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U-Pb zircon dating of Proterozoic igneous charnockites from the Mawson Coast, East Antarctica

Published online by Cambridge University Press:  13 May 2004

David N. Young
Affiliation:
Department of Geology, Australian National University, PO Box 4, Canberra, Australian Capital Territory 2601, Australia Northern Territory Geological Survey, PO Box 2655, Alice Springs, Northern Territory 0870, Australia
Lance P. Black
Affiliation:
Bureau of Mineral Resources, PO Box 378, Canberra, Australian Capital Territory 2601, Australia

Abstract

We report ion-microprobe U-Pb zircon ages from charnockites of a large Proterozoic composite batholith, Mawson Coast, Australian Antarctic Territory. The charnockites crystallized from orogenic magmas of intermediate composition (mainly 54–68% SiO2) intruded into a granulite-facies metasedimentary gneiss sequence between the second and third recognized deformations. A sample of low-Ti charnockite provides an age of 954 ± 12 Ma and a high-Ti charnockite is dated at 985 ± 29 Ma (all ages quoted at 95% confidence). The age difference is not significant at the 95% confidence level. Both these ages were obtained from zircons with igneous zoning and/or morphology and thus are thought to date igneous crystallization. Zircons from a felsic gneiss xenolith within the charnockite have cores of various ages, many from 1.7 to 2.0 Ga, but with other grains between 1.0 and 1.5 Ga and a single 2.5 Ga zircon. These zircon cores are direct evidence for an early to middle Proterozoic age for the supracrustal basement sequence in this mobile belt. Many of these zircon cores are concordant but abundant discordant grains suggest a complex history of multiple Pb-loss events. Zircon rims grew at 921 ± 19 Ma, probably during the post-charnockite deformation (D3). Previously obtained Rb-Sr dates for charnockite of 886 ± 48 Ma and 910 ± 18 Ma were probably also rest during D3. A Rb-Sr isochron date of 1061 ± 36 Ma previously reported for high-Ti charnockite from Mawson Rock is thought to be erroneous, and a new date of 959 ± 58 Ma (consistent with both the igneous and reset dates above) is interpreted from those data.

Type
Papers–Earth Sciences and Glaciology
Copyright
© Antarctic Science Ltd 1991

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