Tectonic evolution of the Paleozoic rocks in southeast Australia using geophysical, geochronological, geochemical and Hf isotope systematics

The Paleozoic geology of the southeast Australia preserves a history of tectonic activities which involves the growth of Cambrian-Triassic orogenic belts, generally known as Tasmanides on the East Gondwana margin. The Tasmanides are divided into five orogenic belts and the evolution of these orogens include igneous activity, long sedimentation cycles and multiple shortening and deformation stages. Lachlan Orogen represents the middle part of these Tasmanides which preserves an interesting record of obduction of ophiolites, deposition of turbidites and multiple rifting and deformation phases. For past few decades, there has been much disagreement over the tectonic evolution of the Lachlan Orogen. This research is an effort to reconstruct the Early-Middle Paleozoic tectonic history of the Lachlan Orogen and western Tasmania in southeast Australia. The goals of this study are to (1). Understand the early Paleozoic tectonic settings in Tasmania and Lachlan Orogen in the context of orogenic phases during middle-late Cambrian, (2). Use the geological record of Tasmania and Lachlan Orogen to test and refine the paleogeographic reconstructions proposed for the deposition of late Cambrian to Early Silurian sedimentary rocks, (3). Test the long-lived Lachlan Orocline rotation hypothesis. These objectives are addressed primarily through (1). Reconstruction of the regional tectonic setting of middle Cambrian igneous rocks in Tasmania and Victoria in the context of geochronology and isotope geochemistry, (2). improving the knowledge of depositional age, stratigraphy, and provenance of late Cambrian to early Silurian strata in Tasmania and Lachlan Orogen, (3). Paleomagnetic testing of the tectonic elements of Lachlan Orocline in context of its rotation during Middle Paleozoic. The Cambrian greenstones in southeast Australia are relics of the proto-Pacific oceanic lithosphere exposed in Victoria and Tasmania. New U-Pb zircon ages and Hf isotope data from three gabbroic rocks and two sedimentary units within these ophiolites indicate that Gabbro rocks from Dookie in Victoria are comparable in age to the McIvor Hill gabbro and Heazlewood River Tonalite from Tasmania. These new results, when combined with previously reported zircon ages, indicate a two-stage development (518-512 Ma and 505-490 Ma) for central Victoria and a single stage evolution (513-509 Ma) for the gabbro and tonalites in Tasmania. The Hf isotopic data indicate Meso-Neoproterozoic continental contamination during Mt Read Volcanics eruption in Tasmania. Zircon Hf vs U/Yb, and U/Yb vs Nb/Nb* ratios suggests that these rocks may have formed during subduction beneath the young and hot oceanic lithosphere. The use of U-Pb detrital zircon data to construct stratigraphic comparisons, tectonic settings, and provenance of late Cambrian to early Silurian sedimentary rocks in Tasmania and Waratah Bay, southern Victoria shows that the Tyennan Region supplied adequate detrital zircons in western and central Tasmania throughout the late Cambrian and early Ordovician time periods. Ordovician sedimentary rocks from Lefroy in the northeast Tasmania have detrital zircon populations derived from distal sources, similar to the rest of the sedimentary rocks in the Lachlan Orogen. The detrital U-Pb signatures in Waratah Bay, south Victoria have mixed Tyennan and Gondwana sources. The transition in detrital zircon sources in west Tasmanian sedimentary rocks suggests that Tasmania docked with mainland Australia during the Cambrian Tyennan Orogeny. Hafnium isotope signatures supports the potential sediment sources for these rock sequences established by the U-Pb detrital data. The hafnium isotope signatures from Waratah Bay in south Victoria shows the supply of detrital sediments from western Tasmania and Macquarie Arc in the Lachlan Orogen verifying the existence of VanDieland during Paleozoic time. Paleomagnetic investigations in the Lachlan Orogen have revealed that components within the Lachlan Orocline have indeed rotated, although the evidence for rotation is limited and subject to substantial statistical uncertainty. Detailed paleomagnetic investigation, assisted by X-ray analysis of the magnetic mineralogy of middle Cambrian to lower Silurian sedimentary and igneous rocks in Victoria and New South Wales indicate four demagnetisation behaviours (type A, B, C, and D). Type A is believed to record original paleomagnetic information (hosted in magnetite), which can be used to test the Lachlan Orocline hypothesis. Samples showing type B and C demagnetisation behaviors retain a stubborn, coercive, thermally stable magnetic paleomagnetic portion that is carried by hematite and represents an overprint and specimens showing type D demagnetisation behavior are influenced by noise. Interpretation of the thermal demagnetisation behaviour is backed by vibrating sample magnetometer data where type A showing a simple hysteresis loop whereas types B and C display very wide hysteresis loops. Detailed SEM mineralogy of the representative samples show the presence of titano-magnetite and hematite as remanence carriers which verifies the presence of an overprint. Two paleopoles of type B and C specimens from the rocks in Tabberabbera Zone and the Rockley Volcanics in Oberon (New South Wales), plot within the confidence limit of poles defining the late Silurian to early Devonian Australian apparent polar wander path. These poles interpreted to be the result of alteration mineralisation produced by fluids driven by tectonic compression during the Bindian (420‚ÄövÑv¨410 Ma) or early Tabberabberan (405‚ÄövÑv¨380 Ma) orogenies.