Rubidium-Strontium Dates from the Shackleton Range Metamorphic Complex in the Mount Provender Area, Shackleton Range, Antarctica

Abstract

The Shackleton Range Metamorphic Complex in the Mount Provender area consists of northwest-trending migmatitic hornblende-bearing gneisses, marbles and calc-silicate rocks, kyanite schist, quartzites and granulites, amphibolites, garnetiferous gneisses, and ultramafic rocks. The calculated whole rock Rb-Sr isochron age for five samples of feldspathic augen gneiss from a layer 25 m thick is $$583 \pm 48$$ m.y. with an initial $$Sr^{87}/Sr^{86}$$ ratio of $$0.7084 \pm 0.0024$$; the calculated age for three samples of granite gneiss in layers in the augen gneiss is $$656 \pm 66$$ m.y. with an initial $$Sr^{87}/Sr^{86}$$ ratio of $$0.7078 \pm 0.0064$$. Biotite, separated from the augen gneiss, gives a Rb-Sr date of $$519 \pm 15$$ m.y. Rb-Sr data for three samples of gneissic granite plot close to a 600 m.y. reference isochron with an assumed initial $$Sr^{87}/Sr^{86}$$ ratio of 0.707. These data indicate Rb-Sr isotopic closure in latest Precambrian to Cambrian time in probable association with regional deformation and metamorphism. The deformational and metamorphic event in the Mount Provender area is roughly contemporaneous with the Beardmore Orogeny (680-620 million years before present) in the Transantarctic Mountains and with 450-600 m.y. metamorphic and plutonic events in southern Africa.