KAr dates and paleomagnetic evidence for Cretaceous alteration of Mesozoic basaltic lava flows, Mesa Range, northern Victoria Land, Antarctica

Abstract

The Mesozoic tholeiite basalts in the Transantarctic Mountains are probably Jurassic in age based on a fragmentary fossil record, on their stratigraphic relation to the older sedimentary rocks of the Beacon Supergroup, and on a few 40 Ar 39 Ar and RbSr age determinations. However, the whole-rock KAr dates of these basalts vary widely between ∼ 100 and ∼ 200 Ma and do not constrain the age of these rocks. New whole-rock AAr dates of flows on the summit of Pain Mesa in the Mesa Range of northern Victoria range from 103 ± 4 to 174 ± 7 Ma and confirm previously published results. However, chemical analyses indicate that the K 2O concentrations of these flows on Pain Mesa decrease with increasing volatile content (loss on ignition) and that most of the low-K flows in the Mesa Range yield low KAr dates. This evidence suggests that K was lost as a result of alteration of the flows by aqueous solutions and that the alteration was accompanied by even greater losses of radiogenic 40Ar. Accordingly, the new KAr dates of these rocks indicate that the age of alteration is <103 ± 4 Ma whereas their crystallization age is > 176 ± 8 Ma. The discordance of KAr dates of the lava flows of the Mesa Range, as well as those at Litell Rocks reported by other investigators, suggests that northern Victoria Land experienced a geologic event during the Cretaceous Period that caused low-grade thermal metamorphism and hydrothermal alteration of the flows. The occurrence of such an event is supported by paleomagnetic evidence indicating that the basalt flows of the Mesa Range and the Litell Rocks as well as sedimentary rocks of early Paleozoic age of the Bowers Terrane in northern Victoria Land, have anomalous virtual geomagnetic pole positions caused by remagnetization during the Cretaceous Period. The tectonothermal and hydrothermal activity in northern Victoria Land during the Cretaceous Period indicated by these results may have been caused by the reactivation of deep crustal faults in the area during the separation of Australia from Antarctica.