Isotopic and lead‐alpha ages of some Australian zircons

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Abstract
Isotope dilution Pb‐U ages have been measured on zircons from three areas in the Australian Precambrian. Three samples, from the Rum Jungle Complex, basement to the Pine Creek Geosyncline, yield an extrapolated age of 2,550 m.y. This result, in conjunction with published evidence concerning the age of intrusive granites, enables preliminary time‐limits, 2,550 m.y. to about 1,700 m.y., to be assigned to the period of sedimentation in this Geosyncline. Results from the Mount Isa area support geological deductions that the oldest phases of the Sybella Granite are remnants of basement rocks, approximating in age to the Ewen Granite at around 1,780 m.y.; that the Wonga Granite may well be derived from sediments older than the tectonic event reflected in the 1,300–1,450 m.y. K‐Ar ages observed in this region; and that the Wimberu Granite may belong to a later event, but older than that indicated by the biotite ages. Zircon from metasedimentary gneiss underlying the Ravens‐thorpe System in Western Australia yields an age, 2,800 m.y., equivalent to that of the intrusive pegmatite at Cattlin Creek. This suggests that the metamorphism dated at 2,800 m.y. caused complete loss of all previously‐accumulated lead from this older material. On the continent‐wide scale, these results fill an important gap in an emergent pattern of overlapping tectonic events, with an overall eastwards progression of activity. The most significant result from a series of Pb‐α ages is that gneissic rocks from the Snowy Mountains in the Tasman Geosyncline may have relict ages greater than that of the dominant Siluro‐Devonian tectonism. A pre‐Devonian age is also observed for zircon extracted from the beach sands of northern New South Wales.

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