Paleo-to-Mesoarchean crustal evolution in the Uauá Block, Northeastern São Francisco Craton, Brazil: New insights from petrography, U–Pb geochronology, Lu–Hf isotopes and trace elements in zircon

Abstract

The timing of Earth's initial crustal growth and reworking through collisional settings remains an issue of extensive debate. Here, we report a Paleo-to Mesoarchean migmatite-gneiss, temporally- and spatially-associated with a convergent plate boundary in the Uauá Block, Brazil. This rock records an amphibolite-bearing tonalitic residue and pyroxene-bearing granitic leucosome. Orthopyroxene indicates high-temperature metamorphism during leucosome generation, while the widespread retrogressed hornblende in the residue indicates conditions corresponding to amphibolite facies. Zircon grains display LREE-depleted magmatic cores characterized by distinct zoning, crystallized at 3325 ± 70 and 3245 ± 8 Ma. They are enclosed by LREE-enriched metamorphic overgrowth zones and rims formed at 3120 ± 7 and 3069 ± 9 Ma, accompanied by an increase in Ti content. The εHf(t) values decrease with time from +1.55 to −7.06 and Lu/Hf ratios (0.001–0.011) are consistent with the evolution of the Paleo-to Eoarchean crustal sources with TDM Hf model ages between 3.56 and 3.73 Ga. Our findings provide direct evidence of crustal reworking, implicating a Paleoarchean protolith submitted, during early Mesoarchean, to a high-temperature metamorphism spatially-associated with a regional collisional setting in the São Francisco Craton. Further, we surmise that convergent tectonic processes may have played an important role in crustal reworking during the Paleo-to Mesoarchean transition. These observations allowed a comparison between the crustal evolution of the Uauá and Gavião Blocks in the northern São Francisco Craton, as well as with other Archean cratons.