Aspectos petrográficos e petrológicos de um gnaisse calciossilicático (meta-rodingito) em alto grau na base da Klippe Carvalhos, Faixa Brasília, borda Sul do Cráton São Francisco, em Minas Gerais

Abstract

Petrographic and petrologic features of a high-grade calcium-silicate gneiss (metarodingite?) at the base of Carvalhos Klippe, Brasília Belt, Southern border of the São Francisco Craton, Minas Gerais State. In the southern region of Minas Gerais State, in the southern border of the Neoproterozoic Tocantins Orogen, at São Francisco Craton south, several ultramafic bodies occur among oceanic metassediments, turbiditic and pelagic to hemipelagic, in medium to high-grade metamorphism and positioned along deep thrust faults associated with granulites, retroeclogites, and basement slivers. Among these ultramafic rocks of controversial origin and with a probable ophiolitic nature, the most important are the Morro do Corisco and Fazenda da Roseta, in the Liberdade region. The Fazenda da Roseta ultramafic body is made up by an association of meta-spinel harzburgite, meta-websterite, serpentinite carbonate, pegmatitic metabronzitite, and a garnet clinopyroxenite with a gneissic fabric, a possible meta-rodingite. This lithotype shows high density, ultramelanocratic character, banded granoblastic gneissic texture, which is constituted by mineral associations with varied proportions of garnet, clinopyroxene, epidote, and hornblende as the association of metamorphic peak, and plagioclase, epidote, ilmenite, magnetite, carbonate, which forms incomplete simplectitic reactions around garnet, among hornblende, with apatite and carbonate, the accessory minerals. Chemically, these rocks show a characteristic enrichment in CaO and major Na2O and SiO2 depletion when is compared with meta-mafic and meta-ultramafic rocks of the region. These rocks are positioned on the fields of meta-mafic rocks related to N-MORB and IAT. It has curved upward patterns for large-ion lithophile elements (LILE) and flat patterns for HFES and heavy rare earth elements (HREE), when compared with the abundance of these elements in N-MORB basaltic compositions. When this rock is analyzed together with the meta-ultramafic rocks of the region, it is possible to observe possible parental relations with meta-websterite, meta-harzburgite and meta-bronzitite, which show similar patterns of LILE, with negative Rb anomaly, and similar REE patterns, with enriched patterns related to chondrite and a Ce strong negative anomaly. From litho-textural and geochemical considerations together with the aloctone character of the associated ultramafics, retroeclogites and deep thrust faults, it is suggested that this (meta)ultramafic rock is a register of a calcium metassomatism (formation of rodingite), a probable meta-rodingite, in a probable oceanic environment, in a former episode to the process of subduction and collision, attributed to the closing of the Brasilíades Ocean, in a scenario of the Brazilian Orogen formation, synchronous to the formation of structural nappes.