THE NATURE OF PALEOPROTEROZOIC ROCKS IN THE SOUTHERN SANGRE DE CRISTO RANGE BASED ON MAPPING THE CULEBRA PEAK QUADRANGLE, COLORADO

Recent mapping of the south half of the Culebra Peak quadrangle, Colorado further shows that a large part of the Sangre de Cristo range in southern Colorado is underlain by two types of Paleoproterozoic rocks—a 1688-1689 Ma quartzofeldspathic gneissic granite that intruded Middle (?) Paleoproterozoic country rocks composed of layered orthogneisses and lesser paragneisses. Both the gneissic granite and the layered gneisses are part of the southern Yavapai Province, one of a series of accreted Paleoproterozoic arc sequences that comprise the basement of the southwestern United States.

The total thickness of the gneissic granite is unknown. Based on outcrop observations, the gneissic granite has been subdivided into three units: equigranular with sparse biotite foliation, augeniferous with distinctive biotite-hornblende foliation, and aplitic with poorly defined quartzofeldspathic foliation. Field observations suggest the augeniferous and aplitic gneissic subunits are superjacent to the equigranular subunit. The contoured average attitude for S1 foliation in gneissic granite is N53E, 40NW. Mesoscopic S2 fabric, while occasionally present, was only observed in float. Farther north in the Trinchera and Ojito Peak quadrangles, other workers have reported a broad, regional S2 fabric that strikes ENE to E-W.

Combined, the para- and orthogneisses are at least 500 m thick and consist of interspersed layers of quartz-biotite-hornblende gneiss, quartz-alkali feldspar-plagioclase-biotite-muscovite gneiss, and amphibolite each of which ranges from 1 to 45 m thick. Based on our observations, the quartz-biotite-hornblende gneiss is the most extensive, while one particular amphibolite layer is the thickest and most homogeneous suggesting that it originated from a basaltic protolith. On the whole, the mineralogy of the layered gneisses is Al-deficient, and thus, they are believed to originate largely from volcanic and/or volcaniclastic, as opposed to pelitic, protoliths. S1 foliation in the layered gneisses is usually parallel or subparallel to compositional layering, and the contoured average attitude for S1 foliation is N71E, 56NW. Crenulated S2 fabrics, ptygmatic quartz veins, and boudinage were observed in amphibolite, quartz-plagioclase-muscovite gneiss, and quartz-biotite-hornblende gneiss, respectively. Average S2 in crenulated amphibolite is N42E, 47NW.