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The Mt Bruce Supergroup of Western Australia was laid down between ca 2,8 Ga and ca 2.2 Ga in the Hamersley Basin, unconformably aver a basement at the alder, granite-greenstone, component of the Pilbara Craton.The Mt Bruce Supergroup consists of three groups: the Fartescue Graup, Hamersley Group and Turee Creek Group in upward sequence. The Hamersley Group, which is divided into eight formations, has a general thickness of -2.5 km, and is characterised by major banded iron-formation (BIF) units. Reported here are SHRIMP U-Pb zircon results (406 grain analyses) tram 13 samples taken from the Hamersley Group and near the top of the underlying Fortescue Group. In combination with SHRIMP results previously published from 12 Hamersley Group samples, the present results provide significant new constraints on the depositional chronology of the group, and suggest that the overage Compacted) depositional rates of each of the main depositional lithologies (BIF, carbonate, shale) were -180 m per million years, 12 m per million years and 5 m per million years, respectively. Some recently published SHRIMP ages from the Joffre Member differ slightly from those that are interpreted from the present data, and it is suggested that the two datasets may be reconciled it non-zero-age Pb loss is taken into account. The total body of zircon U-Pb age data tram the Fortescue and Hamersley Groups is consistent with a model involving continuous accumulation of basin-fill far at least 330 million years, from ca 2780 Mo to the tap of the Hamersley Group at ca 2449 Ma, The word 'continuous' in this context means that there may have been no breaks in deposition longer thon 1 million years. However, this model is not proven, and a major challenge far future work is to measure the length of any proposed non-depositional intervals. [ABSTRACT FROM AUTHOR]

The Ural Volcanics and Mt Hope Volcanics in the Central Lachlan Orogen, New South Wales, overlie non-volcanic, sedimentary, below-wave-base submarine facies in two intracontinental rift basins, the Rast and Mt Hope Troughs. The Ural Volcanics and Mt Hope Volcanics consist primarily of felsic (rhyolite to dacite), coherent facies and associated felsic monomictic breccia facies. These volcanic facies are interpreted to represent submarine lava-sill complexes, which define intrabasinal, effusive, volcanic and shallow intrusive centres. Small felsic intrusions and rare intermediate (monzodiorite) to mafic (gabbro) dykes also occur. The rocks experienced chlorite to biotite greenschist-facies metamorphism. LA-ICPMS U/Pb zircon dating indicates the Ural Volcanics and the Mt Hope Volcanics were broadly contemporaneous, but with mean ages of 421 ± 2 and 411 ± 5 Ma, respectively, the Ural Volcanics are slightly older. Given the differences in the degree of metamorphism, the different ages might represent greater Pb loss for the Mt Hope Volcanics. Major- and trace-element data (variably high K+, Zr, Nb, and Y, among others) is most consistent with A-type, within-plate origins for the igneous rocks, although some loss of immobile elements apparently accompanied metamorphism. The geologic setting, based on mapping, indicates that the Ural Volcanics and Mt Hope Volcanics were erupted and deposited in an intrabasinal environment, best described as an elongate, intracontinental submarine rift. This setting, and apparent absence of a proximal contemporaneous arc, supports contentions that A-type magmas are produced in rift settings. [ABSTRACT FROM AUTHOR]

Following seminal studies in the Lachlan Fold Belt (southeastern Australia), it has become almost axiomatic that metaluminous granites derive from infracrustal precursors, whereas strongly peraluminous plutons have metasedimentary or supracrustal sources, as reflected in the I- and S-type designation. Recently, zircon saturation thermometry has been used to further subdivide I-type granites into high- and low-temperature categories. That low-temperature I-type granites evolved by restite separation from magmas generated in the zircon stability field is implicit in this classification. To explore this hypothesis, we report an ion microprobe U-Pb (zircon) study into three hallmark ‘low-temperature’ Lachlan Fold Belt I-type suites. The combined patterns of zircon age inheritance and bulk rock Zr trends suggest that each suite formed from magmas that were initially zircon-undersaturated, and that fractional crystallisation, not restite unmixing, was the dominant differentiation process. The low temperature status presently applied to these rocks cannot therefore be justified. The inherited zircons in these I-type granites reflect melting and assimilation of metasedimentary rock, and testify to a supracrustal source component. [ABSTRACT FROM AUTHOR]