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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 K2O 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.

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