Sr-Nd-Pb-He-O isotope and geochemical constraints to the genesis of Cenozoic magmas from the West Antarctic rift

The West Antarctic Rift System (WARS) represents one of the major active continental extension zones on Earth. The Ross Sea coast in northern Victoria Land (NVL) is littered with alkalin rift-related igneous products (Middle Eocene^Present).This study characterizes the nature of the magma source involved in the rift process through geochemical^isotopic investigation of Cenozoic basalts from NVL, and provides important constraints for the reconstruction of the tectono-magmatic evolution of the Ross Sea region^WARS. The chemical compositions of the basalts (Miocene^Present) display major and trace element characteristics typical of ocean island basalts (OIB), with strong enrichment in the most incompatible elements. Whole-rock isotopic compositions are in the range 07028^07034 for 87Sr/86Sr, 05129^05130 for 143Nd/144Nd (eNd(t) 48^67), 193^197 for 206Pb/204Pb, 154^156 for 207Pb/204Pb and 387^393 for 208Pb/204Pb, suggesting a HIMUlike (high U/Pb) signature of the mantle source. Determinations of 3He/4He on crushed olivine yielded values between 57 and 72 times the atmospheric ratio, similar to the lithospheric mantle and in the range of mid-ocean ridge basalts (MORB).The d18Ool of olivine separates varies from 492 to 553ø and is positively correlated with Fo content. Integration of our geochemical and isotope data with available geological, geophysical and geochronological data has led to the following reconstruction.The differences in the oxygen isotope values principally reflect the involvement of a heterogeneous mantle source and/or the assimilation of variable amounts of hydrothermally altered crustal rocks from the volcanic edifices. The 3He/4He data allow us to exclude a plume-driven model to explain the continuing rifting process. Based on the evidence of metasomatic processes, we propose a model to generate the mantle source(s) of the Cenozoic basaltic melts of the NVL.This is sublithospheric mantle metasomatized during an amagmatic extensional event that affected theWARS in the Late Cretaceous. During Eocene^Oligocene times, mantle flow warmed the mantle at the edge of the thick Antarctic lithosphere, and the reactivation of old translithospheric discontinuities promoted mantle melting and the rise of magmas as plutons and dyke swarms. From the Late Miocene to Present, the continuing craton-directed mantle flow led to normal faulting of the rift shoulder, which favoured the rise of magmas to build up large volcanic edifices.