Stabilization of the southern portion of the São Francisco craton, SE Brazil, through a long-lived period of potassic magmatism

Although the southern portion of the São Francisco craton in southeast Brazil encompasses one of the largest segments of Meso- to Neoarchaean crust in the South American plate, there is little understanding of the processes leading to cratonization of this region. Our combined field and U-Pb age data show that the crust experienced massive intrusions of potassic (K-rich) granitoids between 2750 and 2600Ma, and that this magmatic event marked the beginning of a fundamental change in the tectonics of the terrain, with the ensuing billion years being dominated by the deposition of a series of thick clastic and chemical sedimentary successions. Thus, deep crustal partial melting and subsequent transport and intrusion of these granites in the upper crust is a likely mechanism by which this segment of the São Francisco crust attained buoyancy to survive recycling. Regional mapping indicates that the potassic granitoids are distributed over an area of ∼25,000km², and are related to one of the most prolific periods of potassic magmatism in the craton. LA-ICP-MS and SHRIMP U-Pb data from 16 samples of potassic granitoids together with published TIMS ages suggest a long lived period of magmatism from 2750 to 2700Ma, and a small pulse of granite crystallisation at ca. 2612Ma. The U-Pb SHRIMP and LA-ICP-MS data also indicate that the Archaean granitoids do not record evidence of U-Pb zircon resetting. Our interpretation is that partial melting and transport of granitoid melts (rich in heat producing elements) to the upper crust gave rise to a thermally stable lower crust and that this lower crustal segment (in the craton area) became sufficiently refractory and resistant to further partial melting.