TECTONIC STYLE INSIGHTS FROM PALEOPROTEROZOIC AMPHIBOLITE GEOCHEMISTRY, BIG THOMPSON AND POUDRE CANYONS, NORTHERN COLORADO FRONT RANGE

A significant portion of North America is part of a Paleoproterozoic accretionary orogen that extends approximately from Arizona to Labrador. This orogen is often subdivided into an older Yavapai Province (1.8-1.7 Ga) and a younger Mazatzal Province (1.7-1.6 Ga). Many models explain formation of both provinces by juvenile arc accretion. However, models that also include tectonic switching, which additionally includes cycles of slab rollback with extensional backarc basin formation and collapse, are now commonly applied. The northern Colorado Front Range around the Big Thompson and Poudre Canyons contain a variety of Yavapai Province rocks, including amphibolite bodies.

The amphibolite bodies are generally elongate with size ranging from meter to kilometer scale. They are predominately concordant to the bedding and foliation in the encasing metasedimentary rock. Deformation fabrics are variably imprinted on the amphibolites and they likely originated as mafic sills, though pillows are reported for a body along the Buckhorn Creek shear zone (BCSZ). Typical amphibolite mineralogy include amphibole and quartz, while plagioclase, biotite, and Fe-Ti oxides are generally present. Geochemically, most amphibolites classify as basalt and all are tholeiitic, their incompatible elements reveal three distinct chemistries. Big Thompson Canyon amphibolites are the most enriched in rare earth elements (REE) with a sloping profile, while high field strength elements are depleted and other large ion lithophile elements are enriched relative to the REEs. Poudre Canyon amphibolites are similar but are less enriched. The body along the BCSZ is only slightly enriched with an overall flat profile.

Discrimination diagrams provide inconsistent results, but the amphibolites most commonly lie in the mid ocean ridge basalt or the island arc tholeiite/basalt fields. This variability, and the chemical patterns discussed above, is consistent with amphibolite protolith generation in an extensional backarc basin. No field evidence supports multiple generations of amphibolites, and no related calc-alkaline magmatism exists. These observations, paired with other regional geochronologic and geochemical results suggest the Big Thompson and Poudre Canyon areas formed through a tectonic switching tectonic style.