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

Abstract

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...