Paper No. 12
Presentation Time: 11:20 AM


SHERVAIS, John W., Department of Geology, Utah State University, 4505 Old Main Hill, Logan, UT 84322-4505, ANDREASEN, Kyle, Utah State University, Logan, UT 84322, BUCHWALDT, Robert, Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139 and HANAN, Barry B., Geological Sciences, San Diego State University, San Diego, CA 92182-1020,

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).