The northern Monashee Mountains lie at the northern termination of a north-south zone of Eocene extension in the southern Omineca Crystalline Belt of British Columbia. Isotopic, structural, and petrogenetic studies in the northern Monashee Mountains document middle to Late Cretaceous regional metamorphism, two episodes of crustal anatexis, and a period of rapid cooling coincident with middle Eocene extension. Partial melting of geochemically and isotopically similar pelitic sources produced peraluminous granites during and postdating regional metamorphism at 100 Ma and 63 Ma, respectively. Rb-Sr muscovite-whole-rock isochrons from the younger granites give an age of 62.5 ±2.0 Ma. Rb-Sr isochron and U-Pb mineral ages are similar, indicating that the 63 Ma granites were emplaced into a terrane that had cooled to 500 °C or less by 63 Ma. Hornblende, muscovite, and biotite K-Ar and apatite fission-track mineral cooling ages from the northern Monashee Mountains are are 76.3 ±5.8 (n = 7), 51.4 ±3.5 (n = 6), 55.1 ± 2.7 (n = 10), and 45.0 ±2.9 (n = 3) Ma, respectively (mean age and standard deviation). Accelerated cooling, beginning with the emplacement of 63 Ma granites and ending at 45 Ma, occurred in response to ramping along a crustal-scale thrust fault followed by offset on the middle Eocene offset on the middle Eocene North Thompson normal fault.

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