The oldest Precambrian rocks in the area of study are represented by abundant inclusions and schlieren of hornblendite, plagioclase-hornblende schist, and plagioclase-hornblende-biotite schist. These inclusions are remnants of a pre-batholithic metamorphic terrane.
The next oldest, and dominant rock in the central Nacimiento Mountains is gneiss that ranges in composition from quartz monzonite to granodiorite. Where strongly sheared, the rock is microcline-biotite-quartz-plagioclase schist. Gradational zones of schist within the gneiss were mapped separately. The gneiss and the schist are cut by metamorphosed spessartite dikes and by leucocratic dikes of various textures and compositions.
Intruding the gneiss in the southeastern part of the mapped area is a red, fine-grained, non-gneissic rock that ranges in composition from granite to quartz monzonite. This is probably the youngest Precambrian rock exposed.
The Precambrian rocks of the Nacimiento Mountains were affected by two periods of major regional metamorphism. The earlier episode is now recognized only by the presence of the ma fie inclusions, which represent the almandine-amphibolite facies of regional metamorphism.
A second period of regional metamorphism after intrusion of a quartz monzonite to granodiorite pluton resulted in a strong northeast-trending lineation and foliation. This regional metamorphism, which formed the gneiss and schist, occurred before emplacement of the fine-grained red granite, and may correspond to the oldest of the three periods of Precambrian deformation described by Bingler (1965) in northern New Mexico.
The present structure of the Nacimiento Mountains is mainly due to Laramide deformation. The central block fits well into previous models of Laramide deformation (Baltz, 1967; Woodward and others, in preparation), but has behaved as a separate block, which was uplifted and tilted about 13° to the east. The uplift occurred along high-angle reverse faults on the west side.
Northeast-trending zones of weakness which developed during the Precambrian continued as zones of weakness through Laramide time. They are, .in places, clearly the directions followed by Laramide faults.
The mechanism of emplacement of the Precambrian batholith is not clear, as overlying Paleozoic strata obscure the relationships between gneiss and the original country rock, and internal structures do not clearly indicate the emplacement mechanism. The abundance of schlieren and mafic inclusions within the gneiss suggests, but does not prove, that piecemeal stoping and assimilation were important. The younger fine-grained red granite truncates gneissic structure and contains inclusions of the gneiss. It may have been emplaced by a combination of piecemeal stoping and forceful intrusion (dilation).
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Lee Albert Woodward
Second Committee Member
J. Paul Fitzsimmons
Third Committee Member
Albert Masakiyo Kudo
Reed, Kent Richard. "Precambrian Geology of the Central Nacimiento Mountains, Sandoval County, New Mexico." (1971). https://digitalrepository.unm.edu/eps_etds/292