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

Abstract

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- P T synthetic systems, and their relatively oxidized character, appear more compatible with an origin by partial melting of graywackes at crustal levels. For both groups of gneisses, primary melts with relatively low Na 2O K 2O ratios can be postulated which are credibly derived by 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.