New Mexico Geological Society Annual Spring Meeting — Abstracts

The Zuni Mountains offer a rare Precambrian exposure into both the Jemez Lineament and the basement of the Colorado Plateau. Proterozoic rocks of the Zuni Mountains are dominantly metaigneous and can be divided into 3 suites: 1) tonalite/granodiorite with mafic enclaves, 2) peridotite that is either a crustal cumulate or an exotic slice of mantle, 3) quartz monzonite and rhyolite that represent shallow-level magmatism at ca. 1.65 Ga (Bowring & Condie, 1982). The peridotite occurs as an elongate lens with sheared margins that was metamorphosed and hydrothermally altered. The sheared margins of the body and the absence of associated mafic rocks imply tectonic emplacement prior to intrusion by the quartz monzonite although distance of transport is unconstrained.

The Zuni Mountains have a remarkable history of igneous and tectonic activity: Arc magmatism (likely 1.75-1.7 Ga), tectonic emplacement of the sheared peridotites, and the intrusion 1.65 Ga granites and rhyolites was followed by a younger shearing event. This shear zone trends WNW for ~10km, dips 60° south and bends to the NNE with a SW -plunging mineral elongation lineation. Shear sense is extensional (top to the SW) often with a dextral component and occurred at temperatures between 400 and 500°C. An unfoliated, megacrystic granite (likely 1.4 Ga) intrudes this package and is bordered by diabase dikes (likely 1.1 Ga) along NW- trending faults. Cambrian syenites and flourospar veins (age unknown) are also present. The area was a NW -trending uplift during the Ancestral Rocky Mountain orogeny and during the Laramide orogeny, indicating long-term Phanerozoic buoyancy. Cenozoic volcanism produced the Mt. Taylor volcanics and the Zuni-Bandera basaltic fields (that contain mantle xenoliths).
Both belong to the NE-trending Jemez Lineament that is underlain by low velocity, (hot) mantle. Documentation of the Zuni Mountain's history may lead to better understanding of long-lived lithospheric-scale structures and their influence on igneous and tectonic activity.