Palaeoproterozoic dome-forming structures related to granulite-facies metamorphism, Jequié block, Bahia, Brazil: petrogenetic approaches

Abstract

Dome structures in the São Francisco craton (Northeast Brazil), have been identified in the regions of Brejões and Santa Inês within the granulitic Jequié block. Domes are mainly composed of CH6 charnockites, which intrude various granulites of the basement. The Brejões dome is surrounded by a rim of supracrustal rocks. The enderbite–charnockite suites CH1 and CH2, which are partially included in the basement and were previously dated ca 2.8–2.7 Ga, are similar to modern calc-alkaline suites and could be derived by partial melting of an enriched tholeiite (degree of fractionation ca 24–23%). An older Archaean component is suggested by negative ɛNd values at 2.8–2.7 Ga which range between −1 and −4 and Nd model ages with an average ca 3.1 Ga. These magmatic suites are ascribed to the amphibolite-facies based on preserved prograde mineral assemblages and results of geochemical modelling. The CH6 rocks formed in domes are of granitic composition. They mainly derive from partial melting of the 2.7 Ga CH2 basement suite. The hercynite-quartz assemblage in supracrustal rocks surrounding the Brejões dome suggests very high temperature (>1000 °C) for the CH6 magmas which would have induced partial melting in adjacent supracrustal rocks, forming garnet-cordierite-bearing granitic melts. Emplacement of domes is constrained by the age of the granulite facies metamorphism in the Jequié block (ca 2.1 Ga) and the average cooling age of monazite which occurred in the syn-dome charnockite and the surrounding heterogeneous granulites of the basement at 2045 ± 2.5 Ma. A southern dome exhibits a younger monazite age of 2026 ± 1 Ma, which may suggest a broader age span for the doming event.