Proterozoic tectonic evolution of the Cimarron Mountains, north-central New Mexico
Jeffrey A. Grambling and R. D. Dallmeyer
The Cimarron Mountains contain three Proterozoic crustal blocks separated by ductile shear zones. The first block consists of greenschist-facies volcanic and plutonic rocks metamorphosed below 450°C; the second, amphibolite-facies quartzite metamorphosed at 500-550°C, 3.5-4.5 kb; and the third, granulite-facies supracrustal and plutonic gneisses metamorphosed at 800-850°C, 7.5-8.5 kb. The three crustal blocks were juxtaposed along two shear zones of different geometry and age. The older ductile shear zone, subhorizontal with top-to-the-southwest movement, separates underlying granulites from overlying amphibolite-facies quartzite. The underlying crustal block records a retrograde P-T path of cooling and decompression that intersects the peak metamorphic conditions of the overlying crustal block. Intersection of the P-T paths accompanied joining of the two crustal blocks across a synmetamorphic, mid-crustal extensional fault at 1400 Ma, as dated by 40Ar/39Ar analyses of hornblende. The younger ductile shear zone, nearly vertical and trending northwest, records right-slip ductile movement overprinted by much younger, brittle, vertical faulting. The early ductile movement juxtaposed greenschists against granulites, at P-T conditions in the greenschist facies. A muscovite 40Ar/39Ar spectrum may date this event at between 1295 Ma and 1100 Ma.
- Grambling, Jeffrey A.; Dallmeyer, R. D., 1990, Proterozoic tectonic evolution of the Cimarron Mountains, north-central New Mexico, in: Tectonic development of the southern Sangre de Cristo Mountains, New Mexico, Bauer, Paul W.; Lucas, Spencer G.; Mawer, Christopher K.; McIntosh, William C., New Mexico Geological Society, Guidebook, 41st Field Conference, pp. 161-170.