Geology of the Precambrian Farmington Complex, Bountiful Peak, Morgan and Davis Counties, Utah

Abstract

The southern portion of the Precambrian Farmington Canyon Complex is located approximately ten miles north of Salt Lake City, Utah, and forms the westernmost ridge of the Wasatch Mountains overlooking Great Salt Lake. The metamorphic complex consists of complexly deformed, high-grade, biotite-quartz-feldspar gneiss, amphibolite, granitic gneiss, migmatitic gneiss, and pegmatitic granite. Evidence for at least three episodes of deformation have been observed. Rare evidence for the first event is seen as a refolded isoclinal fold. Northwest trending mineral lineations and folds are a result of the second event. Interference patterns produced by the superposition of two sets of folds suggests a possible, although minor, third deformational event. The last event produced zones of mylonitization, thrust faults, and shear zones, probably during the pre-Late Cretaceous Sevier Orogeny. The preceeding events occurred in the Precambrian. The Complex has undergone three periods of metamorphism. An Archean granulite facies event has been suggested (Bryant, 1980) but no evidence for this event was observed in this study. Dynamic metamorphism was associated with the Sevier Orogeny. A middle proterozoic event of upper amphibolte grade, coincident with the second deformational event, attained P-T conditions up to and possibly above the second sillimanite isograd, as the pelitic assemblage of sillimanite-K feldspar-quartz- muscovite-plagioclase would indicate. The presence of only a few, sparsely located, coarse-grained, leucocratic pods within the laminated biotite-quartz-feldspar gneiss suggests that conditions were not appropriate for broad scale partial melting. Possible explanations for this lack of partial melting include: a) lack of water, b) temperatures too low, or c) anorthite content too high.