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

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.

[1]  S. Harley.  The origins of granulites: a metamorphic perspective , 1989, Geological Magazine.

[2]  M. Sandiford.  Horizontal structures in granulite terrains: A record of mountain building or mountain collapse? , 1989 .

[3]  G. Clarke.  Structural constraints on the Proterozoic reworking of Archaean crust in the Rayner Complex, Macrobertson and Kemp Land coast, East Antarctica , 1988 .

[4]  I. Wendt,et al.  U-Pb zircon and Sm-Nd model ages of high-grade Moldanubian metasediments, Bohemian Massif, Czechoslovakia , 1988 .

[5]  P. Ollila,et al.  Pyroxene exsolution; an indicator of high-pressure igneous crystallization of pyroxene-bearing quartz syenite gneiss from the High Peaks region of the Adirondack Mountains , 1988 .

[6]  W. Compston,et al.  Early archaean zircon ages from orthogneisses and anorthosites at Mount Narryer, Western Australia , 1988 .

[7]  D. Bridgwater,et al.  Late Archaean ages for the deposition of clastic sediments belonging to the Malene supracrustals, southern West Greenland: evidence from an ion probe UPb zircon study , 1988 .

[8]  L. P. Black,et al.  Chemical evolution of granitic rocks in the East Antarctic Shield, with particular reference to post-orogenic granites , 1988 .

[9]  W. Compston,et al.  Multiple zircon growth within early Archaean tonalitic gneiss from the Ancient Gneiss Complex, Swaziland , 1988 .

[10]  V. Gostin,et al.  Zircon age evidence for the Late Precambrian Acraman ejecta blanket , 1987 .

[11]  D. Ellis.  Origin and evolution of granulites in normal and thickened crusts , 1987 .

[12]  M. McCulloch,et al.  THE RAYNER COMPLEX OF EAST ANTARCTICA - COMPLEX ISOTOPIC SYSTEMATICS WITHIN A PROTEROZOIC MOBILE BELT , 1987 .

[13]  W. Compston,et al.  Four zircon ages from one rock: the history of a 3930 Ma-old granulite from Mount Sones, Enderby Land, Antarctica , 1986 .

[14]  W. Ranson.  Complex exsolution in inverted pigeonite; exsolution mechanisms and temperatures of crystallization and exsolution , 1986 .

[15]  P. Kinny.  3820 Ma zircons from a tonalitic Armîsoq gneiss in the Godthåb district of Southern West Greenland , 1986 .

[16]  J. McLelland.  Pre-Grenvillian history of the Adirondacks as an anorogenic, bimodal caldera complex of mid-Proterozoic age , 1986 .

[17]  S. Chipera,et al.  Garnet-orthopyroxene-plagioclase-quartz barometry: refinement and application to the English River subprovince and the Minnesota River valley , 1985 .

[18]  W. Compston,et al.  Unsupported radiogenic Pb in zircon: a cause of anomalously high Pb-Pb, U-Pb and Th-Pb ages , 1984 .

[19]  W. Compston,et al.  U‐Pb geochronology of zircons from lunar breccia 73217 using a sensitive high mass‐resolution ion microprobe , 1984 .

[20]  M. T. Naney.  Phase equilibria of rock-forming ferromagnesian silicates in granitic systems , 1983 .

[21]  L. P. Black,et al.  Geochemistry of Precambrian gneisses: relevance for the evolution of the East Antarctic Shield , 1983 .

[22]  W. Compston,et al.  Ion microprobe identification of 4,100–4,200 Myr-old terrestrial zircons , 1983, Nature.

[23]  S. Morse.  A partisan review of Proterozoic anorthosites , 1982 .

[24]  J. W. Sheraton,et al.  Enderby land, Antarctica—an unusual Precambrian high‐grade metamorphic terrain , 1980 .

[25]  F. Barker.  Trondhjemite: Definition, Environment and Hypotheses of Origin , 1979 .

[26]  R. F. Emslie.  Anorthosite massifs, rapakivi granites, and late proterozoic rifting of north America , 1978 .

[27]  E. Essene,et al.  Igneous pyroxenes from metamorphosed anorthosite massifs , 1978 .

[28]  R. Steiger,et al.  Subcommission on geochronology: Convention on the use of decay constants in geo- and cosmochronology , 1977 .

[29]  W. Compston,et al.  The statistical assessment of Rb‐Sr isochrons , 1966 .