The relationship between alkaline magmatism, lithospheric extension and slab window formation along continental destructive plate margins

Two distinct groups of Late Cenozoic OIB-like basalts are recognized along the Antarctic Peninsula. Small volumes of undersaturated basanites, tephrites, alkali and olivine basalts and "within-plate' tholeiites, which post-date the cessation of subduction as a result of ridge crest-trench interactions by 40 to <10 Ma, are scattered along the southern part of the Peninsula. At James Ross Island, in the north of the Peninsula, alkali basalts, hawaiites and rare mugearites were erupted synchronously with subduction at the South Shetland Islands trench to the west. Both groups of basalts are similar in terms of their geochemical and isotopic characteristics, although they apparently owe their origin to two distinct combinations of tectonic processes. Neither of these suites of basalts were generated as a result of significant lithospheric extension and passive asthenospheric upwelling on a regional scale. The commonly held assumption that alkaline volcanism along consuming plate margins results from periods of significant inter-arc extensional tectonism is, therefore, not necessarily valid. The unique tectono-magmatic regime resulting from the formation of slab windows is probably the only setting in which small degree melts of MORB source asthenosphere not associated with a plume are generated and preserved in the geological record. -from Authors