The Maimon Formation consists largely of metavolcanic rocks with a small proportion of metasediments. Specific premetamorphic lithologic units that have been identified by a combination of field mapping, petrographic study, and chemical analyses include metabasalt (pyroxene-plagioclase and related mafic plagioclase porphyry), metadacite/metarhyolite (quartz porphyries), undivided metavolcanic rocks (largely plagioclase porphyry; probably metaandesite), and metasediments (including limestone [now marble], iron formation, and carbonaceous shale [now slate]). Although flows are common in the Maimon Formation, agglomerates and tuffs are also present. Rock types and textures suggest that most of the Maimon Formation volcanic rocks were emplaced in a submarine environment and that water depths increased to the southeast. Many rocks have undergone both seafloor and regional metamorphism, and much of the formation consists of the assemblage epidote-chlorite-albite. Less deformed zones throughout the Maimon Formation preserve remnant phenocrysts, and remnant pillows and amygdules are present in the northern part. Foliation, which apparently resulted from regional metamorphism, is widespread but is best developed in the northwestern part of the Maimon Formation, where actinolite is part of the metamorphic assemblage. The Maimon Formation is part of a belt of low-grade metamorphic rocks, including the Amina Schist, that crops out along the northern margin of the Cordillera Central. Whereas chemical compositions of the Maimon Formation support the interpretation that it consists largely of metavolcanic rocks that were altered by seafloor metamorphism, published chemical analyses for the Amina Schist are richer in CaO and could represent volcaniclastic metasediments. Neither the Maimon nor the Amina chemical analyses are as strongly bimodal as the Los Ranchos Formation, with which correlation has been suggested, and the types of mineral deposits in these formations also differ significantly. If these three units are coeval, they must have formed in different environments, with the Los Ranchos Formation forming on a seamount or other elevated seafloor, the Maimon Formation forming in a more typical oceanic island-arc setting, and the Amina Schists forming from detritus derived from these or other rocks.

Precambrian supracrustal rocks of the Cimarron Canyon area consist of quartzite, amphibolite, and a previously undescribed bimodal metavolcanic assemblage with associated metasedimentary rocks. These layered rocks, and the plutons that intrude them, differ on opposite sides of the Fowler Pass fault, a northwest-trending reverse fault of presumed Laramide age. The rocks east of the fault consist of a weakly metamorphosed sequence of felsic and mafic volcanic rocks and interlayered volcaniclastic sediments (now phyllite, chlorite schist and metasiltstone). The felsic rocks, which locally contain well-preserved eutaxitic fabric and bipyramidal quartz phenocrysts, are anomalously low in K 2 O and may represent highly altered tuffs. The metabasalts are tholeiitic and locally show amygdular texture. The metavolcanic rocks were intruded sequentially by small plutons that range in composition from gabbro to granodiorite and granite but do not appear to be comagmatic with granitic rocks west of the fault, the former being appreciably lower in potash and higher in lime and soda. West of the fault, the weakly to strongly foliated granitic rocks contain elongate roof pendants of quartzite (feldspathic and muscovitic near contacts) and smaller pendants of amphibolite of uncertain origin. Stratigraphic relations between the quartzites and the metavolcanic rocks, which lie on opposite sides of the Fowler Pass fault, are not determinable in this area. Lithologic similarity of these rocks to radiometrically dated supracrustal rocks in the Sangre de Cristo Mountains and Tusas Mountains to the west suggests a late early Proterozoic age for the deposition and metamorphism of the Precambrian framework of the Cimarron Mountains.

The Pecos greenstone belt, dated at ca. 1,720 Ma, consists of metamorphosed subaqueous basalts together with locally important felsic volcanic rocks, iron-formation, and metasedimentary rocks, some of volcanic provenance. Volcanic rocks define a compositionally bimodal suite of basalt (80 percent) and dacite-rhyolite (20 percent). Mafic volcanics are mostly fine-grained, massive to well-foliated amphibolite that locally display relict amygdules, pillows, and pillow breccia. Felsic volcanic rocks are mainly porphyritic flows and crystal-rich volcaniclastic units that contain variable amounts of quartz, K-feldspar, and plagioclase phenocrysts. The greenstone belt also includes a compositionally bimodal subvolcanic complex that intrudes, and is locally overlain by, portions of the volcano-sedimentary pile. The subvolcanic complex comprises concordant to discordant, hypabyssal intrusions of tonalite-trondhjemite (65 percent) and diabase-gabbro (30 percent). Ultramafic and mafic rocks with possible ophiolitic affinities constitute the remaining 5 percent of the subvolcanic complex. Greenstone-belt rocks have undergone regional metamorphism of upper greenschist to lower amphibolite grade and show the effects of at least three periods of deformation. The metamorphic sequence is cut by pre- to syn-orogenic granites and quartz porphyries dated at ca. 1,650 Ma and by syn- to post-orogenic granitic rocks dated at ca. 1,500 to 1,450 Ma. Mafic volcanic and subvolcanic rocks define four distinct chemical populations: (1) ultramafic, (2) high-Mg tholeiite, (3) tholeiite, and (4) calc-alkaline basalt (CAB). Geochemically, the high-Mg tholeiites are similar to Archean high-Mg tholeiites, whereas the tholeiites and calc-alkaline basalts are similar to their counterparts in modern arc systems. The ultramafic rocks and various basalt groups cannot be related to each other by closed-system fractional crystallization or batch melting of a single mantle source. Their incompatible-element ratios seem to demand at least three different mantle sources (one depleted). Felsic volcanics can be divided into three groups (one rhyolitic and two dacitic) on the basis of immobile-element abundances. The rhyolites and one dacite group can be produced by fractional crystallization of CAB. The other dacite group (Doctor Creek dacite) is not obviously or easily related, chemically or genetically, to the rest of the Pecos volcanic and subvolcanic rocks. It instead may belong to a younger (ca. 1,700 Ma) felsic-dominated volcanic succession that is widespread in northern New Mexico. Rocks of the Pecos greenstone belt may represent a remnant of a back-arc basin that opened far enough to form oceanic crust and tap a depleted mantle source.