40AR/39AR GEOCHRONOLOGICAL ANALYSIS OF MINNA BLUFF, ANTARCTICA: EVIDENCE FOR PAST GLACIAL EVENTS WITHIN THE ANTARCTIC CRYOSPHERE

The history and chronology of the Minna Bluff volcanic peninsula provides insight into past dynamics of the Ross Ice Sheet. Twenty-five samples of volcanic rock from different elevations on Minna Bluff have been dated using Ar/Ar geochronology. When volcanoes interact with glacial ice the resulting lithofacies can provide useful information on ice thickness that can be used to interpret ice-sheet history. Datable volcanic rocks that bracket obvious glacial unconformities also provide information about glacial history. This information can be combined with data from the ANDRILL marine core to help determine ice-sheet chronology for the Miocene in the McMurdo Sound region. Prior K/Ar dating of the southeastern exposures at Minna Bluff yielded dates ranging from 11 to 7.26 Ma and suggested that the southern tip of the bluff represents the oldest eruptions. New Ar/Ar results from sanidine and amphibole mineral separates and groundmass concentrates, show older ages of 11.67 ± 0.19 Ma in the northern end of Minna Hook (the eastern end of the 50 km long bluff) and the youngest age of 8.28 ± 0.17 Ma from one of the stratigraphically highest lava domes on Minna Hook. A prominent glacial unconformity, which probably represents either over riding by the Ross Ice-Sheet or formation of localized glaciers on the bluff, separates the lower volcanic section (dominated by basanite flows and breccias) from the central volcanic section (dominated by trachyte domes and secondary basanite flows). Multiple pairs of samples were collected from both above and below this unconformity in order to best constrain the period of glacial erosion. The youngest age from beneath the unconformity is 9.74 ± 0.07 Ma and the oldest age above the unconformity is 9.53 ± 0.07 Ma. This leaves between 350,000 and 70,000 years (allowing for errors at two sigma) for the glacial erosion to have taken place.