Meso-Cenozoic calc-alkaline magmatism in the South Shetland Islands, Antarctica: Implications for temporal changes of the mantle sources

Abstract

We present new Sr-Nd-Pb isotope data of the volcanic rocks from Barton and Weaver peninsulars, King George Isalnd (KGI), Antarctica. We interpret these data in combination with previously published trace and isotope data for Meso-Cenozoic volcanic rocks in the South Shetland Islands (SSI) to gain a better understanding of the geochemical evolution of the mantle source region [1]. The studied rocks are from four volcanic islands and range in age from 143 to 44 Ma. They have high abundances of LILE and LREE relative to HFSE, consistent with products of subduction related magmatism. The systematic inter-island variations are recognized from a comprehensive examination of the trace elements and isotopic compositions. The degree of enrichment of Sr-Nd-Pb isotopic compositions decreases towards younger samples, while the ratios of fluid-mobile elements/HFSE (Sr/Yb, Pb/Yb and U/Yb) gradually increase. We suggest that the compositional trends observed in the volcanic rocks of the SSI can be created from the addition of a relatively constant subduction component to temporally varying heterogeneous mantle sources. The higher radiogenic Pb and Sr isotopes and lower 143Nd/144Nd ratios of the older volcanic rocks from Greenwich and Livingston islands compared to younger rocks can be explained by the significant influence of enriched previously metasomatized mantle material rather than fluids or sediment melts from the subducting slab. In contrast, the geochemical nature of the youngest King George Island volcanic rocks suggests a relatively large contribution of a slab-derived fluid component to the magma generation, but a minor role of the enriched component.