The Itabuna-Salvador-Curaçá Orogen revisited, São Francisco Craton, Brazil: New zircon U–Pb ages and Hf data support evolution from archean continental arc to paleoproterozoic crustal reworking during block collision

Abstract

Precambrian orogenic belts are key areas to understand how the continental crust formed and evolved with time, and to constrain when plate tectonics began on planet Earth. The Itabuna-Salvador-Curaçá Orogen, in the São Francisco Craton, is an orogenic belt over 800 km long that is interpreted as the result of collision of four blocks during the Paleoproterozoic. However, its geological history before the final continental collision and the time span of collision are not well constrained. Here, we present new field relationships, in situ zircon U–Pb ages and Hf isotope data for gneisses and granites from the orogen and one of the blocks (Gavião block) that collided during the Paleoproterozoic. Most of the samples were collected along a geological traverse across the northern area of the orogen. The geochronological results indicate ages of 3335 Ma and 3307 Ma for gneisses of the Mairi Complex (Gavião block), 2637 Ma, 2620 Ma and 2581 Ma for gneisses of the Caraíba Complex within the orogen, and ages between 2100 Ma and 2058 Ma for the collisional granitoids from the Itabuna-Salvador-Curaçá Orogen. The slightly negative εHf(t) values (−1.9 and −4.5) for samples of the Mairi Complex suggest contribution from older continental crust in the genesis of the gneisses protoliths. For the Caraíba Complex, positive to negative εHf(t) values (0.9 to −3.7) for the orthogneisses, coupled with similar whole-rock εNd(t) signature from the literature, suggest a continental arc setting for emplacement of the Caraíba Complex orthogneisses. The εHf(t) values (−12.1 and −13.4) for one collisional granitoid indicate it can have originated from the partial melting (reworking) of the Caraíba Complex crust. From the tectonic point of view, the ages of the Caraíba Complex become younger from west to east and this age variation can be interpreted in terms of a continental arc that formed by trench/slab retreat on the edge of the Gavião block. This model is supported also by the occurrence in the Gavião block of coeval (2595 ± 21 Ma) volcanic rocks in the Mundo Novo greenstone belt (interpreted in the literature as formed in a back-arc basin) and some metamorphosed clastic sedimentary rocks of the Saúde Complex with provenance solely from sources older than 2500 Ma. On the other hand, the ages of the collisional granites can be used to constrain the timing and duration of the orogen's evolution. Accordingly, the early collisional granites with ages between 2100 Ma and 2094 Ma set a minimum age for the beginning of collision. The syn-to late-transcurrent granites were emplaced from 2084 Ma to 2060 Ma, and the post-collisional granites from about 2058 Ma until 2027 Ma. Therefore, block collision to form the Itabuna-Salvador-Curaçá Orogen has lasted at least 70 million years.