Crustal reworking and Archean TTG generation in the south Gavião Block, São Francisco Craton, Brazil

Abstract

Origin of early Earth’s felsic crust is a matter of growing debate in which the rock record from old cratons plays a key role. In northeast Brazil, the São Francisco Craton preserves the oldest felsic rocks of South America and understanding its Archean evolution is an important piece of this debate. Here we present new SHRIMP U-Pb zircon ages and whole-rock geochemistry, as well as a large, compiled dataset to discuss the evolution of the south Gavião Block (GB), a major component of this craton. The GB contains Paleo- to Mesoarchean (3.36–3.18 Ga) TTGs, high-K granites, and minor diorites of the Gavião Complex followed by renewed magmatism, reworking and anatexis between 3.27 and 3.22 Ga. The TTG rocks of Gavião Complex can be divided into high-Sr (high pressure) and low-Sr (low- to medium pressure), evidencing that the sodic magmatism likely occurred at different depths diachronically between 3.36 and 3.20 Ga. We report a crystallization age of a ca. 3.35 Ga for an Archean unusual high-K calc-alkaline meta-granite which marks the first potassic magmatism of the Gavião Complex, suggesting reworking of preexisting TTGs. Zircon U-Pb dates from a TTG migmatite constrained an age of ca. 3.27 Ga, interpreted as a metamorphic event within the basement of the GB, stretching back in ca. 120 My the oldest metamorphic age known so far for the whole São Francisco Craton. Additionally, at ca. 2.7 Ga, the GB was intruded by the intraplate, alkaline magmatism of the Caraguataí Suite, marking thus the stability of this Block during Neoarchean. We argue that crustal reworking was the main mechanism responsible for the formation of the GB during the Paleo- to Mesoarchean. The data suggest the GB rocks share an old source possibly including Hadean components. We propose that the formation of the TTGs of GB is a result of the melting of low-K mafic crust at the base of a progressively thicker oceanic plateau.