TIMING OF EXHUMATION, DEFORMATION AND FLUID-FLOW PRESERVED IN MICROTEXTURALLY COMPLEX K-FELDSPAR, SANGRE DE CRISTO RANGE, NEW MEXICO

⁴⁰Ar/³⁹Ar K-feldspar thermal histories and U-Pb dates of hydrothermal epidote veins from Proterozoic basement rocks in the southern Sangre de Cristo Range, New Mexico, indicate Grenville-age exhumation and related brittle deformation and fluid flow, and a Neoproterozoic (ca. 800 Ma) metasomatic event. Delineation of the timing of these processes has important implications for the evolution of the Western USA lithosphere.   ⁴⁰Ar/³⁹Ar step heating analyses of K-feldspar yield maximum apparent ages of ~1050 Ma interpreted as the onset of regional cooling through 300°C.  Cooling, interpreted as exhumation at this time, is supported by U-Pb epidote analyses that date fracturing and fluid flow at 1123±140 Ma. SEM, BSE, and SEM-CL imaging of pegmatitic K-feldspar megacrysts reveals that portions of these grains have experienced an aqueous dissolution-reprecipitation reaction resulting in discrete areas of original clear film perthite and secondary pink patch perthite. Single subgrains of film perthite from two pegmatites yield total gas ages of 956±4 and 969±4 Ma, and 1065±4 and 1036±4 Ma, respectively.  Coexisting patch perthite yield significantly younger ages of 761±3 and 804±3 Ma, and 835±3 Ma, respectively.  While film perthite age spectra and total gas ages are similar to numerous other K-feldspars from the region, the ca. 800 Ma ages for patch perthite subgrains are restricted to the Pecos Valley.  These young total gas ages are interpreted as new K-feldspar growth ages resulting from Neoproterozoic alkali metasomatism likely related to deep circulating crustal fluids during Rodinia rifting. The chemical dissolution of film perthite and reprecipitation of patch perthite occurred at a temperature that did not cause significant thermally induced ⁴⁰Ar* loss from the film perthite, thereby preserving two distinct age regimes within a single megacryst.  These observations reveal the power and requirement of linking detailed petrology and microsampling for argon analyses when dealing with these complex basement rocks. Also, the preservation of the discrete age domains related to two discrete geological processes exemplifies the utility of K-feldspar as a geo and thermochronometer for these complex ancient rocks.