A SHORT INTERVAL OF JURASSIC CONTINENTAL FLOOD-BASALT VOLCANISM IN ANTARCTICA AS DEMONSTRATED BY AR-40 AR-39 GEOCHRONOLOGY/

A continental flood basalt province, the Ferrar Group (Kirkpatrick Bas alt and Ferrar Dolerite), crops out along 3000 km of the Transantarcti c Mountains in Antarctica and is temporally related to the break-up of Gondwanaland. Although a wide range of dates, between 90 and 193 Ma, have been published for the Kirkpatrick Basalt, it is now recognized t hat the young dates reflect non-ideal behavior of Ar in the matrix. In order to refine the geochronology, feldspar separates have been analy zed by the Ar-40/Ar-39 incremental heating method. The main objectives are to constrain the duration of extrusive activity and the timing of volcanism along the outcrop belt. Basalt samples have been studied fr om the three principal outcrop areas, yielding the following apparent ages: central Transantarctic Mountains 176.8 +/- 0.5 Ma; south Victori a Land 176.4 +/- 0.4 Ma; north Victoria Land 176.6 +/- 0.7 Ma. Ages fr om different stratigraphic levels within each area and from the three different areas are not analytically distinct. The data imply that the eruptive activity which produced the Kirkpatrick Basalt occurred with in a short interval of less than about 1 m.y. at 176.6 +/- 1.8 Ma, ove r an area which included more than 1200 km of the Transantarctic Mount ains. The Jurassic volcanism in Antarctica represents a short episode of magmatism, comparable in duration with other well dated continental flood basalt provinces. The linearly extensive outcrop of the Ferrar Province and the rapid eruption of the lavas suggests that lithospheri c stretching exerted a major control on magmatism. The poorly constrai ned age of the Bajocian-Bathonian boundary makes the previously sugges ted connection between Ferrar volcanism and an extinction event at tha t boundary uncertain.