The geology, geochemistry and structure of the Mount Darwin-South Darwin Peak area, western Tasmania

摘要

The Cambrian Darwin Granite intrudes calc-alkaline rhyolites of the Central Volcanic Complexudon the Darwin Plateau, Western Tasmania. Two distinct granite phases are recognised, anudequigranular granite and a granodiorite. A biotite grade contact aureole is preserved in theudCentral Volcanic Complex immediate to the Darwin Granite. Debris flow deposits ofudvolcaniclastic conglomerates and sandstones, and coherent dacite lavas of the Mid - LateudCambrian Tyndall Group unconformably overlie the Darwin Granite and Central VolcanicudComplex, and are in turn overlain unconformably by pebble to boulder conglomerates of theudsiliciclastic Owen Conglomerate.udStratigraphic and structural evidence recognise three deformation periods within the Mt Darwinud- South Darwin Peak area: the Mid - Late Cambrian, Late Cambrian - Early Ordovician, and theudDevonian Tabberabberan Orogeny. Mid - Late Cambrian deformation, evidenced by graniticudand foliated volcanic clasts in basal Tyndall Group conglomerate, indicates catastrophic upliftudand subsequent unroofing of the granite prior to Tyndall Group deposition. This unconformityudrepresents a significant Cambrian hiatus in the southern Mount Read Volcanics. A secondudunconformity between the Tyndall Group and the Owen Conglomerate marks cessation ofudTyndall Group deposition with the onset of deposition of large volumes of siliceous detritus.udThe two Devonian Tabberabberan-related deformations are characterised by, NW and Ntrendinguddextral strike slip faulting and locally intense N-trending cleavage development. TheudDevonian structures dominate the region and have obscured the effects of earlier deformations.udDevonian metamorphism to greenschist facies, characterised by chlorite-sericite assemblages,udoccurs throughout the area with intense development in shear zones.udEpigenetic, granite-related, vein and disseminated Cu-Au mineralisation occurs within andudadjacent to the Darwin Granite. Sulphur isotope data indicates mixing of Cambrian seawaterudwith magmatic-hydrothermal fluids in a hydrothermal convection cell generated duringudemplacement of the Darwin Granite. Depletion of Na20 and CaO along with K20 and FeOudenrichment have occw;red during pervasive K-feldspar, sericite and chlorite alteration.udGeochemical analyses of the Central Volcanic Complex and the magnetite series Darwin Graniteudsuggest that these units were emplaced within an extensional basin (back-arc?), at an Andeantypeudmargin. Granitic magma was generated through partial melting of an upper mantle toudlower crustal source. REE data suggests that the CVC and Darwin Granite are not comagmatic,udand field relationships preclude a genetic relationship between the Tyndall Groupudand the Darwin Granite. Elemental discrimination trends illustrates some similarities betweenudthe Darwin Granite and the most evolved phases of the Murchison Granite. However, theudDarwin Granite displays no Eu anomaly in contrast to the negative Eu anomaly and HREEudenrichment in the Murchison Granite, suggesting that the two granite systems are notudgenetically related.ud