P-T-TIME CONSTRAINTS ON NAPPE EMPLACEMENT IN THE SOUTHERN BRASÍLIA BELT, BRAZIL

The Brasília Belt lies between the São Francisco, Amazonas and Paranapanema cratons in central Brazil. In the southern portion of this belt, the Andrelândia Nappe Complex records metamorphic conditions ranging from greenschist facies at the base to high pressure granulite facies at the top. The uppermost Socorro–Guaxupé nappe (SGN) represents the arc from the overlying plate, and the underlying Três Pontas–Varginha (TPV) and Carmo da Cachoeira (CdC) nappes comprise pelitic sediments metamorphosed in a subduction environment prior to arc collision with the São Francisco Craton. Average P-T calculations on samples taken from these nappes yield maximum P-T conditions of 10.0±1.3 kbar and 1015±68°C for the SGN, 13-16 kbar and 800-950° for the TPV nappe, and 6-9 kbar and 520-580°C for the CdC nappe. For three samples from the TPV nappe, zircons were dated in situ using a Cameca 1270 SIMS. ²⁰⁷Pb/²⁰⁶Pb dates inferred to be from metamorphic zircon overgrowths were culled from the dataset; a probability distribution plot of these dates indicates two ages of zircon growth, at 650±4 Ma and 605±6 Ma (2σ). Monazite from samples taken from the TPV nappe and the CdC nappe was dated in situ using a JEOL/JXA-8900 EPMA and a Cameca 1270 SIMS. Samples from the TPV nappe yielded a range of dates for both EPMA and SIMS monazite analyses between ca. 620 Ma to ca. 560 Ma. Yttrium rich zones in monazite yielded a ²⁰⁷Pb/²³⁵U age of 609±8 Ma and a ²⁰⁶Pb/²³⁸U age of 603±7 Ma. Yttrium poor zones in monazite yielded a 207Pb/235U age of 571±9 Ma and a ²⁰⁶Pb/²³⁸U age of 571±8 Ma. For a Grt-Bt schist from the CdC nappe, the cores of monazite grains included in biotite, in the matrix of the rock, record a (U-Th)-Pb age of 635±6 Ma, a ²⁰⁶Pb/²³⁸U age of 630±13Ma and a ²⁰⁷Pb/²³⁵U age of 623±10 Ma. Rims from these same grains record a (U-Th)-Pb age of 607±3 Ma, a ²⁰⁶Pb/²³⁸U age of 605±8 Ma and a ²⁰⁷Pb/²³⁵U age of 602±6 Ma. The younger monazite age is consistent with the younger U-Pb zircon SIMS age. We speculate that the younger age of monazite and zircon growth (ca. 605 Ma) may correspond with the emplacement of the SGN on top of the nappe stack, whereas the older ages record the peak of subduction-related metamorphism in the TPV (ca. 650 Ma) and the CdC (ca. 635 Ma) nappes. Younger (U-Th)-Pb ages in some samples may relate to overprinting by the Ribera Belt, which we are currently evaluating.