Bodorkos, Simon, Cawood, Peter A., Oliver, Nicholas H.S., and Nemchin, Alexander A. (2000) Rapidity of orogenesis in the Paleoproterozoic Halls Creek Orogen, northern Australia; evidence from SHRIMP zircon data, CL zircon images, and mixture modeling studies. American Journal of Science, 300 (1). pp. 60-82.

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Abstract

Combining U-Pb SHRIMP zircon geochronology with cathodoluminescence imaging enables the resolution of temporally closely-spaced geological events important for understanding tectonothermal processes in the Paleoproterozoic Halls Creek Orogen of northern Australia. The youngest detrital zircon grains from a low-grade quartz-muscovite psammite of the Tickalara Metamorphics have a 207 Pb/ 206 Pb SHRIMP age of 1864+ or -4 Ma, defining a maximum depositional age for the unit. Zircon crystals from a high-grade garnet-biotite metapelite ( approximately 5-10 volume percent leucosome) from the same sequence are considerably more complex, and SHRIMP analyses form a single, large concordant group in the range approximately 1885 to 1830 Ma. The zircon crystals contain three distinct CL zoning patterns, and individual SHRIMP spots show a corresponding variation in 207 Pb/ 206 Pb ages. Concentric oscillatory-zoned zircon dominates pre-1850 Ma ages and is interpreted as detritus from igneous source rocks. Narrow structureless zircon rims infrequently overgrow and truncate the oscillatory zoning. These rims comprise most of the post-1850 Ma analyses and are inferred to be the product of uppermost amphibolite facies metamorphism, reflecting the interaction of a robust pre-existing detrital zircon suite with a very limited melt volume. In addition, some zircon cores of uncertain geological affinity contain large areas devoid of oscillatory zoning with individual analyses clustering around 1850 Ma. Dividing the data into "detrital" and "metamorphic" suites solely on the basis of CL imaging yields an older group of 18 analyses ( 207 Pb/ 206 Pb age = 1867+ or -4 Ma) and a younger group of 10 analyses ( 207 Pb/ 206 Pb age = 1843+ or -4 Ma), not including five SHRIMP spots within areas of unzoned zircon. Mixture modeling of all 33 analyses in the post-1900 Ma data set resulted in a best-fit solution composed of two distinct components: (1) an older group of 19 analyses with an age of 1867+ or -4 Ma, and (2) a younger group of 14 analyses with an age of 1845+ or -4 Ma. These results suggest that the unzoned patches of zircon might be related to metamorphism rather than being detrital cores. Importantly, the ages and proportions of populations predicted by mixture modeling are otherwise very similar to those derived from analysis of CL zoning patterns. These data imply that high-temperature metamorphism occurred in the metasedimentary rocks less than 25 my after the crystallization of the igneous detrital source. Such rapid rates of erosion, deposition, and burial have rarely been proposed for Proterozoic rocks, despite evidence for analogous orogenic processes in the Mesozoic and Cainozoic on a comparable timescale. Careful evaluation of geological and geochronological data in Proterozoic provinces elsewhere may reveal similar patterns, with potential implications for the possible rates of Proterozoic orogenesis and crustal evolution.

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