Precambrian Rocks of the Tusas Mountains

INTRODUCTION Precambrian rocks in the Tusas Mountains range from nearly monomineralic quartzite, in which the principal result of metamorphism has been simple recrystallization, through locally metasomatized schist and gneiss profoundly modified by potash-rich fluids during the intrusion of pegmatite. Texturally, the rocks range from schist and phyllite containing penetrative and pervasive cleavage or schistosity that has completely transposed earlier planar structures, to unfoliated porphyritic granite. Geologic studies of the Precambrian in the Tusas Mountains include an early reconnaissance by Just (1937), and quadrangle mapping by Barker (Las Tablas quadrangle, 1958), Jahns, Smith, and Muehlberger (0jo Caliente quadrangle, 1960), Bingler (La Madera quadrangle, 1965), Muehlberger (Brazos Peak quadrangle, 1968), and Doney (Cebolla quadrangle, 1968). Just (1937) established an early stratigraphic framework by naming the Ortega Quartz i te, the P icur is Basalts, the Vallecitos Rhyolites, the Tusas Granite, and the Petaca Schist. Barker (1958, 1970) subdivided and revised Just's nomenclature. Muehlberger's (1960) comprehensive review of the Precambrian of the Tusas Mountains follows the nomenclature of Barker (1958). Detailed studies include Lindholm's (1964) structural petrologic analysis of the Ortega Quartzite in the type area, Ritchie's (1969) remapping of 12 square miles in the Poso Lake area, and a geologic map of the same area by Stensrud (1970), and Hutchinson's (1968) map of the Hopewell Lake area. Formal st rat igraphic names have been used by other workers for the Precambrian of the Tusas Mountains (Just, 1937; Barker, 1958, 1970; Muehlberger, 1960) and correlations based upon lithology but implying continuity have been made linking distant exposures, in particular from the Tusas Mountains to the southern Sangre de Cristo Mountains (Montgomery, 1953; Miller, Montgomery, and Sutherland, 1963). In this report, stratigraphic units are given lithologic names to avoid generalizations regarding the original continuity of rock masses. This article is a condensed version of the author's discussion of Precambrian rocks of Rio Arriba County (Bingler, 1968). The reader is referred to the 1968 report for a full-color geologic map of the Tusas Mountains.