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.
Abstract New isotopic (Rb–Sr, U–Pb zircon and Sm–Nd) and petrological data are presented for part of an extensive Proterozoic mobile belt (locally known as the Rayner Complex) in East Antarctica. Much of the belt is the product of Mid‐Proterozoic (∼ 1800–2000 Ma) juvenile crustal formation. Melting of this crust at about 1500 Ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived. Deformation and transitional granulite‐amphibolite facies conditions (which peaked at 750 ± 50°C and 7–8 kbar (0.7–0.8 GPa) produced open to tight folding about E–W axes and syn‐tectonic granitoids about 960 Ma ago. Subsequent felsic magmatism occurred at about 770 Ma and not, as has been widely advocated, at 500–550 Ma, which appears to have been a time of widespread upper greenschist facies (400–500°C) metamorphism, localized shearing and faulting. Sm‐Nd model ages of 1.65–2.18 Ga disprove a previously favoured hypothesis that the Rayner Complex mostly represents reworked Archaean rocks from the neighbouring craton (Napier Complex). Models that involve rehydration of the Napier Complex are no longer required, since the Rayner Complex was its own source of water. Two episodes of Proterozoic crustal growth are identified, the later of which occurred between about 1200 Ma and 1000 Ma, and was relatively minor. Sedimentation took place only shortly before Late Proterozoic orogenesis. The multiphase history of the Rayner Complex has resulted in complex isotopic behaviour. Three temporally discrete episodes of Pb loss from zircon have been identified, the earliest two of which are responses to the c. 960 Ma and 540 Ma tectonothermal events. Fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth. Pb loss during the high‐grade event was probably governed by the same process or by lattice annealing. Some zircon suites also document recent Pb loss. Most lower concordia intercepts have no direct geological meaning and are explicable as mixed ages produced by incomplete Pb loss during two or more secondary events. Whereas all zircon separates from the orthogneisses produce U–Pb isotopic alignments, zircons from the only analysed paragneiss produce scattered data, in part reflecting a range of provenance. The 960 Ma event was also associated with the growth of a characteristically low U zircon (∼ 300 μg/g) in rocks of inferred high Zr content. There is ubiquitous evidence for the resetting of Rb–Sr total‐rock isochrons. Even samples separated by up to 10 km fail to produce igneous crystallization ages. Minor mineralogical changes produced by the 540 Ma upper greenschist‐facies metamorphism were sufficient to almost completely reset some Rb–Sr isochrons and to produce open system conditions on outcrop scale, at least in one location.
The St Marys Porphyrite crops out in the St Marys district of eastern Tasmania and is a felsic, quartz porphyrite body which contains the only known extrusive rocks associated with the widespread Devonian granitoids of Tasmania. It consists of a thick (1400 m) sheet of predominantly dacitic, welded, ash‐flow tuff together with a high‐level, vesiculated part of the volcanic feeder. The boundary between these subdivisions is an extension of the eastern boundary of the nearby Catos Creek dyke, a deeper unvesiculated level of the feeder. In the St Marys Porphyrite, the boundary is interpreted as a subsidence fault which threw the extrusive rocks down against their feeder, whilst in the Catos Creek dyke, it was the locus of early magma emplacement as well as of major movement. Ash‐flow tuff in the St Marys Porphyrite is particularly rich in crystal fragments (up to 58% by volume). Its matrix becomes progressively more recrystallized with height above the base of the sheet, thus indicating rapid emplacement and cooling as a single unit. This resulted in poor preservation of tuffaceous textures except near the base. Individual ash‐flows are generally difficult to identify, but flows or parts of flows are locally defined by variations in the proportions of metasedimentary lithic fragments and strongly recrystallized pumice(?) fragments (schlieren). Rb‐Sr isotopic data and major trace and rare earth element chemistry strongly support comagmatism of the St Marys Porphyrite with both Catos Creek dyke and Scamander Tier dyke, which is part of the early I‐type phase of magmatism in the Blue Tier Batholith. Thus, the age of emplacement of the porphyrite body (388 ± 1 Ma) precisely limits the age of early magmatism in the Blue Tier Batholith. It also limits the age of earlier deformation in the country rocks (Mathinna Beds).
The Doradilla prospect area, containing a skarn-type deposit, lies 45 km south-east of Bourke in western New South Wales. Tin and other base metal mineralisation is genetically associated with the highly fractionated I-type Midway granite. In this study, the SHRIMP zircon U–Pb dating technique has been used to determine that the Midway granite and comagmatic quartz–feldspar porphyry dykes are of Middle Triassic age. Hence, it is concluded that the mineralisation is also Middle Triassic in age. No other granites of this age are known in the Bourke region. However, highly fractionated, weakly oxidised to reduced granites of Early to Middle Triassic ages with associated tin, gold and base metal mineralisation occur approximately 500 km to the east-northeast within the New England Orogen. This suggests that the Early to Middle Triassic pulse of magmatism that occurred in northern New South Wales is of greater extent than previously suspected and suggests that similar fertile granites may occur beneath cover within the Bourke–Brewarrina–Byrock region.
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