Magmatic characteristics and geochronology ofudOrdovician igneous rocks from the Cadia - Nevilleudregion, New South Wales: implications for tectonicudevolution

摘要

TheOrdovician volcanic and intrusive rocks of the Cadia –Neville region, in the southern Molong VolcanicudBelt section of the Macquarie Arc in central-western New South Wales, display a temporal progressionudfrom shoshonitic basaltic volcanism (e.g. Mt Pleasant Basalt Member) in the late Darriwilian to Gisbornianud(ca 460 – 453 Ma) to small-volume dacitic medium-K calc-alkaline magmatism (e.g. Copper Hill-typeuddacite) in the late Eastonian to early Bolindian (ca 450 – 445 Ma) to large-volume, high-K calc-alkaline toudshoshonitic, mafic to highly evolved magmatism (e.g. Nullawonga Latite Member and Cadia IntrusiveudComplex) at about 445 – 440 Ma. The two episodes of shoshonitic magmatism are separated by at leastud15 (but up to *20) million years, during which time limestones, associated with regional uplift, wereuddeposited broadly coincident with emplacement of the medium-K calc-alkaline dacitic intrusions.udDespite this, the lavas and intrusions associated with both episodes of shoshonitic magmatism haveudstrikingly similar geochemistry, indicating that a similar mantle source and partial melting processes wereudinvolved over this time interval. The arc-type geochemistry of the volcanic and intrusive rocks in theudCadia–Neville region is very similar to the lavas and intrusions forming the Pliocene Tavua Caldera in VitiudLevu, Fiji. The Pliocene shoshonitic rocks at Tavua Caldera were emplaced following cessation of arcudmagmatism when the arc moved into an extensional phase. By analogy with the Pliocene tectonicudsetting in Fiji, the arc-like signature of the Ordovician magmas of the Macquarie Arc may be due to preprocessingudof the mantle above a subduction zone prior to extension and fragmentation of arcudlithosphere that led to generation of the subduction-modified shoshonite-dominated magmas.