40Ar/39Ar thermochronology of high-pressure granulite nappes in the southern Brasília Belt, Brazil: Implications for nappe exhumation

Abstract

Neoproterozoic orogenesis in the southern sector of the Southern Brasília Belt was the result of subduction and terminal collision between the Socorro-Guaxupé Arc and Paranapanema Block to the west (present coordinates), and the passive margin along the southern portion of the western side of the São Francisco Craton to the east. We report the results of a thermochronologic study of the uppermost passive margin derived high-pressure granulite facies nappes of the Andrelândia Nappe Complex to better constrain the timing of exhumation of these nappes. Based on 40Ar/39Ar biotite plateau ages, the Três Pontas-Varginha Nappe at the top of the Andrelândia Nappe Complex was the first nappe to be exhumed through ∼300 °C during the interval 591 to 567 Ma; this was followed by exhumation of the underlying Carmo da Cachoeira Nappe through ∼300 °C at circa 540 Ma. Initially, the Três Pontas-Varginha Nappe cooled slowly at a rate of <1°C Ma-1 from ∼800 °C at circa 648 Ma to ∼750 °C at circa 590 Ma, but then the rate of cooling increased dramatically to >20°C Ma-1. This increase is attributed to tectonically-driven exhumation during orogenic collapse triggered by regional extension as a far-field effect of eastward subduction of the passive margin on the eastern side of the São Francisco Craton during accretion of a magmatic arc terrane associated with the formation of the Ribeira Belt to the east. The average rate of cooling in the underlying Carmo da Cachoeira Nappe was 4 to 5 °C Ma-1. This slower rate reflects collapse of the nappe stack, which is likely to have led to faster exhumation of nappes higher in the stack. Exhumation of the Andrelândia Nappe through ∼300 °C circa 50 Ma after exhumation in the correlative Carmo da Cachoeira Nappe demonstrates that the two nappes were exhumed separately. The delayed exhumation of the Andrelândia Nappe may be due to reworking during orogenesis associated with formation of the Ribeira Belt. The Andrelândia Nappe records cooling from the bottom up; this was likely a consequence of final emplacement on the colder São Francisco Craton.