Polymetamorphism of the Hyland River Area, Northern Canadian Cordillera and Constraints on the Timescales of Low‐Pressure Metamorphism

ABSTRACT The Hyland River schist–gneiss belt is an elongate, partly fault‐bounded regional‐metamorphic high (approximately 2000 km 2 ) in the northern Canadian Cordillera. It includes a migmatitic core flanked by schist and phyllite and hosts numerous large mid‐Cretaceous intrusions. Regional‐metamorphic grade increases progressively across unfaulted flanks from the chlorite zone to the cordierite + K‐feldspar zone over a structural thickness of ~3.5–4 km. An early stage of regional metamorphism (M 2 ) produced garnet and staurolite (±sillimanite)‐bearing schists. This was followed by low‐pressure, high‐temperature (LP‐HT) regional metamorphism (M 3 ), which resulted in partial melting of deep structural levels. Regional (M 2–3 ) isograds were folded (F 4 ) prior to the emplacement of late‐ to post‐tectonic batholiths. Upright F 4 folds produced kilometre‐scale structural relief, with the result that contact aureoles were developed across steep regional‐metamorphic gradients. We document the timescales involved in the transition from regional LP‐HT to contact metamorphism by integrating mineralogical and microstructural observations with high resolution U–Pb TIMS geochronological data from variably deformed intrusions. Felsic‐intermediate magmatism within the schist–gneiss belt extended over a period of ≥ 7 My. LP‐HT regional metamorphism (M 3 ) overlapped with syntectonic leucogranite and pegmatite intrusions (~112–106.9 Ma) but took place prior to the emplacement of large, late‐ to post‐kinematic batholiths of granite and granodiorite (106.2–105.1 Ma). Folding of regional isograds took place during the short interval of time between peak M 3 metamorphism and formation of the batholiths (< 1 My). The small age difference between regional LP‐HT and contact metamorphism attests to a common magmatic cause and illustrates the short timescales over which overprinting relationships can develop in such settings.