U–Pb isotopic constraints on diachronous metamorphism in the northern Monashee complex, southern Canadian Cordillera

U–Pb isotopic data from the northern Monashee complex, one of the deepest structural exposures in the southern Canadian Cordillera, indicate that the age of metamorphism varies according to structural position in a 6 km thick section. This metamorphism resulted in an unusual sequence in which rocks with the lowest-grade mineral assemblage (kyanite–sillimanite–staurolite–muscovite) are underlain and overlain by higher-grade rocks. Xenotime and monazite U–Pb dates vary progressively from 64 Ma in the structurally highest rocks to 49 Ma in the deepest rocks. Discordant U–Pb ages from Proterozoic and Cretaceous monazite and titanite are used to interpret the thermal significance of the early Tertiary dates. The discordant analyses define linear arrays with lower intercepts that broadly overlap with early Tertiary, and the amount of discordance varies with structural level; it is least in the deeper rocks and greatest in higher rocks. Electron microprobe work showed that the monazite discordance in the deeper rocks resulted from Tertiary mineral overgrowth and recrystallization rather than Pb diffusion. We use previous studies of Pb diffusion and the fact that Proterozoic monazite and titanite suffered only negligible to moderate amounts of diffusive Pb loss to contend that elevated temperatures (c. 600–650 °C are inferred from pelitic mineral assemblages) existed in the deeper rocks for a short duration, perhaps a few million years. The downwards younging 64–49 Ma U–Pb dates are interpreted as closely reflecting xenotime and monazite growth ages rather than cooling ages or substantially reset ages based on the lack of Pb diffusion in monazite and the previously obtained ⁴⁰Ar/³⁹Ar data which suggest that rapid cooling occurred immediately after the U–Pb dates. In addition, growth ages are interpreted as thermal peak ages based on U–Pb dates from coeval kyanite-bearing leucosomes, the consistent nature of the U–Pb dates throughout the study area, and petrographic relationships which suggest that monazite grew before or during development of the syn-metamorphic foliation. These interpretations lead us to conclude that metamorphism was diachronous according to structural level, with higher rocks attaining peak temperatures and cooling rapidly while deeper rocks were heating towards a thermal peak that was attained a few million years later. This thermal scenario requires that higher rocks cannot have been the heat source for the deeper metamorphism, as was previously proposed.