A suite of Early Jurassic metaluminous to strongly peraluminous granitoids occupy a rear-arc position in Palmer Land, Antarctic Peninsula with respect to a Mesozoic–Cenozoic magmatic arc. Fractional crystallization of a range of mafic to silicic magmas yielded feldspar megacrystic granites, orthogneisses and foliated granodiorites. Leucogranites are strongly peraluminous and have the highest 87Sr/86Sr initial ratios of 0.7205–0.7208, somewhat lower than those of older paragneiss. Isotopic and elemental data are used to model generation of leucogranite magma by partial melting of paragneiss, followed by mixing of basalt magma with the crustal melt to form parent magmas of the rest of the granitoids. The crustal partial melting was a result of heat convected by the mafic magma, intruded into an extensional back-arc basin setting contemporaneous with the initial stages of Gondwana break-up. The mafic magma had relatively low 143Nd/144Nd ratios, distinct isotopically from the MORB-like basalt thought to represent the mantle input to granitoid genesis in the north western part of the Antarctic Peninsula. The nature of the mantle source(s) of the enriched basalt is uncertain, but it is isotopically similar to approximately contemporaneous basalts associated with the Karoo mantle plume.