Coupled garnet Lu-Hf and monazite U-Pb geochronology constrain early convergent margin dynamics in the Ross orogen, Antarctica

The Ross orogen of Antarctica is an extensive (>3000km-long) belt of deformed and metamorphosed sedimentary rocks and granitoid batholiths, which formed during convergence and subduction of palaeo-Pacific lithosphere beneath East Gondwana in the Neoproterozoic-early Palaeozoic. Despite its prominent role in Gondwanan convergent tectonics, and a well-established magmatic record, relatively little is known about the metamorphic rocks in the Ross orogen. A combination of garnet Lu-Hf and monazite U-Pb (measured by laser-ablation split-stream ICP-MS) geochronology reveals a protracted metamorphic history of metapelites and garnet amphibolites from a major segment of the orogen. Additionally, direct dating of a common rock-forming mineral (garnet) and accessory mineral (monazite) allows us to test assumptions that are commonly used when linking accessory mineral geochronology to rock-forming mineral reactions. Petrography, mineral zoning, thermobarometry and pseudosection modelling reveal a Barrovian-style prograde path, reaching temperatures of similar to 610-680 degrees C. Despite near-complete diffusional resetting of garnet major element zoning, the garnet retains strong rare earth element zoning and preserves Lu-Hf dates that range from c.616-572Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c.610-500Ma, and retain only vestigial cores. Monazite cores yield dates that overlap with the garnet Lu-Hf dates and typically have low-Y and heavy rare earth element (HREE) concentrations, corroborating interpretations of low-Y and low-HREE monazite domains as records of synchronous garnet growth. However, ratios of REE concentrations in garnet and monazite do not consistently match previously reported partition coefficients for the REE between these two minerals. High-Y monazite inclusions within pristine, crack-free garnet yield U-Pb dates significantly younger than the Lu-Hf dates for the same samples, indicating recrystallization of monazite within garnet. The recrystallization of high-Y and high-HREE monazite domains over >50Ma likely records either punctuated thermal pulses or prolonged residence at relatively high temperatures (up to similar to 610-680 degrees C) driving monazite recrystallization. One c.616Ma garnet Lu-Hf date and several c.610-600Ma monazite U-Pb dates are tentatively interpreted as records of the onset of tectonism metamorphism in the Ross orogeny, with a more robust constraint from the other Lu-Hf dates (c.588-572Ma) and numerous c.590-570Ma monazite U-Pb dates. The data are consistent with a tectonic model that involves shortening and thickening prior to widespread magmatism in the vicinity of the study area. The early tectonic history of the Ross orogen, recorded in metamorphic rocks, was broadly synchronous with Gondwana-wide collisional Pan-African orogenies.