Isotopic constraints on timing of deformation and metamorphism in the Thor–Odin dome, Monashee Complex, southeastern British Columbia.

Abstract

New and existing U–Pb and [superscript 40]Ar/[superscript 39]Ar geochronological data, and oxygen and hydrogen stable isotope data, are combined with structural and metamorphic data from Thor–Odin, the southern culmination of the Monashee Complex. This leads to a new interpretation of the timing of deformation and metamorphism. Amphibolites in Thor–Odin with hornblende [superscript 40]Ar/[superscript 39]Ar dates between ~75–70 and ~51 Ma experienced more [superscript 18]O- and D-depletion than amphibolites with older dates. The younger dates that were previously interpreted as cooling ages, may have resulted from complete or partial Ar loss in the presence of meteoric fluids that were introduced into the rock during extension. Monazite crystals in pelitic schist, quartzite and orthogneiss, which have U–Pb ages younger than [superscript 40]Ar/[superscript 39]Ar hornblende ages in amphibolite in northwest Thor–Odin, may have grown during extension in the presence of fluids. Titanite, xenotime and zircon dates may be interpreted in the same way. Thus, the U–Pb dates that were previously interpreted as representing peak of metamorphism and the hornblende [superscript 40]Ar/[superscript 39]Ar dates that were previously interpreted as representing cooling ages, may be interpreted as reflecting meteoric fluid penetration of the crust during regional extension. This implies that the age of the thermal peak of metamorphism is older than ~75–70 Ma. Migmatisation in a basement orthogneiss in Thor–Odin occurred at ~1.8 Ga. Dissolution rims are preserved in zircon between ~1.8 Ga domains and 52 Ma overgrowths. Because growth of new zircon (and possibly other U–Pb accessory phases) did not take place, any geological event that occurred during the ~1.8 Ga to 52 Ma time interval is not recorded. Cordilleran deformation and metamorphism may have taken place within that time interval, e.g. in the Middle Jurassic and/or mid- to Late Cretaceous, the time of Cordilleran deformation and metamorphism in the rocks overlying the Monashee Complex. The Joss Mountain orthogneiss, west of the Monashee Complex in the Selkirk Allochthon, is dated at 362 ± 13 Ma. F[subscript 3] folding in pelitic schist at Joss Mountain is constrained between ~73 and ~70 Ma. Existing structural, metamorphic and geochronological data in, and close to, the Shuswap Metamorphic Complex in the southern Canadian Cordillera are shown to be consistent with a channel flow model.