Ophiolites are distinctive assemblages of mafic, ultramafic, and felsic igneous rocks that are commonly thought to represent oceanic crust and mantle that has been accreted to a continental margin. The accretion of ophiolite and island arc terranes has been the primary mechanism of continental growth since the Proterozoic.
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 formed by subduction of oceanic plates beneath the Archean Wyoming province. These rocks were modified by a later collisional event circa 1700 Ma and are now amphibolite grade gneisses.
Our reconnaissance studies of the southern part of the Farmington Canyon complex suggest that it may represent an subduction zone accretionary complex that has been overprinted by amphibolite facies metamorphism. 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. The combination of old Nd model ages and Archean inherited zircon components implies that this accretionary complex formed on the SW margin of the Wyoming province, 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 Santaquin arc in the mid-Proterozoic.
This project will first map portions of the Farmington Canyon complex south of Farmington and north of Salt Lake City to locate and sample critical rocks and mineral assemblages, and to select the best areas for more detailed study. The project will then carry out detailed mapping at relatively small scales of one or two carefully chosen areas to demonstrate the distribution of exotic blocks within the gneiss, to compare block distributions to the Franciscan complex, and to carry out more detailed sampling of the blocks. Sampling will focus on the amphibolites (metabasalts), quartzites (metacherts), and ultramafic rocks (mantle tectonites?). Whole rock geochemical studies for major and trace element concentrations will be carried out, using XRF and ICP-MS, to establish their protoliths and tectonic affinities. The mafic rocks will be studied using standard discrimination techniques developed for oceanic crust and island arc lavas. Quartzites will compared to published analytical data for Mesozoic to recent cherts to determine if they are metacherts or detrital, using the signatures of hydrothermal metal deposits. In addition, the project will carry out a reconnaissance study of Sr-Nd-Pb isotopes (6-8 samples) to confirm the provenance of mafic blocks in the assemblage, and to determine whether more detailed studies are warranted. Finally, detailed paleo P-T investigations of garnet-clinopyroxene amphibolites will be carried out to determine if an earlier history of high-pressure metamorphism can be documented in any of the mafic blocks; this investigation will use x-ray mapping of relict mineral phases, reconstruction of primary phase compositions using image analysis techniques, and calculation of paleo P-T-time paths for allowable assemblages, using the electron microprobe.
Shervais, J.W., Significance of Subduction-related Accretionary Complexes in Early Earth Processes, in Reimold and Gibson, editors, Early Earth Processes, Geological Society of America Special Paper, submitted October 2004.
Masters Thesis Kyle Andreasen, Petrology and geochemistry of the Farmington Canyon complex, Utah, M.Sc. Thesis, Utah State University, in progress.