GEOCHRONOLOGY OF CRETACEOUS GRANITES AND METASEDIMENTARY BASEMENT ON EDWARD-VII-PENINSULA, MARIE-BYRD-LAND, WEST ANTARCTICA

Rb-Sr ages of Swanson Formation on Edward VII Peninsula, West Antarctica, indicate a late Ordovician age, 421-432 Ma for regional metamorphism. K-Ar ages of 113-440 Ma, reflect a second thermal metamorphism during emplacement of widespread Cretaceous granites. Rb-Sr ages of five monzogranite/syenogranitic plutons of Byrd Coast Granite are in the range 95-105 Ma (initial Sr-87/Sr-86 ratios 0.710-0.715) and represent stages of crystallization of anorogenic granite (A subtype). These correlate with Byrd Coast Granite (100-110 Ma) farther east in Marie Byrd Land, and with Cretaceous granitoids on the Campbell Plateau and in southern New Zealand. K-Ar mica/hornblende and fission-track apatite/zircon ages indicate that regional cooling began c. 90-100 M.y. ago immediately after granite emplacement. Uplift continued throughout the peninsula during the period 55-100 Ma (late Cretaceous-early Tertiary), associated with regional uplift in the rift-drift stages of Gondwana break-up at the South-west Pacific spreading centre. Apatite fission track ages show that during late Cretaceous-early Tertiary time the peninsula behaved as two blocks. The Alexandra Mountains were exhumed 20 m.y. before the Rockefeller Mountains and are possibly separated by a fault active initially in the mid-Cretaceous or earlier, and later reactivated in the late Cretaceous-early Tertiary. An averaged uplift rate (50-100 Ma) of 0.025 mm yr(-1) is characteristic of the inferred intraplate tectonic setting.