THE GREEN MOUNTAIN ARC–RAWAH BATHOLITH BOUNDARY IN THE SOUTHERN LARAMIE MOUNTAINS OF COLORADO—CORNELIUS CREEK SHEAR ZONE

Geologic mapping in the Laramie Mountains of northern Colorado has identified a significant Paleoproterozoic basement discontinuity. The Cornelius Creek shear zone (CCsz), a brittle fault associated with Mesozoic or younger deformation, is a 20 km segment of a larger east-west-striking fault system that crosses the entire 60 km width of the southern Laramie Mountains. Offset of Paleozoic and Mesozoic rocks on the eastern and western flanks of the range indicate hundreds of meters of displacement. Within the range, offset of Mesoproterozoic intrusive rocks (1.44 to 1.41 Ga) is also hundreds of meters supporting moderate motion on this fault system during or after Laramide uplift. However, a 12 km section of the CCsz at its western end juxtaposes rocks incompatible with such a small offset. A layered sequence of upper amphibolite-grade bimodal, calc-alkaline metavolcanic and metasedimentary units north of the CCsz were deposited from 1.78 to 1.75 Ga as part of the Green Mountain magmatic arc terrane. Subparallel compositional layering, metamorphic foliation, and fold axes strike to the northeast, and dip steeply northwest. Granite of the Rawah Batholith (ca. 1.72 Ga), south of the CCsz contains sparse inclusions of older metamorphic rocks. Foliations in larger inclusions generally strike east-west and dip steeply to the north, discordant with the rocks north of the CCsz. At other exposures across the region, the Rawah Batholith consistently interfingers, injects, and assimilates the intruded wall rocks. Large, irregular wall rock bodies extend deep into the intrusion with concordant foliation indicating very irregular batholith margins. Pre-Rawah intrusive rocks (1.74 to 1.73 Ga) are found interlayered with the older bimodal basement units along the well mapped southern edge of the batholith. North of the CCsz, no intrusive rocks associated with the Rawah batholith or pre-Rawah intrusions have been identified. These local relationships along the CCsz imply post-Rawah, pre-Mesoproterozoic (1.72 to 1.44 Ga) structural juxtaposition that parallels the younger, brittle structure.