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

Abstract

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.