TECTONIC IMPLICATIONS OF LATE PROTEROZOIC-EARLY PALEOZOIC IGNEOUS ROCK ASSOCIATIONS IN WESTERN TASMANIA

In contrast to the mainland Lachlan Fold Belt, Late Proterozoic basement(?) and Cambrian rocks are exposed over large areas of western Tasmania. They offer the possibility of solving the paradox regarding the continental versus oceanic nature of the basement beneath southern Australia, and better understanding the Late Proterozoic and Cambrian tectonic history of this southern part of the Lachlan Fold Belt. Around 600 Ma, attenuation and eventual rifting of Proterozoic continental crust resulted in formation of a thinned passive continental margin transected by small rift basins in which rift tholeiites transitional to MORB accumulated. Eastward-directed intra-oceanic subduction commenced to the east of the passive margin at some time before the Middle Cambrian, forming an oceanic arc with boninites and low-Ti magnesian quartz tholeiites in the forearc region. Continued subduction of intervening oceanic crust between the arc and the passive margin resulted in an arc-continent collision within the Middle Cambrian. One or more extensive sheets of forearc crust, dominated by low-Ti basalts and boninites and their cumulate complements, were overthrust onto thinned continental crust. Continued compression at the suture initiated what was probably a short-lived episode of westward-directed subduction beneath the newly-emplaced allochthon and passive margin basement, and generated medium- to high-K andesites and more felsic rocks of the Central Volcanic Complex of the Mount Read Volcanics. Rebound backthrusting along the eastern side of the Central Volcanic Complex of the Mount Read Volcanics belt exhumed underthrust Proterozoic crystalline crust which now forms the Tyennan region, and a foreland basin half-graben, the Dundas Trough, formed along the western edge of this basement inlier. The upper Mount Read Volcanics are mainly post-collisional high-K to shoshonitic basalts and andesites generated by delayed partial melting of subduction-modified, underthrust passive margin subcontinental mantle. Extension in this post-collision collage of crustal elements led to limited rifting and emplacement of the tholeiitic Henty Dyke Swarm through at least the Central Volcanic Complex. Lower crustal generation of extensive sheet-like intrusive quartz-feldspar porphyrite bodies and associated Tyndall Group felsic explosive volcanism occurred within the Dundas Trough. Uplift and excavation of Tyennan region Proterozoic crystalline crust provided abundant coarse siliciclastic detritus of the Owen conglomerate and correlates which flooded into and gradually filled the Dundas Trough. The major events invoked in the above model to have generated continental crust (allochthon emplacement, reversal of subduction, post-collisional volcanism and graben formation, rebounding and excavation of underthrust crystalline basement, graben filling) all occurred probably within a 20-Ma period. Late Cambrian deformation is well known in the Adelaide Fold Belt (Delamerian Orogeny) and in the New Zealand and Transantarctic Mountains segments of the once-continuous fold belt along the eastern margin of Early Palaeozoic Gondwanaland. It is not recorded from the mainland section of the Lachlan Fold Belt in Victoria, although given the nature of Cambrian outcrops along major Devonian faults in Victoria, this is hardly surprising.