Early Precambrian Evidence of a Primitive Ocean Crust and Island Nuclei of Sodic Granite

Abstract

The primordial crust consisted of a stratiform oceanic-type ultrabasic-basic assemblage, relicts of which are retained at the base of Archaean sequences of eastern Transvaal, Rhodesia, and Western Australia. Commonly isochemical metamorphism renders the chemistry of well-retained segments of the meta-igneous rocks significant to their original compositions. Archaean tholeiitic metabasalts comply with the principal criteria for oceanic tholeiites, but tend to have higher Fe/Mg and Mn, and lower Al, Ti, K/Rb, and Fe+ 3 /Fe+ 2 than average recent oceanic tholeiites. These geochemical characteristics can be interpreted in terms of either shallow-level differentiation and (or) a mantle relatively depleted in certain siderophile and transition elements, which possibly indicates a lesser degree of core segregation in the Archean. Metabasalts occurring within basic to acid calc-alkaline volcanic cycles at higher stratigraphic levels than the ultrabasic-basic oceanic assemblages are chemically distinct from metabasalts incorporated in the latter; these rocks have higher K, Sr, Zr, and Y values than recent oceanic tholeiites, and may have originated from high degrees of melting of an underlying oceanic substratum. A model is suggested whereby the evolution from an oceanic crust to greenstone belts proceeded through the rippling of the oceanic crust and the subsidence of linear zones; partial melting below the troughs resulted in the emergence of sodic granites and in cyclic calc-alkaline volcanism. High Ni/Mg and Cr/Mg ratios in the earliest sodic granites support their derivation from the oceanic crust. The granites display cyclic decrease in Na/K, K/Rb, and Sr/ Rb with time, trends which reflect crustal thickening. The plutonism resulted in the superposition of sialic nuclei on early linear structural trends, and in a consequent formation of spatially discrete volcanic-sedimentary depositories, or greenstone belts, in which sequences show an Alpine-type trend of evolution from ophiolites into turbidites and late-stage conglomerates. Notwithstanding this similarity, important differences between Archaean greenstone belts and Alpine or island-arc belts are indicated. Significant similarities exist between this model and the evolutionary pattern of the Fijian archipelago.