40AR/39AR AGES OF GLACIAL SEDIMENTS FROM INDEPENDENCE HILLS, PATRIOT HILLS, AND MARBLE HILLS OF THE ELLSWORTH MOUNTAINS IN ANTARCTICA: A COMPARISON WITH ZIRCON U-PB AGES

Detrital hornblende grains were collected from sublimation moraines in the Ellsworth Mountains of West Antarctica and were dated for provenance studies. The hornblende crystals are euhedral with weathered faces and are enveloped in clay. Argon isotope compositions and concentrations of single hornblende grains were measured on a Micromass VG5400 noble gas mass spectrometer to determine the age of mineral formation or timing of a thermal resetting event. ⁴⁰Ar/³⁹Ar ages of detrital hornblende grains from Independence Hills, Patriot Hills, and Marble Hills of Ellsworth Mountains in West Antarctica cluster in a population centered on ~300-250 Ma. There are a few scattered ages at 30 Ma, 500Ma, and 750 Ma. In contrast to the hornblende ages, zircon U-Pb ages cluster around 1050 and 650 Ma. The Mesoproterozoic zircon ages are likely to be related to the formation of the supercontinent Rodinia and the Neoproterozoic ages may result from overprinting of these grains during remelting of Mesoproterozoic crusts or primary crystallization during the Pan-African orogeny. The fact that the observed ⁴⁰Ar/³⁹Ar ages of hornblende grains are much younger than the U-Pb ages of zircon grains implies the occurrence of a metamorphic resetting event in the source rocks. Gondwanan tectono-thermal events of the late Permian may have reset the K-Ar clock in hornblende grains but have been insufficient to reset the U/Pb clock in the zircons.