CONTINENTAL SIGNATURE IMPOSED ON THE EVOLUTION OF OCEANIC-ISLAND-TYPE VOLCANOES IN THE MARIE BYRD LAND SECTOR OF THE WEST ANTARCTIC RIFT
MBL volcanoes are composed of a thick (1400-~5000m) section of basalt, surmounted by felsic rocks that make up ~5-10% of total volcano volume. The basalts are OIB in character and felsic rocks include peralkaline trachytes, phonolites, and rhyolites. In all these characteristics they resemble oceanic island volcanoes like Gran Canaria. However in four volcanoes pantellerite and phonolite were erupted coevally from the same edifice, unlike oceanic island examples. The oldest felsic volcano is ~19 Ma and occurs at the dome crest. The remaining 17 become systematically younger, along rectilinear NS and EW chains, toward the perimeter of the dome. It was initially tempting to interpret these chains as hot spot tracks, by analogy with oceanic islands, but this could not account for coeval younging in four opposite directions.
There seem to be three continental signatures stamped on the overall oceanic character of these volcanoes. (1) During dome uplift, felsic volcanism migrated away from the crest, as extension of relict NS and EW fractures was propagated distally, allowing release of magma from crustal magma chambers (JGR 94:7223-7236). (2) The continental lithosphere has acted as a filter, trapping basalt magmas at a variety of depths between ~50 km and the surface, allowing different PT fractionating assemblages to produce a variety of end products that could subsequently be erupted from the same edifice (CMP, DOI: 10.1007/s00410-011-0646-z). (3) The same volcano evolution has been continuously repeated in the same area, but without the final nephelinite stage. This is perhaps an effect of a stationary plate centered over a plume source that has remained too hot to produce the low % melting required to produce nephelinite (in prep).