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Alice Springs, Australia

Kirkland C.L.,Geological Survey of Western Australia | Bingen B.,Geological Survey of Norway | Whitehouse M.J.,Swedish Museum of Natural History | Beyer E.,Northern Territory Geological Survey | Griffin W.L.,Macquarie University
Precambrian Research | Year: 2011

The Scandinavian Caledonides exposes increasingly far travelled nappes upwards (westwards). The Lower- to Middle Allochthon are widely regarded as indigenous to the pre-Caledonian margin of Baltica (Fennoscandia), while higher thrust sheets (e.g. lower Upper Allochthon) have more controversial ancestries. Recent studies have suggested that thick early-Neoproterozoic (Tonian-Cryogenian) metasedimentary sequences hosted in thrust sheets of the Scandinavian, Greenland, British and Svalbard Caledonides represent two cycles of sedimentation along the peri-Rodinia (Laurentia-Baltica) margin of the supercontinent; alimented by erosion of the Grenvillian and Sveconorwegian orogenic belts. To test and expand this model, we report zircon U-Pb and whole rock Sm-Nd data from orthogneisses and siliciclastic metasediments in a transect from the (para)autochthonous Dividal Group, through the crystalline Middle Allochthon Akkajaure Nappes and into the overlying Seve Nappes (lower Upper Allochthon).The Akkajaure Nappe Complex (Middle Allochthon) is dominated by Precambrian crystalline rocks, of granitic to dioritic composition, that yield ages of 1788 ± 6, 1806 ± 15, and 1876 ± 10 Ma. These rocks are thrust over an imbricated basement (Lower Allochthon), which contains rocks of similar age, including a rhyolite dated at 1790 ± 6 Ma. The ages and Nd isotope chemistry of these rocks are identical to those from other Fennoscandian basement units and imply derivation from west of the Lofoten islands. A sample of sediment, originally attributed to the lowest unit of the overlying Seve Nappe Complex, has a detrital population dissimilar to all other Seve-Kalak Nappe samples and has a significant 1800. Ma volcanic component. This sediment was intruded by granite at 1797 ± 4. Ma. Its provenance and rapid depositional timing indicates it is better considered as part of the tectonically captured Fennoscandian basement (Middle or Lower Allochthon).Metasediments from higher Seve nappes, in Jämtland, were deposited after 730. Ma and contain a detrital zircon population indistinguishable from many sediments within the wider North Atlantic Region (NAR). Based on the similarity in detrital zircon age populations, depositional timings, and the apparent lack of material from the local Fennoscandian crystalline basement, these Jämtland Seve sediments are interpreted as peri-Rodinian deposits, which were carried along the passive margin of Baltica during the opening of Iapetus (after 610. Ma). The strong similarity of detritus across NAR sedimentary units and the Rodinian paleogeography this implies, indicates that the exotic versus endemic derivation of nappes is more relevant in terms of which side of Iapetus these rocks were sitting after rifting; Baltica side (endemic) versus Laurentia side or microplate (exotic).The detrital zircon population from the early Cambrian part of the (para)autochthonous Dividal Group is identical to that from the Jämtland Seve and modern river sands flowing off the nappes. This indicates that the Seve Nappes were in sufficient proximity to Baltica by the Cambrian to shed detritus into the Dividal Group. This supports the notion of Ordovician arrival of peri-Rodinian units onto Baltica. © 2011.

Maidment D.W.,Geoscience Australia | Huston D.L.,Geoscience Australia | Donnellan N.,Northern Territory Geological Survey | Lambeck A.,Geoscience Australia
Precambrian Research | Year: 2013

Detrital zircon dating of the Warramunga Formation that hosts iron-oxide Au-Cu-Bi mineralisation in the Tennant Region indicates that this unit was deposited after ca. 1860Ma. Intrusive felsic porphyries within the Warramunga Formation have U-Pb zircon ages of 1847.5±2.4, 1847.4±2.8, 1846.3±2.9 and 1848.9±2.2Ma, consistent with a single magmatic event at about 1848Ma. Peperitic margins of some of these intrusions indicate that deposition of the Warramunga Formation continued to about the time of porphyry emplacement. The Tennant Creek and Hill of Leaders granites have ages of 1851.1±3.5Ma and 1845.5±2.4Ma respectively, indistinguishable from those of the felsic porphyries, and consistent with previous interpretations that the porphyries are the high-level equivalents of the granitoids of the Tennant Supersuite. Felsic porphyry at the White Devil mine with an age of 1847.4±2.8Ma cross-cuts ironstone that hosts mineralisation, and the S1 foliation, indicating that ironstone formation and deformation occurred soon after deposition of the host succession. Alteration associated with Au-Cu-Bi mineralisation overprints the White Devil porphyry, providing a maximum age for mineralisation. Minimum ages of 1851-1847Ma for mineralisation are provided by previous 40Ar-39Ar age determinations, indicating that mineralisation, deformation and magmatism took place during the ca. 1850-1845Ma Tennant Event, contemporaneous with, or shortly after, the last stages of deposition of the Warramunga Formation. Although there is a close temporal association between mineralisation and felsic intrusive rocks of the Tennant Supersuite, the intrusive rocks are only weakly fractionated, suggesting they were not a significant source of Au and Cu. A felsic volcaniclastic rock from the Rover gold field 90km southwest of Tennant Creek yielded a unimodal zircon age group of 1842.3±4.5Ma, allowing correlation with the younger parts of the Warramunga Formation or the unconformably overlying ca. 1840Ma Ooradidgee Group. Although a detailed correlation remains uncertain, the simplest interpretation is that the Au-Cu-Bi mineralisation at the Rover Field occurred at a broadly similar time to that at Tennant Creek. © 2013.

Hollis J.A.,Northern Territory Geological Survey | Hollis J.A.,Geological Survey of Western Australia | Van Kranendonk M.J.,University of New South Wales | Cross A.J.,Geoscience Australia | And 3 more authors.
Lithosphere | Year: 2014

The timing of widespread continental emergence is generally considered to have had a dramatic effect on the hydrological cycle, atmospheric conditions, and climate. New secondary ion mass spectrometry (SIMS) oxygen and laser-ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) Lu-Hf isotopic results from dated zircon grains in the granitic Neoarchean Rum Jungle Complex provide a minimum time constraint on the emergence of continental crust above sea level for the North Australian craton. A 2535 ± 7 Ma monzogranite is characterized by magmatic zircon with slightly elevated δ18O (6.0‰-7.5‰ relative to Vienna standard mean ocean water [VSMOW]), consistent with some contribution to the magma from reworked supracrustal material. A supracrustal contribution to magma genesis is supported by the presence of metasedimentary rock enclaves, a large population of inherited zircon grains, and subchondritic zircon Hf (εHf = -6.6 to -4.1). A separate, distinct crustal source to the same magma is indicated by inherited zircon grains that are dominated by low δ18O values (2.5‰-4.8‰, n = 9 of 15) across a range of ages (3536-2598 Ma; εHf = -18.2 to +0.4). The low δ18O grains may be the product of one of two processes: (1) grain-scale diffusion of oxygen in zircon by exchange with a low δ18O magma or (2) several episodes of magmatic reworking of a Mesoarchean or older low δ18O source. Both scenarios require shallow crustal magmatism in emergent crust, to allow interaction with rocks altered by hydrothermal meteoric water in order to generate the low δ18O zircon. In the first scenario, assimilation of these altered rocks during Neoarchean magmatism generated low δ18O magma with which residual detrital zircons were able to exchange oxygen, while preserving their U-Pb systematics. In the second scenario, wholesale melting of the altered rocks occurred in several distinct events through the Mesoarchean, generating low δ18O magma from which zircon crystallized. Ultimately, in either scenario, the low δ18O zircons were entrained as inherited grains in a Neoarchean granite. The data suggest operation of a modern hydrological cycle by the Neoarchean and add to evidence for the increased emergence of continents by this time. © 2014 Geological Society of America.

Edgoose C.J.,Northern Territory Geological Survey
Episodes | Year: 2012

The Amadeus Basin of central Australia has a depositional history spanning the Neoproterozoic to the Devonian/Carboniferous. It was initiated as part of the Neoproterozoic Centralian Superbasin, which formed in an intracratonic setting related to the break up of Rodinia. Sedimentation continued until the 580-540 Ma Petermann Orogeny, coinciding with the assembly of Gondwana, which resulted in the fragmentation of the superbasin into separate intracratonic basins. The Petermann Orogeny was focused in the Musgrave Province and the southern part of the Amadeus Basin, and involved significant N-directed shortening on largescale structures that involved both the basement and the overlying Neoproterozoic sedimentary rocks. It significantly transformed the basin architecture, with the development of major basin features that controlled subsequent sedimentation. Deposition of Paleozoic successions was largely concentrated in sub-basins and troughs in the N of the basin, where up to 14 km is preserved. Minor events or uplifts punctuated this depositional history and account for local disconformities and absent sections. The 450-300 Ma Alice Springs Orogeny was a multi-phase, intracratonic event concentrated in the Arunta Region and the northern part of the Amadeus Basin. Like the earlier Petermann Orogeny, the Alice Springs Orogeny involved both basement and basin sedimentary rocks, but with overall S-directed movement. Synorogenic sedimentation accompanied Mid-Late Devonian uplift, with Late Devonian-Carboniferous basin inversion terminating sedimentation, and folding the youngest successions. The Amadeus Basin has known reserves of U, minor historic and recent Au production, and is prospective for base metals, especially Cu, and phosphate. The Ordovician succession supports commercial gas production, and the Neoproterozoic succession is considered prospective for oil and gas.

Hollis J.A.,Geological Survey of Western Australia | Wygralak A.S.,Northern Territory Geological Survey
Episodes | Year: 2012

The Pine Creek Orogen comprises a succession of Paleoproterozoic sedimentary and volcanic rocks, unconformably overlying Neoarchean granitic basement and intruded by Paleoproterozoic mafic rocks and granites. The orogen is subdivided from west to east into the Litchfield Province, Central Domain and Nimbuwah Domain, based on the distinct timing and nature of sedimentation, magmatism and metamorphism. The orogen hosts a wide range of commodities, the most important of which are U and Au. Rifting of Neoarchean basement at 2020 Ma led to deposition of clastic, carbonate, and carbonaceous sedimentary and volcanic rocks in a shallow basin. At 1870 Ma, sedimentation in the Nimbuwah Domain was rapidly followed by burial, I-type granitic magmatism (1867-1860 Ma), compressional tectonism and mid-pressure amphibolite-facies metamorphism (1865-1855 Ma). Major U deposits occur in the Nimbuwah Domain within basal Paleoproterozoic strata, close to tectonised contacts with Neoarchean basement. Metamorphism of the Nimbuwah Domain coincided with sedimentation and volcanism in the Central Domain and Litchfield Province at 1863 Ma. This was followed by extensional high-temperature, low-pressure metamorphism (1855 Ma) and associated felsic and arc-related mafic magmatism (1862-1850 Ma) in the Litchfield Province. At or after this time, greenschist- facies metamorphism and upright folding and shearing occurred at upper crustal levels in the Central Domain, generating structural traps for subsequent Au- and Fe-bearing fluids. Almost all Au occurrences are associated with late to post orogenic, I-type Cullen Supersuite granites (1835-1820 Ma). Shortly thereafter, platform sediments were deposited in braided rivers across the orogen. The strong spatial heterogeneity in the distribution of U and Au suggests that the pre-existing crustal architecture of the orogen was a significant factor controlling their distribution.

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