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

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 <600°C at 200 MPa (2 kbar), consistent with rapid cooling or variable thermal structure in the Glen Mohr Shear Zone, a major north-south trending structural feature; and (iii) migmatites with or without garnet, ~660°C at less than 330 MPa (3.3 kbar). The shallow portion of the metamorphic field gradient is interpreted as resulting from conductive heating from the mantle through a thinning crust; this drove widespread biotite-grade metamorphism in the shallow New England Orogen crust (as shallow as ~12 km). Higher-grade rocks are much more spatially restricted. The spatial association of quartzite units centered within two cordierite-bearing high-T domains of the Wongwibinda Metamorphic Complex and an increased abundance of quartz veins above the cordierite isograd suggests heat advection by aqueous fluid locally perturbed the broad conductive heating. On the basis of a spatial and temporal association, fluid was channeled within shear zones, such as the Glen Mohr and Wongwibinda shear zones, and locally infiltrated nearby rocks. Electron microprobe chemical dating of metamorphic monazite in three migmatite samples (296.9±1.5 Ma) indicate that high-temperature–lowpressure metamorphism occurred about 10–12 million years after deposition of the sedimentary rocks (c. 309 Ma). The age determined for two samples of the Glen Mohr Shear Zone (291.5±1.8 Ma) is indistinguishable from ages determined for the post-metamorphic Abroi Granodiorite (290.5±1.6 Ma), indicating that the metamorphic cycle lasted c. 5-10 million years. Garnet grains in schists and migmatites commonly display a flat unzoned interior with narrow (350 nm) Mn-rich rims of variable composition. Unzoned cores are inferred to result from elemental homogenisation at peak metamorphic conditions. The narrow rims are associated with texturally resorbed grain edges that formed during retrograde conditions. The retrograde overprint is nearly pervasive across the complex and is most obvious nearer to shear zones and some intrusive rocks of the Hillgrove Supersuite. U-Pb geochronology of five samples of the Hillgrove Supersuite that show plutonism in the complex involves two pulses: c. 300 Ma and c. 292 Ma, overlapping the age of high-T–low-P metamorphism (296.8±1.5 Ma), and also postdating it. Zircon xenocrysts (≥310 Ma) in the plutonic rocks have U-Pb-Hf isotopic character similar to the Girrakool Beds, indicating that crust similar to the country rocks is the likely source of the xenocrysts. The ¹⁷⁶Hf/¹⁷⁷Hf initial character for zircon for the c. 300 Ma plutons is less radiogenic than those in the c. 292 Ma plutons, illustrating an increasing mantle component in the Hillgrove Supersuite with time. These data are consistent with a rift tectonic setting, where mantle-derived magma is predicted to increasingly migrate to shallower crustal levels over time as the crust thins and becomes hotter (metamorphism), and early partial melting of the metasedimentary crust depletes the source rocks, thus reducing the S-type component in younger Hillgrove Supersuite plutons. It is concluded that an extensional geodynamic setting around the Carboniferous–Permian boundary supports conductive heat transfer as the key driver of regional scale biotite-grade metamorphism. Advective heat transfer via hot aqueous fluids along shear zones is required to drive very local high-temperature–low-pressure metamorphism. The older ages for the Hillgrove Supersuite at c. 300 Ma suggest that some magmatic heat advection is also likely and may be important for the formation of the migmatite samples.The geology and evolution of the Wongwibinda Metamorphic Complex indicate a critical role for slab roll back and extension, evidenced by the presence of S-type granites, during the Late Carboniferous to Early Permian. This geodynamic setting led to local high-temperature–low-pressure metamorphism, S-type granite production, and development of rift basins such as the Sydney-Gunnedah-Bowen system.