K—Ar age, composition, and origin of Mesozoic mafic rocks related to Ferrar Group, Pensacola Mountains, Antarctica

Abstract

A variety of mafic igneous bodies, including the unusually large, differentiated, layered gabbroic Dufek intrusion, and basalt and dolerite dikes and sills, intrude a Permian and older sequence in the Pensacola Mountains, near Weddell Sea, Antarctica. K-Ar age determinations show that the sills (179 ± 5 m.y.), the Dufek intrusion (172 ± 4 m.y.), and probably the dikes (minimum age 169 ± 4 m.y.) were emplaced over a narrow time interval in the Early to Middle Jurassic and are about coeval with the Ferrar Group of the Transantarctic Mountains and tholeiite elsewhere near Weddell Sea. Minerals of the sills and dikes seem to contain variable, generally minor amounts of extraneous ⁴⁰Ar. Pyroxene of the Dufek intrusion lost much ⁴⁰Ar, probably resulting from subsolidus phase-change activity. Inferred dike and sill magmas in Pensacola Mountains were Si02-rich tholeiites chemically like the Ross Sea area Ferrar hypersthene tholeiite, and have comparable anomalously high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7104-0.7126). Marked major and minor-element chemical differences, including Sr isotopic and dispersed-element ratios, exist compared to Jurassic tholeiite of other Weddell Sea areas that are more like South African Karroo tholeiite. The Jurassic Ferrar igneous province in Antarctica is divided into a Transantarctic Mountains subprovince and a Weddell Sea subprovince. The Pensacola Mountains clearly have Transantarctic Mountains affinity. A subprovince boundary, therefore, lies between the Pensacola and Theron Mountains. Both subprovinces are related to late Early Jurassic initial rifting: the Weddell Sea tholeiite to rifting that led to a Late Jurassic spreading center south of Africa; and the Transantarctic Mountains tholeiite to a different, but related, rift zone that failed at an early stage.