Geochemical evolution of Pliocene-Recent post-subduction alkalic basalts from Seal Nunataks, Antarctic Peninsula

Following more than 200 Ma of subduction of Pacific oceanic crust beneath the west coast of the Antarctic Peninsula, subduction ceased by a series of ridge crest-trench collisions. However, magmatism continued after the cessation of subduction with basaltic rocks of the intraplate alkalic association being erupted from centres scattered along the whole length of the peninsula. In the northeast, at Seal Nunataks, a suite of intraplate basalts ranging in composition from tholeiite to alkali basalt was erupted less than 4 Ma ago, almost synchronously with the cessation of subduction in that area (4–6 Ma). LREE-enrichment [(La/Yb)n 4.7–11.5] but consistency of HREE abundances for all the basalts suggest they were generated by partial melting in the garnet stability field of the mantle. A number of trace-element ratios [e.g. Zr/Nb (4.9–8.4), Hf/Ta (1.5–3.3), Sr/Nb (15–26), Ti/Nb (390–800)] are likely to have been fractionated during partial melting/melt extraction, although they do exhibit some correlation with isotope ratios, suggesting variations in these ratios may have been controlled by both partial melting and source heterogeneity. However, Th/Ta (0.87–2.31), La/Th (5.4–10.5) and K/Rb (275–1330) ratios also exhibit considerable variations within the suite and show strong correlations with 87Sr/86Sr ratios (0.7028–0.7033). These trace-element-isotope covariations are explained by a model involving mixing of a LILE-, 87Sr-depleted end-member, broadly similar to the source for non-Dupal OIB and N-type MORB, and upper-mantle material with high LILE/HFSE and 87Sr/86Sr ratios. This high-LILE/HFSE, high-87Sr component may represent mantle material which was enriched during the previous 200 Ma of subduction-related magmatism within the Antarctic Peninsula. Post-subduction high-Mg andesites (‘bajaites’), which are associated with the cessation of subduction long other continental margins, are apparently absent within the Antarctic Peninsula. This is likely to be a result of differences between the plate configurations along the Antarctic Peninsula and other continental margins prior to and during the cessation of subduction.

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