THE FARMINGTON CANYON COMPLEX, UTAH: AN OBDUCTION MELANGE ON THE MARGIN OF THE WYOMING PROVINCE

The Farmington Canyon Complex is an allochthonous Paleoproterozoic terrane comprising amphibolite grade migmatitic felsic gneiss, amphibolite, ultramafic schist (meta-komatiite), and quartzite, cross-cut by younger, post-orogenic intrusions of granite and pegmatite. The gneisses have chemical compositions consistent with formation from greywacke or wacke protolith, and contain zircon with Paleoproterozoic ages. Quartz-rich rocks associated with the amphibolites and ultramafic schists display no primary internal structure but the presence of Archean detrital zircon suggests a clastic origin. Amphibolites and ultramafic rocks typically occur as slab-like blocks overlain by quartzose sediments. Amphibolites also occur as dikes that cross-cut quartzite and are transposed parallel to foliation in the surrounding gneiss. Amphibolite block protoliths are tholeiitic basalts with MgO 7-12%, FeO* 10-15%, and TiO₂ 0.5-2%, whereas the ultramafic rocks are meta-komatiites similar to those in the Superior province, with MgO 24-32%, FeO* 10-11%, and TiO₂ < 0.4%. Amphibolite dikes are tholeiitic Fe-Ti basalts with MgO 4-8%, FeO* 12-19%, and TiO₂ 1-3%. Trace element systematics for both suites of meta-tholeiite and meta-komatiite are similar to OIB, but their Nd isotopic compositions are more depleted than OIB, suggesting a continental influence.

We suggest that the amphibolites and komatiites represent an oceanic plateau formed by melting an OIB or plume-like source, possibly during continental rifting. It’s sedimentary cover was silicified during prolonged exposure on the seafloor. The oceanic basalt-komatiite-quartzite assemblage was tectonically intercalated with passive margin sediments of the Wyoming craton margin during collision with the Santaquin arc, as the arc partially over-rode the craton margin. The resulting schüppenzone represents an obduction melange (Shervais et al 2011, GSA Sp Paper 480).