FURTHER SIGNS OF AN ENRICHED MANTLE SOURCE UNDER THE NEOPROTEROZOIC ARAÇUAÍ-RIBEIRA MOBILE BELT

Abstract

Isolated small bodies of Neoproterozoic coronitic gabbros crop out in the states of Espírito Santo (Jacutinga and Itaoca) and Rio de Janeiro (Amparo), in the central portion of the Araçuaí-Ribeira Mobile Belt. The rocks from Jacutinga and Itaoca are mainly gabbronorites and show a wide range of modal/textural varieties recording crystal-setting processes normally found in layered intrusions. The Amparo intrusion is formed by a coronitic rock, also present as layers in Jacutinga and Itaoca. The primary mineral assemblage consists of olivine, opx, cpx, plagioclase, ilmenite, Ti-magnetite and sulphides. Coronitic overgrowths related to the olivine-plagioclase reaction form concentric rims of opx, amphibole and simplectitic amphibole-spinel around olivine cores. They are all very similar in shape and composition pointing towards similar physical-chemical crystallization conditions. Different geothermometric measurements yielded crystallization temperatures around 800°C, which are close to those calculated for primary opx-cpx pairs (800/940°C). The absence of regional deformational and metamorphic paragenesis is compatible with their intrusion into the middle/ lower crust during a late collisional phase. A long lasting cooling environment at a late magmatic stage induced the sub-solidus reactions. Low K sub-alkaline signatures with clear enrichment in some incompatible elements such as Ba, Sr and LREE associated with high 87 Sr/ 86 Sr initial ratios (around 0.706-0.708) are anomalous chemical characteristics of these gabbros. Enriched mantle types (EM1, EM2 and HIMU) with high Sr-initial ratios are frequently found in the Southern Hemisphere. The chemical anomalies obtained in this study could be related to largescale phenomena, similar to the Dupal anomaly.