Precambrian basement at Molodezhnaya Station, East Antarctica

Abstract

The Precambrian basement exposed around Molodezhnaya Station (lat 67°40′S, long 45°50′E) in western Enderby Land, Antarctica, consists largely of well-layered, in part migmatitic, gneisses and of plutonic gneisses. The most abundant rock types in the well-layered gneisses are pyroxene gneiss, hornblende gneiss, garnet-biotite gneiss, garnet-pyroxene gneiss, and quartzo-feldspathic gneiss. The plutonic gneisses crop out in three bodies, each of a different rock type: charnockitic, granodioritic, and enderbitic. Field relations show that the charnockitic gneiss locally cuts the compositional layering in the well-layered gneisses and that it was emplaced prior to granulite-facies metamorphism and folding. Total-rock Rb-Sr ages calculated on seven samples of quartzo-feldspathic gneiss range from 460 ± 250 to 2,120 ±155 m.y. (assumed initial Sr 87 /Sr 86 ratio of 0.715). Eight samples of charnockitic gneiss give an isochron age of 987 ± 60 m.y. and an initial ratio of 0.7109 ± 0.0015. Seven samples of granite give an isochron age of 512 ± 155 m.y. U-Th-Pb data on monazite, sphene, and allanite from pegmatites indicate an age of crystallization of 500 to 550 m.y. for the pegmatites. Rb-Sr ages of 460 and 465 m.y. were obtained on biotites from gneiss. The geochronologic data are consistent with the following sequence of events: (1) Formation of the well-layered gneisses, possibly 2,000 m.y ago. (2) Plutonism, granulite-facies metamorphism, and folding, 1,000 m.y. ago. (3) Dikes of granite and pegmatite, amphibolite-facies metamorphism, and faulting, 500 to 550 m.y. ago. (4) Secondary mineralization, possibly 400 to 500 m.y. ago. The event 1,000 m.y ago appears to have been important in this part of Antarctica, and some Antarctic charnockites may have been emplaced at that time.