Trench-proximal volcanism following ridge crest-trench collision along the Antarctic Peninsula
On the basis of multichannel seismic reflection, magnetics and bathymetry data, a Pleistocene-Holocene (<0.1Ma) volcanic seamount has been identified at a location very close to the shelf edge off the west coast of the Antarctic Peninsula. The seamount overlies the subducted trace of a transform fracture zone which divides segments of the margin along which collision took place 3.1 and 6.0 Ma ago. New trace element and isotopic data from dredged samples demonstrate that the seamount appears to be predominantly formed of relatively primitive undersaturated alkali basalt. The basalts have geochemical signatures that are indistinguishable from ocean island basalts and some continental alkali basalts, (e.g. Th/Ta 1.0–1.5, Rb/Nb<0.5, 87Sr/86Sr 0.70265–0.7028, 143Nd/144Nd approximately 0.5129) and are totally lacking in any evidence for interaction with subduction-enriched mantle. Geochemical similarities with other, more trench distal, postsubduction alkalic basalts along the Antarctic Peninsula are striking, strongly implying that all the postsubduction basalts were derived from a chemically similar asthenospheric source region. The basalts were most likely to have been generated as a result of the formation of a slab window beneath the Antarctic Peninsula following ridge crest-trench collision. Subduction component-free subslab asthenosphere upwelled into the incipient void left by the continued sinking of the leading plate following collision, and decompressional melting resulted. This type of trench-proximal volcanism following ridge crest-trench collision differs from that in other locations where calc-alkaline volcanism persisted or ophiolite obduction occurred.