Origin of trondhjemitic biotite—quartz—oligoclase gneisses from the Venezuelan Guyana Shield

Precambrian Research, 3 (1976) 317--342

317

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

ORIGIN OF TRONDHJEMITIC BIOTITE--QUARTZ--OLIGOCLASE GNEISSES FROM THE VENEZUELAN GUYANA SHIELD

THOMAS W. DOUGAN

Department of Geology, Allegheny College, MeadviUe, Pa. (U.S.A.) (Received July 3, 1975; accepted October 10, 1975)

ABSTRACT Dougan, T.W., 1976. Origin of trondhjemitic biotite--quartz--oligoclase gneisses from the Venezuelan Guyana Shield. Precambrian Res., 3: 317--342. Biotite--quartz--oligoclase gneisses constitute the dominant lithology in a 3-b.y.-old metamorphic assemblage in the Venezuelan Guyana Shield. The assemblage includes basaltic amphibolites as well as granitic gneisses, iron formation, and other metasedimentary lithologies. Consideration of major- and trace-element compositions indicates that both the biotite gneisses and basaltic amphibolites are meta-igneous. The amphibolites have oceanic tholeiite compositions. Two groups of biotite gneisses can be differentiated by chemical criteria. Both groups have major and trace-element compositions which allow their derivation as partial melts of tholeiite compositions at mantle depth, as has been suggested for similar rocks in other areas. However, the compositional correspondence of the gneisses with low variance liquidus loci in low-PT synthetic systems, and their relatively oxidized character, appear more compatible with an origin b y partial melting of graywackes at crustal levels. F o r both groups of gneisses, primary melts with relatively low Na 2O/K= O ratios can be postulated which are credibly derived b y melting of graywackes; reasonable fractionation processes can be hypothesized to explain the compositional variations within each group. Although stratigraphic relations in the Venezuelan Guyana Shield are uncertain, geologic relations in and near the map area allow, as valid working hypotheses, stratigraphic sequences which parallel those which have been recognized in better-known Archean terranes.

INTRODUCTION

The problem of antecedence of quartzo-feldspathic gneisses with Na~O/K20 ratios exceeding unity is a recurring one which became classic through the work of the Engels, among others (Engel and Engel, 1953, 1958; Engel, 1956), in the Adirondacks. In these and other studies (Buddington and Leonard, 1962; Coombs, 1965; Chesworth, 1970; Young, 1971; Hanson and Goldich, 1972; Arth and Hanson, 1972; Shaw, 1972), antecedents as diverse as quartz keratophyres, graywackes, sodic tuffs, altered volcanics, volcanogenic sediments, and intrusive magmas derived by partial melting of amphibolites or eclogites have been postulated.

318 The relatively sodic character of Archean terranes has been much emphasized (Eade et al., 1966; Eade and Fahrig, 1971; Glikson, 1971, 1972; Glikson and Sheraton, 1972; Engel et al., 1974). One model for early evolution of sialic crust postulates essentially sodic Archean protocratons developed in island-arc or oceanic environments, with subsequent differentiation processes resulting in increasing K20/Na20 ratios. Wynne-Edwards and Hassan (1972) have described gray quartz--oligoclase gneisses as among the most abundant but least studied of continental rocks. This report describes a sequence of such gneisses from the Precambrian Shield in Venezuela, and discusses observations which may refine opinion regarding the origin of such rocks. The gneisses have been dated at approximately 3 b.y.BP {Hurley et al., 1968, table II, M.I.T. No. 6708) and the data presented provide an additional test for Archean crustal models. GEOLOGIC SETTING The sodic gneisses being considered constitute part of a strongly metamorphosed stratigraphic assemblage on the Caroni River northeast of Cerro Bolivar in the northcentral Guyana Shield {Fig. 1). The regional geology of this part of the shield has been described by Kalliokoski {1965). The structure and stratigraphy of the map-area are summarized in Fig. 1. The area is fault-bounded to the north and west; the northern boundary, the Ciudad Piar-Guri Fault, is a major zone of cataclasis at least several hundred kilometers in length. The Imataca Complex, a sequence of predominantly acid pyroxene granulites and granitic gneisses characterized by the presence of iron formation, lies north of the Ciudad Piar-Guri Fault and west of the map-area (Fig. 1B). The map-area is the western end of a biotite--quartz--oligoclase gneiss--amphibolite terrane at least 100 km in length lying south of the Ciudad Piar-Guri Fault. Detailed mapping in ~his terrane east of the map-area (Chase, 1963) indicates a lithologically monotonous sequence of quartz-oligoclase gneisses and amphibolites in alternating layers up to several kilometers thick. Amphibolite is considerably more abundant than in the area being considered. The gneisses were interpreted by Chase as phacolithic trondhjemite intrusions; the amphibolite was correlated by Kalliokoski (1965) with the Carichapo amphibolite which, elsewhere, exhibits pillow structure. The stratigraphic assemblage in the map-area appears lithologically more varied than that elsewhere in the gneiss--amphibolite terrane, and contains iron formation lithologically and petrographically identical to that of the Imataca Complex proper. The sequence is predominantly biotite--quartz-oligoclase gneisses with subordinate granitic gneisses, amphibolites, magnetite-quartz--iron formation, cordierite--sillimanite--biotite--quartz--plagioclase gneiss, pyroxene--garnet--quartz--anorthite gneiss, and minor laminated calc--silicate quartzites. One type of amphibolite is internally stratified, with interlayers of clinopyroxene--anorthite gneiss and quartzite, and is interpreted as being of sedimentary origin. A second type of amphibolite is mineralogically-

319 63o00

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Fig. 1. Location and generalized geology of map-area. B summarizes the regional geology in the vicinity of the map-area. Vertical lining symbolizes the Imataca Complex, with iron formation indicated by broken lines. Solid shading represents Carichapo amphibolite, horizontal lining the Yuruari Formation of the Pastora Series, blank areas are undifferentiated quartz--feldspar gneisses. Faults indicated by linear s-pattern (after Kalliokoski, 1965). C summarizes the geology of the map area. Solid bands are basaltic amphibolites, dotted pattern indicates metasedimentary lithologies, vertical lining, granitic gneisses, and diagonal lining, cataclasites of Ciudad Piar-Guri Fault zone. Blank areas are biotite--quartz--oligoclase gneisses with thin interlayered amphibolites. Other symbols as in B or conventional. Numbers in C refer to locations of samples analyzed, Tables I and II.

h o m o g e n e o u s a m p h i b o l i t e , sensu strictu, f o r m s layers 0 . 1 - - 2 0 m t h i c k i n t i m a t e l y i n t e r s t r a t i f i e d w i t h b i o t i t e - - q u a r t z - - o l i g o c l a s e gneisses, is m o r e f r e q u e n t in o c c u r r e n c e t h a n t h e m e t a s e d i m e n t a r y t y p e , a n d is o f basaltic c o m p o s i t i o n . T h e m o s t p e r s i s t e n t a n d t h i c k e s t layers o f this t y p e o f a m p h i b o l i t e are i n d i c a t e d in Fig. 1C. T h e p r i m a r y c o n c e r n o f this r e p o r t is w i t h t h e b i o t i t e gneisses a n d t h e basaltic a m p h i b o l i t e s . A n a l y s e s o f o t h e r lithologies h a v e b e e n given e l s e w h e r e ( D o u g a n , 1 9 7 4 ) in a discussion o f m e t a m o r p h i s m o f t h e rocks. I n f e r r e d m e t a m o r p h i c

320

conditions were 725--800 ° C, PT = 5--6 kbar, PH~O < PT in the northwest corner of the area, 650--700 ° C, PT = 5--7 kbar, PH20 approximating PT, in the south and east.

Petrography and field relations of biotite gneisses and basaltic amphibolites Biotite--quartz--oligoclase gneisses ("biotite gneisses") are typically leucocratic and have sparse non-perthitic microcline, greenish-brown biotite, and untwinned oligoclase. Cordierite, sillimanite, and garnet occur locally in mafic schlieren. Biotite gneisses rarely have more than 10 mode % K feldspar, and are not intergradational with granitic gneisses containing 20--35% K feldspar. Minor, more mafic, variants of biotite gneiss are hornblende + biotite-quartz--calcic oligoclase and diopsidic augite--+hornblende+quartz+andesine gneisses, both variants lacking K feldspar. Several textural facies of biotite gneiss can be recognized. (1) Fine-grained, non-migmatitic, equigranular, strongly foliated gneisses with a gneissic lamination reflecting modal variation in biotite. (2) Fine- to medium-grained, equigranular gneisses with foliation and gneissic lamination weakly developed a n d c o m m o n l y migmatitic, with discordant leucocratic quartz--oligoclase segregations. (3) Medium- to coarse-grained gneisses, with gray oligoclase crystals up to 1--2 cm in length in a medium-grained biotite--quartz--oligoclase matrix. Foliation and gneissic lamination are imperceptible or weak and discordant leucocratic quartz--oligoclase segregations are ubiquitous. Two compositionally distinct groups of biotite--quartz--oligoclase gneisses, designated A and B, are defined below. The two groups are mineralogically and texturally similar, and cannot be systematically differentiated on geologic or petrographic bases. Some petrographic generalizations are valid regarding the two groups, but recognition of such generalizations is dependent on initial discrimination of the two groups on a chemical basis. (a) Hornblende+biotite and pyroxene+hornblende variants of biotite gneisses generally belong to group B compositions; group A gneisses only rarely have hornblende and/or pyroxene. Conversely, group A gneisses locally have sillimanite and/or cordierite, which are of rare occurrence in group B gneisses. (b) Group B gneisses most commonly belong to textural facies 3, described above, group A gneisses to facies 1 and 2; both groups, however, occur in all three textural facies. Chase (1963) also recognized three facies of biotite gneisses similarly characterized, and interpreted field relations as indicating successive generations of syntectonic trondhjemite intrusion in the order listed. Similar field relations in the map-area can be as credibly attributed to varying degrees of metamorphic recrystallization, anatexis, tectonic brecciation, and dilatational intrusion. For example, amphibolite layers are frequently broken into angular fragments separated by discordant biotite gneiss veins. The intrusive biotite gneiss is

321 usually similar petrographically to that which envelops the amphibolite layer, and the veins themselves typically exhibit a foliation paralleling their margins or contacts. This relationship could be attributed to injection of a viscous biotite-laden melt into the amphibolite; alternatively, however, it could also result by tectonic disruption of competent amphibolite and solid intrusion of relatively plastic biotite gneiss. Leucocratic quartzo-feldspathic segregations in migrnatitic biotite gneisses are better attributed to anatexis than to intrusion of a late-trondhjemite magma. In migmatitic granitic gneisses, the segregations are invariably granitic, with abundant K feldspar; in biotite gneisses, the segregations are quartz--oligoclase, with sparse K feldspar. Such fine stratigraphic selectivity is difficultly reconciled with magmatic intrusion but is wholly consistent with local anatexis of differing bulk compositions. Amphibolites are mineralogically homogeneous, strongly foliated and nonlaminated. In the northwest corner of the area, they contain orthopyroxene, augite, brownish-green hornblende, and complexly twinned labradorite; south and eastward, pyroxenes disappear and the mineralogy is minor quartz, untwinned or simply twinned andesine, and grass-green hornblende. Amphibolite, as well as iron formation, behaved competently during deformation. In outcrops, amphibolite is commonly brecciated and injected by biotite gneiss. On a larger scale, detached segments of amphibolite and iron formation layers have been separated along strike by distances of up to 6--8 km (Fig. 1C). In contrast, biotite gneisses are attenuated on fold limbs and thickened at crests. This could be attributed either to plastic flow during deformation or, alternatively, to a structural control on trondhjemite intrusion. Similarly the fragmentation and separation of competent lithologies could be attributed with equal facility to tectonic brecciation or to catazonal intrusion along planes of stratification. Neither these nor any other field observations permit unequivocal conclusions as to the origin of the gneisses. ANALYTICAL

METHODS

Hand-specimen-sized samples were crushed, homogenized and split for atomic absorption and X-ray fluorescence analysis. For absorption analysis La20 was added to splits, which were then dissolved in aqua regia--HF solutions in Paar containers, diluted to a concentration of 1/100 in boric acid solution, and analyzed for all major and trace elements listed (Tables I and II) excepting Zr. Solutions were further diluted by a factor of 10, and the analyses repeated. With the exception of Si and A1, all absorption analyses are reproducible within + 0.4% (maximum standard deviation in replicate splits and in differing dilutions). Severe interference occurred with Si and A1; for these elements, only X-ray determinations were utilized. Analytical curves were based on the USGS standards G1, Wl, G2, GSP 1, AGV 1, and BCR 1 and the "usable values" of Abbey (1972). For splits used for X-ray analyses, 3 g of sample were fused with 1 g lithium

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metaborate, crushed, mixed with chromatographic cellulose to bring the total weight to 5 g, and pressed on cellulose-backed pellets. Determinative curves were based on the USGS standards and the values of Abbey. All elements except Na, Cr, Ni, Co and Li were determined. X-ray analyses of replicate splits had a maximum imprecision of _+0.6% (SiO2). X-ray and absorption determinations of elements analyzed by both methods agreed within 3% and, for such elements, values listed are averages of the two determinations. M A J O R A N D T R A C E - E L E M E N T COMPOSITION O F BIOTITE GNEISSES

Biotite--quartz--oligoclase gneisses are oversaturated, slightly peraluminous, and sodic, with K20/Na20 ratios ranging from 0.7 to 0.2. All binary variation diagrams examined which utilize a variation parameter incorporating SiO~, exhibited large scatter, suggesting absence of systematic compositional variation. However, by other compositional criteria, two distinct chemical facies of biotite gneisses can be defined. Fig. 2 plots 37 analyzed gneisses in terms of K20, Rb, and Zr and clearly delineates two compositional groups, one with higher K20/Rb ratios than the other. These two groups of gneisses are subsequently designated as group A and group B, respectively. Once the two groups of gneisses are so separated, it is apparent that they differ systematically in other aspects of major and trace-element composition. Thus, for example, in ~ Fe203--CaO--Alks and A1203--~ Fe203--Alks diagrams (Fig. 3) the two groups define distinct, though merging, compositional fields.

RbxlO0

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Fig. 2. K20--Rb X 100--Zr X 100 plot of biotite--quartz--oligoclase gneisses. Solid circles are group A gneisses, o p e n circles are group B gneisses.

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In these and other major element diagrams not incorporating SiO2, group A gneisses exhibit well-defined compositional trends, in contrast to the apparent absence of systematic variation in SiO2-based binary diagrams. To test the possibility that a regular compositional variation is masked in such binary diagrams by a non-systematic or independent variation in SiO2, the analyses were recalculated on an SiO2-free basis and plotted using the weight ratio A1203/Na20 as a variation parameter (Figs. 4 and 5). This ratio is a function of the ratio total normative feldspars: normative albite. In these plots, despite the fact that the recalculation of analyses would amplify analytical inaccuracy and imprecision, it is apparent that: (a) group A gneisses exhibit a welldefined compositional variation; and (b) less defined trends are present in group B gneisses, although compositional scatter remains large. Variation in trace-element composition of the gneisses is summarized in a series of standard plots in Fig. 6. These indicate that the two groups of gneisses differ systematically not only in major elements but trace elements as well. Group A gneisses, for example, are on average (Table I) higher in CaO and A1203, lower in total iron, have greater K/Rb, Sr/Ba and V/ZFe ratios, and greater Rb, Zr, and Li contents than group B gneisses. ANTECEDENCE OF BIOTITE-QUARTZ--OLIGOCLASEGNEISSES Several independent evidences suggest an igneous antecedent for both groups of gneisses.

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Major element compositions The only credible sedimentary protolith for quartzose biotite gneisses would be sandstones, more specifically, graywackes, the only sandstone with K20/ Na20 less than unity. The slightly per aluminous character of the gneisses, the sporadic occurrence of garnet, sill•man•re, and cord••rite in the rocks, their stratigraphic association with pelitic units and with basaltic, possibly metavolcanic, amphibolites, would all be consistent with this interpretation. However, eight of nineteen analyzed group A gneisses, and seven of eighteen analyzed group B gneisses fall outside or at the edge of A1203--Alks and K20-Na20 compositional fields of graywackes delineated by Middleton (1960). Since these fields are based on a large number of analyses, it would be necessary to postulate compositionally atypical or unusual graywackes as antecedents for such gneisses, although the majority of both groups fall within the graywacke fields. Shaw (1972) combined the most successful of chemical criteria used to discriminate igneous and sedimentary protoliths in a discriminant function (DF), incorporating factors for most major elements. Positive DF

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values indicate probable igneous antecedence, negative values a sedimentary parentage. DF values for group A gneisses have a range and mean of -2.1 to + 7.4 and + 3.1, respectively; only two of nineteen analyzed specimens have negative values. Since group A gneisses are compositional intergradational, an igneous origin is indicated for the entire group. Group B gneisses have a DF range and mean o f - 0 . 7 8 to + 4.4 and + 1.9, respectively; only three of eighteen analyzed specimens have negative values. A largely igneous origin is indicated, though a mixed sedimentary--igneous protolith is also possible.

331

Trace-element compositions Shaw (1972) delineated compositional fields of igneous and sedimentary rocks in terms of selected trace elements (Fig. 7). Although Ca/Sr and 104V/ FM ratios of analyzed gneisses fall largely in the areas of compositional overlap, their Ni and V contents plot mostly outside the sedimentary field and within the igneous field. A significant number of analyses fall outside both fields as regards V contents. Although these trace-element data are somewhat equivocal, the weight of evidence favors an igneous origin for both gneiss groups.

Fe--Ti oxide phases Fe--Ti oxide phases of biotite gneisses have been described elsewhere (Dougan, 1974). The gneisses contain ilmeno-magnetite and/or ilmenite--hematire exsolution intergrowths, commonly partially replaced by rutile and hematite. Ilmenite--hematite ratios in intergrowths are completely variable, indicating an original crystallization above a solvus crest at 950°C (Carmichael, 1961) or near 800°C (Lindsley, 1973). Assuming that hematite--rutile replacements are metamorphic in origin, and that no cation loss occurred during replacement reactions, original crystallization temperatures were estimated by applying the Buddington--Lindsley geothermometer (Buddington and Lindsley, 1964) to analyzed ilmeno-magnetite and hematite--ilmenite intergrowth pairs. T--P02 values so estimated are in the range 800--I050°C, in the vicinity of the M H buffer. These temperatures exceed those inferred for metamorphism in the northwest corner of the area (725--800 °C) and considerably exceed inferred metamorphic temperatures in the south and east (650--700°C). These and other data indicate that the Fe--Ti oxide phases are relict magmatic crystallates and that, consequently, the gneisses are metaigneous. COMPOSITION AND ANTECEDENCEOF AMPHIBOLITES Homogeneous amphibolites lacking internal stratification are interpreted as meta-igneous. Major and trace-element analyses are listed in Table II. In terms of Z Fe203--Alks--MgO, the amphibolites fall within the compositional range of tholeiites as defined by Irving and Baragar (1971) and are essentially coincident with oceanic tholeiite averages (Fig. 8). Miyashiro (1974) and Miyashiro and Shido (1975) delineated compositional fields of basalts, including that of abyssal tholeiites, in terms of Z FeO/MgO ratio and TiO2, Z FeO, Cr, V and Ni; the amphibolites are coincident with oceanic tholeiites in these plots (Fig. 9) and in most aspects of composition (Table II). Glikson (1972) compared oceanic tholeiites and Archean metabasalts; in agreement with systematic differences noted by Glikson, the amphibolites have lower A1 and generally higher Rb, Ba and Ni than recent oceanic tholeiites.

332 ~_Fe203

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Fig. 8. Z F%O~--Alks--MgO plots of basaltic amphibolites, group A and group B gneisses, representative graywacke fields, and average basalts. Group A and group B gneisses indicated by solid clots and open circles, respectively; circled symbols are gneisses with negative values of the Shaw (1972) discriminant function. Long- and short-dashed lines are visually fitted variation curves for group A and group B gneisses, respectively. Dash-dot curve demarcates tholeiitic (convex side) and calc-alkaline (concave side) volcanics, after Irving and Baragar (1971). Compositional fields designated Bal. and Fran. are graywacke fields, calculated from analyses given by Chappell (1968) and Ernst (1971) respectively; other graywacke averages or composites plot within, or closely adjacent to, these fields. Analyzed basaltic amphibolites are indicated by solid circles. Circled dots are basalt averages, as follows: A, B, C, oceanic tholeiite averages; D, mid-Atlantic Ridge tholeiites; E, average high-alumina basalt, Japan (after various workers, in Glikson, 1971). ORIGIN O F BIOTITE---QUARTZ----OLIGOCLASE GNEISSES

Accepting an igneous antecedent for both groups of gneisses, the question of origin of the magmas arises. Their most probable derivations are by: (1) partial or total melting of graywackes; (2) fractional crystallization and/or contamination of basaltic magmas; and (3) partial melting of amphibolite or eclogite of oceanic tholeiite composition. The biotite gneisses considered here approximate in mode of occurrence and in major and trace-element compositions meta-igneous gneisses elsewhere which have been considered in detail as regards origin of the magmas (Condie and Lo, 1971; Arth and Hanson, 1972; Glikson and Sheraton, 1972; Hanson and Goldich, 1972). These studies attributed trondhjemite melts to partial melting of amphibolite or eclogite at mantle depths, a conclusion based in part on trace element suites and the experimental generation of trondhjemite melts from such rocks (Green and Ringwood, 1968; Lambert and Wyllie, 1970, 1972). It is n o t intended here to duplicate the argumentation of these studies; in summary m a n y

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Fig. 9. Comparison of compositions of basaltic amphibolites with those of basalts. Basaltic amphibolites indicated by solid dots. Basalt compositional fields from Miyashiro and Shido (1975).

aspects of the biotite gneisses, including much of the major and trace-element data, are consistent with derivation by partial melting of oceanic tholeiite compositions. However, the biotite gneisses exhibit several characteristics not noted elsewhere which may assist in resolving the problem of the origin of such rocks. It appears premature, in the case of the biotite gueisses considered here, to exclude an origin by partial melting of graywackes. In particular, two compositional attributes of biotite gneisses cannot be easily reconciled with partial melting of amphibolite or eclogite at mantle depth, but suggest melting of graywackes at crustal depth. (1) One argument against derivation of trondhjemites by melting of graywackes is the inference, based on some experimental results, that this would require temperatures exceeding 1000°C at crustal depth (e.g., Arth and Hanson, 1972; Hanson and Goldich, 1972), an apparently unacceptable situation. However, Engel et al. (1974} have on other bases postulated Archean temperatures as great as 1500°C at depths as shallow as 5 km. Moreover, trondhjemite melts in equilibrium with biotite and amphibole can be generated by partial melting of graywackes at low PT and 700--750°(] in the presence of chloride solutions (Kilinc, 1972). If the biotite gneiss magmas were generated by equilibrium or fractional melting of graywackes at crustal depth, their compositions should be con-

334 trolled by liquidus or solidus geometries of relevant systems. The most important constituents of both graywackes and the biotite gneisses are SIO2, A1203, Na20 and Z Fe, CaO and K~O being subordinate. Analyzed biotite gneisses are plotted in relevant, synthetic ternary diagrams (anhydrous, P T = 1 atm) in Fig. 10. Although group A gneisses trend across the plagioclase--quartz cotectic (Fig. 10b), a general correspondence of gneiss compositions and low-variance liquidus loci is apparent. The effect of increased PT and PH20 on the position of these loci is incompletely known. Assuming that this effect is significant, the correspondence could have two interpretations; either the gneisses originate by partial melting at relatively low crustal pressures, or their compositional correlation with low-pressure liquidus loci is merely coincidence. In the Al203--SiO2--Na20 diagram, for example, available data indicate that increasing P T and/or PH~O shift the relevant liquidus loci toward Ab (e.g., Boettcher and Wyllie, 1969) which would diminish their cgmpositional coincidence with the gneisses. Similarly, if the gneisses originated by partial melting of eclogite or amphibolite, the correspondence would again best be assigned to coincidence; partial melts of such rocks, which are compositionally remote from the systems figured, would not be expected to exhibit a control by liquidus geometries of these systems. If the correlations apparent in Fig. 10 are not merely coincidental but causal, they would be consistent with derivation of the gneisses by partial melting of graywackes at crustal pressures. (2) It would be expected, if the gneisses were derived by melting of oceanic tholeiite compositions, that they would be characterized by relatively low Fe203/FeO ratios. An average ratio for oceanic tholeiites is 0.15 (Engel et al., 1965). Although some gneisses have ratios as low as 0.16, most are considerably more ferric. The average ratio of twelve specimens analyzed for both Fe203 and FeO is 0.7 and in some specimens, Fe203 exceeds FeO. It is unlikely that the gneisses were oxidized by interlayered lithologies since these are much subordinate in volume and generally are more reduced than the gneisses. The oxidized character of biotite gneisses thus appears to be a primary feature and is better explained by partial melting of sediments than of relatively reduced tholeiites. The genesis of the two distinct groups of gneisses is a problem not easily resolved. The continuous compositional variations present in each group, though less defined in group B than in group A, suggest that some process or processes of differentiation were operative in each group. One problem with deriving trondhjemitic melts by partial melting of graywackes is that such melts should have greater K20/Na20 ratios than the parent graywacke (e.g., Arth and Hanson, 1972). Although some volcanic graywackes have much lower K20/Na20 ratios than the biotite gneisses (e.g., ChappeU, 1968, Table III), graywackes generally have greater K20/Na20 ratios than most of the biotite gneisses, whether group A or B. Consequently, of the range of gneiss compositions, those most credibly postulated as the primary melts subjected to differentiation would be those with lowest Na20/K20 ratios. Figs. 8 and 11 summarize several tests of this hypothesis. Although most of the biotite gneisses have lower

335

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Fig. 10. Plots of group A gneisses (solid dots) and group B gneisses (open circles) in the synthetic ternary systems which most closely approximate the gneisses in composition. Liquidus geometries are for anhydrous, P T = I a t m , conditions. The SiO2--F%O 3-NaA1Si30 s diagram was constructed from data of Bailey and Schairer (1966).

336

Fe203/Alks ratios than graywackes (Fig. 8), the two gneiss groups define compositional trends in this diagram which merge with graywacke compositions at high Z Fe203/Alks ratios. Additionally the group A and group B gneisses with negative, i.e., metasedimentary, values of the Shaw discriminant function plot coincident with the graywacke compositions. These relationships could be attributed to an extensive, possibly nearly complete, melting of graywackes, with the compositional trends towards lower ZFe203/Alks ratios being generated by differentiation of the melts so formed. Combining this argument with the previous postulate regarding K20/Na20 ratios, it would be expected that the initial high Z Fe203/Alks melts would also exhibit the lowest Na20/K20 ratios. Fig. 11 ~lots Z Fe203/Alks ratios

O o o

ID LO-

\

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0

0 0

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Fig. 11. ZFe2Os/Alks versus Na20/K20 plots for group A gneisses (solid circles) and group B gneisses (open circles). Range of graywacke Na20/K20 ratios indicated by bars, as follows: A, range of Na20/K20 of graywackes figured by Middleton (1960); B, lower portion of Na~O/K20 range of volcanic graywackes analyzed by Chappell (1968), range extends to Na20/K20 = 15.8; C, range of Na20/K20 ratios of 32 Franciscan graywackes (Ernst, 1971). Curve is a visual fit for group A data.

against Na20/K20 ratios for each group of gneisses. In group A gneisses the lowest Na20/K20 ratios, approximately 1.5, coincide with high 2; Fe203/Alks ratios, as predicted, and are at the lower end of the graywacke range. Group B gneisses exhibit large scatter in this plot, although a general decrease in Na20/K~O with increase in Z Fe203/Alks is apparent. In group A gneisses, postulating as primary melt that composition with greatest A1203/Na20 ratio (Fig. 4), the compositional variations towards lower A1203/Na20 ratios (and greater Na20/K20 ratios) could be generated by separation of phlogopitic mica, an Fe--Ti oxide, and a relatively calcic plagioclase in appropriate proportions, producing compositions depleted in K20, MgO, Fe and TiO2 and enriched in sodic plagioclase component. Trace-element data appear generally consistent with this hypothesis. The increase in Ca/St with increasing N a 2 0 / C a O ratio, or with decreasing A I 2 0 3 / N a 2 0 ratio (Fig. 4), for

337

example, is in agreement with a plagioclase fractionation (Heier and Taylor, 1959; Taylor, 1965). Taylor (1965) proposed the Ba/Rb ratio as a most sensitive index of fractionation processes. Since Rb is preferentially incorporated in biotite and Ba in K feldspar, the decrease in Ba/Rb with decrease in A1203/Na20 and K20 (Fig. 4) is consistent with removal of biotite. Major-element variation in group B gneisses is much tess regular than in group A gneisses, and if the variation is a result of a crystal fractionation process, the mechanism would have to be considerably more elaborate in detail than that suggested for group A gneisses. Again assuming a primary melt of lowest Na20/K20 ratio, i.e., greatest A1203/Na20 ratio (Fig. 5), the sympathetic decrease in CaO, MgO, and Z Fe, coupled with increase in K20, in the direction of decreasing A1203/Na20 is inconsistent with major separation of biotite but could be attributed to removal of hornblende, which is presently common in more calcic compositions of group B gneisses. This would create some problems as regards the trace-element data. For example, since hornblende incorporates K in preference to Rb (Nagasawa and Schnetzler, 1971), removal of hornblende should result in decreasing K/Rb ratios with decreasing A1203/Na20; in fact K/Rb ratios exhibit no systematic variation. Arth and others (1974) have suggested fractionation processes similar to those suggested above to account for compositional variation in a Finnish Precambrian hornblende gabbro--biotite--trondhjemite suite. The mechanisms proposed appear generally applicable to the Venezuelan trondhjemites and additional data may permit resolution of problematical details. These include the apparent lack of regularity in SiO2 variation in group A gneisses and the fact that, although a plagioclase fractionation appears necessary to explain the nature of compositional variation in group A gneisses (Fig. 4), the compositional trend in these gneisses is at a high angle to the plagioclase--quartz cotectic in the SiO2--Ab--An system (Fig. 10). Although compositional variation within each group of gneisses could be attributed to crystal fractionation processes, the ultimate origin of the two distinct series is problematical. Wynne-Edwards and Hassan (1972) suggested that anatexis of quartz--oligoclase gneisses simply generates melt compositions approximating that of the original gneiss. In the Case of the biotite--quartz-oligoclase gneisses considered here, both gneiss groups are approximately coincident with liquidus minima in relevant ternary systems (Fig. 10) and possibly the simplest hypothesis would be to postulate one group of gneisses as an anatectic derivative of the other. However, group B gneisses have greater Z Fe and MgO contents than group A gneisses, which argues against deriving the former as partial melts of the latter; although most trace-element data would be consistent with such an origin. Conversely, group A gneisses have generally greater CaO contents and CaO/Na20 ratios than group B gneisses, which weighs against their originating as partial melts of group B compositions, a conclusion also supported by trace-element differences. K/Rb ratios of group A gneisses, for example, are approximately double those of group B gneisses. Thus, any hypothesis deriving either group of gneisses

338

by anatexis of the other would require resort to numerous special circumstances and it appears preferable to postulate two distinct original melts. Further speculation on conditions for derivation of such melts is not justified by available data. REGIONAL STRATIGRAPHIC IMPLICATIONS

Numerous models have been proposed for development of Archean sialic crust and for Archean igneous cycles (Wilson et al., 1965; Goodwin and Shklanka, 1967; Goodwin, 1968; Glikson, 1970, 1971; Glikson and Sheraton, 1972; Glikson and Lambert, 1973; Engel et al., 1974). The tentative state of understanding of relations in the Venezuelan Shield prohibits strict correlations with evolutionary sequences recognized in better known Archean terranes. Some observations, however, appear justified to place the rocks discussed above in the context of current opinion regarding Archean volcanism and shield evolution. Suggestions advanced below are largely speculative and best regarded as working hypotheses. The stratigraphy and stratigraphic uncertainties of the north-central Guyana Shield have been discussed by Kalliokoski (1965). In the vicinity of the area concerned in this report, two major stratigraphic assemblages are recognized. The Imataca Complex (Fig. 1) is a series of granitic gneisses and predominantly acid-intermediate pyroxene granulites characterized by the presence of iron formation. Unpublished analytical data indicate that the Imataca Complex, at least in the area immediately north of the map area, is a metamorphosed, Calc-alkaline, predominantly salic, possibly metavolcanic sequence, with only minor metasediments. The complex is intruded by small, discordant granites, one of which is dated at about 2 b.y. BP (Posadas and Kalliokoski, 1967; Hurley and others, 1968). The Imataca Complex is bordered on the south by the Pastora--Carichapo Assemblage, consisting of an older series of metavolcanic amphibolites and minor metasediments, the Carichapo Formation, and a younger series of graywackes, volcanic wackes, and mafic pillow lavas, with minor metacherts, tuffs, and manganiferous metasediments, the Pastora Series. Mafic and ultramafic intrusions occur within the Pastora--Carichapo terrane (Kalliokoski, 1965). Trondhjemite gneisses, similar to those discussed in this report, occur interlayered with Carichapo amphibolites, at least in the northern portion of the Pastora--Carichapo terrane, and were interpreted as syntectonic intrusions (Chase, 1963); the extent of occurrence of such gneisses is not well known. Kalliokoski and others regard the Pastora--Carichapo Assemblage as unconformably overlying the Imataca, an interpretation based on regional relationships and the generally higher metamorphic grade of the Imataca. Relations in the map-area suggest the converse. Iron formation units in the map-area are lithologically and petrographically identical with Imataca iron formation and are interpreted as such. The interpretation of the elongate fold outlined by iron formation in the southern half of the map-area (Fig. 1C) as a synform

339 would imply that the Pastora--Carichapo Assemblage extending eastward from the map-area without apparent structural break conformably underlies at least part of the Imataca. Radiometric dates are ambiguous and do not resolve this problem. The Imataca Complex is in excess of 3 b.y. old (Hurley et al., 1968; Gaudette et al., 1972). Hurley et al. cite Rb--Sr ages of 1500--2100 m.y. for Carichapo assemblages (Table I) but record a 3130-m.y.-age for a sample from the biotite--quartz--oligoclase gneiss assemblage in the map-area (Field No. 8614, Table II). If this gneiss is correlative with that interlayered with and possibly intruded into Carichapo amphibolites elsewhere, the Carichapo would also exceed 3 b.y. in age. Relationships in the map area and adjacent areas allow the following as valid working hypotheses, in addition to that of Kalliokoski: (1) The Carichapo amphibolites and interlayered trondhjemite gneisses together constitute a metavolcanic sequence conformably overlain by the Imataca Complex. The generally higher metamorphic grade of the Imataca does not weigh heavily against this interpretation; trondhjemite gneisses in the map area are as high grade as the Imataca (Dougan, 1974), and regional variations in grade may reflect lateral rather than vertical gradations. (2) The Carichapo amphibolites are older than Imataca; trondhjemites conformably intrude both the Carichapo amphibolite and at least the lower portion of the Imataca series. These models imply the following magmatic sequences, in respective order: (A) Basaltic, possibly ophiolitic-trondhjemitic magmatism, succeeded by more potassic calc-alkaline volcanism, followed by late granitic intrusion. (B) Basaltic volcanism, followed by calc-alkaline volcanism, with subsequent or concomitant trondhjemite intrusion and later granitic intrusion. The parallelism between these possibilities and recognized sequences in other Precambrian terranes, as for example by Glikson (1971, 1972), is apparent. The delineation of two distinct groups of trondhjemite gneisses in the map-area opens other possibilities, including combinations of A and B involving two periods of trondhjemitic magmatism. SUMMARY

Biotite gneisses and basaltic amphibolites of the map-area are meta-igneous. The amphibolites have oceanic tholeiite compositions. The biotite gneisses are divisible by chemical criteria into two groups. Group A gneisses exhibit a regular compositional variation attributable to a fractionation process involving removal of biotite and plagioclase. The more irregular compositional variation in group B gneisses may result through a crystal fractionation process: if so, the process must have been more elaborate in detail than that in group A gneisses. Although the gneiss compositions could be partial melts of oceanic tholeiites at mantle depth, some features are more easily reconciled with melting of graywackes at crustal levels. Primary melts can be postulated which are credible derivations of graywackes.

340

Considerable uncertainty exists regarding regional stratigraphic relationships in the Venezuelan Guyana. Geologic relations in the map-area permit working models paralleling crustal evolutionary sequences recognized in better-known Archean terranes. ACKNOWLEDGEMENTS

The original field work upon which this study is based was carried out some time ago, in 1960--1961, with the support of the Ministerio de Minas e Hidrocarburos of Venezuela. This field work was part of a study of the Venezuelan Guyana Shield under Dr. J.O.K. Kalliokoski, now of Michigan Technological University. The analytical data presented were obtained during a leave of absence supported by Allegheny College in 1974.

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