GEOCHEMISTRY OF THE METAVOLCANOSEDIMENTARY AND GRANITOID ROCKS OF THE ONOT GREENSTONE BELT

Abstract

Petrogeochemical data are presented on the rock associations of the Onot greenstone belt (GB) located in the southeastern Sayan Region. The infracrustal plagiogneisses and plagiogranitoids of the Onot GB are analogous to the oldest granitoids of the Archean granite-greenstone terrane (GGT). The geochemical modeling showed that parental tonalite-trondhjemite melts can be derived from a metatholeiite source at P ∼ 16 kbar in equilibrium with a garnet-amphibole residue. The supracrustal complex consists of komatiite-basalt, rhyolite, tholeiite-basalt, tuffaceous-terrigenous, carbonate, and iron-rich quartzite rock associations. Metarhyolites are strongly enriched in HFSE, REE, and radioactive elements and are comparable to A-type granites. They may have originated through tonalite melting at a low pressure in an extensional environment during mantle plume upwelling. Amphibolites correspond to TH1-type low-Ti high-Mg tholeiites and komatiite basalts. The high contents of Fe, Mg, Cr, Ni, U, Th, and LREE and the high La/Yb ratio of the terrigenous sediments indicate a contribution of both infracrustal and supracrustal components. The bimodal volcanism, specific composition of metarhyolites, and contribution of infracrustal the TTG component in the geochemistry of metasediments indicate an ensialic rifting origin of the belt. The chemical composition of the sialic basement and metavolcanosedimentary rocks of the GB suggests an independent crustal evolution of the Onot segment prior to its conjugation with the adjacent Sharyzhalgai granulite-gneiss complex.