Rian A. Dutch

Cambrian reworking of the southern Australian Proterozoic Curnamona Province: constraints from regional shear-zone systems

Palaeoproterozoic to early Mesoproterozoic metamorphic rocks of the Curnamona Province, southern Australia, are crosscut by a system of regional-scale shear zones that locally dominate the terrain. Combined metamorphic and geochronological data from localities across the southern Curnamona Province indicate that the peak metamorphic shear-zone assemblages formed during the Cambrian (c. 500 Ma) Delamerian Orogeny, and not during the waning stages of the c. 1600 Ma Olarian Orogeny as has been previously asserted. A combination of monazite chemical U–Th–Pb and garnet Sm–Nd geochronology indicates that shear-zone fabrics formed between 497 and 517 Ma. Peak metamorphic conditions obtained from prograde garnet–staurolite–biotite–muscovite–chlorite–quartz assemblages are between 530 and 600 °C at pressures of around 5 kbar. The apparent absence of significant up-pressure prograde paths recorded by the mineral assemblages, together with modest (10–20%) Delamerian shortening, suggests that attainment of burial to depths of around 18 km was largely a function of pre-Delamerian sedimentation over the interval from c. 700 to 530 Ma. The spatial association between the pattern of basement metamorphism and reactivation during the Delamerian Orogeny is interpreted to reflect in part the distribution of pre-Delamerian sedimentation, and highlights the potential importance of pre-orogenic processes such as basin development in controlling the style and pattern of later terrain reactivation and reworking.

A Proterozoic Wilson cycle identified by Hf isotopes in central Australia: Implications for the assembly of Proterozoic Australia and Rodinia

Current models for the assembly of Proterozoic Australia suggest that the North Australian craton (NAC), West Australian craton (WAC), and South Australian craton (SAC) had amalgamated by at least 1.6 Ga, with possible rafting and reattachment of the SAC by ca. 1.3 Ga. In this scenario, the younger (1.2–1.1 Ga) Grenvillian-aged Musgrave Province of central Australia, which separates all three cratons, has been considered postcollisional to intracratonic. However, new and recent U-Pb and Lu-Hf isotopic analyses of zircons from the Musgrave Province indicate continuous active-margin magmatic activity between 1.7 and 1.2 Ga. A distinctive inverted U-shaped pattern of the Hf array for this 500 m.y. period is evidence of part of a Proterozoic Wilson cycle, with subduction initiation at 1.7 Ga and eventual ocean closure by 1.2 Ga. We estimate that the cycle began at 2.2 Ga. Overlap of the Musgrave zircon age spectra and Hf isotopic array with the along-strike Albany-Fraser orogen (AFO) suggests derivation of the Musgrave Province from the WAC, not the NAC or SAC as previously thought. The Musgrave Province link to the WAC confirms that Australia did not assemble until at least early Grenvillian time (ca. 1.2 Ga). Moreover, because the SAC was part of the much larger Mawson continent, the 1.2 Ga collision was of transcontinental magnitude similar to that of the type-Grenville orogen in Laurentia. This favors an Australia-Mexico (AUSMEX) configuration at 1.2 Ga, rather than the southwestern United States and East Antarctica (SWEAT) or Proterozoic Australia–western United States (AUSWUS) models. The Musgrave-AFO marks a major, underestimated phase of Rodinian assembly.

High-grade Paleoproterozoic reworking in the southeastern Gawler Craton, South Australia

SHRIMP U–Pb geochronology and monazite EPMA chemical dating from the southeast Gawler Craton has constrained the timing of high-grade reworking of the Early Paleoproterozoic (ca 2450 Ma) Sleaford Complex during the Paleoproterozoic Kimban Orogeny. SHRIMP monazite geochronology from mylonitic and migmatitic high-strain zones that deform the ca 2450 Ma peraluminous granites indicates that they formed at 1725 ± 2 and 1721 ± 3 Ma. These are within error of EPMA monazite chemical ages of the same high-strain zones which range between 1736 and 1691 Ma. SHRIMP dating of titanite from peak metamorphic (1000 MPa at 730°C) mafic assemblages gives ages of 1712 ± 8 and 1708 ± 12 Ma. The post-peak evolution is constrained by partial to complete replacement of garnet–clinopyroxene-bearing mafic assemblages by hornblende–plagioclase symplectites, which record conditions of ∼600 MPa at 700°C, implying a steeply decompressional exhumation path. The timing of Paleoproterozoic reworking corresponds to widespread deformation along the eastern margin of the Gawler Craton and the development of the Kalinjala Shear Zone.

Age constraints on the timing of iron ore mineralisation in the southeastern Gawler Craton

SHRIMP U–Pb zircon data obtained from a magnetite gneiss deposit in the southeast Gawler Craton indicate that the protoliths were deposited between 1750 and 1735 Ma. Previously, these iron-rich gneisses were thought to have been a metamorphosed banded iron formation and part of the Neoarchean to early Paleoproterozoic Sleaford Complex. Detrital zircon data provide maximum depositional ages between 1765 Ma and 1740 Ma, with ϵNd(1750) values between –4.6 and –3.3. Metamorphic zircon ages are 1735–1725 Ma, indicating that the magnetite gneiss was formed during the 1730–1690 Ma Kimban Orogeny. The pelitic mineralogy of the magnetite gneiss and presence of detrital zircon demonstrate that the protoliths were not a pure chemical sediment. Correlation with the greenschist facies Price Metasediments in the southern Gawler Craton suggests a series of iron-bearing basins were developed prior to the onset of the Kimban Orogeny and suggest that metamorphism played a role in ore formation.

Orogen-parallel flow during continental convergence: Numerical experiments and Archean field examples: COMMENT and REPLY COMMENT

[Duclaux et al. (2007)][1] use the Eyre Peninsula region of the eastern Gawler Craton (South Australia) as their primary field example of Archean-aged orogen-parallel flow as envisaged from the perspectives of numerical models. While we would like to make it clear that we do not contest the notion

Further evidence for two metamorphical events in the Mawson Continent

Abstract In this study, in situ and erratic samples from George V Coast (East Antarctica) and southern Eyre Peninsula (Australia) have been used to characterize the microstructural, pressure–temperature and geochronological record of upper amphibolite and granulite facies polymetamorphism in the Mawson Continent to provide insight into the spatial distribution of reworking and the subice geology of the Mawson Continent. Monazite U-Pb data shows that in situ samples from the George V Coast record exclusively 2450–2400 Ma ages, whereas most erratic samples from glacial moraines at Cape Denison and the Red Banks Charnockite record only 1720–1690 Ma ages, consistent with known ages of the Sleaford and Kimban events, respectively. Phase equilibria forward modelling reveals considerable overlap of the thermal character of these two events. Samples with unimodal 1720–1690 Ma Kimban ages reflect either formation after the Sleaford event or complete metamorphic overprinting. Rocks recording only 2450–2400 Ma ages were unaffected by the younger Kimban event, perhaps as a result of unreactive rock compositions inherited from the Sleaford event. Our results suggest the subice geology of the Mawson Continent is a pre-Sleaford-aged terrane with a cover sequence reworked during the Kimban event.