U-PB GEOCHRONOLOGICAL CONSTRAINTS ON THE PALEOPROTEROZOIC CRUSTAL EVOLUTION OF THE UPPER GRANITE GORGE, GRAND-CANYON, ARIZONA

Citation
Dp. Hawkins et al., U-PB GEOCHRONOLOGICAL CONSTRAINTS ON THE PALEOPROTEROZOIC CRUSTAL EVOLUTION OF THE UPPER GRANITE GORGE, GRAND-CANYON, ARIZONA, Geological Society of America bulletin, 108(9), 1996, pp. 1167-1181
Citations number
52
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Geosciences, Interdisciplinary
ISSN journal
0016-7606
Volume
108
Issue
9
Year of publication
1996
Pages
1167 - 1181
Database
ISI
SICI code
0016-7606(1996)108:9<1167:UGCOTP>2.0.ZU;2-C
Abstract
New field observations and U-Pb zircon and monazite ages are used to o utline a geologic history of Paleoproterozoic rocks beneath the Colora do 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 pe raluminous granite dikes, that are variably deformed and metamorphosed . New U-Pb ages indicate that layered supracrustal rocks were deposite d or erupted over at least 8 m.y., between 1750 +/- 2 and 1741 +/- 1 M a. 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 (Ru by granodiorite), and 1713 +/- 2 Ma (Horn diorite). Both supracrustal and plutonic rocks are intruded by 1698-1662 Ma peraluminous granite a nd 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 fab rics formed between 1730 +/- 3 and 1698 +/- 1 Ma. Group 2 fabrics form ed between 1713 +/- 2 and 1685 +/- 1 Ma. The timing of group 3 fabric formation is poorly constrained, although the 1662 Ma Phantom pluton c ontains shear bands that offset pegmatitic dikes that crosscut the plu ton. Metamorphic grade varies across the transect from lower amphiboli te facies to lower granulite facies. We suggest that single-crystal U- Pb ages (1706-1697) of metamorphic monazite, from a 1741 Ma supracrust al rock, directly date the timing of lower granulite-facies metamorphi sm in the eastern Upper Gorge. U-Pb ages (1685-1680 Ma) of granite dik es that postdate the development of leucosomal pegmatite pods in migma titic supracrustal rocks in the eastern Upper Gorge are consistent wit h this suggestion. The Paleoproterozoic geologic history of the Upper Granite Gorge is similar to that of the Yavapai province in central Ar izona 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 Fives Chasm pluton, the oldest known rock from the southwestern United States, has a Pb isotopic sign ature similar to that of rocks in the Mojave province. Second, the Tun a Creek granodiorite, a member of the 1710-1750 Ma are-related plutoni c 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 comp onents participated, at least locally, in the development of the 1730- 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.