Is the Southern Farmington Canyon Complex a Late Archean/Early Proterozoic Accretionary Complex?

Utah State University, Logan UT

Investigators

Abstract

One of the fundamental questions in the planetary evolution of Earth centers on when modern style plate tectonic processes, driven by the sinking of dense lithospheric plates, became the dominant mode of thermal convection and crustal deformation. This project examines rock assemblages in the Wasatch Mountains of northern Utah that may represent a late Archean to early Proterozoic accretionary complex that formed by subduction of oceanic plates beneath the western margin of North America. These rocks, part of the southern Farmington Canyon complex, were modified by a later collisional event circa 1700 Ma and are now represented by amphibolite grade gneisses. The southern Farmington Canyon complex contains blocks of mafic metavolcanic rock (amphibolite, pyroxene amphibolite, garnet amphibolite), ultramafic rock, and quartzite - some of which may represent metachert associated with mafic metavolcanic rocks - in a matrix of quartzo-feldspathic gneiss with a chemical composition similar to greywacke. Radiogenic isotopic studies yield old Neodymium model ages and Archean inherited zircon components which suggests that this accretionary complex formed on the southwestern margin of the Wyoming craton, in conjunction with a coeval continental margin arc represented in part by the northern portion of the Farmington Canyon complex (orthogneiss, migmatite, pegmatite). This contintental margin arc apparently collided with the volcanic Santaquin arc during Middle Proterozoic time. To better understand the geologic evolution of this area, we are conducting geologic mapping and geochemical analysis of parts of the Farmington Canyon complex south of Farmington and north of Salt Lake City. Sampling is focused on the amphibolites (metabasalts), quartzites (metacherts), and ultramafic rocks (mantle tectonites?). Whole rock geochemical studies for major and trace element concentrations are being carried out in order to establish the protoliths of the metamorphic rocks and the tectonic setting in which they originated. Detailed paleo pressure-temperature investigations of garnet-clinopyroxene amphibolites are being used in order to determine if an earlier episode of high-pressure metamorphism is preserved in any of the mafic blocks. This study is providing critical information bearing on the nature of Archean/Mesoprotoerozoic tectonic events along the western margin of the ancient North American continent.