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    SHRIMP U-Pb zircon geochronological evidence for Neoarchean basement in western Arnhem Land, northern Australia [查看] JulieA.Hollis、ChrisJ.Carsonb、LindaM.Glass
    The Pine Creek Orogen, located on the exposed northern periphery of the North Australian Craton,comprises a thick succession of variably metamorphosed Palaeoproterozoic siliciclastic and carbonate sedimentary and volcanic rocks, which were extensively intruded by mafic and granitic rocks. Exposed Neoarchean basement is rare in the Pine Creek Orogen and the North Australian Craton in general.However, recent field mapping, in conjunction with new SHRIMP U–Pb zircon data for six granitic gneiss samples, have identified previously unrecognised Neoarchean crystalline crust in the Nimbuwah Domain, the eastern-most region of the Pine Creek Orogen. Four samples from the Myra Falls and Caramal Inliers, the Cobourg Peninsula, and the Kakadu region have magmatic crystallisation ages in the range 2527–2510 Ma. An additional sample, from northeast Myra Falls Inlier, yielded a magmatic crystallisation age of 2671±3 Ma, the oldest exposed Archean basement yet recognised in the North Australian Craton. These results are consistent with previously determined magmatic ages for known outcropping and subcropping crystalline basement some 200km to the west. A sixth sample yielded a magmatic crystallisation age of 2640±4 Ma. The ca. 2670Ma and ca. 2640Ma samples have ca. 2500Ma metamorphic zircon rims, consistent with metamorphism broadly coeval with emplacement of the volumetrically dominant ca. 2530–2510Ma granites and granitic gneisses. Neoarchean zircon detritus, particularly in the ca. 2530–2510Ma and ca. 2670–2640Ma age span, are an almost ubiquitous feature of detrital zircon spectra of unconformably overlying metamorphosed Palaeoproterozoic strata of the Pine Creek Orogen,and of local post-tectonic Proterozoic sequences, consistent with this local provenance. Neoarchean zircon is also a common detrital component in Palaeoproterozoic sedimentary units across much of the North Australian Craton suggesting the existence of an extensive, if not contiguous, Neoarchean crystalline basement underlying not only a large part of the Pine Creek Orogen, but also much of the North Australian Craton.
    The Palaeoproterozoic evolution of the Litchfield Province, western Pine Creek Orogen, northern Australia Insight from SHRIMP U–Pb zircon and in situ monazite geochronology [查看] ChrisJ.Carson、KurtE.Wordena、IanR.Scrimgeour、RichardA.Stern
    The Litchfield Province, located on thewestern margin of the Palaeoproterozoic Pine Creek Orogen, northern Australia, comprises variablymetamorphosed pelitic and psammitic sediments, extensively intruded by voluminous granitoids and mafic rocks. SHRIMP U–Pb in situ monazite geochronology of undeformed granulite-facies migmatitic Hermit Creek Metamorphics (ca. 730 ◦C, ca. 5 kb) and mid-amphibolite-facies Fog Bay Metamorphics (ca. 600 ◦C, ca. 4 kb) indicates mid- to high-grade metamorphism affected these rocks at ca. 1855 Ma. SHRIMP U–Pb analysis on low Th/U metamorphic zircon rims from the Fog Bay Metamorphics also indicate metamorphism at ca. 1855 Ma. The undeformed nature of the Hermit Creek and Fog Bay Metamorphics suggest that metamorphism was not accompanied by significant penetrative strain. These observations suggest that the elevated geotherm at this time may have been due to conductive and advective thermal input within a thinned lithosphere during intra-crustal extension and rifting.Metamorphism at ca. 1855Ma in the Litchfield Province is similar in age to that reported fromthe Hooper Orogen and the Western and Central Zones of the Halls Creek Orogen, but not with the Eastern Zone.Furthermore, there appears no evidence for the ca. 1845–1835Ma Halls Creek Orogeny in the Litchfield Province, suggesting that the inferred suture in the Halls Creek Orogen between the Kimberley Craton and the proto North Australian Craton, may not extend into the Litchfield Province and that the Litchfield Province may have been contiguous with, or forms part of, the Kimberley Craton. The upper-amphibolitefaciesWelltree Metamorphics (ca. 700 ◦C, ca. 4 kb), in contrast, preserve monazite grains that are aligned with a penetrative biotite fabric and are dated at 1813±3 Ma, thus recording a previously unrecognised tectono-thermal event in the Pine Creek Orogen.
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