Rare-earth and LIL element fractionation in high-grade charnockitic gneisses, south Norway

Rare-earth and LIL element fracfionation in high-grade charnockitic gneisses, south Norway D. FIELD, S. A. DRURY & D. C. COOPER

L1THOS

Field, D., Drury, S. A. & Cooper, D. C. 1980: Rare-earth and LIL element fractionation in hitch-grade charnocidtic gneisses, south Norway. Lithos 13, 281-289. Oslo. ISSN 0024-4937. High-Fe, intermediate-acid, charnocldtic gneisses in the Arendal-Trom0y area of the Svecofennian terrain of southeast Norway comprise two chemic~ly contrasting zones - one with normal large-ion-fithophile (LIL) element characteristics, and the other l,IL-deficient. The LIL-deficient varieties also have low Y.REE, commonly with positive Eu anomalies. The norma~-LIL rocks are enriched in XREE, exhibit fractior~ated patterns and have negative Eu anomalies. Modelling shows tha~ both the LIL and REE patterns are consistent with an essentially primary fr~tionation process involving the separation of cumulus (LIL-deficien0 phases from an~lesitic-dzcitic magma e,,lplaccd directly under the high-grade conditions, with the norm~d-LIL rocks crystallising from the residual melt. This process is interpreted as a deep-seated componem of the magma system which culminated in the emplacement of some higher level rap~kivi granite late in the Svecofennian event. The model presented does not require anortho:dte to be part of the same magma system.

D. Field, Department of Geology, University of Nottingham, Nottingham, U.K. S. A. Drury, Departmen: of Earth Sciencez, T&,.Open University, Milton: Keynes, U.K. D. C. Cooper, Department of Geology, Universi~. of Nottingham, Nottingham, U.K. Present address: Institute of Geological Sciences, Keyworth, No~:tingham, U.K.

Hypotheses concerning the chemical evolution of the lower continental crust rely heavily on data from Precambrian granulite facies rocks. Their abundances of K, Rb, Cs, U and Th, and their K/Rb, Rb/Sr~ Ba/Rb and BalSr ratios have figured largely in various interpretations. Lambert & Heier (1968) first recognised in ~he Australian craton that granulite facies charnockitic gneisses with intermediate to high pressure assemblages were characterised by low K, Rb, Cs, U, Th, RblSr and high K/Rb, Ba/Sr compared with lower pressure granulites and amphibolite facies gneisses from terrain widely separated in space and time. Discussion continues on the relative merits of various mechanisms whereby such fractionations may have arisen. There have been three main suggestions, all of which centre on the origin of largeion-lithophile (LIL) deficient charnockitic gneisses:

(1) :['hey are refractory residues of granite melting, LIL elements having been selectively partitioned into migrating partial melts (e.g. Fyfi' 1973). (2) LIL elements were purged to higher crustal levels, together with H 2 0 during high grade metamorphism involving dehydration (e.g. Sheraton et al. 1973).

(3) LIL-deficient charnockites crystallised directly from rising magmas at deep crustal levels, with the LIL elements selectively partitioned into residual hydrous magma (Holland & Lambert 1975). None of these models has been rigorously tested, presumably partly because of the supposed relative ease of redistribution of LIL elements under high-grade conditions. As yet there are few published charnockite analyses which incorporate full data for the rare-earth elements (REE) which are widely regarded as being (relatively) immobile under metamorphic conditions, and hence more amenable to mathematical modelling. The available REE data are for rocks from Scandinavia (Green et al. 1972; Ormaasen 1977; Hubbard & Whitley 1978, 1979), Scotland (Drury 1978; Muecke et al. 1979; Tarney et al. 1979; Tarney & Saunders 1979), Poland (Tarney & WindMy 1977) and Spain ~Drury, in press). In the ,M'endal-.Trom~y district of south Norway there is a clear spatial/genetic relationship between LIL-deficient and nornial-LIL charnockitic gneisses, the extreme fracfiom~tior~ in this suite being related to the iml~osi|ion of the pyro×enic assemblages. New data for the REE enable models for this fractionati=,n to be tested and applied to LIL element patterns.

LITHOS 13 0980)

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Geological and geochemical setting The study area is situated in the ArendalTromey region of southern Norway, which forms part of the Bamble sector of the Fennoscandian Shield (Fig. 1). In this region there is a 15 km wide belt of granulite facies rocks including metasediments, metabasic sheets and charnockitic gneisses. Mineral associations suggest a low to intermediate pressure regime. For ease of reference, a zonal scheme (A-D) is used to subdivide the local ten'ain (Fig. 1). Zone A comprises amphibolite facies assemblages, and is delineated from zone B by a well defined orthopyroxene isograd in metabasic rocks (Field & Clough 1976). In zones A and B, acid to intermediate gneisses are characterised by hornblende-biotite assemblages, whilst in zones C and D they are charnockitic, containing orthopyroxene. Those in zone C have ortho-

pyroxene-hornblende-biotite-K-feldspar bearing assemblages. Zone D, whie!~i contains the highest grade rocks in the area, is dominated by charnock'tic gneisses which are virtually devoid of K-feldspar, hornblende and bic~tite - they are nearly anhydrous. They fall in the compositional range tonalite-trondhjemite (Cot, per & Field 1977). The chamockitic gneisses from zone D (the Tromey gneisses) show extreme LIL-depletion (Cooper & Field 1977) (Table !) with very low Rb/Sr (mean < 0.05, n = 79) and very high K/Rb (mean > 1300). Charnockitic gneisses of identical fie!d setting from zone C have normal LIL affinities, with values approximating to upper crustal averages (mean Rb/Sr= 1.2, mean K/Rb= 290, n =- 40; Field, unpublished data) (Table 1). The charnockitic gneisses from zones C and D are part of the same Sr isotope system, and their low initial sTSr/86Sr rat~.o (0.70345 _+0.00014) indicates that they separated together from a low Rb/Sr source, region not more than 50 Ma before their whole rock Rb-S~"isochron age of 1536_+ 26 Ma (Field & E',/lheim 1979). They are interpreted as new additions to the continental crust, and not as reworked older gneisses or melts of high Rb/Sr metasediments. There is strong evidence that their Sr isotope systems were subsequently disturbed by a relatively low grade event at about 1060 Ma (Field & R~heim 1979). The combi~led structural, mineralogical and isotopic evidence shows that the geochemically contrasting cbarnockite gneisses of zones C and D were formed during the same event as that which imposed their granulite facies mineralogies. The chemical variability reflects fractionation associated with this high grade event, and is described and discussed in the light of this evidence.

Geochemical data Eight analyses, six from zone D and two fi'om zone C, are given in Table 1, together with mean analyses for the two zones. The REE were determined at tl~e Open University by instrumental neutron activation analysis, following procedures described by Paul et al. (1975). The other trace elements and the major elements were determinecl at Nottingham following procedt~res described by Cooper & Field (1977). Variability ~f the LIL elements and the REE is

LITHOS

Rare-earth and LIL element fractionation

13(1980)

283

Table 1. Major, LIL and rare-earth element analyses for eight ¢harnocldtie gneisses from Arendal-Tromey. D435

SiO2 AlsOa TiO2 FezOs MgO CaO NazO K20 MnO

D430

D35

D409

D423

D439

C408

C409

74.78 ~2.67 0.31 3.29 0.12 3.95 3.71 0.32 0.10 0.05 0.12

72.41 14.64 0.31 3.02 i.66 2.61 4.75 0.59 0.06 0.09 0.35

63.39 15.11 0.83 8.17 3.48 1.85 6.34 0.35 0.10 0.28 0.23

69.51 13.99 0.39 5.75 0.91 4.57 3.92 0.34 0.09 0.07 0.22

73.76 13.01 0.44 3.57 0.72 1.65 5.8~ 0.21 0.04

67.t]6 13.55 0.97 6.48 1.27 3.12 3.03 4.12 0.07

0.10

0.19

H~O +

74.44 13.05 0.38 3.53 0.50 3.65 3.40 0.20 0.04 0~04 0.29

0.19

0.36

69.68 1~.33 0.70 4.34 i.00 2.33 3.21 4.02 0.04 0.09 0.38

Total

99.52

99.42

100.49

100.13

99.76

99.49

99.77

99.83

1 158 156 1660 156 0.006 0.987 156 3.5 4.2 2.2 0.5 0o71 0.6 0.12 0.15 !.13 0.22 13.3 0.9 4.3

2 119 72 1330 36 0.017 0.605 168 5.5 8.2 5.2 1.6 0.94 2.2 0.46 0.51 3.59 0.6'; 28.9 0.59 1.5

3 126 250 1630 83 0.024 1.984 74 6.9 11 5.6 1.2 0.50 1.1 0.17 0.10 0.74 0.14 27.2 3.6 1.3

2 154 93 1450 47 0.013 0.604 76 12 24 18 4.2 1.6 4.1 0.73 0.37 2.8 0.46 67.9 2.3 I. I

2 163 91 1410 46 0.012 0.558 96 8.3 16 13 3.9 0.86 5.9 1.2 ~.! 1.2 1.2 58.2 0.55 0.55

2 48 I 11 870 56 0.042 2.3!3 261 15 33 24 6.3 1.1 7.8 1.5 1.3 8.6 1.4 100.6 1.3 0.46

146 134 755 234 5.17 1.089 5.634 564 71 165 93 20 2.6 20 3.4 1.9 I! 1.6 389.5 5.8 0.32

156 107 696 215 4.46 1.457 6.504 473 90 200 98 20 2.a 17 2.5 0.99 5.4 0.65 436.9 6.2 0.26

P205

Rb Sr Ba K/Rb Ba/Rb Rb/Sr BadSr 7.x La Ce Nd Sm Eu Gd Tb Tm Yb Lu X REE Cer~/Vbn Eu/Eu*

Average Zone D 68.35 13.83 0.53 6.01 1.90 3.64 4.67 0.47 0.09 0.12 0.23

< 9t

149 296 > 1300~ > i05 ~ < 0.05 * 1.6 133

Average Zone C 67.48 13.82 0.84 5.65 '.07 7,'.88 3.21 4.25 0.07 0.26 0.49

139 163 896 290 8.58 1.15 6.10 478

Pretixes C and D refer to zone C and zone D samples respectively. Average analyses for zone D 0~ = 79} at,,.; "
expressed in Figs. 2A, 3 and 4A. The LIL element distributions have been discussed previously (Cooper & Field 1977). Chondrite-normalized plots of l he REE in the eight analysed charnockite gneisses (Fig. 2A) reveal the following features: (1) A large variation in X REE. The two zone C samples have by far the highest concentrations. Even among the petrographically similar zone D samples, La ranges from 10 to 45 times chondritic abundances and Lu from 4 to 40 times. (2) Variable enrichment of light REE relative to heavy REE. The two zone C samples show stroag enrichment in light REE. Amongst the

zone D samples, CeN/YbN ratios are low (0.63.6), and variable. Three of the samples show enrichment of heavy REE relative to li£ht REE (CeN/YbN< l). The overall low CeN/YLN ratios distinguish this group of Proterozoic rocks from felsic granulite facies rocks of Archaean age (Green et al. 1972; Drury 1978; Muecke et al. 1979; Tarney & Satmders 1979) and fr,~m Archaean gneisses and plutonic granitoids in general (e~g. Arth & Ha,lson 1975; O'Nions & Pankhurst 1978). They ~re also quite different from the patterns shown by the tonalites and trondhjemites from the classic Proterozoic suite of the Uusikaupunki-Ka~nti area, soutt~west Finland (Arth et al. 1978). In terms of o*~erall

284

Field et al.

LITHOS 13 (1980)

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Fig. 2. (A). REE in charnockRic gneisses normalised relative to the average of 10 ordinary chondrites (Nakamura 1974). Prefixes C and D refer to zone C and zone D samp}es, respectively. (B). Chondrite normalised plots of results of the equilibrium fractionation model discussed in text. F' is the fraction of melt remaining.

REE patterns, l:he TromCy rocks are more comparable with the sou~h Californian batholith ~CTowell et al. 1965) and Phanerozoic andesites (e.g. q horpe et al. 1976). (3) A large variation in Eu relative to the abundance of other REEs, as expressed by Eu/Eu* ratios (0.3-4.3). There is a clear inverse relationship between the magnitude of Eu anomalies and E REE (Table 1), which is illustrated by the graph of log Eu/Sra ~,. log Sm (Fig. 3). The two normal-LIL charnockitic gneis~es of zone C have strong neg~tive Eu anomalies. The three samples with distinct postfive Eu anomalies and low E REE are LIL-deficient charnockitic gneisses from zone D. These are further examples (also Green et al. 1972; Drury 1978; Tarney & Saunders 1979) which show ~hat it is not only the anorthosites of high-grade terrains that have significant positive Eu anomalies (cf. Tarney & Windley 1977:157). Sample D ~ 9 has no significant Eu anomaly, plots entirely with the field of Phanerozoic calcalkaline andesites and dacites (Thorpe et al. 1976) and approximates the average REE dis-. tribution quoted by Condie (1976) for dacites.

The wide spectrum of patterns shows that the REE, like the LIL elements, are strongly fractionated.in this suite. If the profiles are reflecting magmatic processes (see introduction), the range from lowE REE with positive Eu anomalies to high XREE with negative Eu anomalies by itself suggests that feldspar fractionation had an important influence: during both partial melting and fractional crystallisation, separation of feldspar characteristically generates positive Eu anomalies in the residuum or cumulates, and produces negative Eu anomalies in the melt fraction (e.g. Hanson 1978).

Trace element modelling Rare-earth elements Various mathematical modelling schemes (e.g. Arth 1976; Pearce & F/lower 1977; Hanson 1978) have been applied by us to test the hypothesis of feldspar-dominated crystal fracfionation of a precursor magma as a possible toe's.as of generating the REE distributions as eithe~r cumuilates or residffal melts. The models used were:

Rare-earth and L I L element fractionation

LITHOS 13 (1980) ~) = Liquid r.m~l~sitia~s OxF'I 0.g

to .~kP g

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Average Phanm',~c clklcite Sample flo.C409 C ~,08 ~ F ' = O. 3 D&3,

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0 01

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•.~,ld~g 1

10

100

Sm IPF.:I

Fig. 3. Pint of EulSm against Sm for charnoekitic gne~sses. Also shown are vectors for the equi:ibrium fractionation model discussed in the text showing liquid and cumulate compositions derived from average Phanerozoic dacite (Condie 1976) for various values of F', the fractior., of melt remaining, and for different values of D~u, the bulk distribution coefficient for Eu between crystals and melt. For reference, melt and crystal fractionation vectors for important single minerals are shown as solid and dashed lines.

(a) Abstraction of leuconorite (plagioclase65, orthopyroxene30, ilmenites) from both average Phanerozoic andesite and basalt with a 20 times chondritic, fiat REE distribution. (b) Progressive crystal~isation under low PH,o (granulite facies) conditions of average Phanero:,.oic dacite. Variants of each of these models proved capable of generating REE patterns similar to lhose in the zone C charnockitic gneisses, if they were residual melts in such systems. However, only

285

model (b) also produced REE patterns in cumulates that matched the REE-depleted patter-as with positive Eu anom;dies found in many of~he zone D charnockitic gneisses. This model is evaluated in detail. The parent magma is assumed to have the REE composition of average Phanerozoic dacite (Condie 1976). Following Eanson (1978), ~ae mineral/melt distribution coefficients used in the calculations have been taken from Arth's (1976) compilation of phenocryst-maU'ix partition coefficients for dacite and rhyolite compositions (T~ble 2). Although use of these data initially takes no formal account of experimental studies concerning the possible effects of variations in physical and chemical p~ameters on partitioning (e.g. Irving 1978), the similarity between the data for co-existing min(ral pairs in dacites and ..... from Scotland some pyroxene grant~,'"*-~ (Muecke et at. 1979) indicates that they are a realistic first approximation. The crystallising phase was given a composition of plagioclase~o, quartz30, orthepyroxenes, oxides5, approximr,~. ing to the modes of the zone D samples. Both equilibrium crystaUisation and Rayleigh fractional crystallisation models were applied to the data, but on13, the former results are reported as that process is likely to more closely approach conditions expected in plut~mic environme~lts where crystallising phases may be in total equilibrium with the melt (Arth 1976). The equation used is CLICk= l/IF' +D~ ( i - F ' ) ] (Arlh 1976, equation I 1), where C~ is the concentr~tL)a in the parent magma, Cn is the concentration in the fractior ated liqr i6, F' is the fraction of liq~ i d remaining and D~ is the bulk distribution coef~cient given by Ds = E ~!X ~x Kd ~. A'~ is the weight fraction of a given mineral i in the precipitating phase and K d ~ is the rqineral/melt distribution

Table 2. Mineral/melt p~rition coefficients used in modelling. All are for dacite melt compositions, except ov:hopyroxene and clinopyroxene, which are for rhyofitic compositions. D~-,ta from Arrh's (1976) ¢omp httion.

Ce Sm Eu Gd Yb K Rb Ba Sr

Plagioclase

Orthopyroxene

Clinopyroxene

Hornblerde

Garnet

0.24 0.13 2.17 0.09 0.08 0.263 0.048 0.36 2.84

0.15 0.27 0.17 0.34 0.~6 0.0023 0.t)027 0.0029 0.0085

0.50 1.61 1.56 1.85 1.58

0.90 3.99 3.44 5.48 4.g9

0.35 2.66 |.50 ~0.5 39.9

286 Field et al. coefficient for a given trace element for mineral i. The concentration of a trace element in the residue, C~, relative to, the parent C~, is calculated from the relationship Cs/C~=CtJC~x Ds, which is from the definition of a distribution coefficient. Fig. 2B shows chondrite-normalized REE patterns in residual melts and cumulates for fractions of remaining melt (F') from 0.9-0.1. The log Eu/Sm v. log Sm diagram (Fig. 3) shows vectors for residual melts and cumulates in the model, vectors for equilibrium crystallisation of various individual minerals, and the compositions of the charnockitic gneisses (see Pearce & Flower 1977, for details of such graphs). The patterns in residual melts after 70-90% crystallisation closely parallel those in rocks C408 and C409, and the general features of the data are encompassed by the model pattern in both Figs. 2 and 3. Two main points of deviation require explanation: (a) The Eu anomalies in the rocks are not as large as predicted by the model. This is clear from comparison between Figs. 2A and 2B, and is reflected in the lower angle locus of Eu/Sm v. Sm in the rocks compared with the model vectors for residual melts and cumulates (Fig. 3). Reducing the bulk distribution coefficient for Eu from 1.27 to 0.5 gives vectors for both cumulates and residual liquids which closely parallel the locus of Eu/Sm for the rocks. It also reduces the Eu/Eu* ratios towards those observed. This reduction would correspond to a partition coefficient of 0.85 for Eu in plagioclase, compared to ~he quoted value of 2.11 for phenocryst/matrix partition in dacites (from Arth 1976). The experimental work of Drake (1975) and Drake & Weill ~1975) has indicated that Eu behaves differently from the other rare earths in that partitioning is strongly dependent on oxygen fugacity (cf. Mysen et al. 1978) and their data suggest that the lower' value of 0.85 would probably be more in accord with deep-seated crystallisation under moderate fc,.~conditions. Perhaps it is significant that this value of 0.85 is the same as that derived for l-lu in the plagioclase of the presumed parental monzonorite to the Rogaiand anorthosite complex in southwestern Norway (Duchesne et al. 1974), albeit that this magma system ISi()~-~ 48%) is of different composition from that under consideration here. ~b) Thre~: of the zone D samples show enrich-

LITHOS 13 (1980)

ment in HREE relative to LREE (Ces/YbN ~<1.0) (Fig. 2A), and another has a value for CeN/Yb Nof 1.3. This feature could be explained in cumulates by the presence of an early crystallising mineral such as garnet or zircon, both of which concentrate HREE. However, this has not happened in the present case for although some of 'the charnockites do contain up to 5 % gaxnet, this mineral is not a constituent of the analysed samples. Also, Zr tends to increase with REE content (Table 1), suggesting a preferential concentration of zircon in the residual melts rather than in the cumulates. The most likely cause of the HREE enrichment is by contamination with the metasediments with which the charnockites are associated. These rocks are usually highly garnetiferous and irregular assimilation might well have produced HREE enrichment in the parental magma of the charnockites, particularly if the assimilated material was a garnet-rich residue of an earlier partial rnelting episode.

K, Rb, Ba and Sr These elements have been modelled as trace components in exactly the same fashion a,i the REE. It is possible to treat K as a trace element because of its very low abundance and the absence of K-feldspar in the zone D samples. Using the same equilibrium crystallisation model and crystallising and abstracting the assemblage plagioclase60, quartz30, orthopyroxenen, oxiides~ from the composition of average Phanerozoic dacite gives the trace element profiles for cumulates and residual liquids in Fig. 4B. This diagram has been devised to highlight chen~ical differences between rocks in terms of the LIL elements. Comparison with data from the eight selected samples from zones C and D (Fig. 4A) shows ~hat the modelling data satisfactorily explain the marked contrasts between zones C and D in terms of K, Rb, Ba, Rb/Sr, Ba/Sr, Ba/Rb and K/Rb, It is not only the general shapes of the profiles which are consistent with these rc~cks representing cumuJtates (zone D) and residm~i liquids (zone C) fi'om a parental magma wi~ dacitic proporfior~ality between the chosen elements, but also, wi~h the single exception of Sr, ~he actual values arz remarkably similar. All analysed samples (n =: 119) from zones C and D fi~ within the model envelope of Fig. 4B.

Rare-earth and L I L element fractiona~ion

LITHOS 13 (19110)

287

o i x

1000

SQ~ple C/,,09 C ¢08 Dl,39 D/.09 D ¢23 D 35 D/.30 D¢3'i

100

/o.''.

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#

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Fig. 4. (A). Abundances of LIL elements and ~,aluesof important interelement ratios in charaockitic gneisses. Symbols as in Fig. 2A. (B). Results of equilibrium fractionation n~odelling for LIL elements and ratios.

Discussion The close match between the data and the results of modelling equilibrium cryslallisatbn, together with the Rb-Sr isotope data (Field & R[heim 1979), supports the hypothesis th;~.t the geochemically diverse ArendaI-Tromey charnockitic gneisses are part of the ~;ame fractionation system, and that their chemical differences were imparted very soon after t~e separal!ion of the precursors from their source: region, probably at mantle depths. Because modelling of part~ial melthg processes can also produce sinihr REE and ElL element p~.tterns to those exhibited by the Arendal-TromCy rocks (i.e. zone D= residua, zone C = ~aelts), these data do not by themsLflves rule out this possibility as a means of gene~'ating the observed fractionations. Nevertheless, this would seem unlikely because (a) the 87Sr/%r initial ratio of 0.70345_+0.00014 requfires that partial melting could only have occurred in a rock suite already characterised by low Rb/Sr, an~ (b) no field evidence has been recogr~ised, such as veins of normal-LIL material in LIL-de-

ficient material, which could be used to support this alternative. Another possibility is that there was hydrous fluid transfer between the two ~:ones: several authors (e.g. Sheraton et al. 197~; Drury 1973; Heier 1973; Collerson & Fryer ii978) have ascribed the peculiarities of LIL-deficient charnockitic gneisses to crustal differentiation through purging of hydro fluids carrying LIL elements during granulite facies metai,lorphism of more hydrous precursors. In view of the data presented here, retention of this type of metasomatic model for the Arendal-Tromey rocks (cf. Cooper & Field 1977) wou|d not only have to explain the very close spatial association between the normal-LIL and LIL-deficient charnockitic gneiss groups, neither of which is water saturated, but it would also require that any metasomatic transport of K, Rb, etc. was accompanied by selective hydro~ls fluid transfer of all REE except Eu, i.e. the rare" earth elements were mobile during the high grade metamorphic event. Whilst the modelling results do not formally preclude the possibility of metasomatic transfer

28;8 Field et al. of the REE, in our view the simplest explanation of :,he field relations and geochemical diversity of the charnockitic gneisses is that they represent syn-kinematic concordant intrusions which underwent crystallisation and fractionation trader low pH20 and relatively high pCOz conditions (Touret 1971; Hoefs & Touret 19751, and that the less hydrous cumulates were separated from the residual magma by tectonic filter pressing. This model is compatible with bo~h the clear relationship between geochemistry and structural level and with the evidence that there was only a short (<~ 50 m.y.) time lapse between the separation of the gneiss precursors from a low Rb/Sr source region and the development of their present mineralogies (Field & Rhheim 1979). Any contamination of the precursor magma by radiogenic Sr from the garnet-rich supracrustal envelope would not only lead to the observed HREE enrichment in early crystallising phases, but would also reduce this time lapse to insignificance. On this available evidence, the crystal fractionation model is preferred in subsequent discussion. The REE patterns of the normaI-LiL charnockific gneisses (zone C) are closely similar to those from the Proterozoic rapakivi granites of ~outhwest Sweden (Hubbard & Whitley 1978, 1979; Koljonen & Rosenbei'g 1974). Moreover, the Arendal-Trom#y charnockites show a strong Fe-enrichment trend similar to that which characterises rapakivi granites (Cooper & Field 1977). The rapakivi granites of the Baltic Shield were emplaced after the ~. 1.65 Ma Svecofennian event, and possibly represent significant additions to the crust at that time. Hubbard & Whitley ( i978, 1979), have presented convincing field and geochemical evidence for a comagmatic link between some normal-LIL charnockites K.,O ~ 4.6%) and a higher levell rapakivi granite, the Torpa granite (1.45 Ma) in the nearby terrain at Varberg, southwest Sweden; they have not recognised a LlL-deficient charnockite component of this suite In S.E. Norway there is a comagmatic link between LIL-deficient and normabLIL charnockites, in rocks of broadly similar age. These combined data suggest that the same process operated in both terrains, and that there is a comagmatic link between LIL-deficient charnockites, normai-LIL charnockites and at least some rapakivi granites. Whereas Hubbard & Whiiley ( 1978, 1979) suggested that the parent magma of the Varberg charnockites was derived

LITHOS 13(1980) by melting c,f supraerustals in contact with an unseen anorthosite intrusion, the new data from S.E. Norway favour a deeper low Rb/Sr source region for the (andesite-dacit¢) magma. This underwent fractional crystallisation during emplacement at depth, producing LlL-deficient cumulates and leaving a residual melt phase which crystallised as normal LIL charnockite and rapakivi granite at successively higher structural levels. The model we have presented does not require the involvement of anorthosige which, significantly, does not outcrop either in S.E. Norway or in S.W. Sweden. Whereas there may be charn0ckites of several different origins, there is now a growing body of data indicating t h e importance of fractional crystallisation dur;ng the generation of new continental crust (e.g. Ormaasen 1977; Drury 1978 and herein). The crystallisation history of any rising andesite-dacite magma will be affected by, inter alia, the prevailing geothermal conditions, the rate of ascent and perhaps most of all by the composition of the vapour phase. For instance, in a high pCOz environment - as indicated tbr ~he Arendal rocks by the fluid inclusion studies (To~lret 1971; Hoefs & Touret 1975) - pHzO is correspondingly reduced, thereby depressiag the stability of hydrous minerals so that early crys:allising phases are anhydrous (Heier 1973). In a deep-seated, ~yn-kinematic environrrent such as that under consideration here, the result is a suite of charnockitic rocks emplaced as sheets parallel to the existing planar fabrics ,icf. Bridgwater et al. 1974). The additional evidence provided by the Arendal rocks is that these processes can also involve separation of the early, anhy~irous phases a~ LIL, REE-deficient cumulates, with the more hydrous residual melt crystallising at high~:.~r structural levels to give rocks with normal-LIL contents and fractionated REE patterns. A c k n o w l e d g e m e n t s . - Th~:aks are due to Phil Potts and Oiw en

Williams-Thorpe of the Open University for performing neutron activation analyse~ and to John Eyett of Nottingham University for some of t~,e XRF :analyses. Josie Wilkinson drew the figures, and secretarial assistance vas given by Joan Waugh and Jean Pearson.

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L~AOS 13 (~9~0)

Rare-earth and LIL element fraciionation

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