U-Pb geochronologic constraints on the Paleoproterozoic crustal evolution of the Upper Granite Gorge, Grand Canyon, Arizona

Abstract

New field observations and U-Pb zircon and monazite ages are used to outline a geologic history of Paleoproterozoic rocks beneath the Colorado Plateau in the Grand Canyon of northern Arizona. The Upper Granite Gorge of the Grand Canyon exposes three lithotectonic units, including supracrustal rocks, mafic to intermediate-composition plutons, and peraluminous granite dikes, that are variably deformed and metamorphosed. New U-Pb ages indicate that layered supracrustal rocks were deposited or erupted over at least 8 m.y., between 1750 ± 2 and 1741 ± 1 Ma. Mafic to intermediate-composition plutons intruded the supracrustal rocks at 1741 ± 1 Ma (Zoroaster granite), 1737 ± 1 Ma (Grapevine Camp granite), 1730 ± 3 Ma (Trinity granodiorite), 1717 ± 1 Ma (Ruby granodiorite), and 1713 ± 2 Ma (Horn diorite). Both supracrustal and plutonic rocks are intruded by 1698–1662 Ma peraluminous granite and pegmatite dikes that locally form 50% of the rock volume. U-Pb ages of foliated plutonic rocks and crosscutting granite dikes bracket the timing of fabric development for three groups of fabrics. Group 1 fabrics formed between 1730 ± 3 and 1698 ± 1 Ma. Group 2 fabrics formed between 1713 ± 2 and 1685 ± 1 Ma. The timing of group 3 fabric formation is poorly constrained, although the 1662 Ma Phantom pluton contains shear bands that offset pegmatitic dikes that crosscut the pluton. Metamorphic grade varies across the transect from lower amphibolite facies to lower granulite facies. We suggest that single-crystal U-Pb ages (1706–1697) of metamorphic monazite, from a 1741 Ma supracrustal rock, directly date the timing of lower granulite-facies metamorphism in the eastern Upper Gorge. U-Pb ages (1685–1680 Ma) of granite dikes that postdate the development of leucosomal pegmatite pods in migmatitic supracrustal rocks in the eastern Upper Gorge are consistent with this suggestion. The Paleoproterozoic geologic history of the Upper Granite Gorge is similar to that of the Yavapai province in central Arizona. However, unlike the juvenile crust that characterizes the Yavapai province, rocks in the western Upper Gorge preserve evidence for pre-1.8 Ga crust. First, the 1840 ± 1 Ma Elves Chasm pluton, the oldest known rock from the southwestern United States, has a Pb isotopic signature similar to that of rocks in the Mojave province. Second, the Tuna Creek granodiorite, a member of the 1710–1750 Ma arc-related plutonic suite, contains inherited zircons with minimum ages of 1998 and 2178 Ma and exhibits whole-rock Pb isotopic signatures similar to rocks of the Mojave province. These data indicate that pre-1.8 Ga crustal components participated, at least locally, in the development of the 1740–1710 Ma arc-related magmatic suite and suggest that a boundary between reworked crust and juvenile crust may be present in the western Upper Gorge.