AR-40 AR-39 GEOCHRONOLOGY OF FERRAR DOLERITE SILLS FROM THE TRANSANTARCTIC MOUNTAINS, ANTARCTICA - IMPLICATIONS FOR THE AGE AND ORIGIN OF THE FERRAR MAGMATIC PROVINCE/

The Ferrar Dolerite constitutes the hypabyssal phase of the tholeiitic Ferrar Group of Antarctica. Sills with compositions representing most of the range of geochemical variation of the Ferrar Dolerite, and sep arated by distances of as much as 1400 km, have been analyzed by the A r-40/Ar-39 method on feldspar and biotite separates. The Ar-40/Ar-39 a ges for five individual sills range from 176.2 to 177.2 Ma and show no significant difference. These ages reflect crystallization at 176.7 /- 1.8 Ma (where the uncertainty includes provision for systematic unc ertainty in the age of the neutron-fluence monitor calibrated relative to MMhb-1 at 513.5 Ma). Combining data from these sills with previous determinations on coeval lavas and underlying pyroclastic units indic ates an age of 176.6 +/- 1.8 Ma for the Ferrar tholeiitic rocks as a w hole. The duration of magmatic activity was less than approximate to 1 m.y. By extension, other rocks in the Ferrar magmatic province, which occur from southeastern Australia, along the Transantarctic Mountains to the Theron Mountains. are inferred to have this age. The short dur a;ion of magmatic activity as well as the consistent pattern of geoche mical variation and distinctiveness of the Ferrar racks suggest that m agmas were transported laterally by an extensive dike swarm which is i nferred to have originated in the Weddell Sea sector of the province.