Magmatismo máfico Ediacarano no extremo sul do Orógeno Brasília Meridional: etrologia e implicações geodinâmicas

Abstract

In the Southern Brasília Orogen, in the southern portion of the Tocantins Province, which originated from the Neoproterozoic continental collision of the São Francisco-Congo and Parapanema plates, there are several occurrences of metamafic and ultramafic rocks of controversial origins. Among these rocks, the ultramafic body of Fazenda da Roseta (UFR) stands out, in the region of Liberdade, south of Minas Gerais, located at the base of Klippe Carvalhos, and tectonically positioned amid high pressure granulitic metasediments, associated with the Andrelandia Nappes System. In this body, a high metamorphic metasomatic rock, object of this study, is considered as a product of the interaction of a mafic dyke with host hazburgitic rocks that composes the UFR. Such metasomatic rock is defined as olivine-granofels, and presents chemo-textural characteristics that record multiple metamorphic stages. Such stages are characterized by domains that present granoblastic, polygonal equidimensional granular, sympctitic and decussate textures, and are constituted by different modal proportions of amphibole, olivine, orthopyroxene, spinel, chlorite, magnetite and ilmenite, where mineral assemblages of the MASH System predominate (MgO–Al₂O₃–SiO₂–H₂O). This rock, classified as a blackwall zone, according to field relations and in discriminating geochemical diagrams, is positioned in sub-alkaline basalt fields. The geochemical patterns are sub-horizontalized for the high-field strength elements (HFES) and heavy REEs (ETRP), and there is an enrichment for the large-ion lithophile elements (LILE) and light REEs (ETRL), relative to their abundance. elements in comparison compositions to N-MORB basalts. Analysis of zircons using the U-Pb method (SHRIMP) indicate a crystallization age of 611 Ma, when, possibly, crustal melts generated during the exhumation of the subducted continental crust, metasomatized the overlying lithospheric mantle, producing mafic magmas. Geothermobrarometric calculations and textural characteristics indicate that the thermal peak occurred around 954⁰C, at pressures ranging from 7.5 to 9 kbars, with an hourly trajectory. The granofels olivine, as well as the mafic dyke, are inferred to be part of the first mafic magmatism edicaryan recorded in the OBM.