Abstract A Rb‐Sr age of 897 ± 9 m.y. is obtained for dolerite from the Stuart Dyke Swarm in the southern part of the Arunta Block, Northern Territory. The dyke swarm presents an older age limit for the unconformably overlying Heavitree Quartzite, basal formation of the Amadeus Basin sequence. This limit is consistent with all isotopic data with the exception of previously determined glauconite ages from the Vaughan Springs Quartzite, a correlative of the Heavitree Quartzite in the Ngalia Basin.
Abstract The Arunta Inlier is part of a major ensialic Proterozoic mobile belt in central Australia. It comprises three latitudinal tectonic Provinces (Northern, Central and Southern) which have markedly different lithological, metamorphic and structural histories, and are bounded by major deformed zones. Tectonic activity migrated back and forth within the Inlier, and gradually lessened in intensity and extent. Large‐scale movements were localized along the bounding deformed zones from early on. Deformation and metamorphism were concentrated in the Central Province at about 1800–1750 Ma, and then at about 1700 Ma moved to its margins. Subsequent activity at 1700–1600, 1500–1400, 1050–900 and 400–300 Ma was concentrated in the Southern Province and in a belt which extended NW across the entire Inlier from the E part of the Southern Province. The tectonic evolution of the Arunta Inlier resulted from six cycles of crustal extension and compression. The first (before 1800 Ma), and possibly the second (1800–1750 Ma) caused rifting and then limited subduction of subcrustal lithosphere. The later cycles suggest repeated thermal activity and thrust‐faulting in relatively narrow crustal belts above a long‐lasting elongate plume in the mantle. Key words: tectonicsmetamorphismA‐subductionensialic riftingPrecambrian mobile beltsArunta Inliercentral Australia
Abstract Small mesothermal vein quartz‐gold‐base‐metal sulfide deposits from which some 20t of Au‐Ag bullion have been extracted, are the most common gold deposits in the Georgetown region of north Queensland—several hundred were mined or prospected between 1870 and 1950. These deposits are mostly hosted by Proterozoic granitic and metamorphic rocks and are similar to the much larger Charters Towers deposits such as Day Dawn and Brilliant, and in some respects to the Motherlode deposits of California. The largest deposit in the region—Kidston (>138t of Au and Ag since 1985)— is substantially different. It is hosted by sheeted quartz veins and cavities in brecciated Silurian granite and Proterozoic metamorphics above nested high‐level Carboniferous intrusives associated with a nearby cauldron subsidence structure. This paper provides new information (K‐Ar and Rb‐Sr isotopic ages, preliminary oxygen isotope and fluid‐inclusion data) from some of the mesothermal deposits and compares it with the Kidston deposit. All six dated mesothermal deposits have Siluro‐Devonian (about 425 to 400 Ma) ages. All nine of such deposits analysed have δ18O quartz values in the range 8.4 to 15.7‰. Fluid‐inclusion data indicate homogenisation temperatures in the range 230–350°C. This information, and a re‐interpretation of the spatial relationships of the deposits with various elements of the updated regional geology, is used to develop a preliminary metallogenic model of the mesothermal Etheridge Goldfield. The model indicates how the majority of deposits may have formed from hydrothermal systems initiated during the emplacement of granitic batholiths that were possibly, but not clearly, associated with Early Palaeozoic subduction, and that these fluid systems were dominated by substantially modified meteoric and/or magmatic fluids. The large Kidston deposit and a few small relatives are of Carboniferous age and formed more directly from magmatic systems much closer to the surface. Key words: Etheridge Goldfieldfluid inclusionsgold depositsmetallogenic modelquartz veinsoxygen isotopesisotopic dating
Protolith zircon in high‐grade metagranitoids from Queensland, Australia, partially recrystallized during granulite‐grade metamorphism. We describe the zircon in detail using integrated cathodoluminescence, U–Pb isotope, trace element and electron backscatter diffraction pattern (EBSP) analyses. Primary igneous oscillatory zoning is partially modified or obliterated in areas within single crystals, but is well preserved in other areas. A variety of secondary internal structures are observed, with large areas of transgressive recrystallized zircon usually dominant. Associated with these areas are recrystallization margins, interpreted to be recrystallization fronts, that have conformable boundaries with transgressive recrystallized areas, but contrasting cathodoluminescence and trace element chemistry. Trace element analyses of primary and secondary structures provide compelling evidence for closed‐system solid‐state recrystallization. By this process, trace elements in the protolith zircon are purged during recrystallization and partitioned between the enriched recrystallization front and depleted recrystallized areas. However, recrystallization is not always efficient, often leaving a ‘memory’ of the protolith trace element and isotopic composition. This results in the measurement of ‘mixed’ U–Pb isotope ages. Nonetheless, the age of metamorphism has been determined. A correlation between apparent age and Th/U ratio is indicative of incomplete re‐setting by partial recrystallization. Recrystallization is shown to probably not significantly affect Lu–Hf ages. Recrystallization has been determined by textural and trace element analysis and EBSP data not to have proceeded by sub‐grain rotation or local dissolution/re‐precipitation, but probably by grain‐boundary migration and defect diffusion. The formation of metamorphic zircon by solid‐state recrystallization is probably common to high‐grade terranes worldwide. The recognition of this process of formation is essential for correct interpretation of zircon‐derived U–Pb ages and subsequent tectonic models.
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