K-FELDSPAR 40AR/39AR THERMOCHRONOLOGY OF WESTERN USA PRECAMBRIAN LITHOSPHERE: IMPROVED UNDERSTANDING OF SYSTEMATICS TOWARDS MORE ACCURATE GEOLOGICAL MODELS

Thermochronology is a key component of multi-disciplinary efforts to evaluate lithospheric evolution. ⁴⁰Ar/³⁹Ar K-feldspar thermochronology is sensitive to the temperature range between 300 and 150°C and therefore has the potential to document the timing and rates of exhumation of the Western USA Precambrian basement.  In general, regional K-feldspar thermochronological efforts from the Rocky Mountains support Grenville-aged denudation (ca. 5 km) that is associated with formation of new structures and/or reactivation of existing crustal weaknesses. Younger ages of ca. 800 Ma record renewed exhumation and/or discrete fluid flow events perhaps related to fault activity. Accurate tectonic interpretation of the ⁴⁰Ar/³⁹Ar results is highly dependent upon adherence to simple volume diffusion of ⁴⁰Ar* under conditions of slow-cooling or discrete episodic thermal events. Several K-feldspars break from the regional age patterns and yield highly complex age spectra or apparent ages that are contradictory between individual data sets thus suggesting either more complex local thermal histories or microtextural modification of the K-feldspars following initial argon closure. In order to better determine if or when K-feldspar are providing accurate thermal histories we have undertaken a program of BSE, SEM and CL analyses along with microsampling of subgrains for argon thermochronology to better link microtextural evolution to argon behavior. In so doing we have determined that multiple generations of K-feldspars may exist in many Rocky Mountain Precambrian rocks and simple extraction of thermal histories under a model that only considers age variation related to temperature variation requires more scrutiny. By fully utilizing a combined approach of sample characterization and microsampling methods we are poised to more fully exploit the potential of K-feldspar thermochronology towards refining geological models for western USA lithospheric evolution.