Cooling and exhumation of the Shuswap Metamorphic Core Complex constrained by 40Ar/39Ar thermochronology

摘要

The exhumation history of the Shuswap Metamorphic Core Complex (Shuswap MCC) is deciphered from structural analysis and 40Ar/39Ar thermochronology. With the exception of a few samples contaminated by excess argon, the analytical results indicate a consistent range of early Tertiary 40Ar/39Ar ages throughout the area. Hornblende yields ages ranging from 59–54 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 40Ar/39Ar ages, combined with existing U/Pb ages on zircons and monazites, 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 (700 °C to 300 °C), from crystallization of zircons through closure of argon diffusion in hornblende and micas, between 56 and 48 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 350 °C to 150 °C and indicates a period of thermal stability from 48 to 45 Ma, and a cooling event at 45 Ma, except for the K-feldspars sampled in the immediate footwall of the Columbia River fault, which indicate rapid cooling at 33 Ma. These cooling events are attributed to exhumation accommodated by local activation of high-angle normal faults.