Cooling and exhumation of the shuswap metamorphic core complex constrained by ~(40)Ar/~(39)Ar thermochronology

摘要

The exhumation history of the Shuswap Metamorphic Core Complex (Shuswap MCC) is deciphered from structural analysis and ~(40)Ar/~(39)Ar thermochronology. With the exception of a few samples contaminated by excess argon, the analytical results indicate a consistent range of early Tertiary ~(40)Ar/~(39)Ar ages throughout the area. Hornblende yields ages ranging from approx59-54 Ma, muscovite and biotite ages are clustered between 49.5 and 48 Ma, and are consistent with the timing of closure of K-feldspars, typically ranging from 50 to 43 Ma, except for samples in the immediate footwall of the Columbia River fault that yield ages as young as 26 Ma. These ~(40)Ar/~(39)Ar ages, combined with existing U/Pb ages on zircons constrain the cooling history at several localities in the Shuswap MCC between Trans Canada Highway 1 and the Thor- Odin dome. The migmatitic core of the complex is affected by a rapid cooling event (approx 700 deg C to approx 300 deg C), from crystallization of zircons through closure of argon diffusion in hornblende and micas,between approx 56 and approx48 Ma. Based on our previous structural analysis, we propose that this first period of rapid cooling follows an exhumation event associated with the formation of the Shuswap MCC by activation of a low-angle detachment and ductile thinning of a previously thickened and partially molten crust. Multidiffusion-domain thermal modeling of K-feldspar constrains the lower temperature history from approx 350 deg C to 150 deg C and indicates a period of thermal stability from approx48 to approx45 Ma, and a cooling event at approx 45 Ma, except for the K-feldspars sampled in the immediate footwall of the Columbia River fault, which indicate rapid cooling at approx 33 Ma. These cooling events are attributed to exhumation accommodated by local activation of high-angle normal faults. Therefore, we distinguish two periods of exhumation in the Canadian Cordillera: (1) Exhumation in Paleocene time related to crustal thinning and tectonic denudation by activation of detachment zones at approx60-50 Ma, which corresponds to gravitational collapse of the thickened and partially molten Cordilleran crust; and (2) Exhumation related to Eocene regional extension, Basin-and-Range style, at approx 45 Ma, with reactivation of the eastern boundary fault of the core complex, the Columbia River fault, in Oligocene time at approx 33 Ma.