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 consistent with accumulation in an island arc or immature back-arc basin. However, the trace element chemistry of basalts and the lack of abundant andesite may be inconsistent with an arc setting and the bimodality of the volcanic suite suggests the presence of at least somewhat older continental crust. The age span of the volcanism indicates the terrane is coeval with the older volcanogenic terrane of the southwestern United States and suggests that an active volcanotectonic zone extended at least as far northeast as central Colorado.

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