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

A continental flood basalt province, the Ferrar Group (Kirkpatrick Basalt and Ferrar Dolerite), crops out along 3000 km of the Transantarctic 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 that the young dates reflect non-ideal behavior of Ar in the matrix. In order to refine the geochronology, feldspar separates have been analyzed by the 40Ar/39Ar 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 from the three principal outcrop areas, yielding the following apparent ages: central Transantarctic Mountains 176.8 +/- 0.5 Ma; south Victoria Land 176.4 +/- 0.4 Ma; north Victoria Land 176.6 +/- 0.7 Ma. Ages from 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 within a short interval of less than about 1 m.y. at 176.6 +/- 1.8 Ma, over an area which included more than 1200 km of the Transantarctic Mountains. 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 lithospheric stretching exerted a major control on magmatism. The poorly constrained age of the Bajocian-Bathonian boundary makes the previously suggested connection between Ferrar volcanism and an extinction event at that boundary uncertain. A continental flood basalt province, the Ferrar Group (Kirkpatrick Basalt and Ferrar Dolerite), crops out along 3000 km of the Transantarctic 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 that the young dates reflect non-ideal behavior of Ar in the matrix. In order to refine the geochronology, feldspar separates have been analyzed by the 40Ar/39Ar 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 from the three principal outcrop areas, yielding the following apparent ages: central Transantarctic Mountains 176.8 +/- 0.5 Ma; south Victoria Land 176.4 +/- 0.4 Ma; north Victoria Land 176.6 +/- 0.7 Ma. Ages from 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 within a short interval of less than about 1 m.y. at 176.6 +/- 1.8 Ma, over an area which included more than 1200 km of the Transantarctic Mountains. 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 lithospheric stretching exerted a major control on magmatism. The poorly constrained age of the Bajocian-Bathonian boundary makes the previously suggested connection between Ferrar volcanism and an extinction event at that boundary uncertain.