TECTONIC HISTORY OF THE PROTEROZOIC COLORADO PROVINCE, SOUTHERN ROCKY MOUNTAINS

The Proterozoic ocean-arc assemblages in the Colorado Province record two major orogenies: (1) an older Paleoproterozoic episode from collision and subsequent subduction along the north-vergent Cheyenne Belt with the Archean Wyoming craton followed, after a stable interval of ~250 m.y., by (2) a Mesoproterozoic intracratonic orogeny involving coeval deformation, metamorphism, and granite plutonism. The older disturbance, herein named the Colorado Orogeny, was a protracted (1.78-1.65 Ga) gneiss-forming episode with associated amphibolite grade metamorphism and syntectonic calc-alkalic granodioritic plutonism. Two type localities for the Colorado Orogeny are proposed: the "Medicine Bow event", adjacent to the Cheyenne Belt, characterized by steep folds of general east-west trend, subparallel to the trace of the Cheyenne Belt; and the "Boulder Creek event", in the central Front Range, characterized by open to closed folds generally oriented at high angles to the Cheyenne Belt. The former took place during the interval 1.78-1.75 Ga, whereas the latter culminated at ca. 1.72 Ga. Both events occurred in a high temperature-low pressure metamorphic environment, at depths on the order of 25-30 km. Sillimanite is a common metamorphic mineral; kyanite is virtually absent. The younger disturbance, herein named the Berthoud Orogeny, mainly produced steep northeast-trending ductile shear zones and related shear folds, pervasive remetamorphism, and widespread ~1.4 Ga alkali-calcic (A-type) and peraluminous granitic plutons. During this episode of contractional deformation, the crust was reheated to temperatures somewhat lower than those obtained during the older orogeny; sillimanite and garnet re-crystallized locally. Probably the shear folds developed in a transpressive contractional stress field. The ~1.4 Ga plutons are interpreted as having been generated in high stress segments of the shear zones; the melts moved upward and along the shears and were localized by zones of weakness along the structures. The shear zones were reactivated repeatedly during Phanerozoic time, and had an important role in the development of the overall geologic framework of Colorado, including localizing the hypabyssal plutons responsible for the ore deposits of the Colorado Mineral Belt.