GEOCHEMISTRY OF THE MT-WRIGHT VOLCANICS FROM THE WONOMINTA BLOCK, NORTHWESTERN NEW-SOUTH-WALES

The Mt Wright Volcanics are located in the Wonominta Block of northwes tern New South Wales. Detailed regional mapping has shown that the blo ck is a composite tectonic unit and that the metavolcanic rocks descri bed as the Mt Wright Volcanics may have been emplaced at different tim e from Late Proterozoic (northern section: Packsaddle, Nundora) to Ear ly Cambrian (southern section: Mt Wright). Geochemical investigations, including major and trace elements, as well as analyses of relic clin opyroxene, show that the rocks have affinities with alkali basalt with light-rare-earth-element-enriched compositions. An intra-plate extens ional environment (such as rift- and/or plume-related) is considered m ost likely for the formation of the rocks. Though metamorphosed to var ious degrees, the rocks apparently retain much of their primary Sr iso topic character (initial Sr-87/Sr-86 about 0.7032) and, apart from the ir age, resemble the Tertiary intraplate volcanism in eastern Australi a. The Nd isotope analyses yield remarkably similar results between th e two sections of the Mt Wright Volcanics, with Nd-143/Nd-144 between 0.51260 to 0.51271 and epsilon(Nd)(T) 4.7 +/- 0.4 (calculated to 525 M a). A kaersutite-bearing xenolith found in the northern section of the volcanic sequence has a Nd isotope composition more depleted than its hosts with epsilon(Nd)(T) of 7.7. The isotope results suggest that th e Mt Wright Volcanics were derived from a depleted mantle source witho ut significant crustal contribution. It is proposed that the Mt Wright Volcanics possibly represent the products of a rifting event that led to the breakup of the Proterozoic supercontinent during Early Cambria n in eastern Australia.