Early Palaeogene mafic–intermediate dykes, Robert Island, West Antarctica: Petrogenesis, zircon U–Pb geochronology, and tectonic significance

Abstract

Geochemical, geochronological, and Lu–Hf isotopic data for mafic–intermediate dykes on Robert Island, West Antarctica, are systematically investigated in order to decipher their petrogenesis and the genetic relationship between the South Shetland Islands and the north‐western Antarctic Peninsula. These dykes are enriched in large‐ion lithophile elements with significantly negative Nb and Ta anomalies, and low (Hf/Sm)N and (Nb/La)N and high Ba/La and Ba/Th ratios, with high εHf(t) values of +8.2 to +14.2. They were derived from partial melting of depleted mantle metasomatized by slab‐derived fluids. Precise ages of 55–54 Ma for the dykes confirm the occurrence of Early Eocene magmatism on Robert Island, corresponding to the final peak of subduction‐related magmatism. The available data suggest that Early Palaeogene (62–54 Ma) igneous rocks on the South Shetland Islands and the north‐western Antarctic Peninsula are products of contemporaneous and different lithofacies magmatism with precursors originating from depleted mantle in a magmatic arc setting. Furthermore, the Early Palaeogene igneous activities were most likely associated with slab rollback and extension in fore‐arc regions resulting from the decreasing convergence rate for the subduction of the Phoenix Plate beneath the Antarctic Plate at ~60 Ma.