New Mexico Geological Society Annual Spring Meeting — Abstracts

Precambrian rocks of the Tusas Mountains of northern New Hexico can be divided into two main Iithologic units. The older unit ranges from schistose quartzite with interlayered metarhyolite, amphibolite, pelite, and metaconglomerate to a section dominated by mafic volcanics. The younger unit is a masslve cross-bedded quartzite ranging from several hundred to more than a thousand meters in exposed thickness. It is proposed that these two units correlate with the Vadito Group and Ortage Quartzite of the Ortage Group as mapped in the Picuris, Truchas, and Rio Mora uplifts. Rocks in the Tusas Mountains show evidence of at least two deformations with a minor overprint of one or more younger generations of folds.

Viridine, Mn-andalusite, occurs in a 50 to 100m-thick horizon at or slightly above the base of the massive Ortega Quartzite throughout the Tusas Mountains. This horizon probably represents a continuation of the Mn-rich layer that occurs just below the Ortega Quartzite in the Picuris and Rio Mora uplifts. The presence of the viridine in the Tusas supports the proposed stratigraphic correlation within the range and with the other Precambrian uplifts of northern New Mexico.

Kyanite, andalusite, and sillimanite occur in the Ortega Quartzite in the Tusas Mountains. Kyanite is the most common Al₂SiO₅ polymorph. Sillimanite occurs alone and with kyanite in the southern parts of the range whereas andalusite coexists with kyanite in several samples from the northern part of the range. The relative distribution of kyanite and sillimanite suggests that isograds may be subhorizontal, planar surfaces. similar to those described in the Picuris, Truchas, and Rio Mora uplifts. Such a geometry would constrain metamorphic conditions throughout the range to be not far removed from the aluminum silicate triple point at approximately 500°C and 3.75 kb. The Tusas Mountains, therefore, further increase the unusually large area of relatively constant metamorphic conditions in northern New Mexico.