Abstract

Following the cessation of subduction along the Antarctic Peninsula, there was a change in the character of the magmatism from calc-alkaline to alkaline. A Miocene-Pliocene suite of basanites, tephrites and alkali and olivine basalts from Alexander Island, south-west Antarctic Peninsula, was erupted between 18 and 30 Ma after the cessation of subduction. These basalts exhibit incompatible trace element characteristics similar to those reported for continental alkali basalts and OIB. Major and trace element variations are consistent with an origin by low but variable degrees of partial melting of a garnet lherzolite source with residual garnet ± clinopyroxene, followed by limited fractional crystallization of olivine and clinopyroxene. Sr- and Nd-isotope compositions cover a limited range of 87Sr/86Sr 0.70275-0.70343 and 143Nd/144Nd 0.51298–0.51286. These ratios are consistent with the derivation of the basalts from a depleted source region broadly similar to that of MORB and non-Dupal OIB.

Correlated LILE/HFSE-isotope trends demonstrate that the olivine basalts are enriched in the LILE and 87Sr but depleted in 143Nd relative to other Alexander Island samples. The LILE-enrichment is attributed to the inheritance of a subduction component from mantle material which previously constituted the mantle wedge during Mesozoic subduction along the Antarctic Peninsula.

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