BASEMENT EXHUMATION, FAULT REACTIVATION, AND K-METASOMATISM IN THE SOUTHERN SANGRE DE CRISTO RANGE, NEW MEXICO: 40AR/39AR INSIGHTS INTO NEOPROTEROZOIC TECTONISM AND CRUSTAL FLUID FLOW

⁴⁰Ar/³⁹Ar thermochronology of metamorphic and plutonic K-feldspar from crystalline rocks between Santa Fe and Las Vegas, NM indicates the southern Sangre de Cristo basement has experienced a common thermal history. In general, ⁴⁰Ar/³⁹Ar age spectra are dominated by Neoproterozoic apparent ages that record cooling from ~300 to 150°C between 1000 and 800 Ma. This is interpreted as basement exhumation from ~10 km depth to a few kilometers or less. In contrast, K-feldspar in the subsurface of the Las Vegas basin, east of the range-bounding Montezuma fault zone, suggest later exhumation between ca. 700 and 500 Ma. Discrepancies between basement cooling histories from opposing sides of the Montezuma fault confirm a Proterozoic ancestry of this structure, and indicate the fault was active during Grenville tectonism and was reactivated during late Neoproterozoic Rodinia rifting. Detailed thermochronology of metasomatic K-feldspar in the Pecos River Valley corroborates a complex history of Neoproterozoic brittle deformation and associated hydrothermal fluid flow. Amphibolite, tonalite, and granodiorite lithologies are crosscut by K-feldspar ± epidote veins, exhibit in situ K-feldspar replacement of sodic plagioclase, and are locally brecciated and strongly metasomatized. Metasomatic K-feldspar yield complex age spectra that support at least three punctuated K-feldspar growth events before ~1100 Ma, between ~850 and 900 Ma, and ~410 Ma. K-feldspar from N-NE trending breccia zones comprises the 850-900 Ma population constraining brecciation and associated alteration at this time. U-Pb epidote geochronology of one sample indicates the presence of two generations of epidote with dates of 1197±130 Ma and 720±80 Ma that overlap temporally with K-feldspar dates. Metasomatism between ~850 and 900 Ma coincides with exhumation of the Sangre de Cristo block and provides evidence for a link between regional tectonism and localized faulting and fluid flow during the Neoproterozoic.