Proterozoic geology of supracrustal and granitic rocks in the Caballo Mountains, southern New Meixco
— Paul W. Bauer and Richard P. Lozinsky

The western slope of the Caballo Mountains in southwestern New Mexico contains early Proterozoic supracrustal and plutonic rocks that are unconformably overlain by lower Paleozoic sedimentary rocks to the east, and are in fault contact with rift-related Cenozoic sediments to the west. Supracrustal rocks consist of biotite–hornblende–plagioclase amphibolites that probably represent metabasalts, and quartz–plagioclase–Kfeldspar– biotite gneisses that may represent metamorphosed felsic volcanic rocks. No primary igneous textures are preserved. Amphibolitic and fine-grained metaigneous xenoliths in the felsic gneiss suggest that felsites postdate amphibolites. Plutonic rocks include an older, coarse-grained, porphyritic biotite granodiorite in the east-central map area, and a younger, medium-grained pink granite in the northern and southern map areas. Both granitic bodies intrude, but are separated by, the supracrustal sequence.

All Precambrian plutonic and supracrustal rocks are penetratively deformed and have sustained amphibolitegrade regional metamorphism. Excellent evidence for three fabric-forming deformational events is recorded in supracrustal rocks. The earliest recognized deformation is represented by a strong schistosity and gneissic layering (S₁) in felsites. The second event tightly folded this fabric around steep, approximately east-trending axial surfaces in felsites. S₁ and S₂ are parallel or subparallel throughout the area. The final event is a nonpenetrative cross-folding that formed small crenulations in amphibolites. Although both plutonic bodies contain a penetrative foliation parallel to that in the country rock, and it is not known whether the granitic rocks preserve S₁, S₂, or both, several lines of evidence suggest pre-D, crystallization of the granodiorite.

Similar Proterozoic lithologies that have undergone similar deformational histories are exposed in nearby uplifts. Additional mapping and geochronology will be required to determine the manner in which these rocks relate to those of the Caballo Mountains.