Petrological and geochemical data are reported for a series of Late Cretaceous-Middle Miocene volcanic, hypabyssal and intrusive rocks from King George Island (KGI) and from nearby Ridley Island, South Shetland Islands. Major element data indicate a calc-alkaline, basic to intermediate composition for the analysed samples. Although emplaced on a continental margin, the KGI rocks generally display low abundances of incompatible trace elements, close to those typically observed in calc-alkaline suites erupted in intraoceanic island arcs. A few samples have a significant negative Ce anomaly. Many incompatible elements define smooth positive trends on interelemental variation diagrams which suggests that magmas erupted at different times on KGI maintained a rather constant composition in terms of incompatible element ratios. Geochemical modelling, based on Sr isotope ratios and incompatible element ratios, suggests that the primary calc-alkaline magmas of KGI were all generated in an upper mantle modified by addition of small amounts of pelagic sediments dragged down by subduction processes.