Composition and thermal evolution of the lithospheric mantle beneath the Ribeira Belt, SE Brazil: evidence from spinel peridotite xenoliths

This study reports the first geochemical and Pb isotopic data for mantle xenoliths from beneath the Neoproterozoic Ribeira Belt, southeastern Brazil. The cm-sized spinel peridotite xenoliths are hosted by a Cretaceous lamprophyre dike that intruded high-grade metamorphic rocks. Major- and trace-element compositions of the main minerals indicate that the xenoliths derive from a shallow fertile mantle that has undergone a low degree of melt extraction (2–9% partial melting). On the basis of modeled isochemical phase diagrams for lherzolites, pressure and temperature conditions are inferred to vary from 1300–1350 °C and 17–19 kbar (fertile composition) to 1330–1430 °C and 17–23 kbar (relatively depleted composition), which correspond to high geothermal gradients of 65–80 mW/m 2 . Temperatures of last equilibration calculated based on the average REE content of pyroxenes for the same lherzolite samples vary from 1233 ± 56 °C to 1085 ± 42 °C, while conventional thermometry ( T BKN ) yields average values of 807 and 755 °C, indicating re-equilibration at lower temperatures. Pb isotope ratios of clinopyroxene define a mixing line that intercepts the Stacey-Kramers two-stage terrestrial Pb evolution curve at ca. 200 Ma. Linear regressions yield two errorchrons of 56 ± 75 Ma and 571 ± 99 Ma (95% confidence level). These results combined with the ages and tectonic settings of host rocks are suggestive of an overprint of a younger tectono-thermal event, most likely related to the opening of the South Atlantic Ocean, over a mantle previously equilibrated during the Precambrian development of the Ribeira Belt.