GEOCHEMISTRY AND GEOLOGIC RELATIONS OF META-BASALTS AND META-KOMATIITES OF THE FARMINGTON CANYON COMPLEX, WASATCH MOUNTAINS, UTAH

The Paleoproterozoic Farmington Canyon Complex is an allochthonous felsic migmatite gneiss terrane, with locally abundant granite, amphibolite, ultramafic schist and quartzite. Metamorphic grade is amphibolitic with greenschist facies overprint on shear zones. Non-deformed granites and pegmatites are interpreted to be post-orogenic. The gneisses have intermediate chemical compositions consistent with formation from greywacke/andesite protoliths, and contain zircon with Paleoproterozoic dates. 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 both occur as boudinaged layers and lenses within the gneiss, commonly in association with quartzite layers and as dikes that cross-cut the enclosing gneiss and/or have been transposed parallel to banding and foliation. 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 metatholeiites and metakomatiites imply formation by extensive melting of an enriched source region followed by extensive fractionation. Zircons separated from the amphibolites are ca 1.9 Ga and are likely metamorphic.

We propose that the volcanic rocks formed in an intra-oceanic setting. Sedimentary cover derived from an adjacent continental area was silicified during prolonged exposure on the seafloor, prior to incorporation of the proto-oceanic crust into a greywacke-dominated basin, either through subduction and/or collision with the Santaquin arc. In either case, oceanic crust and sediment were incorporated into a schüppenzone with continental sediments. Final metamorphism occurred ca 1.8 Ga (Hedge et al 1983; Nelson et al 2002).