PROTEROZOIC METAMORPHISM OF THE VALLECITO CONGLOMERATE, NEEDLE MOUNTAINS, SOUTHWEST COLORADO

The Vallecito Conglomerate is a member of a suite of Proterozoic basement rocks that have been exposed in the Needle Mountains Complex, southwestern Colorado. This metasedimentary unit lies within the transition zone between the Proterozoic Mazatzal and Yavapai provinces, and is part of the regional 1.8-1.4 Ga quartzite phenomenon that is recorded in basement rocks throughout the southwestern United States. Recent identification of metapelite layers at the top of the Vallecito Conglomerate provides information that allows for determination of the conditions at which the rocks in the Needle Mountains were metamorphosed. Mineral assemblages in the contact aureole of the 1.4 Ga Eolus Granite range from andalusite + biotite + muscovite + garnet at approximately two km from the contact to sillimanite + k-spar at about 1 km. This petrographic data also provides evidence for two phases of deformation because included foliations within k-feldspar are at an angle to matrix foliations. Metamorphic conditions at 2 km from the pluton were approximately 500°C -550°C and 3-5 kbars based on garnet-biotite thermometry and GASP barometry. This is consistent with mid-crustal depths and may be another example of regionally pluton-enhanced deformation at 1.4 Ga.