Characteristics of the lithospheric mantle beneath northeastern Borborema Province, Brazil: Re-Os and HSE constraints on peridotite xenoliths

The northeastern part of the Borborema Province, NE Brazil is noteworthy for its history of crust formation from the Archean to Cenozoic, which certainly resulted in chemical changes in the lithospheric mantle beneath. Here we examine the chemical evolution of the subcontinental lithospheric mantle (SCLM) in the northeastern Borborema Province using the Re-Os isotopic system combined with major and trace element, including highly siderophile element (HSE) data for 23 mantle xenoliths. The xenoliths were sampled by the Cenozoic basalts of the Macau Volcanic Field (MVF) and classified in two groups - G1 and G2 based on chemical and textural differences. The xenoliths from both groups are moderately refractory with olivine Fo content ranging from 88 to 91. The Primitive Upper Mantle (PUM)-normalized HSE patterns for the Gi xenoliths are kinked, showing relative enrichments in Re and Ru consistent with variable degrees of both partial mantle melting and metasomatic enrichment. By contrast, HSE patterns for the G2 xenoliths are generally flat, but with increasing depletions from Pd to Re, consistent with single-stage melt depletion. Initial Os-187/Os-188 ratios for both groups range from 0.11544 to 0.12930. The ratios are similar to those of modern abyssal peridotites, suggesting that the SCLM beneath northeastern Borborema was recently derived from convecting upper mantle. Osmium model ages range from similar to 0 to 1.7 Ga, with peaks at <= 0.2 Ga between initial Os-187/Os-188 ratios of 0.125 and 0.126 in both G1 and G2 xenoliths. The representative Os T-RD model age of 1.7 Ga corresponds to a Late Paleoproterozoic magmatic event and suggests the presence of Paleoproterozoic remnants in the SCLM beneath northeastern Borborema Province. From the combination of these new Re-Os data and those from literature, we conclude that lithospheric extension and thinning from the onset of the opening of the Equatorial Atlantic resulted in asthenospheric (juvenile material) upwelling in the Mesozoic (<= 0.2 Ga) that presumably partially replaced older SCLM. Lithospheric impingement due to the asthenospheric upwelling likely triggered mantle melting that accounts for both the multi-stage and single-stage HSE patterns of G1 and G2 xenoliths, respectively. This melting event was coeval with the generation of the Rio Ceara-Mirim tholeiitic magmatism which occurred during the Equatorial Atlantic opening in the Cretaceous. The upwelled asthenosphere into the resulting gaps proportional to the lithospheric thinning in the Mesozoic became cooler and thermally and rheologically re-equilibrated as a rigid lithospheric mantle since the Cretaceous. Therefore, the SCLM beneath northeastern Brazil remained non-convective since the Mesozoic and probably vertically stratified geochemically. Another major event recorded only by the G1 xenoliths is the metasomatic changes induced by melt percolation from the LREE-enriched melts that were genetically associated with the Cenozoic MVF basalt magmatism. (AU)