Precambrian rocks of the Mora-Rociada area, southern Sangre de Cristo Mountains, New Mexico
J. Michael O'Neill
Early Proterozoic crystalline rocks of the Mora-Rociada area consist of layered metasedimentary and metavolcanic rocks intruded by synkinematic mafic to intermediate rocks and postkinematic granite. The layered rocks are divided into three units: (1) a basal unit consisting of quartz-feldspar-mica gneiss and migmatite interlayered with minor knotted sillimanite schist and micaceous quartzite; (2) a middle unit consisting of quartzmuscovite schist and metabasalt and containing minor interlayers of quartzite, chlorite schist, quartz-biotiteandalusite schist and hornblende and biotite gneiss; and (3) an upper unit consisting of micaceous quartzite and felsic metavolcanic rock locally interlayered with metabasalt, calc-silicate rock, marble and iron formation. Poorly preserved crossbedding and graded bedding indicate that the section is upright. The supracrustal rocks are exposed in three large, southwest-plunging anticlines. Pervasive Precambrian shearing, isoclinal folding and penetrative southwest-plunging crenulation axes and mineral lineations characterize the lower part of the sequence and decrease in intensity upsection. Younger folds are present but are restricted to areas adjacent to younger intrusive bodies and structures of Laramide age.
The synkinematic intrusive rocks range in composition from noritic to tonalitic based on major element geochemistry. Small, coarse-grained norite plugs are closely associated with and locally gradational into hornblende- plagioclase gabbro. The gabbroic bodies are lenticular with their long dimensions subparallel to the foliation of the host rocks. They are commonly well-foliated, fine- to medium-grained near their margins, becoming more coarsely crystalline and locally pegmatoidal near their centers. Tonalitic rocks are locally gradational into gabbro and consist of quartz and plagioclase with variable amounts of hornblende and biotite. Middle Proterozoic postkinematic granite and pegmatite have intruded all rocks, locally refolding the layered sequence.
- O'Neill, J. Michael, 1990, Precambrian rocks of the Mora-Rociada area, southern Sangre de Cristo Mountains, 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. 189-199.