have b e c o m e available for a c c o m m o d a t i o n elsewhere - not in this case within the i m m e d i a t e l y adjacent Sr-bearmg minerals (as m closed total rock systems) but In biotite-poor, plagloclase-rich rocks elsewhere within the same locahty

43 Empmcally, the same model can be apphed on a larger scale to the data for Series 4, 6, 8 (net loss of 87Sr by leaching of biotite) and 11 (net increase of 87Sr in biotite-poor rocks), but with the added lmphcation that any Sr leached from biotite must have been redistributed on a scale larger than the individual outcrops If this did occur, there remains the more general, and yet unsolved, problem as to why the available 87Sr was not wholly taken up m locally available Srbearing minerals as happens when mineral resetting occurs within closed total rock systems (cf [20,30]) The results also have important Implications concerning the interpretation of Rb-Sr isotope data based on regionally collected samples (1) As a group, and without considering any other samples, the data points for locahtles 6, 8, 11 and 12 form a linear array with a slope intermediate between the primary and secondary ages (Fig 5) (cf [20]) This situation arises owing to a combination of samples with reduced 87Sr/a6Sr ratios (higher Rb/Sr - Series 6, 8), samples with enhanced 87Sr/86Sr ratios (low Rb/Sr - Series 11) and unaltered samples (Series 12) Had the original samphng involved only these locahties, and the slgmficance of the secondary alteration remained undetected, there would have been a very real posslblhty of ascribing a (non-existent) geological event to this (spurious) age, particularly as there Is geological ewdence for only a single high P-T event and one of the primary alms was to date this (2) The data from the widely-spaced, reset, localltles 6 and 10, plot along the same 1060-m y reference lsochron (as do the partmlly reset samples from localities 8 and 11) (Fig 5) If taken alone, these data would gwe the appearance of an apparent rehomogenlsation of the ISOtope system on a regional scale However, since quite different results have been obtained from locahtles close to one another (e g 10, 11, 12), this cannot have happened The illusion of a regional rehomogemsatlon is created because the total range in 87Rb/a6Sr ratios at locahties 6, 8, 10 and 11 is limited ( 0 - 2 ) , in fact, the range within locahty 10 overlaps with all of the others Consequently, each o f the sub-volumes appears to be in equlhbr/um [31] The Series 4 samples define a parallel lsochron, but with a much higher initial 875r/86Sr ratio, simply because these have been collected from a sub-volume where the 8VRb/86Sr variation is m a higher range ( 4 - 6 ) The net result is a pair of parallel, but stepped,

secondary lsochrons, similar to those described elsewhere for altered Tasmanlan eclogltes [32] It is our experience from this study that had we not collected sufficient samples to be able to define individual locality isochrons, we should not have been able to make such a strong case for the 1536-m y age Moreover, without careful investigation of the relatively minor secondary mineral alteration, we might well have wrongly correlated the younger (~1060 m y ) age to the charnocklte event As it is, we have found no evidence for there having been a high-grade regional metamorphic event during the socalled Sveconorweglan (Grenvllhan) orogeny which has been presumed to have affected thts part of the Fennoscandlan shield The more general point IS that Rb-Sr data can easily be misinterpreted if the possible effects of even seemingly minor secondary alterations are ignored - particularly if samples are collected on a regional basis in order to obtam a spread in the Rb/Sr ratios

Acknowledgements We acknowledge the skilled technical assistance of T Enger (Oslo), J Eyett and J Wilkinson (Nottingham) and the secretarial help of J Pearson and P Blankley We thank J C Roddlck and P N Taylor for their critical reviews This work was supported by NATO grant No 1391

References 1 D C Cooper and D Field, The chemistry and ongms of Proterozolc low-potash, high-iron, charnockltlc gnelsses from Trom~by, south Norway, Earth Planet Scl Lett 35 (1977) 105 2 D Field and P W L Clough, K/Rb ratios and metasomatism m metabasltes from a Precambnan amphlbohtegranuhte transition zone, J Geol Soc London 132 (1976) 277 3 D Field, S A Drury and D C Cooper, Rare earth and large 1on hthophfle element fractlonatlon in high-grade charnockltlC gnelsses, south Norway (m preparation) 4 K O Kratz, E K Gerhng and S B Lobach-Zhuchenko, The isotope history of the Precambnan of the Balnc Shield, Can J Earth So 5 (1968) 657 5 T F W Barth and J A Dons, Precambrlan of southern

44

6

7

8 9

10

11

12 13

14

15

16

17 18

19

Norway, m Geology of Norway, P Holtedahl, e d , Nor Geol Unders 208 (1960) 6 tt R Wynne-Edwards and Z1a-U1 Hassan, Intersecting orogenic belts across the North Atlantic, Am J Scl 268 (1970) 289 R K O'Nlons, R D Morton and H Baadsgaard, Potassmmargon ages from the Bamble sector of the Fennoscandian shield in south Norway, Nor Geol Tldsskr 49 (1969) 171 N H Magnusson, Age determmatmns of Swedish Precambrmn rocks, Geol Foren Stockholm Forh 82 (1960) 407 R K O'Nlons and H Baadsgaard, A radmmetrlc study of polymetamorphlsm in the Bamble region, Nor~ ay, Contrlb Mineral Petrol 34 (1971) 1 R K O'Nmns and K S Heler, A reconnmssance Rb-Sr geochronological study of the Kongsberg area, south Norway, Nor Geol Tidsskr 52 (1972)143 S B Jacobsen and K S Heler, Rb-Sr isotope systematlcs in metamorphic rocks, Kongsberg sector, south Norway, Llthos 11 (1978) 257 A Bugge, Kongsberg-Bambleformasjonen,Nor Geol Unders 146 (1936) J A W Bugge Geological and petrographical investigations In the Kongsberg-Bamble lormatmn, Nor Geol Unders 160 (1943) I C Starmer, The geology and evolution ot the southwestern part of the Kongsberg series, Nor Geol Tldsskr 57 (1977) 1 R J Pankhurst and R K O'Nions, Determination of Rb/Sr and 87Sr/86Sr ratios of some standard rocks and evaluatmn of X-ray fluorescence spectrometry in Rb-Sr geochemistry, Chem Geol 12 (1973) 127 R K O'Nlons and R J Pankhurst, Secular varlatmns m the Sr- ISotope composition of Icelandic volcanic rocks, FarthPlanet ScI Lett 21 (1973)13 D York, Least squares fitting of a straight line with correlated errors, Earth Planet Scl Lett 5 (1969) 320 C Brooks, S R Hart and I Wendt, Realistic use of twoerror regressmn treatments as applied to rubidium-strontium data, Rev Geophys Space Phys 10 (1972) 551 L P Black, T H Bell, M J Rubenach and I W Wlthnall,

20

21 22

23 24

25

26

27

28

29

30 31

32

The slgmficance of Rb-Sr total rock ages m a multiply deformed and polymetamorphIc terrain, U S Geol Surv Open File Rep 78-701 (1978) 41 R W Page, Response of U-Pb zircon and Rb-Sr total rock and mineral systems to low grade regional metamorphism in Proterozolc Igneous rocks, Mt Isa, Australia, J Geol Soc Aust 25 (1978) 141 C J Hawkesworth and M A Momson, A reduction in 87Sr/86Sr during basalt alteration, Nature 276 (1978) 381 S Moorbath, Age and isotopic constraints for the evolution of Archaean crust, in The Farly History of the Earth, B F Wlndlcy, ed (Wiley, London, 1976) 351 S Moorbath, Ages, isotopes and evolution ot PrecambNan continental crust, Chem Geol 20 (1977) 151 D Brldgwater and K D Collerson, The major petrological and geochemical characters of the 3600 m y Ulvak gneisses from Labrador, Contrib Mineral Petrol 54 (1976) 43 D Bndgwater and K D Collerson, On the origin of early Archaean gnelsses a reply (to A Y Ghkson), Contnb Mineral Petrol 62 (1977) 179 K D Collerson and B J Fryer, The role of fluids In the formation and subsequent development of early continental crust, Contrlb Mineral Petrol 67 (1978)151 D C Cooper, The geology of the area around Eydehavn, south Norway, Ph D Thesis, UmversJty of Nottingham (1971) S R Hart, A J Erlank and E J D Kable, Sea floor basalt alteration some chemical and Sr isotopic effects, Contrlb Mineral Petrol 44 (1974) 219 R K O'Nions, S R Carter, R S Cohen, N M Evenson and P J tlamllton, Pb, Nd and Sr isotopes in oceanic lerromanganese deposits and ocean floor basalts, Nature 273 (1978) 435 K Bell and J Blenklnsop, Reset Rb/Sr whole rock systems and chemical control, Nature 273 (1978) 532 J C Roddlck and W Compston, Strontium isotopic equilibration a solution to a paradox, Earth Planet Scl Lett 34 (1977) 238 A R~thelm and W Compston, Correlations between metamorphic events and Rb-Sr ages m metasedlments and ecloglte from western Tasmania, Llthos 10 (1977) 271