“On-the-Rocks” Field Trip Proterozoic Geology of the Front Range

Title: “On-the-Rocks” Field Trip Proterozoic Geology of the Front Range

Leaders: Jack Reed and Bruce Bryant, USGS

Date: Saturday, August 11, 2001

Publication: The Outcrop, July 2001, p. 17-18

The oldest rocks in the Front Range are apparently the products of arc-magmatism and sedimentation in arc-related basins which were part of an extensive terrane that was accreted to the Wyoming craton over an interval of 130 m.y. beginning at about 1,790 Ma. These deposits were complexly deformed and metamorphosed at high temperature and relatively low pressure to upper amphibolite facies, although locally they were not as highly metamorphosed. Locally mineral assemblages suggest metamorphism in the granulite facies. A variety of Early Proterozoic calc-alkaline plutons, chiefly granodiorite and monzogranite, but ranging from peridotite to syenite, cut the supracrustal rocks. These plutons range from concordant and strongly foliated to discordant and only weakly foliated. Brief widespread mid-crustal reheating at 1.4 Ga was accompanied by intrusion of dikes, stocks, and irregularly discordant plutons of weakly foliated two mica granite and monzogranite. The youngest major group of Proterozoic igneous rocks in central Colorado comprises the 1,092-1,074 Ma Pikes Peak batholith, which crops out from Pikes Peak near Colorado Springs to north of the North Fork of the South Platte River southwest of Denver. The Proterozoic rocks are cut by an anastomosing swarm of northeast-trending zones of ductile deformation and cataclasis having long and complicated movement histories and apparently localizing Laramide and younger plutons and associated mineral deposits of the Colorado mineral belt. The field trip will visit some exposures of rocks illustrating facets of this long and complex geologic history in and near Coal Creek and Clear Creek canyons northwest and west of Denver.

Trip participants will meet at the Cold Springs Park ‘n Ride at 6th Ave. and Simms St. at 8:00 AM. Participants are responsible for their own food and drink, as well as appropriate field gear and clothing. The trip will involve light walking at some stops. To register call Jeanette Dubois at 303-799-0854.