GEOCHEMISTRY AND TECTONIC SETTING OF SOME NEOPROTEROZOIC AND EARLY CAMBRIAN VOLCANICS IN WESTERN NEW-SOUTH-WALES

Geological and geochemical investigation of rocks previously mapped as the Mt Wright Volcanics in western New South Wales shows that two ver y different magma series are present in this region. An earlier transi tional alkaline basalt-trachybasalt-trachyandesite-trachyte-alkali rhy olite suite, dated by SHRIMP U-Pb on zircons at 586 +/- 7 Ma, is overl ain and intruded by a calc-alkaline basalt-andesite-dacite suite. The latter is faulted against the Early Cambrian Cymbric Vale Formation, c onsisting mainly of vitric tuffs and pelitic sedimentary rocks for whi ch a zircon SHRIMP U-Pb age of 525 +/- 8 Ma has been previously report ed. The calc-alkaline suite hosts intercalated Early Cambrian archaeoc yathan limestones that contain the same faunal assemblage as basal Cym bric Vale Formation limestones, suggesting that these two units may be closely associated temporally. The transitional alkaline suite at Mt Wright is compositionally very close to the basalt-trachybasalt-trachy te suite of the Nundora-Packsaddle-Mt Arrowsmith region, extending nor thwest of Mt Wright for more than 150 km. Our study confirms the Neopr oterozoic age for the Kara beds to which these alkaline volcanic seque nces have previously been assigned. We propose a continental rift sett ing for the eruption of these lavas and note their pronounced composit ional similarities to the basaltic to alkali rhyolite lava piles in th e Afar Rift, bordering the Red Sea. The calc-alkaline suite is more di fficult to assign to a particular tectonic setting of eruption. Despit e a broad compositional similarity to medium-K calc-alkaline lavas in mature island are or active continental margin settings (e.g. Japan, M exico), the thin pile of aphyric and sparsely plagioclase-phyric coher ent lavas of the Wit Wright calc-alkaline suite are unlike typical lav a breccia-pyroclastic-dominated are sequences. By analogy with well-kn own calc alkaline lava sequences in the Basin and Range extensional se tting in western USA, we suggest that the calc-alkaline suite rocks at Mt Wright formed by partial melting of subcontinental lithospheric ma ntle in an immature continental rift around 525 Ma. This extensional e pisode may record an early stage of development of the Kanmantoo Troug h. Later magmatism associated with more advanced extension generated t he tholeiitic basalts and dykes, some approaching MORE compositions, k nown from the Kanmantoo sections of eastern South Australia and wester n Victoria.