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 Nunatak, are virtually constant in major, trace element and isotopic composition over this entire age span. Their high FeO-low Al203 character indicates melting of garnet peridotite at about 140 km depth. There is no evidence for the involvement of ancient continental lithosphere or MORB asthenosphere in the magmatism. Isotopically, the lavas show the highest 206 Pb/ 204Pb ratios (up to 20.7) of any of the Cenozoic volcanism 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 Tubuai and Mangaia. he other localities along the Hobbs Lineament, the earliest volcanism, which is coeval with that at Coleman Nunatak, is of shallower derivation (∼ 110 km), and isotopically like the oceanic FOZO end-member (206Pb/204Pb ∼19.5). The trace-element patterns are similar to those at Coleman, but less enriched in the most incompatible elements by a factor of two. Modeling of the trace element data is consistent with a uniform mantle source composition, depleted in major elements, but hydrous and mildly enriched in the incompatible and LREE. Inversion for the bulk distribution coefficients of the source mantle reveals a spidergram with a marked negative Ti anomaly and marked positive anomalies for K, Sr, Zr and HE From this modeling, the extent of melting at Coleman is inferred to be ∼ 1.6%, as compared 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 basalts from the Coleman locality). ZO component is prevalent as a mixing end-member in WARS volcanism from numerous other Marie Byrd Land (MBL) and Northern Victoria Land (NVL) localities. It is also the main constituent of the three nearby oceanic plumes (Balleny, Scott, Peter I islands). The HIMU component is at best a minor constituent of these oceanic plumes, but is present at several other MBL and NVL localities, as well as in pieces of Zealandia which were adjacent to this coast 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 accompanying volcanism. This ‘fossil-plume’ proto-lithosphere is now being sampled during WARS extension. Likely mechanisms for the volcanism relate either to small-scale convection associated with strong basal topography of the lithosphere (such as that recorded by the Hobbs Lineament volcanism), or to emplacement of a new plume, which may in part be driving the extension.