GEOCHRONOLOGY AND GEOCHEMISTRY OF PALEOZOIC INTRUSIVE ROCKS IN THE ROCKVALE REGION, SOUTHERN NEW-ENGLAND OROGEN, NEW-SOUTH-WALES

Palaeozoic intrusive rocks of the New England Batholith from the Rockvale district in the southern New England Orogen form three distinct associations: (i) the Carboniferous Rockvale Adamellite, a member of the Hillgrove Suite of deformed S-type granitoids; (ii) a small I-type igneous complex on the northwestern margin of the Rockvale Adamellite: several members of this complex have similar chemical compositions to the most mafic members of the Moonbi Suite of New England Batholith I-types; and (iii) a suite of dyke rocks ranging in composition from calc-alkaline lamprophyre through homblende and biotite porphyrite to aplite. Ion-microprobe U-Pb zircon analysis indicates intrusion of the Rockvale Adamellite at 303 +/- 3 Ma (weighted mean Pb-206/U-238 U-238 age; 95% confidence limits). Preliminary investigation of zircon inheritance within the Rockvale Adamellite is consistent with chemical and isotopic indications of derivation of New England Batholith S-type granitoids from a relatively juvenile protolith. Deformation of the Rockvale Adamellite occur-ed after complete crystallization of the pluton and prior to emplacement of dykes and 1-type intrusives. K-Ar biotite and homblende ages show broadly synchronous intrusion of I-type magmas and lamprophyre dykes at ca 255 Ma, indicating that mantle magmatism associated with lamprophyres was contemporaneous with the crustal production of 1-type melts. Chemical similarities between the most mafic Moonbi Suite members and calc-alkaline lamprophyres may also indicate a direct mantle contribution to some 1-type magmas.