Contrasting metamorphic conditions in the Neoproterozoic collision-related Nappes south of Sao Francisco Craton, SE Brazil

Metamorphic conditions are described for three major tectonic entities on the basis of geothermobarometry in a huge Neoproterozoic nappe complex that verges toward the southern border of the Sao Francisco craton. The uppermost Socorro-Guaxupe Nappe, represented by its granulite facies basal portion, yields a maximum temperature and pressure of 890 degreesC and 11 kbar. Its metamorphic evolution is consistent with heating at the base of the crust as a result of an abnormally high geothermal gradient, probably due to underplating by the lithospheric mantle. The underlying Tres Pontas-Varginha Nappe yields two somewhat distinct P-T paths, both characterized by peak assemblages in the kyanite stability field. The basal kyanite-bearing granulites show higher peak pressure values (15 kbar at 840 degreesC) and a trajectory that continues in the kyanite stability field, whereas the upper sillimanite granulites show higher temperatures (880 degreesC at 13 kbar) and a steeper path toward the sillimanite stability field. Data for the Carmo da Cachoeira nappe reveal a steep trajectory, in which the elevated maximum pressure (18.5 kbar at 820 degreesC) was obtained from a garnet amphibolite that lies along its basal contact. The inverted metamorphic pattern previously observed across these sequences is confirmed by our thermobarometric data, which reveal that the highest temperatures were attained toward the top of the pile. (C) 2002 Elsevier Science Ltd. All rights reserved.