This study is aimed to increase the knowledge on the nature and age of the crust of the Robertson Bay terrane through the study of crustal xenoliths brought to the surface by alkaline Cenozoic lavas. Petrographic and geochemical data allow to distinguish three different types of xenoliths: (A) K–feldspar-rich with alkaline geochemical affinity, thought to represent cumulates linked to the Cenozoic igneous activity; (M) lower crustal mafic granulites, with geochemical affinity variable from island-arc tholeiite to calc-alkaline; (F) lower crustal felsic granulites found as both (F1) K-rich, with euhedral microcline, and (F2) low-K types, with geochemical composition varying from granite- to metasedimentary-like. Furthermore, U–Pb dating of zircons by laser-ablation ICP-MS from the F2 sub-type (concordant ages clustering at ~490 and 360Ma) suggests the formation of juvenile crust during the Ross convergence process. These results reveal the composite nature of the lower crust of the Robertson Bay terrane.