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

• Published in: Journal of Metamorphic Geology
• Main Authors: Hagen‐Peter, G., Cottle, J. M., Smit, M., Cooper, A. F.
• Other Authors: National Science Foundation, Deutsche Forschungsgemeinschaft
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2016
• Subjects: Pacific; Antarc*; Antarctica
• Online Access: http://dx.doi.org/10.1111/jmg.12182; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fjmg.12182; https://onlinelibrary.wiley.com/doi/pdf/10.1111/jmg.12182; https://onlinelibrary.wiley.com/doi/full-xml/10.1111/jmg.12182; https://onlinelibrary.wiley.com/doi/am-pdf/10.1111/jmg.12182

Abstract The Ross orogen of Antarctica is an extensive (>3000 km‐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 ~610–680 °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. 616–572 Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c . 610–500 Ma, 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 ...