Anatomy of the Alto Alegre gneiss dome, São Francisco Craton, Brazil: A geological record of transpression along a Palaeoproterozoic arc-continent collision zone

• Palaeoproterozoic arc-continent collision.
• Exhumation of gneiss dome along terrane boundaries.
• Large-scale folding of Archean high-grade complex.
• Gneiss dome formed by crustal folding driven by transpressional tectonics.

Gneiss domes are crustal-scale structures that can be formed in different tectonic settings. In recent collisional orogens they occur along terrane boundaries either within, or on the edge of deformation belts. The Alto Alegre gneiss dome was recognized in the northern São Francisco Craton along the limit of Archaean-Palaeoproterozoic terranes. It consists of a large N-S-elongated elliptic structure that is composed of migmatite-gneisses and concentric, metre-sized amphibolite sheets. In its central zone, the structure is intruded by irregular pink granite stocks. Geochemical data show that the amphibolites are similar in composition to mafic dykes that crosscut the Archaean Santa Luz high-grade Complex (paleocontinent) and unlike basalts of the Palaeoproterozoic Rio Itapicuru greenstone belt (arc). SHRIMP U–Pb zircon geochronology reveals ages of 3012 ± 25 Ma and 3073 ± 51 Ma for gneisses and 2082 ± 5 Ma for pink granite within the Alto Alegre dome, and 3061 ± 44 Ma for Santa Luz gneiss. Field, structural, geochemical, and geochronological data support a model in which the Alto Alegre dome is part of the Santa Luz gneiss complex. The dome was formed by large-scale folding and transpression during the late stage of Palaeoproterozoic arc-continent collision.