Tectonic domains and exhumation history of the Omineca Belt in southeastern British Columbia from 40Ar/39Ar thermochronology

A large geochronological data set comprising Ar-40/Ar-39 and K-Ar (hornblende, muscovite, biotite, and K-feldspar), Rb-Sr (muscovite), fission track (zircon and apatite) and U-Pb (zircon and monazite) dates has been compiled for the southern Kootenay Arc and western Purcell anticlinorium in the Omineca Belt of the Canadian Cordillera in southeastern British Columbia. New Ar-40/Ar-39 data for hornblende, muscovite, biotite, and alkali feldspar are presented and combined with data from other studies. We integrate these data with recent advances in the geology of the region to define three partially fault-bounded domains with differing geological and exhumation histories, here termed the western, central, and eastern domains. The western domain is characterized by (1) late synkinematic Jurassic plutons with hornblende, muscovite, and biotite Ar-40/Ar-39 plateau dates between 170 and 165 Ma, some of which are within error of the U-Pb zircon dates for these plutons, and (2) late Early Cretaceous (118-102 Ma) plutons commonly with concordant mica Ar-40/Ar-39 plateau dates of a similar age range, indicating rapid cooling following emplacement of both suites. The central domain is bounded by regional-scale normal faults (Gallagher and Midge Creek faults, Blazed Creek/Next Creek faults, and Purcell Trench fault) and contains superposed Early and Late Cretaceous zones of Barrovian metamorphic rocks and several mid- to Late Cretaceous, post-kinematic plutons. The transition from the western domain into the central domain is characterized by Ar-40/Ar-39 mica age spectra showing a progression of increasing thermal overprinting. Along the north-south length of the central domain, biotite and muscovite yield Paleocene to Eocene K-Ar and Ar-40/Ar-39 plateau dates between 66 and 40 Ma. The eastern domain consists of (1) a southern portion that occurs in the hanging wall of the Purcell Trench fault, comprising mid-Cretaceous intrusions of the Bayonne magmatic suite emplaced into biotite zone metasedimentary rocks of the Mesoproterozoic Belt-Purcell Supergroup in the western Purcell anticlinorium, and (2) a northern portion that shows a continuous transition with the northern part of the central domain north of the terminus of the Purcell Trench fault. Cretaceous igneous rocks in the southern portion of the eastern and western domains have Ar-40/Ar-39 mica plateau dates that are <9 Myr younger than U-Pb zircon dates, indicating rapid cooling shortly after emplacement Ar-40/Ar-39 step-heating reveals that there was a mid- to Late Cretaceous thermal disturbance in the eastern domain, possibly related to emplacement of younger plutons at deeper crustal levels and the Late Cretaceous Barrovian metamorphic event recorded in rocks of the central domain, such that biotite with dates 更多