Cretaceous arc volcanism of Byers Peninsula, Livingston Island, Antarctica : new petrological, geochemical and isotope data

Abstract

Abstract On Byers Peninsula, the largest summer ice-free area in the South Shetland Islands, a thick Upper Jurassic-Lower Cretaceous sedimentary sequence and penecontemporaneous igneous activity records the progressive emersion of a volcanic arc. The oldest rocks are represented by voluminous basaltic flows intercalated within late Jurassic shallow marine sediments. Following the emersion, Lower Cretaceous strata were deposited. Welded and nonwelded dacitic to rhyolitic ignimbrites are exposed locally at the base of the continental succession. However, the majority of the outcrops consists of Lower Cretaceous basaltic to basaltic andesite plugs and dykes, representing the roots of a volcanic arc, which form most of the topographic highs of the peninsula. Byers Peninsula lavas are porphyritic with mineralogical associations typical of orogenic series. Major, trace and rare-earth element data show that all the rocks are relatively poor in K2O, enriched in large ion lithophile elements and that they have a clear subduction imprint (Nb-Ta anomaly). Furthermore, a tholeiitic tendency is shown by the REE patterns of some basaltic lavas. Sr- Nd- and Pb isotopic data support an origin from the sub-arc asthenosphere. Byers lavas have isotope signatures similar to that of the South Sandwich intra-oceanic arc. The enriched nature of most of the samples, revealed by Pb isotope ratios and Th/Yb enrichment, indicates that the MORB-like mantle source has been modified by fluids and sediments derived from the subducting slab. No clear temporal chemical change is observed during the evolution of this near-trench volcanic arc.