Continuous convergence along the paleo-Pacific margin of Australia during the Early Paleozoic: Insights from the Running River Metamorphics, NE Queensland

Abstract

New geochronological findings, using zircon dating, supported with geochemical and structural data from the Running River Metamorphics (RRM), Thomson Orogen of the Tasmanides, have been used to reinvestigate the Paleo-Pacific margin of Australia during the Early Paleozoic. The RRM comprises a greenschist to amphibolite facies metamorphic succession of gneisses, amphibolite, and quartzite that is intruded by the Ordovician Falls Creek Tonalite. Protoliths of the gneisses and tonalite are interpreted to be emplaced at ~472 and ~457 Ma, respectively, whereas a maximum depositional age of the protolith of the quartzite is constrained to ~471 Ma. Geochemically, the gneisses and tonalite have calc-alkaline signature, and represent S-and I-type granitoids, respectively, that formed in a continental arc setting. The amphibolites have oceanic island basalt-like affinity, and together with the coeval quartzite are interpreted to represent a relict seamount. Five deformation events were identified in the RRM. D1 and D2, and D3 are interpreted to be subduction- and collision-related, respectively. D4 most probably accompanied the movement along the NE segment of the Clarke River Fault, and resulted in rotation of the earlier fabrics into the dominant NE orientation. The timing of tectonism can be constrained between the intrusion of the S-type gneisses at 472 Ma and the intrusion of the undeformed migmatites at ~437 Ma. The common stratigraphy, tectono-metamorphic history, geochemistry, and overlapping dates of the different provinces of the Thomson orogen suggest that following the Delamerian orogeny, the paleo-Pacific margin of Australia has been a convergent plate boundary during Middle Ordovician to Middle Silurian. Our results imply that convergence has been underway along the eastern margin of Australia during the Cambro-Silurian times and hence pushing the boundary of the Delamerian orogeny to the end of Ordovician in the northern part of the Thomson Orogen.