The petrogenesis and setting of Archaean metavolcanics from Karnataka State, South India

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setting of Archaeam metavolcanics S. A. DRURY

Depi. of Earth Sciences The Open University, Milton Keynes MK7 6AA, United Kingdom (Received January 8, 1982; accepted in revisedjxm

November 12, 1982)

Abstract-Major and trace element data for ultrabasic to silicic metavolcanics from five Archaean voicanic-sedimentary belts in South India are summarized. Their total aheration to greenschist and amphiboiite facies assemblages poses probiems for petrogenetic interpretation of these data. However, in the absence of unmetamorphosed lavas, some interpmtation is essential. The data suggest that volcanism in each belt was petrogenetically similar. The presence of both light rare-earth element-enriched and -depleted lavas in individual vokzanic sequences together with variations in Zrfv ratios is interpreted as a consequence of fractionation within uhramanc mantie diapirs, induced by upward vapour migration. C&her chemical variations probably resulted from fracdonal crystalhxation of mahc phases from parcmal high Mg0, high Cr magmas, the influence of plagioclase fmctionation only being noticeable in late andesites and dacites. The consistent depletion of ah basaltic samples in the high field strength elements Nb and Ta relative to the low field strength element Th and to La further emphasises the importance of a vapour phase in their petrogenesis. in this regard they resemble modern erogenic basalts. The favaured tcctottic setting is that of ensialic, multiple back-arc basins above a thinner lithosphere than in modern back-arc environments, which rode over a shallow-angled subduction zone. mTRUI3UcxTUN METAVCXXXNIC rocks occur as prominent

comw nents of the majority of Archaean volcanic-sedimentary belts in the low grade terrain of Karnataka State, South India (see RADHAKRISHNA and VASUIIEV, 1977). They range in composition from peridotitic (MgO > 20%) through high MgO basaltic (MgO from 20% t&l 2%), basaltic (MgO < 12%) to andesitic and rhyodacitic fNAQVi and HUSSur$ 1973; Bt-wsKAR I&o and NAQW, 1978; NAQVI ef al., 1978; ANANTA IYER and VASUDEV, 1979; BHASKAR RAO and DRUW, 1982; DRURY, 1981, 1982a). The be&s have been subdivided into two sets by SWAMI NATH et 471.ff976), on the basis of differences

in metamorphic grade and structural complexity: the Sargur Supergroup comprising highly deformed masses enveloped by younger gneisses (dated at 3.6 3.1 Ga by BECKIHSALErrt al., 1980 and unpublished data), and the Dharwar Supergroup which are less deformed, autochthonous belts resting unconformably upon the gneisses. The Dharwar Supergroup has been further subdivided by SWAMI NATH d a/. (t976) into a @atformal sequence-Bababudan Group-and the supposedly youngest ‘geosynclinal’ sequence-Chitradurga Group. Baring in mind the rapid and complex facies changes both laterally and verticahy, within and between these belts, the great variation in tectonic level and metamorphic grade, and the as yet unresolved structu~ ~rn~~e~ty of the craton. it is possible that the subdivision merely reflects varying strain and metamorphism superimposed on a single, stratigraphically complex, post-gneiss volcanic-sedimentary association (DRURY. 1982b). in this paper, geochemical analyses of 102 sampfes from five belts (Fig. 1) are used: ( 1) to test the hy317

pothesis that the metavokanics are geneticahy related and only one vokanic-sedimentary association is represented by the three-fold subdivision, (2) to investigate the petrogenesis of Indian Archaean volcanism, (3) to evaluate the possibie tectonic setting of Archaean vokank-sedimentary behs in South India.

The samples were coilected from five separate belts: Kudremukb-West Coast (DR~RY, 1981), Bababudan tBlw_ RARKAQ and DRURY, 198 I X Shigegudda (DRuRY, f 982a), Holenarsipur (DRLJRY,1982a) and Kolar (unpubhshed data on samples of K. SHIV KUMAR obrtsinedby the author at this labmatory).Collectively, these represent belts assigned to both Sargur and D&war Supergroups by SWAMINATH cz d f t 976). Metavolcanic lava flows, dated at 3.0 Ga by an Sm-Nd method, form a thick unit at the base of the Kudremukhl+‘esf Coast belt, which rests unconformably upon gneisses dated at 3.3 Ga by an RbSr method (ENWRY ei ul., in prep.). The lavas are interieaved with ctioritic schists, prob ably representing basic t&s, sparse quartaites and coame amphibohte sills. They am overlain by band& iron formations interbedded with femtginous chlorite schists, succeeded by semi-pehtic gneisses which have miic turbidite strucmres. The belt is structurally complex, the upper units being isociinahy folded in large nappes which have suffered refolding by structures associated with a major N-S sinistral shear belt of presumed Arcbaean age (DRr_rRYand I-&XT, f9LQ DRURY, f98f Fig. i 1.The samples dmmssed are undeformed but pervasively metamorphosed at greenschist to lower amphibolite f&es. The Bababudan belt contains tbe most extensive and complete sequence of metavolcanics in South India. The stratigrapby- has been described by SwAMr NA’t-fi ei at. (1976), and the metavolcanics are found in the lower part of the sequence. The lowest Kalasapma suite comprises undeformed peridotitic to tholeiitic flows interbanded with detrital quart&es and &lo&e schists overlying the basal ohgomict conglomerate at the unconformity with 3.36 Ga old gneisses (BECKR‘ISALF. ei of., 1980). The Kalasapma suite is represented here by samples from the small Shigegudda

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FIG. 1. coOl~cpt sketchmap of Karnamka fa#k RADHAWSHNAand VASOLEV,1977) showing the main Archaeaa vokani~-&imentary belts as dark tone. Blank amas are Archaean gneisses and granites (sensu Iazo), tight tones in&ate post-Archaean cover.

beit which is an outlier of the Bababudan belt proper (DRURY, 1982a). This suite is uncan6ormably overlain by tholeiitic basalt to basahic-andeeite Bows of the LingadaMli suite. The upper volcanic unit, the Samaveti suite comprises th*tic baa& to thlsic Bows interbamM with quart&s (IJHAK&RRAO and DWRY, 1982). These are succe&d by a thick unit of handed ironstones and chforitic schists. The Holmarsiipi4r

belt is a smah high& defomr~ tridentshaped mass, enveloped by 3.36 to 3.10 Ga oid gneisss (BECKINSALEez al., 1980; Mzvr+SMr~ ef al., in prep.). Its relations to these gneisms are obscure. The stmdgmphy of the beh is virtually unknown because of large scale poiyphase dsformation, however SWAMINATH H ul. ( 1976)sug~~t~~r~D~~~~~nt.~~ arated by a deformed unconformity. Samples from HOIemusipurrepmsemboththesenotional supergrouprr(DRURY, 1982a). They comprise amphibolite f&es talc-tmmolite schists and amphibolitm. which are associated with thin met&t& and quart&s, thick pllraluminous pelites, and ferruginous cNoritic schists. The Koiur belt is dominati by U&dy deformed, steeply dipping meta-volcanic lava Bows, frequently showing orig-

inal stsuctures, in association with thin cherty horizons and ultramatic schists. The belt is bordered by metasedimentary units comprising banded ironstones, aluminous schists and deformed poiymict conglomerates. The belt’s relationship to the envefoping gneisses is obscufod by intense shearing and mylonitimtion paraM to the margins, and it has been assigned by various authors to both Sargur and Dharwar SUpergroUps. The foregoing descriptions raise three important difficulties relating to the interpretation of geochemical data from the rocks in question: 1. The pervasive rn~rno~~rn of ali samples and the intense deformation of some has erased all evidence whether the lavas crystallized from crystaifree liquids or not. All samples are assumed to have been aphyric lavas. The hydrous alteration probably mobilized strongly low field strength (MS) elements, such as Rb, K. Ba. Sr and Fb, and they are not used in the following discussions. High field strength

Archaean metavolcanics from South India

(HFS) elements, such as Zr, Hf, Ti, P, Nb and Ta, and the rare-earth elements (REE) are widely regarded as being immobile. However, it has been demonstrated (CONDIE et ul., 1977) that some of these elements were mobilized to varying degrees during hydrous and carbonic alteration of Archaean lavas in Barberton. In the complete absence of pristine Archaean volcanic rocks throughout South India, it is therefore nece%%uy to assume that, though HFS element movement may have occurred, it has not been so extreme as to ‘scramble’ totally geochemical trends related to petrogenetic processes. 2. Most, if not all the Archaean volcanic suites in South India were emplaced through older sialic crust. Evidence from modern continental volcanics (e.g. FRANCIS et al., 1980), suggests that reaction between crust and mantle-derived magmas may result in contamination of lavas. Late-Archaean disturbance of Pb and Sr isotope systems in South India lavas (N. H. GALE, pets. commun., 1980) means that such contamination is impossible to evaluate in this case. 3. Whereas many Archaean lavas bear some geochemical resemblance to modem lavas in diverse tectonic settings, there is not a single well-documented case of strdtigraphic or structural data allowing independant coniirmation of a uniformitarian tectonic setting derived from comparative geochemistry. Various theoretical arguments (e.g. DRURY, 1978; SMITH, 198 1) can be mustered to imply strongly that Archaean tectonics were significantly different to modem plate tectonics. Because of these unavoidable d&cult&s, conclusions from the geochemical data relating to petrogenesis and tectonic setting have a low degree of confidence. ANALYSES

Major and trace element data for all 102 rocks are published for Kudremukb (DRURY, 1981), Bababudan (BHASKAR RAO and DRURY, 1982), Holenarsipur and Shigcgudda (DRURY, 1982a), and available from the author for Kolar. All major and some trace elements were analysed using Phillips PW 1450 XRFs at the Universities of Birmingham and Nottingham. REE, SC, Hf, Ta and Tb data were obtained by instrumental neutron activation analysis at the Open University. Analyses of representative metavolcanics f?om each of the belts are given in Table 1 together with data on estimated precision and theoretical detection limits for each trace element. Estimated precisions are based on repeated analyses of various international standards MAJOR ELEMENTS The majority of metavolcanics exhibit a strong iron enrichment trend from primitive highly magnesian lavas to iron-rich tholeiitic basalts (Fig. 2) with the exception of the upper lava series in the Bababudan belt. The latter show only moderate iron enrichment and a trend towards more alkaline liquids, the most highly evolved rocks being Fe-rich andesites and Fe-rich da&es. These rocks are also distinguished from the bulk of volcanics by their higher TiOz/ Al& ratios (Fig. 3) and marked depletion in Cr, Ni and Co (NAQVI and BHAWAR RAO, 1978) as indicated in Fig. 4. The volcanics from the lower sequence in the Shigegudda belt and the other belts show low Ti02/A120, ratios. This

319

duality is evident in many Archaean terrains (e.g.ARNDT el al., 1977; VILJOENand VIUOEN, 1969; NAL,DRE~ and TURNER, 1977) and is best documented from Finland (JAHN et nf.. 1980) where a komatiitic-tholeiitic suite (low Ti02/A1203) was distinguished from a more strictly tholeiitic suite (higher Ti02/A1203). The bulk of the Indian volcanics are therefore of komatiitic-tholeiitic affinities. Whether the Bababudan tholeiitic to &cite suite is clearly separate on petrogenetic grounds depends upon an examination of other geochemical criteria. The mafic and ultramafic volcanics have a spread of MgO contents from 4 to 32 per cent and MgO/ZFeO ratios from 2.3 to 0.4. Diagrams of A1203, Ti02 and CaO versus MgO all show considerable scarier, but are similar to data from Archaean metavolcanic sequences in Africa (HAWKESWORTHand O’NIONS, 1977), Australia (NESBITTand SUN, 1976) and Finland (JAHN m al., 1980). The most highly evolved rocks, tbe andesites and dacites of the Bababudan belt, bave lower CaO and A1203 than the associated basalts. TRACE ELEMENTS

The approach used with the trace element data is to begin by examining elements that are likely only to reflect melting and fractionation, whereby possible lines of liquid descent can be evaluated. These can then be applied to variations in elements that possibly reflect heterogeneities within the source mantle. Finally, elements that appear to behave differently in magmagenesis at different modem tectonic settings are employed as a means of characterizing the possible settings of the volcanic-sedimentary belts thems&KS.

Cr-MgO and Ni-MgO diagrams These diagrams reveal clearly the different influences of the magnesian minerals olivine, py-roxene and spine1 during magmatic evolution, because of their great differences in mineral-melt distribution coe5cients for Cr and Ni. All the m&c and ultramafic samples lie in the same fieids and are part of the same clear trends on semi-logarithmic Ni-MgO and Cr-MgO diagrams (Fig. 4). Abundances of Ni and Cr, although positively correlated with variations in MgO, do not show simple linear relationships. The trends on both NiMgO and Cr-MgO diagrams are virtually identical to those displayed by other Archaean komatiitic-tholeiitic associations (NELSBI-I-Iand SUN, 1976; HAWKESWORTHand O’NIONS, 1977; JAHN el uf., 1980). Gradual increase in MgjNi ratio with MgO is reflected by a markedly curved trend of Ni versus MgO

(Fig. 4a). In more magnesian rocks, Ni shows considerably more scatter than do the Cr data. This scatter is unlikely to reflect analytical errors. The Cr-MgO diagram (Fig. 4b) shows a variation in Cr/Mg ratio, but unlike the Ni-MgO data, the variation can be resolved into two components. one characterized by a 5at trend for MgO > 12 per cent, the other showing a rapid decrease in Cr with decreasing MgO for the basaltic rocks. Moreover, the Cr contents of the high MgO rocks extend to values in excess of the estimate for the Archaean mantle (3000 ppm. SUN and NESBITT, 1977).

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Archaean metavolcanics from South India

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FIG. 2. Diagram of Nafi + &O: total Fe (as FeO): MgO for all samples. Dots: Kudremukh: inverted f&d triangles: 3ababudan; inverted open triangks: S~g~ud~ filled triangles: Holenarsipur; squares: Kolar.

CY-Y diagram The petrogenetic usefulness of plots of Cr against Y for mafic and ultramafrc volcanics has been discussed by PEARCE (1980) (Fig. 5). Neither Cr nor Y appear to be involved in processes that create mantle heterogeneities, which are thdught to involve the movement of fluids and melts containing only those elements that are imcompatible with gamet-lherzohte mineralogy. Both Cr and Y are strongly partitioned into pyroxene and spine& and garnet respectively. Thus it is justifiable to assume that most primary magmas produced during the Earth’s history had sources with similar Cr and Y abundances, as-

sumed here for reference to be those estimated for the Archaean mantle by SUN and NESBITT (I 977). Furthermore, both Cr and Y are HFS elements and widely regarded as being among the most immobile elements used in petrogenetic studies. Genetic pathways are shown on the diagram, for reference, and comprise a melting trend and a fractional crystallization trend descending from the melting trend, after PEARCE (1980). The field of Indian Archaean metavolcanics is a continuum, from peridotitic, high Cr, low Y rocks through high MgO basaltic compositions to basaltic andesites. The scatter may conceal separate trends from different belts. The high-Cr rne~~~dotit~

14 A1203 % FOG.3. Ti02-A&O3 diagram. Symbols as Fig. 2.

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have Y, as well as other incompatible eh?ment abundances above those of chondrites. The trend intersects PEARCE’S ( 1980) meiting curve at around 30 per cent melting, but continues to much higher Cr and lower Y values. Thus, in common with many Archaean volcanic suites. a spectrum of liquids, &ozn prinlitive to highly evolved, was erupted, even within sir&e vokanic sedimentary belts such as Holenamipur, Kok and Kudremukh. In the Babe&tan belt, some primitive liquids were erupted in the lower sequence G&igegudda), but the higher m&c Iavas are strongty evolved. TiO- Zr, Y- Zr and Nb Zr diagrams These diagram (Fig. 6) wert or&inaUy devised by PEARCE and CANN (1973) as a means of exnpiricaUy discriminating between basaltic lavas of di%rent teetonic setting. More recent data on modem basaks

[e.g. SAUNDERS et crl., 1980) reveais that they are neither pncise nor consistent. However they have important uses in petrogenetic stud% since the dement pairs disptay wide variations in solid-Iiquid partition coefficients for the minerals generally involved in magma fractionation. Vectors representing liquid trends imposed by up to 50 per cent fractional crysWizatiozk of various minerals (&er -WE and NORRY, 1979) are shown for reference. The ukramafk to mafic metavolcanics show a clear trend of incxeasing ‘IX&, Y and Nb with increasing Zr. &ted to decreasing Mg0 and scatter about the avemge ZrjTi, Zr/Y and Zr/Nb ratios for the Archaean mantle, similar to Archaean metavolcanics from A~suaiia{N~s~i~~ and SUN, ! 9761. Sikic lavas kom the upper sequence in the &&abudan belt show cieariy as Y, Nb and Zr-enriched rocks, but are strongly depleted in Ti.

Archaean metavolcanics from South India

323

RAO and DRURY, 1982) similar to that in the Cenozoic basaltic andesite-andesite-ignimbrite association ofthe American Cordillera (e.g. THORPE trtal., 1979). B/Y-Zr

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RG. 5. 0-Y dkgrams (see PEARCE, 1980, Fig. 11). For referena, trends are shown reflceting meit composition for

dikent degrees of source melting (bold line) and fractional wstalkation of mafk phases (fine line). Tbe first trend was derived by &AWE (1980) using a model of equilibrium partial melting of a source with olivine, ortbopyroxone clinopyroxene and plagioclase in the proportions 6:2: 1:1, which melted in the proportions 3: 14~4. The large asterisk represents a model source with Cr and Y abundances of the Arcbaean mantle estimated by SW and NESBITT(1977). The fkactiomd crystalhxation trend was derived by PEARCE (1980) assuming an arbitrary parent composition on the melting trend and using a model for fractional crystahiz&on of olivine + spinet f ortbopyroxene+ clinop~xene. The proportions of these mine& have negligible effects on tbe slope or extent of tbe trend.

REE Patterns showing both light REEenricbment and slight depletion relative to flat chondrite-normalized patterns occur (Fig. 7) although the enriched type is predominant. Ultramafrc and mafic examples of both rypes are present. However, all the analysed samples from the Kudremukh and Bababudan belts are light REE-enriched, the depleted types being restricted to the Kolar. Holenarsipur and Shigegudda belts (1266) where they are associated with enriched varieties. Total REE abundances increase with decreasing Cr. Ni and MgO. although there is much scatter. Rare-earth element patterns for the most highly evolved lavas in the upper Bababudan volcanic sequence (Fig. 7b), ranging from tholeiitic basal& through basaltic andesites to dacites, show a regular increase in ZREE with increasing Si02. This is paralleled by a trend of increasingly negative Eu anomalies, and progressive depletion in Sr (see BHASKAR

diagram

PEARCE:and NORRY ( 1979) used the Zr/Y-Zr diagram, in conjunction with evidence on fractional crystallization constrained by Cr data, to model the variation observed in modem basaltic lavas. The Indian data are scattered (Fig. 8) but the majority define a trend encompassed by two of PEARCEand NORRY’S ( 1979) petrogenetic pathways which reflect equilibrium partial melting of an Archaean mantle source with a garnet-free assemblage (I) or a similar source containing garnet (II). Similar distributions characterize Archaean metavolcanics from Australia (NESBI~ and SUN, 1976) and Greenland (MCGREGOR and MASON, 1979). The most magnesian compositions plot close to the values estimated for the Archaean mantle and seem to fall on the same trend as basaltic compositions. However, PEARCE and NORRY (1979) showed that fractional crystallization models for Zr/Y and Zr covariations produce trends that are more nearly parallel to the Zr axis on Fig. 8. Such trends are followed more closely by modem basaltic suites than melting trends. Clearly, the data on Fig. 8 cannot be considered as reflecting any simple model. LIL-HFS element diagrams An important empirical observation on modem basahs is that there is a marked contrast in the ratios of LFS elements to HFS elements between erogenic and non-erogenic settings (SAUNDERS er al., 1980) basalts from erogenic settings being consistently and strongly depleted in HFS elements such as Ta and Nb. compared with non-erogenic bar&s with similar LFS element contents. Indian Archaean metavolcanics are depleted in Ta, as reflected by linear La-Ta and Th-Ta diagrams (Fig. 9). They fall on such clear linear trends that there is little doubt that abundances of these three elements have not been disturbed by metamorphism. The trends are closely similar to those revealed by modem basalts for erogenic settings. but bear no resemblance to those in modern basalts from non-erogenic settings. A similar but more scattered trend is shown by less precisely determined La, T’h and Nb data from all samples. Plots of the Indian data on other discriminant diagrams, such as Th-Hf-Ta (WOOD et al., 1979), those using Zr, Ti, Y and Nb (PEARCE and CANN, 1973) and Th/Yb-Ta-Yb (PEARCE et al., 198 1) also show strong resemblance to modem basalts from erogenic settings. It is interesting to note that precise La, Hf, Th and Ta determinations for Archaean metavolcanics from high grade terrains (unpublished data of the author for 2.9 Ga Scottish Archaean granulites) do resemble those from modem non-or~enic settings.

324

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Correlation between belts The sampIes from Holenarsipur and Kolar have been suggeskd by some to have been emplaced before sialic crust formation in the South Wian craton (SWAMI NATH et (II., 1976) possibiy as primitive oceanic-type crust (NAQW et of., 1978). Those from Kudmmukb, Bababudan and Shigegudda are accepted by aii modern workers as post-dating the dominant gneisses of the craton, that is. they had an ensialic setting. The variation diagrams (Figs. I! to 9) do not permit

a division of the metavolcanics into two such distinct groups on geochemical grounds. Apart from the LFS HFS diagrams in Fig. 9. geochemicai trends among the sampks are diffise, either through element mob&y during alteration or, conceivably in some cases. contamination by pre-existing siak crust. Consequently, while it is impossible to separate suites from different belts, the data are inadequate to assign metavolcanics from all the belts to a single cogenetic suite of lavas. Nevertheless, clear geochemical trends are present in all the diagrams to which members of ail suites belong. Bearing in mind the uncertainties of element mobility during metamorphism and the possibility

325

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of crustal contamination with some elements, it is necessary to attempt some interpretation of petrogenesis and tectonic setting for these metavolcanics. There is no material of better quality throughout South India. 10.0

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FIG. 8.Zr/Y-Zr diagram (see PEARCE and NORRY,1979). Symbols as in Fig. 2, stars: Bababudan silicic rocks. The two reference curves represent melts produced by models of equilibrium partial melting of Archaean mantle source (asterisk: SUN and NESBI~, 1977) containing either: curve 1, olivine (60%)+ orthopyroxene (20%) + clinopyroxene (10%) + plagioclase (10%); or curve II, olivine (60%) + otthopyroxene (20%) + clinopyroxene ( 10%)+ garnet ( 10%) and extend to 5 per cent melting at their Zr-rich extremities. Trends nz&cting closed system fractional crystallization of mafic minerals k plagioclase would be near-horizontal vectors (see PEARCE and NORRY, 1979,Fig. Sa).

Petrogenesis Major element diagrams (Figs. 2 and 3) reveal correlations that suggest that the major controlling phases during magmagenesis were magnesian, but cannot indicate whether the variations reflect different degrees of melting, fractional crystallization or several unrelated parent magmas. The covariation of Ni with MgO in ultramafic to mafic samples (Fig. 4a) must mainly reflect petrogenetic control by olivine, which is the only rockforming silicate which strongly partitions Ni. Another possible influence is the removal of Ni from magma by immiscibie sulphide liquids, and it has been noted (DRURY, 1982a) that some ultramafic samples from Holenamipur have unusually low Ni concentrations compared with their fellows, which may indicate local Ni-rich sulphide segregations. The diffise curved trend of Ni versus MgO (Fig. 4a) can be explained (see JAHN et al., 1980) by the strong temperature dependence of Kod, (LEEMAN, 1974) ranging from 1 to 10 in the range 1600” to 12OO”C, corresponding to peridotitic and basaltic melt temperatures respectively (GREEN, 1975). The near-mantle abundances of Ni in the most ultramalic samples probably reflects high temperature melting of peridotitic magmas where PAi ZT 1.0. The full trend on Fig. 4a could reflect either temperature-dependent, differential partial melting of a mantle source, or con-

326

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FIG. 9. a) La-Ta diagram; b) Th-Ta diagram. Symbols as in Fig. l tinuous fmctionation of otivine during cooling of primary, high-MgO liquids. The stepped trend in the Cr-MgO diagrams (fig. 4b) is similar to that noted for Finnish Archaean metavolcanics (JAHN et al., 1980). and is difficult to explain other than by fractional crystallization. The slow decrease in Cr for MgO greater than 12 per cent could result from the dominance of olivine removal from an ultrabasic liquid together with minor pyroxene and/or spinel. The more rapid decrease in Cr and sharp increase in slope for samples with less than 12 per cent Mg0, implies the dominance of pyroxene It spine1 fractionation in basaltic magmas. The very high Cr contents of ultramatic rocks co&d imply that they were alI pyroxene + spinel-rich cumulates. Their high contents of incompatib elements militates against a cumulate origin to some extent, but this could reflect trapped liquid. Similar, primary Cr-enrichment in high temperature melts could have resuited from total melting of pyroxene and spine1 in the source leaving a dunite residue (GREEN, 198 1) whose Kc, would be 0.6 to 0.8 (AKELLA d al., 1976). In this account of Ni. Cr and MgO variations I agree with the interpretation by JAHN et al.. (1980) for a broadly similar set of Finnish metavolcanic suites. The strong trend on a Cr-Y diagram (Fig. 5) definitively resolves whether the metavolcanics derived from a series of primary melts formed by varying degrees of mantle melting or by progressive fractional crystallization from t&ram&c liquids. Quite clearly, the trend cuts at a high a&e the model line for liquid composition derived by melting. The trend of steep decline in Cr with increasing Y indicates the dominance of fractionation of olivine + pyroxene f spine1 from primitive liquids. There is no sign of any influence by plagioclase fractionation. where Y increases rapidly with small decrease in Cr PEARCE, 1980. Fig. 1I ). This is probably because the evolving liquids

were highly magnesian and required more than W weight per cent separation of mafic phases before plagioclase became a liquidus phase. Although the trend intersects PEARCE'S (1980) model melting curve at around 25-35 per cent melting, this is possibly insignificant. as the trend continues to higher Cr and lower Y values. As explained above. the probability that Archaean peridotitic magmas separated from pure magnesian olivine residues (GREEN, 198 1J at high (cu. 1600°C) temperatures wouid result in high0 primary magmas. The most ultramatic samples possibly represent around 60 per cent melting of mantle. A model of mafic mineral fractionation from parental ultrabasic magmas explains adequately the overall trend of Y, Nb and i!r enrichment (Figs. ba and b). There is no sign of amphibole fractionation which would produce strong Nb and Y depletion in highly evolved liquids. The strong Ti depletion and Zr enrichment in silicic samples from Bababudan can only be explained by fractionation of an Fe-Ti oxide phase, such as titaniferous magnetite, late in the evolution of the local magmas. it may be purely fortuitous, but there is a clear spatial and stratigraphic correlation in many South Indian greenstone belts between silicic lavas and pyrociastic rocks. and titaniferous magnetite-rich sandstones which could be volcanoclastic in origin. Rare-earth element data (Fig. 7), which show an increase in ZREE with decreasing MgO, Ni and Cr. are consistent with the view that matic mineral fractionation dominated evolution of all the suites. However, the depletion of silicic samples in Eu indicates that plagioclase fractionation became important when the magmas involved had become highly evolved, as it seems to have done in the case of Cenozoic bdtandesite-ignimbrite associations in the Andes (THORPE ef ai.. 1979).

Archaean metavolcanics from South India

The most important feature of the REE data is that they show a close spatial association of both LREE-enriched and LREEdepieted lavas of both ultrabasic and basic composition in some of the belts, which is common in other Archaean terrains. Preliminary Nd isotopic data (this laboratory) indicate eNd for LREE-enriched Kudremukh samples, which is close to zero. To a limited degree, this militates against a view that long-lived Archaean mantle heterogeneity beneath South India produced the dichotomy (see SUN and NESBITT, 1978). Data from other Archaean terrains (DEPAOLO and WASSERBURG. 1976; HAMILTON et al., 1977, 1978. 1979) also indicate CN~for Archaean volcanics close to zero, implying that variations in LREE must have been produced very shortly before or during magmatic evolution. Many of the LREE-enriched samples have low Sc/ Hf ratios compared to those showing LREEdepletion. Garnet has high KHREEand high J?, and the data may be compatible with the influence of garnet either in the source or on a liquidus phase during fractionation of-the LREE-emiched samples (JAHN et al., 1980). However, the LREE-enriched samples have nearly flat HREE patterns, whereas the infhtence of garnet should produce strong and increasing depletion in the progressively heavier REE. Distribution coefficients for partitioning of REE between mantle minerals and Hz0 (MYSEN, 1978) and COz (WENDWNDT and HARRISON, 1979), suggest that at depths greater than 70 km in the mantle a vapour phase may become enriched in REE, particularly LREE. Its removal might produce LREEdepletion in the mantle, whereas the strong pressure dependence of the distribution coelIicient implies that its migration to higher levels might enrich rocks there in LREE while leaving HREE with essentially flat patterns (HANSON, 1981). As well as possibly explaining the origin of long term mantle heterogeneities, this process could also occur during the rise and evolution of a mantle diapir, thereby producing coeval and, in a broad sense, cogenetic magmas with contrasting REE patterns. Lavas depleted in LREE may have formed deep in such a diapir or early in its rise, those with enriched LREE having formed at high levels. Such bimodality may also be explained by dynamic melting (LANGMUIR et al., 1977) in a rising homogeneous mantle diapir. Large degrees of melting may produce LREEdepleted magmas, the residue providing LREEemiched magmas if melted at a later stage (HANSON, 198 1). However, both basic and ultrabasic samples described here show both LREE enrichment and depletion, implying that degree of melting is not the main control on REE composition of the magmas. Variations of Zr/Y ratio with Zr (Fig. 8) reveal a complex picture. Some of the ultramafic samples suggest high degrees of melting of SUN and NESBITT’S (1977) Archaean mantle. Others have much higher ~r/u ratios than is consistent with such a model. More evolved, basaltic samples show a similar range

327

of Zr/Y ratios, Aside from complete ‘scrambling’ of data by alteration, the simplest explanation for the spread is that the samples represent melting from sources with a range of Zr/Y ratios followed by fractional crystallization of mafic phases, with or without feldspars. This would produce a range of roughly horizontal trends, according to the models of PEARCE and NORRY (1979, Figs. 4 and 5) each beginning from a melting curve pamllel with trend I on Fig. 8. As outlined in the description of the Cr-Y diagram, it can be argued that the Earth’s mantle has a homogeneous Y distribution. In that case, variable Zr/ Y ratios in the source of the metavolcanics in each Indian greenstone belt would reflect variation in Zr content of the source mantle. Both long term source heterogeneity or differential metasomatism of the source could account for this. In the latter model, the presence of some agent, such as F released by breakdown of mantle mica or amphibole, (LLOYD and BAILEY, 1975), which forms volatile complex ions with Zr, would be required in the vapour phase.

Tectonic setting The depletion in the HFS elements, Nb and Ta and enrichment in the LFS element Th displayed by all the ultramalic to mafic samples (e.g. Fig. 9) is similar to the patterns in modern basalts from orogenie settings such as island arcs and back-arc basins, (SAUNDERSet al., 1980). In modem erogenic basalts the LFS element enrichment is ascribed to the in5uence of LFS-enriched vapour phases rising from dehydrating subducted lithosphere into the overlying mantle wedge and adding mobilized LFS elements to it (RINGWOOD, 1974; THORPE m al., 1979). This LFS-emiched wedge is then supposed to become the source region for modem erogenic basal& being induced to melt by high &a. High PO2 in the wedge allows the stabilization in the residue of oxide phases such as magnetite or ilmenite which have high Kr,.~b, so that melts derived from the wedge are strongly depleted in these elements (SAUNDERS et al., 1980). The contrast with modem LFS-enriched basalts from non-erogenic settings may probably be ascribed to the absence of mantle permeated and metasomatized by migrating vapour phases in these settings. However, the similarities between the South Indian rocks and modem erogenic basalts cannot justify a syllogistic conclusion that they occupied tectonic settings identical to those present today. An idea of the true nature of their setting rests on data about Archaean crustal evolution in South India. The bulk, if not all the greenstone belts of South India developed upon continental crust which was emplaced between 3.4 to 3.1 Ga ago (BECKINSALE et af., 1980; REEVES-SMITHet al., in prep.). This crust was subsequently thickened and reworked to form late Archaean granulites, plutonic granites and granodiorites at 2.55 Ga (REEVES-SMITH et al., in prep.) after a lengthy period of deformation involving highlevel nappes, major transcurrent shearing, and east

to west imbrication by listtic thrust-shears (DRURY. 1982b). The mod& for Archaean volcanicity introduced by WEAVER and T>&#tr%Y ( 1979) invOtVi~g the tt’&@titQ of m&e diapirs fmm subducted shIbsis palticularly attractive, and could be supported by the data. It would explain the metavolcanics’ par& simikity to modern back-arc basin basalts and the source for the vapor phase th8t appears to have played an important role in the bhnodality of some of their ge+ chemical charac~Wcs. If that was the case, there still remains the problem of ~~~ ~hemi~y similar magmas over a very gmat width (a450 km) of continental crust (Fig. I+probably much wider if the effkts of later cmstaI ~~~~ an: removed. lXapirs gewmted from steeply dipping subduction zones would first be expected to enter only a narrow overlying zone as in modem sing& back-arc basins. Second, d&rent trace element patterns would result from diRerent depths of formation. One explanation of the para&1 ‘swarm’ of Indian v&anic-sedimentary belts is that the controlling subduction zone was shallow-angled (DRURY, 1978) involving thin, hot iithosphete so that diapirs were triggemd over a wide afi# &om similar depths. The South Idian beits might therefore be relics of multiple, en&a& backarc basins.

1. Afchatpn meulvolcanic m&s from South India form a single geochemicai population, which may indic&e that either a hgk major qfck of magmatic activity is nprescnted by variably d&rmed and metamorphosed vokanikx&i.mentary be& or sevet& cycles involving similar sources and p@mgenetic proW#es are represented. 2. The variations in REE patterns and ZrfY ratios in the whok population and in si@e suites resulted from t&e m&&on of vapour phases within source diapirs. Prim&e liquids emanating from deep kvefs at the surf&e. They show evinthcdiapirsem idence of high degrees of partial melting from a source similar to &mates of Amhaean mantie wbictr had been leached of LREE by a migratittg vapour phase, and are Cr-rich sag a dun&c residue a&r me&n& The bulk of the basaltic lavas ate more evolved through fractionation of m&c phases from the parental uluabasic liquid. Their ~ommnn LREE enrichment and variable ZrfY ratios reflect metasomathnn at shallow depths in the diapirs by an enriched vapour phase. 3. Minor a&es&es and da&es, the youngest metavolcanics, arc depleted in Ti and Eu and probably reflect fractionation of piagioch~ and Fe-Ti oxides in a shallow magma chamber. 4. The consistent depktion in the HFS elements Nt, and Ta, and etichment in Th ofbasaltic samples. further emphasizes the roie of a vapour phase in their evoiution. In this regard they bear stmng mmblance to modem ba&s in orogenic settings.

5. The favoured tectonic setting for the magmatism is that suggested by WEAVER and TARNE’~’ f l979)diapirism above a sh&low-angkd subduction zone, and magma emplacement through dder crust into multiple back-arc basins. Acknowi&crn&%w--‘The 6eid work warisuppocted by NERC (GR31366Sj and the author was seat& &ded by being ace comp~zdcd in the fted by S. J&as& (DM~, ~Kama&a Stat&. S. M, Nawi INGRIj. R. W. Halt and G. R. ReevesSmi& (Open U&&y). k. Shiv KUM~ (Department of Atomic Energy, India) kindly supplied the Kdar sampies, Help with an&es was &en by P. J. Pufts, 0. Wil&msThoroe. P. Webb 0Duen Utivexsitvl. P. K. Harvev (Nottin&m University) &d 5. W. We&x (Birming&m‘Uniwsityj. Typing was by D. Whyte. The paper was improved and s&a& on the basis of commems by 5-M. f&m. B. J. Fryer and F. A. Frey. lluEFERENcEs

AKEuA I.. Wiw.uds R. f. and MVIQNS0. (!976) Sotutility of Cr, Ti and Al in co&sting &4ne, spinet and liquid at iatm. Proc. .L.unar Sci ConJI:7th. 2, 1f79+11%. ANANTA IYER G. V. and VAS~DEVV. N. ( i 9793 &&emistry of the ArcI&%- mctavokank rocks d-K.&r and Hutti gokiFie&, Katnataka, India. 1. fTec&SQC.It&u to, 419-432. AKNM N. I-., NALD@QT A. J. @&dFVKE D. R. (1977) Komatiiticand iron-rich tb&i& of Mmso Township, Nor&t&St untario* J. P@roL is. 319-369. 5sxt~sws R. D., DRURY S. A. &i WOLTR. W. (1980) 336oM yr oki gtwwz? from the So&I Indian craton. Ivature 283, 469470. 5ruslu~ R40 Y. J. and NAQW S. kl. (1978) Geochemisw of metawicanim t%om the B8k&wian schist be& a fate ~~~y~v~~~~e~ fadis. h Arc~~~~~ (eds. B. ‘$?~~~iey aad S. M. Naqvij pp. 325. Elsetier. B?wiua RAo Y. J, and DRURY S. A. (1982) Iscompatible trace element geocbcmistry of Amkaew aetavoleanic rock3 from the Bababudan vale belt, Kanrotalra .I. Ge&. Sot, It&a 23, i-t 2. CoK. C., VUJO~N M. J. and &AtLE E. J. D. (1977) E&cts of lnltcmtion on ekmcnt d.istributio~s in Afcbiwn thoIeiites &urn the B&es@& gseenstone be&, South Africa Cow&~ MitkmiL Pm&. $4 7549. DRURY S. A. ( 197%)5&!&cFiiictorsin A&ta@n geotectonics. In Archman Getxhentt&y feds. 8. F. Windley and S. M. Naqvi) EIbevier. MtURY S. A (1981) &o&emistry of Archaeari metavolcanic rocks &om the K&em&b wza, Kamataka. J Geoi &x. fndia,ZZ 405-416. DRURY S. A. f 1982aj burns of A&wean metavoicanic rocks from the Hoknarsipur and Sbigegudda vatcam3scdimeaury belts of Kamarak~ South India. Precar&r&n Res. {in prrtss). D&WRY S. A. (1982b3A rc&M tectonic SW@of the Archaean Chitradm grcestanebelt. Karnata& baaedon W&at interpreta&n. J. Gwf. Sex: M’iu fin press). DRURY S. A. and HOLT R. W. I198OjTbc tectonic fmmework of t&e south x%x%%%% cmtos: a recon~ iffvalving Landsat imagery ~~~~~~jc~ 65, T 1-T I 5. -pAOLo D. J. aad WASS~RBURGG. J. (1976) Nd isotope variations and petro8cnetic models. Geophys Res. Lrr6. 3,249-252. FRANCISP. W., THORPE R. S.. MOORBATHS_ KRETZSCHMAR c. A. at& %[email protected]. (r98@) Stromiom isotope evidence for cm&alanon of c&-alk&ne volcattic

I.Bcks

&Ott%

cm

Gab,

notthwcst

Earth Plamt. Sci. Lett. 48.257-269. GWEN D. H. (197% Gcncsis of Arc&&n

4qentina

p&d&tic

mag-

Archaean metavolcanics from South lndia mas and constraints on Archaean geothermal gradients and tectonics. Geoiom 3, 15- 16. GREEN D. H. ( 1981) Petrogenesis of Archaean ultramafic magmas and implications for Archaean tectonics. In Precambrinn PIale Tectonics (ed. A. Kroner), pp. 469. Elsevier. HAMILTONP. J.. O’NIONSR. K. and E~ENSENN. M. (1977) Sm-Nd dating of Archaean basic and ultrabasic rocks. Eanh Planet. Sci. Leff. 36, 263-268. HAMILTONP. J., O’NIONS R. K., EVENSENN. M.. BRIDGEWATERD. and ALLAARTJ. H. (1978) Sm-Nd isotopic investigations of Isua suprac~stais and implications for mantle evolution. Nature 272, 41-43. HAMILTONP. J., EVENSENN. M., O’NIONS R. K., SMITH H. S. and ERLANI( A. J. (1979) Sm-Nd dating of Onverwacht group volcanics, southern Africa. Nature 279,298300. HANSON G. N. (198 1) Geochemical constraints on the evolution of the early continental crust. Phil. Trans. R. Sot. Lond A 301,423-442. HAWKE~WORTHC. J. and G’NIONS R. K. (1977) The petrogenesis of some Archaean volcanic rocks from southern Africa. J. Petrol. 18, 487-520. JAHN B.-M.. AUVRAY B.. BLAIS S., CAPDEV~LAR., CORNICHETJ., VIDAL F. and HAMEURTJ. ( 1980) Trace element geochemistry and petrogenesis of Finnish greenstone belts. J. Petrol. 21, 201-244. LANGMUIRC. H.. BENDER J. F., BENCE A. E.. HANSON G. N. and TA~OR S. R. (1977) Petrogenesis of basalts from the FAMOUSarea: Mid-Atlantic Ridge. Eurih Phznfl. Sci. Leu. 36, 133-156. LEEMANW. P. ( 1974) Experimental determination of partitioning of divalent cations between olivine and basaltic liquid. Unpubl. Ph.D. Thesis, University of Oregon. Part II, pp. 23 l-303. LLOYDF. E. and BAILEYD. K. (1975) Light element metasomatism of the continental mantle: the evidence and the consequences. Phys. Chem. Earth 9, 389-416. MCGREGORV. R. and MASON B. (1977) Petrogenesis and geochemistry of metabasahic and metasedimentary enclaves in the Amitscq gneisses, West Greenland. Amer. Mineral. 62, 887-904. MYSENB.-D. (1978) Experimental determination of crystalvapour prrtition coe5cients for rare-earth elements to 30 kbar pressure. Carnegie Insl. Wash. Yearb. 78,689-695. NAKAMURAN. (1974) Determination of REE, Ba, Fe. Mg, Na and K in carbonaceous and ordinary chondrites. Geechim. Cosmochim. Acta 38, 757-775. NALDRET A. J. and TURNER A. R. (1977) The geology and petrogenesis of a greenstone belt and related nickel sulphide mineralization at Yakabinde. Western Australia. Precambrian Res. 5, 43-103. NAQVI S. M. and HUSSEINS. M. (1973) Relation between trace and major element composition of the Chitradurg metabasalts, Mysore, lndia. and the Archaean mantle. Chem. Geof. 11, 17-30. NAQVIS. M. and BHASKARRAO Y. J. (1978) Geochemistry of metavolcanics from the Bababudan schist belt: a late Archaean/early Proterozoic volcano-sedimentary pile from India. In Archaean Geochemistry(eds. B. F. Windley and S. M. Naqvi). pp. 325-341. Elsevier. NAQVI S. M.. VISWANATHANS. and VISWANATHAM. N. (1978) Geology and geochemistry of the Holenarsipur

329

schist belt and its place in the evolutionary history of the Indian Peninsula. In Archaean Geochemistry (eds. B. F. Windley and S. M. Naqvi). pp. 109. Elsevier. NE~BIT-~R. W. and SUN S. S. (1976) Geochemistry of Archaean spinfex textured peridotites and magnesian and low magnesian tholeiites. Eunh Plana. Sci. Letr. 31,433453.

PEARCEJ. A. ( 1980) Geochemical evidence for the genesis and eruptive setting of lavas from Tethyan ophiolites. In Proc. Inremational OphioliteSymposium, Cyprus. 1979. pp. 261. Geological Survey Department, Nicosia. PEARCEJ. A. and CAM J. R. (1973) Tectonic setting of basic volcanic rocks determined using trace element analyses. Earth Planet Ser. Lett. 19, 290-300. PEARCE 1. A. and NORRY M. J. (1979) Petrogenetic impiications of Ti. Zr. Y and Nb variations in volcanic rocks. Contrib. Mineral. Petrol. 69, 33-47. PEARCEJ. A.. ALABASTERT., SHELTONA. W. and SURLE M. P. (1981) The Oman ophiolite as a cretaceous arcbasin complex: evidence and implications. Phil. Trans. Roy. Sot. Lond. A 300, 299-317. RADHAKRJSHNA B. P. and VASUDEVV. N. (1977) The Early Precambrian of the southern Indian Shield. J. Geol. Sot. India 15, 439-456. RINGWOODA. E. (1974) Petrological evolution of island arc svstems. J. Geol. Sot. L.ond. 130. 183-204. SAUNDERSA. D., TARNEY J., MARSH N. G. and WOOD D. A. ( 1980) Ophiolites as ocean crust or marginal basin crust: A geochemical approach. In Proc. International OphioliteSymposium, Cyprus, 1979. pp. 193. Geological Survey Department, Nicosia. SMITH J. V. (1981) The first 800 million years of Earth’s history. Phil. Tram Roy. Sot. Land. A 301, 401-422. SUN S. S. and NESBIT?R. W. ( 1977) Chemical heterogeneity of the Archaean mantle, composition of the Earth and mantle evolution. Earth Planet. Sci. Lett. 35, 429-448. SUN S. S. and NESBITT R. W. (1978) Petrogenesis of Archaean ultrabasic and basic volcanics from rare-earth elements. Conrrib. Mineral. Petrol. 65, 301-325. SWAMI NATH J.: RAMAKR~SHNANM. and VISWANATHA M. N. (1976) Dhatwar stratigraphic model and &unataka Craton evolution. Rec. Geol. Surv. India 107, 149175. THORPE R. S.. FRANCISP. W. and MOORBATHS. (1979) Ram-earm and strontium isotope evidence concerning the petrogenesis of North Chilean ignimbrites. Earth Planet. Sci Len. 42, 359-367. VIIJOEN M. J. and VILJOEN R. P. (1969) The geology and geochemistry of the lower ubramafic unit of the Onverwacht Group and a proposed new class of igneous rock. Spec. Pub/. Geol. Sot. S. Afr. 2, 55-85. WEARER B. L. and TARNEY J. (1979) Thermal aspects of komatiite generation and greenstone belt models. Nature 279,689-692. WENDLANDTR. F. and HARRISONW. J. (1979) Rare-earth partitioning between immiscible carbonate and silicate liquids and CO2 vapour: results and implications for the formation of light rare-earth enriched rocks. Contrib. Mineral. Petrol. 69. 409-419. WOOD D. A.. JORON J.-L. and TREUIL M. (1979) A reappraisal of the use of trace elements to classify and discriminate between magma series erupted in different tectonic settings. Earth Planet. Sci. Let; 45, 326-336.