HOBBS COAST CENOZOIC VOLCANISM - IMPLICATIONS FOR THE WEST ANTARCTIC RIFT SYSTEM

Basaltic lavas were erupted from a 40-km-long lineament near the Hobbs Coast of Marie Byrd Land, Antarctica, over the period from 11.7 m.y. to 2.3 m.y. ago. The lavas from the southernmost locality, Coleman Nun atak, are virtually constant in major, trace element and isotopic comp osition over this entire age span. Their high FeO-low Al2O3 character indicates melting of garnet peridotite at about 140 km depth. There is no evidence for the involvement of ancient continental Lithosphere or MORE asthenosphere in the magmatism. Isotopically, the lavas show the highest Pb-206/Pb-204 ratios (up to 20.7) of any of the Cenozoic volc anism associated with the West Antarctic rift system (WARS). This HIMU isotopic signature is also clear in the trace element patterns, which closely mimic end-member HIMU basalts from the oceanic islands of Tub uai and Mangaia. From the other localities along the Hobbs Lineament, the earliest volcanism, which is coeval with that at Coleman Nunatak, is of shallower derivation (similar to 110 km), and isotopically like the oceanic FOZO end-member (Pb-206/Pb-204 similar to 19.5). The trace -element patterns are similar to those at Coleman, but less enriched i n the most incompatible elements by a factor of two. Modeling of the t race element data is consistent with a uniform mantle source compositi on, depleted in major elements, but hydrous and mildly enriched in the incompatible and LREE. Inversion for the bulk distribution coefficien ts of the source mantle reveals a spidergram with a marked negative Ti anomaly and marked positive anomalies for K, Sr, Zr and Hf. From this modeling, the extent of melting at Coleman is inferred to be similar to 1.6%, as compared to similar to 3.2% during the earliest volcanism elsewhere on the lineament. With time, the volcanism from these other localities progresses to greater depth, becomes more HIMU in character , and lower in extent of melting (i.e., approaches the character of ba salts from the Coleman locality). The FOZO component is prevalent as a mixing end-member in WARS volcanism from numerous other Marie Byrd La nd (MEL) and Northern Victoria Land (NVL) localities. It is also the m ain constituent of the three nearby oceanic plumes (Balleny, Scott, Pe ter I islands). The HIMU component is at best a minor constituent of t hese oceanic plumes, but is present at several other MBL and NVL local ities, as well as in pieces of Zealandia which were adjacent to this c oast of Antarctica prior to fragmentation of Gondwana. We propose that this HIMU mantle source was emplaced under Gondwana lithosphere prior to breakup, as a large weak plume head, with Little or no accompanyin g volcanism. This 'fossil-plume' proto-lithosphere is now being sample d during WARS extension. Likely mechanisms for the volcanism relate ei ther to small-scale convection associated with strong basal topography of the lithosphere (such as that recorded by the Hobbs Lineament volc anism), or to emplacement of a new plume, which may in part be driving the extension. (C) 1997 Elsevier Science B.V.