Mineralogy, petrology, and origin of the Pedra Branca Suite: a tonalitic-trondhjemitic association with high Zr, Ti and Y, Carajás Province, Amazonian Craton

Abstract

The Pedra Branca suite (2.75 Ga) is located in the Canaa dos Carajas domain in the southeastern Amazonia Craton. It diverges from typical TTG in mineralogical and geochemical terms, by the presence of hornblende and clinopyroxene and because it has a high content of HFSE (Zr, Y, Ti, and Nb). It belongs to the low-K subalkaline series, which varies from metaluminous to peraluminous, and are mostly calc and ferroan granitoids. Amphibole is calcic and classified as ferroan-edenite, and hastingsite. Plagioclase is mainly oligoclase. The Fe/(Fe + Mg) ratios found in the amphiboles indicate that these granitoids were formed under high to moderate fO(2) conditions. Geobarometric calculations suggest pressures between 9.3 and 71 Kbar for the origin and 4.8-53.4 Kbar for the emplacement. Geothermometric calculations suggest initial crystallization temperatures between 945 and 862 degrees C, and the water content in the magma is estimated to be higher than 4 wt%. The magma source was defined as tholeiitic continental gabbro melted in an extensional setting (Carajas Rift) with geochemical features similar to diabase from Nova Canada (PA). The Pedra Branca magma was originated by partial melting (similar to 28%), leaving a residue with plagioclase (An40), hornblende, clinopyroxene, and may or may not have magnetite.