TECTONIC SIGNIFICANCE OF AN EARLY PROTEROZOIC OPHIOLITE FRAGMENT IN THE COLORADO FRONT RANGE: REMNANT OF AN OCEANIC FRACTURE ZONE?

Tectonically intermingled amphibolite, metagabbro, metachert, and carbonate-hosted mélange occur within the Proterozoic Buckhorn Creek shear zone, located west of Fort Collins in northern Colorado. The shear zone strikes E-W, is subvertical, and is up to 1 km wide. Mylonitic amphibolite is the dominant rock type within the shear zone, but pillow structures are well preserved in local low-strain zones. Metagabbros and clinopyroxenites contain shallowly plunging, lineated clinopyroxenes that record high-temperature, dextral strike-slip deformation. Secondary amphiboles are aligned in a downdip lineation and record subsequent S-up contractional deformation at lower amphibolite facies conditions. Ferruginous metacherts up to 1 m-wide occur in the amphibolite section. The association of pillow basalt with gabbro, clinopyroxenite, chert, and mélange is consistent with an ophiolitic origin for this rock package. Whole-rock analyses of major and trace elements are similar to samples from modern fracture zones and plume-modified mid-ocean ridges where plagioclase ± clinopyroxene ± Cr-spinel fractional crystallization is common. The lack of a consistent internal stratigraphy within the Buckhorn Creek zone is also consistent with observations from modern-day fracture zones with anomalously thin crustal sections. We propose that the Buckhorn Creek shear zone initiated as a fracture zone in ca. 1.8 Ga juvenile oceanic crust of the Early Proterozoic Colorado province that was subsequently heavily modified by arc volcanism and sedimentation. Contractional features are likely related to 1.4 Ga deformation that reactivated the shear zone. A similar origin may also apply to other Proterozoic shear zones in the Colorado Front Range, such as the Moose Mountain shear zone 30 km to the south. Our model can explain the juvenile nature of the Colorado province crust, the spacing between shear zones, and the long-lived histories of these zones of crustal weakness.