The Zuni Mountains of western New Mexico offer a rare view into the Precambrian basement of both the Jemez Lineament and the Colorado Plateau. Proterozoic rocks of the Zuni Mountains are dominantly metaigneous and can be divided into three suites: 1) peridotite that is either a crustal cumulate or an exotic slice of mantle, 2) tonalite/granodiorite with mafic enclaves, 3) quartz monzonite and rhyolite that represent shallow-level magmatism at ca. 1.65 Ga. The peridotite exists in an elongate lens and several discrete pods that were metamorphosed and hydrothermally altered to the assemblage serpentine – hornblende – chlorite – talc – dolomite – actinolite – spinel. The absence of associated mafic rocks implied tectonic emplacement prior to intrusion by the quartz monzonite, although distance of transport is unconstrained. The 1.65 Ga suite has a calc-alkaline trend and is interpreted to be part of an arc complex. Tectonic emplacement of the peridotite, arc magmatism, and the intrusion of arc-related 1.65 granites and rhyolites were followed by a younger shearing event. A 10 km long shear zone trends WNW, dips 60 degrees south and has a SW-plunging mineral elongation lineation. Shear sense is extensional (top-to-the-SW), typically with a dextral component. Microstructural data and mineral assemblages suggest that shearing occurred at temperatures between 400 and 500 degrees Celcius. Timing of movement of the shear zone is estimated to be 1440 Ma based on Ar/Ar geochronologic data. An unfoliated, megacrystic granite (likely 1.4 Ga) intrudes this package and is bordered by diabase dikes along NW-trending faults. The diabase yields an Ar/Ar whole rock age of 1100 Ma, which is interpreted to be the time of emplacement. Syenites that were previously thought to be Cambrian are at least 1100 Ma. Fluorspar veins (age unknown) are also present. The area was a NW-tending uplift during the Ancestral Rockies Orogeny and during the Laramide Orogeny, indicating long-term Phanerozoic buoyancy. Cenozoic volcanism produced the Mt. Taylor volcanics and the Zuni-Bandera basaltic fields. Both belong to the NE-trending Jemez Lineament that is underlain by low velocity, presumably 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. A tentative conclusion is that the igneous activity that took place at 1.4 Ga (?) and during the Cenozoic was controlled by a NE-trending structural grain that is interpreted to be a deeply rooted lithospheric anisotropy. This NE-trend is not well expressed in the upper crust; 1.65/1.4 Ga foliation patterns are NW and NE-trending, 1.1 Ga faults, diabase dikes, and Paleozoic uplifts were NW-trending, and Cenozoic volcanism was NE-trending.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Proterozoic evolution, Proterozoic, Zuni Mountains, Shear zone, Peridotite, granite, rhyolite
Strickland, Diana. "Proterozoic evolution of the Zuni Mountains, New Mexico: Peridotites, granites, rhyolites, shear zones, and a reactivation history through the present." (2000). https://digitalrepository.unm.edu/eps_etds/84