The evolution of the Wongwibinda High-T-Low-P Metamorphic Complex, New England Orogen, NSW, Australia

Thesis by publication.Includes bibliographical references.Pt. 1. Metamorphic geology of the Wongwibinda Metamorphic Complex -- pt. 2. Zircon U-Pb-Hf characteristics of the rocks.The Wongwibinda Metamorphic Complex is a high-temperature–low-pressure metamorphic complex located in the southern New England Orogen, northeastern NSW. It is characterised by a local steep metamorphic field gradient (up to ~100°C km⁻¹) in variably metamorphosed accretionary wedge turbidites of the central Tablelands Complex. The metamorphic rocks are surrounded to the north, east and south by S-type granite plutons of theHillgrove Supersuite (Abroi Granodiorite and the Rockvale and Tobermory monzogranites) and the Wongwibinda Fault forms a major structural boundary on the east of the complex. The western boundary is considered to be ~25-30 km to the west where any boundary, possibly with the Sandon Beds is concealed by overburden including Tertiary Basalts.A comparison of (i) detrital zircon U-Pb geochronology, (ii) Hf isotope character, and (iii) trace element composition, across variably metamorphosed rocks of the complex, demonstrates that the Wongwibinda Metamorphic Complex comprises metamorphosed Girrakool Beds, the protolith to the Wongwibinda Metamorphic Complex, exposed to the west of the complex. U-Pb geochronology identifies a maximum deposition age of c.309 Ma for the metamorphosed equivalents of the Girrakool Beds, on the basis of the youngest grains. The majority of detrital zircon ages are c. 320–350 Ma, peaking at c. 330 Ma, with few Proterozoic and Archean grains. These data point to the western Keepit magmatic arc, exposed in the Tamworth Belt, as the likely volcanic provenance for the Girrakool Beds. The Hf isotope data for c. 320-350 Ma detrital grains become less radiogenic over the 30 million year record, representing a short-term isotopic reversal of the overall trend common to external accretionary orogens. Volcanic activity in the Keepit Arc is inferred to decrease rapidly at c. 320 Ma based on a drop in the abundance of <320 Ma zircon grains in sedimentary detritus. This decrease is interpreted as coinciding with the onset of trench retreat and slab roll back that led to the Late Carboniferous to Early Permian period of extension, thus ending volcanic activity in the Keepit Arc.Thermocalc P-T pseudosections are used to determine the P-T conditions of metamorphism and to derive field gradients. The metamorphic field gradient in the Wongwibinda Metamorphic Complex varies, being less than 15–23°C km⁻¹ in the transition from sub-biotite grade to biotite-grade metaturbidites in the west of the complex, and increases to greater than 50°Ckm⁻¹ and possibly over 100°C km⁻¹ in amphibolite-grade cordierite-bearing rocks and garnet-bearing migmatites in the east. The km-scale zones of cordierite-bearing rocks (up to 5 km wide) exhibit a range of textures that include: (i) cordierite–K-feldspar–spotted hornfels, 570–620°C at 250 MPa (2.5 kbar); (ii) sheared cordierite–K-feldspar–augen schists, two samples with variable temperatures of 670-690°C or