OS, SR, ND, PB, O ISOTOPE AND TRACE-ELEMENT DATA FROM THE FERRAR FLOOD BASALTS, ANTARCTICA - EVIDENCE FOR AN ENRICHED SUBCONTINENTAL LITHOSPHERIC SOURCE

Os, Sr, Nd, Pb and O isotopes and trace element data are reported for basaltic andesite and andesite whole rocks and, in part, for selected mineral separates from the Jurassic Ferrar flood basalt province. Radi ogenic Sr(> 0.709), unradiogenic Nd (epsilon(Nd) = -3 to -5), and radi ogenic Pb isotopes, as well as low Nb/La ratios of 0.4-0.6 and Nb/La r atios between 0.45 and 0.6 are found for all rocks including our most primitive sample (Mg# = 71.9). This indicates involvement of either co ntinental crust or enriched lithospheric mantle in magma genesis. Re-1 87/Os-188 correlates strongly with Os-187/Os-188, with an age of 172 /- 5 Ma, in agreement with published Ar-Ar data, Initial Os-187/Os-188 of 0.194 +/- 0.023 is close to the range of typical mantle values for MORE, OIB and lithospheric mantle and much lower than that of contine ntal crust. delta(18)O values between 5 parts per thousand and 7 parts per thousand were obtained on fresh bulk samples, separated plagiocla ses and clinopyroxenes. Sr-O and Sr-Os isotope mixing calculations bet ween depleted mantle peridotite or mantle melts and crustal material r ule out assimilation involving basalts with low Os concentrations, and simple binary mixing or pure AFC processes involving picrites. AFC pr ocesses, combined with continuous replenishment of picritic magmas, ca n explain the isotopic data, provided the crustal end-member has high Sr-87/Sr-86 and low delta(18)O values. However, lower crustal samples displaying these characteristics are absent in the Ferrar region, and are also unlikely to impart the sediment-like trace element patterns o bserved in the Ferrar data. A more likely explanation is a lithospheri c source enriched by subducted sediments. A contribution to Ferrar mag matism from a plume cannot be distinguished.