GEOCHEMICAL EVOLUTION OF RIFT MAGMAS BY PROGRESSIVE TAPPING OF A STRATIFIED MANTLE SOURCE BENEATH THE ROSS-SEA-RIFT, NORTHERN VICTORIA-LAND, ANTARCTICA

Source compositions of Neogene-Quaternary volcanic rocks from the McMu rdo Volcanic Group of the Ross Sea Rift in Northern Victoria Land, Ant arctica are constrained by Nd-Sr-Pb isotopes and trace element ratios in near-primary basalts. The rocks erupted along the western rift marg in (Victoria Land Basin) and the western rift shoulder (Transantarctic Mountains). Near-primary basalts show no evidence of crustal contamin ation, suggesting that their initial Nd-Sr-Pb isotopes reflect the com position of their mantle sources. The initial isotope ratios of near-p rimary basalts range from about Sr-87/Sr-86 = 0.70281 to 0.70504 and N d-143/Nd-144 = 0.51269 to 0.51291 (epsilon(Nd(t)) = 1.3-5.5). The Pb-2 06/Pb-204 ratios vary between 19.3 and 20.1. Our results, in conjuncti on with data from the literature [1-3], suggest the involvement of thr ee mantle source components (or their mixtures) during the formation o f Ross Sea magmas: depleted MORB-type mantle, an enriched mantle compo nent (EM), and a component akin to HIMU. The involvement of these mant le components during magma genesis correlates with tectonic setting: w hereas MORB-type compositions are restricted to Recent within-rift bas alts, EM and HIMU isotope signatures dominate off-rift magmas from the western rift shoulder and Marie Byrd Land respectively. Basalts from the western rift shoulder show temporal isotopic and trace element var iations from EM towards HIMU-type signatures between 15 and 5 m.y. On the other hand, within-rift and Marie Byrd Land basalts changed from H IMU-type towards MORB-type compositions through time. These temporal g eochemical variations together with the respective tectonic settings o f magmatism suggest that the mantle beneath the Ross Sea Rift is strat ified in the order MORB- to HIMU- to EM-type sources. It appears that during rift evolution the EM- and HIMU-type sources are consumed and d epleted asthenospheric MORB-type mantle rises progressively into the m elting region. This suggests that EM resides in the mantle lithosphere . The HIMU-type component may be related to the head of an active or, alternatively, 'fossilized' mantle plume attached to the base of the l ithosphere beneath the Ross Sea area.