Pre-Cambrian Crystalline Rocks of North-Central Utah

Four classes of rocks are recognized, namely: metamorphic sedimentary rocks, metamorphic silicic igneous rocks, injection gneisses, and metamorphic mafic rocks. The sedimentary rocks are now metaquartzites, quartz schists, arkosites, conglomerate schists, and possibly metamorphosed graywackes. The pebbles in the quartz-chlorite conglomerate schist were definitely derived from a pre-existing siltstone, and the metaquartzites may possibly represent older sediments that have been re-worked. Other rocks such as the arkosites were probably derived directly from some near-by pre-existing igneous rock. After sediments of considerable thickness were deposited they were thoroughly injected by sills and dikes and also intruded by a few small stocks. The igneous material consists of granite, quartz monzonite, and pegmatite. The injection occurred during a prolonged orogeny which resulted in widespread development of foliation and various cataclastic structures. The recrystallization, cataclastic features, and mineral composition of the Farmington Canyon complex, as the ancient crystalline rocks are here designated, indicate that the metamorphism was of the low- to medium-grade dynamic type. This main period of metamorphism occurred in early or Middle pre-Cambrian time and was more intense than any deformation since. Later crustal disturbances affected very little the structures left in the rocks by this early metamorphism, except locally in fault zones.