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 Pacific margin of the AP. In Marie Byrd Land (MBL), by contrast, alkaline basaltic volcanism has been semi-continuous from 25-30 Ma to the present, and occurs in the West Antarctic rift system. Together, these Antarctic tectono-magmatic associations are analogous to the Basin and Range, Sierran, and Coast Range batholith provinces. Unlike the western US, however, basaltic rocks throughout WA have uniform geochemical characteristics, with especially narrow ranges in initial⁸⁷Sr/⁸⁶Sr (0.7026-0.7035),¹⁴³Nd/¹⁴⁴Nd (0.51286-0.51299), and La/Nb (0.6-1.4) ratios, suggesting very limited liput from "old" subcontinental lithosphere or crustal sources during magma genesis. However, there are significant differences in the relative and absolute abundances of the LILE (large-ionlithophile elements), and these divide WA into two provinces. Basalts from the AP region have unusually high K/Ba and K/Rb ratios (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 distributions within the range for OIB (ocean-island basalt) (K/Ba <50, Ba/Nb 5-20). This geochemical contrast is accompanied by a three-fold increase in the age range of volcanic activity and a three orders of magnitude increase in the volume of eruptive products, within MBL. The regional differences in geochemistry, and in the volume and duration of volcanic activity, are best explained by a plume-related origin for MBL basalts, whereas alkaline magmatism in the AP is causally related to slab window formation following the cessation of subduction. Plume activity has alreadybeen proposed to explain tectonic doming and associated 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 system suggest uniformly minimal extension throughout this region during late Cenozoic time. Differences in crustal thicknesses can be explained by early Cenozoic or pre-Cenozoic extension, but restraint on extension is suggested by the size of the region and the implied size of the plume. The c. 95% encirclement of the Antarctic plate by mid-ocean ridges and transforms restrains extension on a regional scale, leading to nonadiabatic plume rise and correspondingly little decompression melting.