Petrologia e isótopos de Sm-Nd da formação ferrífera bandada anfibolítica de Cristália (Minas Gerais-Brasil)

Abstract

A fine grained green and red amphibolitic banded iron formation (BIF), with dark green bands (rich in Fe-hornblende, almandine, magnetite and some quartz) and sometimes light green ones (with grunerite, almandine, quartz and magnetite) is found in Cristália (MG-Brazil) region. Mineral chemistry data obtained from electron microprobe supported the use of phase diagrams of the CFMASH and MnCFMASH systems, which allowed determining that metamorphism undergone by such rock would have occurred in minimum conditions of amphibolite facies, under 3.5 kbar and about 600 °C. Assuming that the garnet porphyroblasts have been formed during the metamorphic peak which generated the amphibolitic BIF of Cristália and that garnet (GRT) is in equilibrium with the isotopic reservoir (whole rock, WR) from which it formed, a Sm-Nd isochron (WR-GRT) age of 2018 +/- 130 Ma possibly corresponding to this metamorphism has been found. It is suggested here that this metamorphism is associated with a regional orogenic process, the dynamics of which would have led to the inversion of an ocean basin where the BIF protolith would have deposited. The strongly negative value of εNd(0) (-27.3) for the BIF suggests supply of terrigenous material from a continental source area, with average TDM age of 3200 Ma, into the marine basin where its protolith sedimented.