Pb isotopic constraints on the evolution of the South Adamastor paleo-oceanic remnants, SW Gondwana paleocontinent

Abstract

This research presents the first whole-rock Pb isotope data for Tonian rocks from the Brazilian sector of the Punta del Este Terrane (amphibolites, magnesian schists and quartz-syenitic leucosomes of the Arroio Grande Ophiolite, and amphibolite and metagabbro xenoliths within granitoids), Dom Feliciano Belt. These rocks represent metamorphic remnants of the South Adamastor paleo-ocean. The new Pb isotope data suggest that these rocks are derived from a crustally-contaminated, depleted MORB mantle source, corroborating previously published hypotheses based on elemental geochemistry and Sr-Nd isotope data. The comparison with their Uruguayan (amphibolites of the Paso del Dragón Complex, Punta del Este Terrane) and Namibian (metagabbros and metabasalts of the Chameis Subterrane, Marmora Terrane) counterparts suggests that these rocks represent: (i) distinct regions (e.g. mid-ocean ridge, seamounts, trench) of the South Adamastor back-arc paleo-oceanic lithosphere, which were juxtaposed together during the closure of the basin; (ii) and/or, along the back-arc ridge, the magmatism was chemically heterogeneous.