EXHUMING PRECAMBRIAN BASEMENT: PARSING THERMAL AND ALTERATION HISTORIES FROM 40AR/39AR K-FELDSPAR DATA

Determining timing and rate of mid-crustal Precambrian rock exhumation relies heavily on thermochronology data. High temperature data between 500°C and 300°C describes early stages of cooling between 1.4 and 1.2 Ga, whereas low temperature data (i.e. AFT – 110°C) reveals exhumation following Mesozoic-Cenozoic burial. The intervening billion year history is best recorded by ⁴⁰Ar/³⁹Ar K-feldspar thermochronology because it has the potential to reveal a broad temperature range between 300 and 150°C. Regional K-feldspar thermochronological efforts from New Mexico 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 may 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 diverge from 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. Modern, high sensitivity noble gas mass spectrometers allow tiny (<0.1 mg) K-feldspar fragments to be dated for ⁴⁰Ar/³⁹Ar thermochronology. Individual fragments from larger K-feldspar crystals from the 1.4 Ga Sandia granite located near Albuquerque, NM yield widely ranging ⁴⁰Ar/³⁹Ar age spectra and bulk ages. Bulk ages are typically between 700 and 1200 Ma with the majority of individual spectra yielding age gradients between 800 and 1100 Ma. In many cases thermal histories from individual fragments are not compatible, suggesting recrystallization. Microprobe imaging reveal variation in microtextures, but predicting young versus old related to a discrete microtextures has not been accomplished. This talk will reveal the challenges of linking microtexture to argon thermal history and demonstrates the vast potential for revealing both thermal and fluid histories as stages in the exhumation path of mid-crustal rocks.