EBOR-VOLCANO AND CRESCENT COMPLEX, NORTHEASTERN NEW-SOUTH-WALES - AGEAND 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 intr aplate volcanism in eastern Australia. Geochronological results indica te 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 an d cover an area of 480 km(2). In mid-Miocene time, the Ebor Volcano wa s conservatively 45 km across and of low shield form, standing at leas t 800 m above basement. Subsequent erosional retreat of the Great Esca rpment has removed about 90% of its volume. Flows of olivine- and quar tz-normative tholeiitic basalts grading to icelandite form the bulk of the volcano and were erupted between 19.6 and 19.2 Ma. An alkaline an d transitional basaltic lava suite is locally associated, with the maj ority of occurrences towards the volcano's western margin. Many of the latter are basanite in composition and are interpreted to have been e rupted 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 abou t 19.2-19.0 Ma and are locally accompanied by fragmental and epiclasti c equivalents. The Crescent Complex is a high-level intrusion, tempora lly related to the tholeiitic suite of lavas and centrally located in volcano reconstructions. It is a strongly differentiated gabbro-monzod iorite-monzonite-monzogranite stock with coincident magnetic and gravi ty anomalies. Like many other eastern Australian central volcanoes, Eb or displays characteristic compositional diversity, with strongly frac tionated derivative magmas. With other central volcanoes in the region (Tweed, Nandewar, Warmmbungle), it may be a manifestation of the posi tion of a fixed melting anomaly (hot spot) in the sub-lithospheric man tle during the mid-Tertiary.