A Proterozoic Volcano-Plutonic Terrane, Gunnison and Salida Areas, Colorado

Abstract

Early Proterozoic supracrustal rocks near Gunnison and Salida, Colorado include tholeiitic basalt, dacite to rhyolite, and intercalated sedimentary rocks. These were intruded by essentially synchronous gabbroic sheets, folded, and intruded by major plutons ranging from quartz diorite to granite. Preservation of primary volcanic and sedimentary features is good to excellent; sedimentary rocks in the Gunnison area are mostly turbidite wackes whereas near Salida they are mostly near-source volcaniclastic rocks. U-Pb age measurements on zircons indicate that the earliest bimodal volcanic rocks erupted between 1770 and 1760 m.y. ago and were intruded by granitic plutons between 1760 and 1750 m.y. ago. Renewed bimodal volcanism occurred between 1740 and 1730 m.y. ago and was followed by a period of deformation producing two fold sets before 1725 m.y. ago when the first of a series of younger plutons was emplaced. The compositions and features of the sedimentary rocks and their association with volcanics are con...