Geochemistry constraints of Mesozoic-Cenozoic calc-alkaline magmatism in the South Shetland arc, Antarctica

Abstract

Geochemical data from basalts, basaltic andesites, and andesites of the Mesozoic-Cenozoic (143-44 Ma) from Livingston, Greenwich, Robert, King George, and Ardley Islands of the South Shetland archipelago, Antarctica, are presented. The rocks have variable SiO2 Of approximately 46-61 wt%, Al2O3 of 15-26 wt%, and total alkali (K2O + Na2O) of 2-6 wt%. Most samples have low Mg#, Cr, and Ni, which indicates that they have undergone significant fractional crystallization from mantle-derived melts. The presence of olivine cumulatic in the samples from Livingston and Robert Islands explains some high MgO, Ni, and Cr values, whereas low Rb, Zr, and Nb values could be related to undifferentiated magmas. N-MORB-normalized trace element patterns show that South Shetland Islands volcanic rocks have a geochemical pattern similar to that found for other island arcs, with enrichment in LILE relative to HFSE and in LREE relative to HREE. The geochemistry pattern and presence of calcic plagioclase, orthopyroxene, Mg-olivine, and titanomagnetite phenocrysts suggest a source related to the subduction process. The geochemical data also suggest magma evolution from the tholeiitic to the calc-alkaline series; some samples show a transitional pattern. Samples from the South Shetland archipelago show moderate LREE/HREE ratios relative to N-MORB and OIB, depletion in Nb relative to Yb, and high Th/Yb ratios. These patterns probably reflect magma derived from a lithospheric mantle source previously modified by fluids and sediments from a subduction zone.