Petrogenesis of the late-orogenic Bravo granite and surrounding high-grade country rocks in the Palaeoproterozoic orogen of ltabuna-Salvador-Curaca block, Bahia, Brazil

摘要

In the Sao Francisco Craton, in Bahia state, Brazil, four Archaean crustal segments collided during the Palaeoproterozoic to form the important ltabuna-Salvador-Curaca Orogen (ISCO). Granulites were formed in the central, deepest part of the ISCO while rocks in amphibolite to greenschist facies were formed at the borders. During the uplift of the orogen shear zones, faults and fractures provided channels for the intrusion of granites and syenites into the granulitic country rocks which reached higher crustal levels. This article reports a study of the Bravo granite with a zircon U-Pb SHRIMP age of 2060 +/- 6 Ma, and of its granulite host rocks formed from Archaean and Palaeoproterozoic protoliths in the Bravo and Tanquinho regions. The metamorphic peak occurred between 2064 +/- 6 Ma and 2060 +/- 6 Ma in the Bravo region. The Bravo granite is mostly metaluminous and alkaline due to high K2O contents (4.03-5.95%) which confer high-K to shoshonitic affinities. It is slightly younger than syenites of the orogen (Itiuba, 2084 +/- 9 Ma; Sao Felix, 2098 +/- 1 Ma) which are cut by late felsic veins chemically similar to the Bravo granite. The granulitic rocks form a peraluminous, medium-K calc-alkaline TTG suite. It is proposed that residual liquids represented by the vein rocks and the Bravo granite escaped from the syenitic cumulates, and migrated upwards through crustal discontinuities formed during the uplift of the ISCO.