GEOCHEMISTRY OF THE MOUNT-WINDSOR VOLCANICS - IMPLICATIONS FOR THE TECTONIC SETTING OF CAMBRO-ORDOVICIAN VOLCANIC-HOSTED MASSIVE SULFIDE MINERALIZATION IN NORTHEASTERN AUSTRALIA

The Mount Windsor subprovince is an important relic of Upper Cambrian to Lower Ordovician sedimentation and volcanism in the northern part o f the Tasman orogenic zone. Volcanic-hosted massive sulfide mineraliza tion occurs at several stratigraphic horizons within the volcano-sedim entary package and one major mine is operational within the belt. Majo r and trace element data and Nd isotope ratios are presented for the l east altered coherent units from the three major volcanic-bearing form ations in the Mount Windsor subprovince. The data are used to discrimi nate four major phases of volcanism and related intrusive activity der ived from three isotopically discrete sources and to assess the geodyn amic setting in which the volcanism occurred. The earliest phase of ma fic volcanism has minor and trace element characteristics suggesting a n alkaline intraplate or rife association and it was probably produced by partial melting of attenuated subcontinental lithospheric mantle. The overlying Mount Windsor Formation silicic volcanics have Nd isotop e characteristics (epsilon(Nd(480 Ma)) = -4.7 to -12.8) that suggest t hey were produced by partial melting of underlying Precambrian crustal rocks. Mafic volcanics of the overlying Trooper Creek Formation inclu de a low TiO2 suite Md a high TiO2 suite with a range of distinguishin g chemical characteristics but similar Nd isotope ratios (epsilon(Nd(4 80 Ma)) = 3.8-2.3), which indicate derivation from relatively depleted asthenospheric mantle variably modified by subduction processes. The high TiO2 suite is also represented by abundant intrusions within tire underlying volcanic package. The more silicic volcanics in the Troope r Creek Formation appear to be cogenetic with their mafic associates b ut have varying Nd isotope ratios, which suggest progressive crustal i nteraction with increasing SiO2 content. Comparisons with modern volca nic compositions and ore depositional environments suggest that the vo lcanic and sedimentary units within the Mount Windsor subprovince were deposited in a back are basin developed by extension of continental l ithosphere along the eastern Australian margin in the Late Cambrian an d Early Ordovician. Mineralization and volcanic deposits of similar ag e farther north in the Tasman orogenic zone suggest that this basin ma y have had a north-south orientation, although there is no clear evide nce remaining of the original are front deposits.