PALEOPROTEROZOIC CRUST GENERATION IN THE NORTHERN COLORADO FRONT RANGE

Juvenile arc accretion is often considered the default model that generates continental crust. However, this model may generate little continental crust. Further, modern juvenile island arc systems are all actively experiencing, or have experienced, some backarc extension and/or fully developed sea floor spreading. This suggests that a model of tectonic switching, which includes backarc extension in addition to juvenile arc accretion, more commonly generates continental crust.

The Paleoproterozoic rocks of the northern Colorado Front Range, part of the Yavapai province, offers a natural laboratory for the study of continental crust generation. The Yavapai province is part of an upwards of 1000 km wide northeast trending swath of crust generated at 1.8-1.6 Ga. Datasets and interpretations from many has led to the following tectonic model for the Poudre Basin, which includes the rocks in and around the Big Thompson and Poudre Canyons. The oldest rocks are remnants of 1.78 Ga arc crust left in the basin when the juvenile Green Mountain Arc, found to the north, split during slab rollback along a north dipping subduction zone. This formed the Poudre Basin and the Denver Arc to the south. Deposited on the remnant arc rocks is a sequence of turbidites with maximum deposition ages ranging from 1.78 Ga to 1.74 Ga. This sequence was intruded at 1.78 Ga by mafic magmas in an extensional backarc. Also intruding the turbidites are 1.74 Ga tonalitic rocks formed by the partial melting of shallowly subducted oceanic lithosphere and 1.73 Ga granodioritic rocks formed by normal subduction zone mantle flux melting. The intrusion of the felsic rocks marks the end of ~40 million years of sedimentation and the start of basin compression. The Poudre Basin was isoclinally folded and metamorphosed up to migmatitic conditions at relatively low-pressure at 1.72 Ga.

Work continues to better constrain various aspects of the tectonic switching model for this region of the Yavapai Province. A larger goal of the work is to determine whether the tectonic switching model may be more widely applied across this and other accretionary orogens that produced continental crust.