Long-lived anatexis in the exhumed middle crust of the Torngat Orogen: Constraints from phase equilibria modeling and garnet, zircon, and monazite geochronology

Middle and lower orogenic crust exposed at the Earth's surface represent an exceptional opportunity to investigate accretionary and collisional processes in various tectonic settings. In particular, deciphering their metamorphic history in terms of evolving P-T-t-D paths enables an assessment of lower crustal behavior and the role of granulitic terranes throughout orogeny. In this paper, we use field work, average P-T thermobarometry, phase equilibria modeling, and zircon and monazite UPb dating coupled with detailed LuHf and SmNd garnet dating to investigate the style, the timing, the duration, the extent, and the significance of metamorphism in the lower plate of the Torngat Orogen, a transpressional orogen that is part of the larger-scale Paleoproterozoic Trans-Hudson Orogen in Canada. Supracrustal rocks in the region record clockwise P-T paths with peak conditions gradually increasing from 8.7 kbar at 815 °C to 11 kbar at 915 °C from west to east, and homogeneous near-solidus retrograde conditions at ~6–8 kbar and ~ 775–850 °C. LuHf garnet geochronology indicates that prograde metamorphism was ongoing at c. 1885 Ma, marking the onset of crustal thickening earlier than previously thought. Zircon and monazite UPb ages suggest protracted growth during melt crystallization along the retrograde metamorphic path between c. 1859 and 1811 Ma. The minimum duration of anatexis is estimated to be ~50 Myr. We show that the metamorphic effect of the Torngat orogeny is wider than previously thought, resulting in a large mid-pressure, high-temperature domain with widespread anatexis. Our dataset provides an example of the utility of multi-geochronometer dating for deciphering the metamorphic history of granulitic terranes.