New Mexico Geological Society Annual Spring Meeting — Abstracts

Detailed structural mapping and microstructure studies indicate rocks in the Capilla Peak area of the Manzano Mountains, central New Mexico, are multiply deformed. Thermochronologic studies have been focused on minerals from the Blue Springs schist and the microstructurally complex amphibolite layers within the SevilIeta metarhyolite. Inclusion-rich anhedral actinolitic amphiboles are overgrown and are crosscut by foliation-forming blue-green amphiboles. The blue-green amphiboles define a strong tectonic foliation, the youngest fabric in the amphibolite, which is parallel to the regional foliation trend. Our studies show that the ductile deformation and associated mineral growth that formed this foliation overprint an older, probably ca. 1650 Ma ductile fabric.

⁴⁰Ar/³⁹Ar geochronologic analyses on multiple minerals were conducted to constrain the timing of the observed deformation events. Along with a bulk amphibole concentrate, an actinolitic amphibole and a blue-green amphibole were individually separated from amphibolite samples ML 10-8 and ML 14-18. The anhedral actinolitic amphiboles and bulk separates yield complex ⁴⁰Ar/³⁹Ar age spectra characterized by age gradients increasing from ~200 Ma to 1600 Ma. These samples have total gas ages ranging from ~1110-1290 Ma. The euhedraI blue-green amphiboles have less complex spectra and overall older ages. In particular, ML 14-18 blule-green amphibole is less plagued by young apparent ages and yields a plateau age for 80% of the gas release of 1406 ± 16 Ma (2σ). Based on this result, it is proposed that temperatures at ca. 1400 Ma were sufficiently high to reset the actinoIitic amphiboles with respect to argon (at least 400°C), and this blue-green amphibole grew ca. 1400 Ma.

Six muscovites from the Blue Springs schist and two biotites from amphibolite layers within the Sevilleta metarhyolite also were separated and analyzed. The muscovite separates show variable age spectra with age gradients ranging from ~200 Ma to 1400 Ma. The muscovite samples were collected over a vertical section of greater than 1.5 km (35-50°C, assuming a geotherm of 25°C/km). Assuming samples within the traverse have similar argon closure temperatures, apparent ages can be used to estimate uplift and/or cooling rates. For instance, the structurally deepest sample yields an age of ~1320 Ma, whereas the structurally highest sample yields an age of ~1380 Ma. This 60 Ma age discordance thus cooresponds to an average cooling rate of (0.3-1)°C/Ma.

Biotite from ML 10-8 yields a plateau age of 1277±8 Ma whereas ML 6-10 biotite gives an age of ~1400 Ma. This age discordance probably reflects variation in closure temperature coupled with apparent slow cooling since the two samples are located at the same structural level.

Taken together, the geochronologic and structural data are interpreted to support a model where regional deformation, metamorphism and mineral growth occurred at ca. 1400 Ma. These data suggest that regional temperatures of at least 400°C existed ca. 1400 Ma followed by a period of protracted cooling. The deformation associated with this metamorphism is clearly an event not confined to the margins of 1400 Ma Priest pluton the Manzano mountains.