U/PB GEOCHRONOLOGY AND STRUCTURAL ANALYSIS OF PROTEROZOIC ROCKS IN THE SIERRA NACIMIENTO REGION, NORTHWESTERN NEW MEXICO

The Sierra Nacimiento and the San Pedro Mountains contain two large Laramide basement uplifts, both bounded on the west by the north-striking, west-directed Nacimiento reverse fault. New SHRIMP mean ²⁰⁷Pb/²⁰⁶Pb zircon ages show that the oldest rocks were significantly metamorphosed and intruded about 1,700-1,690 Ma, and were again intruded by granitoid rocks during a second magmatic event about 1,450-1,425 Ma. These ages are similar to those of magmatic/metamorphic events observed along the southern boundary of a broad transition zone between the Proterozoic Yavapai and Mazatzal crustal provinces of the Western U.S. In the Sierra Nacimiento uplift, the oldest basement rocks comprise metatuff (1,687±22 Ma), micaceous quartzite (1,697±10 Ma on detrital zircons), and minor mafic metavolcanic rocks as inliers and roof pendants in a moderately to strongly foliated orthogneiss (1,695±14 Ma) that is correlated with the San Miguel gneiss. The orthogneiss is approximately coeval with a relatively undeformed, pink, coarse-grained, locally flow-foliated monzogranite (1,696±10 Ma) that contains distinctive purplish-gray quartz and crops out extensively in the San Pedro Mountains uplift. In the southwest Sierra Nacimiento, the San Miguel-equivalent orthogneiss is intruded by a dark-gray, locally foliated, coarse-grained granodiorite (1,453±9 Ma) that is, in turn, intruded by a coarse-grained, pink, mostly porphyritic, massive to strongly flow- and shear-foliated biotite-microcline-plagioclase monzogranite (1,447±8 Ma). This monzogranite is widespread in the southern Sierra Nacimiento (mapped as “gneiss” in the Gilman quadrangle by Woodward and others, 1977). The last major intrusion was the regionally extensive Joaquin granite (1,424±9 Ma), which was intruded by small, undated plutons of leucogranite and muscovite granite. Foliations in gneissic rocks of both the ~1,700- and ~1,400-Ma suites strike consistently about N70ºE and dip about 60ºS, with a locally strong, down-dip lineation, interpreted as due to contractional shearing. The foliation is approximately parallel to the mylonitic foliation in the Nacimiento Creek shear zone (NCSZ) in the southern San Pedro Mountains, which is on trend with the regional Jemez lineament. However, the NCSZ has a nearly horizontal linear fabric with dextral shear sense.