150 Myr of Episodic Metamorphism Recorded in the Yukon-Tanana Terrane, Northern Canadian Cordillera: Evidence from Monazite and Xenotime Petrochronology

Abstract

AbstractWe used in situ laser-ablation split-stream petrochronology to target monazite and xenotime associated with specific metamorphic index minerals in the Florence Range metamorphic suite in the northern Canadian Cordillera. Based on this petrochronological approach, we reconstruct the polyphase metamorphic evolution of the Yukon-Tanana terrane in northwestern British Columbia. Our new data reveal a complex P–T–t path, requiring three metamorphic episodes and passing through the kyanite, sillimanite, and andalusite stability fields over 150 Myr from the Permian to the Early Cretaceous. The earliest preserved metamorphic event is cryptic, reaching the kyanite stability field (>600°C, >0.6 GPa) at ca. 270–240 Ma followed by retrograde metamorphism at ca. 240–215 Ma. Renewed garnet growth occurred during a second prograde metamorphic event at ca. 195–185 Ma. Garnet breakdown, possibly linked to decompression and/or cooling in the suprasolidus stability field of sillimanite and K-feldspar (>675°C, <0.8 GPa), is well-documented at ca. 185–170 Ma. Finally, a third metamorphic episode is characterized by the local growth of andalusite, then cordierite, and reached >600°C below 0.3–0.4 GPa after ca. 120 Ma. This study illustrates the importance of analyzing petrochronometers in their textural context, especially inclusions in porphyroblasts, and the complementary P–T–t information that can be extracted from monazite and xenotime. Our new data are compatible with two separate contractional events involving the Yukon-Tanana terrane during the Permian–Triassic and Late Triassic to Early Jurassic, followed by Cretaceous contact metamorphism.