Isotope age constraint for the Blue Dyke and Jardine Peak subvertical intrusions of King George Island, West Antarctica

Abstract

The Blue Dyke and Jardine Peak are subvertical hypabyssal intrusions cutting a stratiform volcanic sequence in the Admiralty Bay area on King George Island (South Shetlands, Antarctica). The rocks are porphyritic, crystal−rich basaltic andesites. Tiny zir− con crystals were used for single grain SHRIMP U−Pb dating. The mean ages calculated for the zircon populations from both intrusions indicates Late Oligocene (Chattian) formations. Zircon grains from the Blue Dyke gave the mean age of 27.9±0.3 Ma, whereas those from the Jardine Peak are slightly younger displaying the mean age of 25.4 ± 0.4 Ma: a Late Oligocene (Chattian) crystallization age the inferred of both these intrusions. These are much younger than previous Eocene K−Ar and Ar−Ar ages for such rocks and suggest that formation of the King George Island intrusions can be related to tectonic processes that ac− companied the opening of the Drake Passage. Key wor ds: Antarctica, King George Island, Late Oligocene, U−Pb SHRIMP dating.