New Mexico Geological Society Annual Spring Meeting — Abstracts

Precambrian amphibolite, tonalite, and granodiorite basement lithologies in the southern Sangre de Cristo Range, New Mexico are extensively K-metasomatized, crosscut by Kfeldspar + epidote veinlets, and exhibit microtextures indicative of pervasive in situ replacement of primary igneous Na-plagioclase by K-feldspar + epidote. ⁴⁰Ar/³⁹Ar thermochronology of metasomatic K-feldspar in the Pecos River valley yields complex age spectra with a diversity of apparent ages ranging from ~420 Ma to older than 1100 Ma that could suggest several discrete metasomatic episodes. Interestingly, coexisting plutonic microcline and metasomatic K-feldspar do not yield similar age spectra or apparent ages. In fact, K-feldspar from one highly metasomatized 100 m² outcrop preserve the regional postGrenville (ca. 1100 Ma) cooling signature similar to unaltered samples across the Sangre de Cristo Range while metasomatic K-feldspar exhibit undualatory behavior and total gas ages of ca. 800 Ma, 700 Ma, 600 Ma, and 420 Ma. The ability of the K-feldspar to retain distinct apparent ages and argon distributions despite their proximity does not favor a high temperature (>200°C) regional event during the Paleozoic. In another metasomatized location, older metasomatic ages may reflect locally higher temperature fluids, as K-feldspar from a ca. 1.4 Ga pegmatite is completely reset to an apparent age of ca. 800 Ma. The alternative, which is preferable given the complications of sustaining a hydrothermal system with discrete high and low temperature regimes on the meter scale, is that the intense chemical environment during metasomatism is responsible for partial to complete resetting of older metasomatic and igneous K-feldspar. This research addresses the ability to interpret Neoproterozoic K-feldspar growth ages from hydrothermally and chemically induced argon loss in older K-feldspar. Distinction between these mechanisms is critical in order to reconcile accurate geologic thermal histories while recognizing the effects of postcrystallization chemical and/or hydrothermal alteration.