Evidence for change in crust formation process during the Paleoarchean in the São Francisco Craton (Gavião Block) : coupled zircon Lu-Hf and U-Pb isotopic analyses and tectonic implications.

Abstract

The continental crust growth/evolution processes and the tectonic regime through Eo- and Paleoarchean times are enigmatic due to the scarcity of preserved crust. The Gaviao ̃ Block, S ̃

ao Francisco Craton (Brazil), contains exposed and well-preserved Eo-Paleoarchean crust remnants, providing a rare opportunity to investigate these issues. Here, we describe new U-Pb ages and Hf isotope data of 3.51–3.4 Ga tonalites and diorites from the Gaviao ̃ Block and compare these with previously published Lu-Hf data from Hadean/Eo-Paleoarquean zircons from the Gavi ̃ ao Block and other primitive cratons. The Eo- to Paleoarchean evolution of the Gaviao ̃ Block is registered by ca. 360 Myr of continuous magmatic events from ca. 3.66 Ga to 3.30 Ga. From the available Hf data, we interpret that each of the events younger than 3.6 Ga registers a new juvenile addition that assimilated older crust, whereas the rocks older than 3.6 Ga are exclusively formed through the reworking of a Hadean, and to a less extent early Eoarchean crust. The shift in the crust generation process with the input of juvenile material into the Gavi ̃ ao Block has been documented within the ~ 3.8–3.5 Ga time in other primitive cratonic complexes

including the Wyoming, Pilbara, Kaapvaal, Slave, Singhbhum, and Yilgarn. As documented in these other cra- tons, our data suggest that a shift in the Hf isotope record to rocks younger than 3.6 Ga reflects a transition from

stagnant-lid to mobile-lid tectonics in the crust formation process of the S ̃

ao Francisco Craton. This change in the geodynamic regime appears to have been global at ca. ~ 3.8–3.5 Ga and facilitated the extraction of juvenile melts, crustal reworking, evolved magmatism, and the production of stabilizing melt-depleted lithospheric mantle.