Contrasting P-T-t paths reveal a metamorphic discontinuity in the New Quebec Orogen: Insights into Paleoproterozoic orogenic processes

The large scale Paleoproterozoic Trans-Hudson Orogen exposed in Canada is recognized to record one of the first complete Wilson cycle. Although it is considered as a prototype of modern accretionary orogen, the strain pattern, thermal state, and architecture in its wedge are still poorly constrained. Part of it, the accretionary New Quebec Orogen (NQO, 1.82-1.77 Ga) resulted from oblique collision between the Superior Craton lower plate and a composite upper plate including the Kuujjuaq Domain (KD) and the Core Zone, and exposes supracrustal sequences of the Rachel-Laporte Zone (RLZ), a continental forearc basin that evolved in a collisional foreland basin upon collision. We performed an integrated analysis, using petrography, phase equilibria modelling and Lu-Hf garnet, and U-Pb zircon, monazite and rutile chronology on supracrustal rocks in the central segment of the New Quebec Orogen. On the one hand, the RLZ experienced a tight clockwise metamorphic evolution, with a peak at 650 degrees C/0.7 GPa. Prograde garnet and monazite growth is estimated at 1804 +/- 8 Ma and 1796 +/- 4 Ma respectively. On the other hand, the metamorphic peak for restitic granulites from the KD is estimated at 790 degrees C/0.72 GPa and follows isothermal decompression. Geochronology yields ages of 1836 +/- 5 Ma for prograde garnet growth, and 1807 +/- 4 Ma for crystallization of zircon from anatectic melt, which was followed by rapid cooling recorded by rutile at 1798 +/- 16 Ma. We propose that the Lac Turcotte Fault that separates both domains has acted as a tectonometamorphic discontinuity that limits contrasting P-T-t paths. We suggest sequential burial of supracrustal sequences along an apparently continuous Barrovian sequence, with progressive recycling of inner zones in the foreland basin and subsequent involvement of the latter in the orogenic wedge.