New Mexico Geological Society Annual Spring Meeting — Abstracts

Ar-Ar dating of hornblende and muscovite separates from 47 samples from Proterozoic rocks of northern New Mexico documents a complex Mesoproterozoic thermal history for northern New Mexico. In most areas of northern New Mexico, hornb1endes give plateau ages of 1.43-1.35 Ga, indicating cooling through about 500°C at this time. This suggests that the well-documented regional uiple point metamorphism (500-550°C, 3-4 kbar) occurred at ca. 1.4 Ga. This metamorphism was superposed on previous Paleoproterozoic amphibolite grade rocks that were residing at 3-4 kbar (10-15 km), as suggested by: 1. disturbed hornblende Ar spectra (nearly one third of samples) that contain pre-1.4 Ga age increments and are interpreted to reflect partial resetting of Paleoproterozoic minerals; 2. numerous 1.4 Ga U-Pb mineral ages in high grade blocks; and 3. field studies of syntectonic 1.7-1.65 Ga plutons.

Aureoles of 1.4 Ga plutons that were emplaced at 3-4 kbar in the Sandia and Manzano Mountains show nearly concordant 1.4 Ga hornblende and muscovite dates, suggesting rapid cooling to ambient conditions of about 300°C at depths of 10-15 km. This corresponds to a 1.4 Ga geothermal gradient of about 25-30°C/km. Somewhat younger, 1.37-1.31 Ga, dates on muscovite throughout much of the region and on hornblende in high grade blocks could represent a second "event" associated with 1.35 Ga magmatism. However, no 1.35 plutons are known in New Mexico and spread of dates seems better explained by cooling due to erosional unroofing following 1.4 Ga plutonism. 1.1-0.9 Ga (Grenville) tectonism locally reset or perturbed muscovite ages and may record further (Grenville) unroofing of middle crustal rocks combined with variable reheating.

Different tectonic blocks record different apparent thermal histories suggesting differential Mesoproterozoic uplift and/or thermal histories. Blocks with the highest metamorphic grades (both P and T) have the youngest mineral ages for both hornblende and muscovite, suggesting that Ar resetting and resulting apparent ages are at least in part due to differential unroofing (1.4-1.0 Ga) of different levels of the middle crust with its Mesoproterozoic (post-1.4 Ga) thermal structure. Regional metamorphism at 1.4 Ga was probably driven by heat from fluid and magma advection into the crust due to upwelling of asthenosphere in southern Laurentia. This metamorphic event was regional in character and enhanced by plutons, but not restricted to pluton aureoles. Variable cooling ages suggest that the 1.4 Ga magmatic/thermal event initiated protracted regional (differential) denudation and cooling of middle crustal rocks of the Southwest.