Isotopic ages and palaeomagnetism of selected magmatic rocks from King George Island (Antarctic Peninsula)

A range of mafic to intermediate extrusive and intrusive igneous rocks from the central part of King George Island were sampled for isotopic and palaeomagnetic studies. Single-grain U-Pb dating of zircon from basalts to trachytes from the upper part of the section gave Eocene ages (53.0+ or -0.7 to 47.8+ or -0.5 Ma), consistent with a whole-rock (super 40) Ar- (super 39) Ar age of 52.7+ or -0.6 Ma. These ages correspond to the first stages of the opening of the Drake Passage. A Late Cretaceous (super 40) Ar- (super 39) Ar age (75.4+ or -0.9 Ma) was obtained only from one basalt sample from the bottom part of the section (Uchatka Point Formation). These new age determinations change substantially the existing stratigraphic chart of King George Island. Palaeomagnetic poles show a marked departure from the East Antarctic apparent polar wander path, reflecting anticlockwise rotation of the rocks after the Early Eocene. The mean age of zircon grains from a basaltic lava flow overlying the tillite at Herve Cove (48.9+ or -0.7 Ma) and the normal polarity magnetization of surrounding basalts allow correlation of the tillite with the upper part of the C22 polarity chron (49.4-48.6 Ma). The consistency in age, crystal morphology, and U and Th contents strongly suggests that the zircon records the magmatic event. However, derivation of the zircon from an older magma and a slightly younger age for the tillite cannot be totally excluded.