Precambrian rocks of the Taos Range and vicinity, northern New Mexico

INTRODUCTION Precambrian rocks underlie much of the area in the Taos Range and vicinity northeast of Taos, New Mexico (fig. 1). In this region, the Precambrian terrane is intruded by Tertiary granitic plutons and overlain unconformably by upper Paleozoic and Mesozoic sediments and by Tertiary volcanics. It also is intruded extensively by Tertiary rhyolite and latite dikes. In general, exposures of Precambrian rock are poor with the only semi-continuous exposures occurring on the high mountain ridges. Precambrian rocks underlie most of the region from Rio Lucero on the south to Red River on the north (fig. 1). They also extend northward from the Moreno Valley and underlie part of the Cabresto Creek drainage. Part of the Taos Range south of Rio Hondo was mapped in reconnaissance manner by Gruner (1920) who recognized most of the important Precambrian rock types in the area. McKinlay (1956, 1957) mapped Precambrian rocks in the northern part of the range as part of quadrangle mapping. Much of the Taos Range was mapped by Clark and Read (1972) as part of their study of the Eagle Nest region. These investigations served to define the distribution of the major Precambrian rock types. This investigation presents results of detailed mapping of Precambrian rocks in the Taos Range and vicinity by the author and students of the New Mexico Tech field course during 1976-77 and 1977-78. Also assisting with the course were F. J. Kuellmer and A. J. Budding. Major Precambrian rocks in the area broadly fall into three categories: metasedimentary, metavolcanic and granitic rocks. In addition, minor diabase dikes of probable Precambrian age occur in some parts of the area. The metasedimentary terrane underlies much of the western part of the area and some of the area east of Red River (fig. 1). The metavolcanic terrane underlies several small areas roughly coincident with the crest of the Taos Range. Granitic rocks fall into two categories: granite (including quartz monzonite) and tonalite-trondhjemite. Tonalite-trondhjemite is intruded into the metavolcanic successions and partly into the metasedimentary successions in the crestal part of the Taos Range. These intrusive bodies are similar to the subvolcanic complex of Robertson and Moench (this guidebook). Major granite plutons occur in the southern and northeastern part of the area. Although the same rock types recognized by McKinlay (1956, 1957) were recognized by our mapping, we were unable to verify the map distributions proposed by McKinlay. We were unable to substantiate the four-fold subdivision of Precambrian rocks proposed by Clark and Read (1972). In addition, some of the lithologic types proposed by these authors are either nonexistent (i.e., granulite) or very minor (i.e., migmatite). Although radiometric dates are not available from Precambrian rocks in the Taos Range, dates of similar rocks of Precambrian age in the Tusas and Picuris ranges probably give an indication of the general age of Precambrian rocks in the Taos Range. The oldest plutonic rocks recognized in northern New Mexico are tonalites, trondhjemites and granodiorites, ranging in age between 1.65 and 1.7 b.y. (Barker and others, 1974; Brookins, 1974; Fullagar and Shiver, 1973; Long, 1974). All dates are recalculated with X87 Rb = 1.42x10-llyr-1 and X235U = 0.985x10-9yr-1. Somewhat older (1.7-1.75 b.y.) felsic volcanic rocks have been reported from the Tusas and Nacimiento mountains (Barker and Friedman, 1974; Brookins, 1974). One or more periods of high-K granitic plutonism and regional metamorphism also are recorded between 1.3 and 1.4 b.y. B.P. (Callender and others, 1976; Gresens, 1975). Mineral deposits in Precambrian rocks of this region have been summarized by Schilling (1960). Most deposits are Laramide or Tertiary in age. Quartz veins bearing minor amounts of sulfides and gold occur in many Precambrian rocks throughout the area. Small deposits of graphite and iron oxides of Precambrian age occur at a few locations in the region.