TECTONIC CONTROLS ON THE GEOCHEMICAL COMPOSITION OF CENOZOIC, MAFIC ALKALINE VOLCANIC-ROCKS FROM WEST ANTARCTICA

Cenozoic, mafic alkaline volcanic rocks throughout West Antarctica (WA ) occupy diverse tectonic environments. On the Antarctic Peninsula (AP ), late Miocene-Pleistocene (7 to < 1 Ma) alkaline basaltic rocks were erupted < 1 to 45 million years after subduction ceased along the Pac ific margin of the AP. In Marie Byrd Land (MBL), by contrast, alkaline basaltic volcanism has been semi-continuous from 25-30 Ma to the pres ent, and occurs in the West Antarctic rift system. Together, these Ant arctic tectono-magmatic associations are analogous to the Basin and Ra nge, Sierran, and Coast Range batholith provinces. Unlike the western US, however, basaltic rocks throughout WA have uniform geochemical cha racteristics, with especially narrow ranges in initial Sr-87/Sr-86 (0. 7026-0.7035), Nd-143/Nd-144 (0.51286-0.51299), and La/Nb (0.6-1.4) rat ios, suggesting very limited input from ''old'' subcontinental lithosp here or crustal sources during magma genesis. However, there are signi ficant differences in the relative and absolute abundances of the LILE (large-ion-lithophile elements), and these divide WA into two provinc es. Basalts from the AP region have unusually high K/Ba and K/Rb ratio s (50-140 and 500-1500 respectively) and marked Ba depletion (Ba/Nb = 2.5-8.0; Ba ppm 66-320) relative to MBL basalts, which have LILE distr ibutions within the range for OIB (ocean-island basalt) (K/Ba < 50, Ba /Nb 5-20). This geochemical contrast is accompanied by a three-fold in crease in the age range of volcanic activity and a three orders of mag nitude increase in the volume of eruptive products, within MBL. The re gional differences in geochemistry, and in the volume and duration of volcanic activity, are best explained by a plume-related origin for MB L basalts, whereas alkaline magmatism in the AP is causally related to slab window formation following the cessation of subduction. Plume ac tivity has already been proposed to explain tectonic doming and associ ated spatial patterns of volcanism in MBL. Most MBL geochemical traits are shared by the volcanic rocks of the western Ross Sea, suggesting that a large plume head underlies the West Antarctic rift system. The uniformity of basalt compositions throughout WA and the entire rift sy stem suggest uniformly minimal extension throughout this region during late Cenozoic time. Differences in crustal thicknesses can be explain ed by early Cenozoic or pre-Cenozoic extension, but restraint on exten sion is suggested by the size of the region and the implied size of th e plume. The c. 95% encirclement of the Antarctic plate by mid-ocean r idges and transforms restrains extension on a regional scale, leading to non-adiabatic plume rise and correspondingly little decompression m elting.