EBOR-VOLCANO AND CRESCENT COMPLEX, NORTHEASTERN NEW-SOUTH-WALES - AGE AND GEOLOGICAL DEVELOPMENT

The Ebor Volcano in northeastern New South Wales is one of a number of central volcanoes formed during the period of extensive Tertiary intraplate volcanism in eastern Australia. Geochronological results indicate a Miocene age (ca 19-20 Ma) for the volcano, which was constructed on Late Palaeozoic to Early Mesozoic rocks of the New England Orogen. Its remnants are dominated by lava flow sequences up to 400 m thick and cover an area of 480 km(2). In mid-Miocene time, the Ebor Volcano was conservatively 45 km across and of low shield form, standing at least 800 m above basement. Subsequent erosional retreat of the Great Escarpment has removed about 90% of its volume. Flows of olivine- and quartz-normative tholeiitic basalts grading to icelandite form the bulk of the volcano and were erupted between 19.6 and 19.2 Ma. An alkaline and transitional basaltic lava suite is locally associated, with the majority of occurrences towards the volcano's western margin. Many of the latter are basanite in composition and are interpreted to have been erupted from a centre at Round Mountain at about 48 Ma, thus being part of the earlier Doughboy province. Several felsic domes, sills, dykes and flows were emplaced late in the development of the volcano at about 19.2-19.0 Ma and are locally accompanied by fragmental and epiclastic equivalents. The Crescent Complex is a high-level intrusion, temporally related to the tholeiitic suite of lavas and centrally located in volcano reconstructions. It is a strongly differentiated gabbro-monzodiorite-monzonite-monzogranite stock with coincident magnetic and gravity anomalies. Like many other eastern Australian central volcanoes, Ebor displays characteristic compositional diversity, with strongly fractionated derivative magmas. With other central volcanoes in the region (Tweed, Nandewar, Warmmbungle), it may be a manifestation of the position of a fixed melting anomaly (hot spot) in the sub-lithospheric mantle during the mid-Tertiary.