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 2600 Ma, 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 ∼25,000 km2, 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 2700 Ma, and a small pulse of granite crystallisation at ca. 2612 Ma. 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.

► A large segment of meso-Achaean crust within the São Francisco craton was transformed into a stable cratonic unit at ca. 2700 Ma. ► The cratonization occurred during multiple events of potassic granitoid emplacement between 2750 and 2700. ► The magmatism marks a fundamental change in the tectonics of the craton, with ensuing billion years dominated by clastic and chemical sedimentation.