Age, geochemistry and Sr-Nd-Pb isotopic compositions of alkali volcanic rocks from Mt. Melbourne and the western Ross Sea, Antarctica

Abstract

New K/Ar ages and geochemical and isotope data (Sr, Nd, Pb) of submarine samples from the Terror Rift Region and subaerial lavas from Mt. Melbourne Volcanic Field (MMVF) in the western Ross Sea, are presented. The MMVF samples are classified into Groups A and B based on their temporal and spatial distribution. All samples are alkaline, ranging from basanite to trachybasalt, and exhibit the Ocean Island Basalt (OIB)-like patterns of trace element distribution, with a prominent depletion in K and Pb. They exhibit an HIMU-like isotopic signature (206Pb/204Pb = 18.510？19.683, 87Sr/86Sr = 0.70300？0.70398, 143Nd/144Nd = 0.51284？0.51297) and trace element affinities (Ce/Pb = 25？35, Nb/U = 45？60, Ba/Nb = 5？13, La/Nb = 0.5？0.9). New K/Ar ages and geochemical data, combined with published data, show no correlations between age and composition of Cenozoic basalts in NVL. The Terror Rift submarine lavas (0.46？0.57 Ma) display a distinct trend, with more primitive geochemical characteristics (higher MgO (7.2？9.8 wt.%) and CaO (9.9？11.9 wt%) and stronger HIMU signature (higher 206Pb/204Pb and less radiogenic 87Sr/86Sr and 143Nd/144Nd ratios) than those of MMVF basalts. Results from a rare earth element (REE) model suggest that the Terror Rift submarine lavas are derived from small degrees (1？2%) of partial melting of an amphibole-bearing garnet peridotite mantle source. Despite the distinctly different ages and locations of the MMVF Group A (0.16？0.33 Ma) and B (1.25？1.34 Ma) basalts, they show similar geochemical and isotopic features, indicating the sharing of common mantle sources and magma processes during magma generation. Incompatible trace element ratios (e.g., Ba/Nb = 6.4？13.2, La/YbN = 14.4？23.2, Dy/Yb = 2.2？3.0) and isotopic compositions of the MMVF Group A and B volcanics suggest derivation from higher degrees (2？5%) of partial melting of a garnet peridotite source and strong influence of an EMI-type mantle source. The stronger HIMU signature of the Terror Rift submarine lavas appears to be related to smaller degrees of partial melting, suggesting preferential sampling of the HIMU component in the less partially melted rocks from the Cenozoic NVL magmatism. In contrast, the higher degree of MMVF A and B magmas can be explained by greater interaction with heterogeneous lithospheric mantle, resulting in a diluted HIMU signature compared with that of the Terror Rift submarine lavas. We assume that HIMU- and EMI-type mantle components incorporated in the Cenozoic NVL magmas originated from sub-continental lithospheric mantle metasomatized by plume or subduction-related fluids prior to the breakup of Gondwanaland.