GENETICALLY DIVERSE BASALT GEOCHEMICAL SIGNATURES DEVELOPED IN THE RIO DAS VELHAS GREENSTONE BELT, QUADRILÁTERO FERRÍFERO, MINAS GERAIS, BRAZIL

Abstract

On the basis of major and trace element analysis, the metabasalts of the Nova Lima Group, Rio das Velhas Greenstone Belt, are divided into tholeiites and magnesian tholeiites. Using rare-earth element patterns, the rocks are subdivided into five geochemical populations (I-V). The chemical variations demonstrated on binary variations diagrams, using major and trace elements, proves this division. Population I is comprised of magnesian basalts and represents the most primitive terms of the sample population, with high MgO, Ni and Cr. Populations II, III, IV and V are differentiated terms evolved from compositions like those of Population I, with low compatible-element contents. The geochemical behavior, represented principally by the trace elements, shows that Population I may have formed by the partial fusion of a mantle-plume source (P-MORB). The other populations are differentiated by fractional crystallization from a melt of composition similar to that of Population I. Populations IV and V present geochemical patterns reflecting mixing processes that have been caused by fractional crystallization together with contamination. The Nova Lima Group metabasalts are derived from an ocean-floor environment, as evidenced by structure (pillow lavas) and lithological associations (banded iron formation and metachert). The geochemical characteristics indicate that at least some of the metabasalts were formed by a mantle plume. These characteristics permit the identification of a submarine plateau environment. That some mafic-rock populations show evidence for contamination and the occurrence of felsic volcanic rocks suggest the presence of an island-arc or back-arc type environment.