Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California

Abstract

Research Article| April 01, 1973 Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California RICHARD LEE ARMSTRONG; RICHARD LEE ARMSTRONG 1Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 Search for other works by this author on: GSW Google Scholar JOHN SUPPE JOHN SUPPE 2Department of Geology, University of California, Los Angeles, California 90024 Search for other works by this author on: GSW Google Scholar Author and Article Information RICHARD LEE ARMSTRONG 1Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520 JOHN SUPPE 2Department of Geology, University of California, Los Angeles, California 90024 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (4): 1375–1392. https://doi.org/10.1130/0016-7606(1973)84<1375:PGOMIR>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation RICHARD LEE ARMSTRONG, JOHN SUPPE; Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California. GSA Bulletin 1973;; 84 (4): 1375–1392. doi: https://doi.org/10.1130/0016-7606(1973)84<1375:PGOMIR>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract K-Ar dates for 123 mineral separates from 91 samples of granitic rocks, and 3 samples of Orocopia and Vitrefax Schist from Nevada, western Utah, the Mojave Desert, the Colorado Desert, and the northern and eastern Peninsular Ranges of southern California, range in age from 218 to 19 m.y.; however, most of the dates lie between 160 and 50 m.y. A majority of the hornblende and muscovite dates are nearly concordant with, but generally a few m.y. older than, coexisting biotite dates; some are highly discordant.The oldest plutons gave Late Triassic to Early Jurassic dates (El Paso and San Bernardino Mountains, 218 to 194 m.y.). Jurassic igneous activity reached a culmination slightly before 150 m.y. ago over a large area in northern Nevada and western Utah and supplied ash to the Morrison Formation of the Colorado Plateau. Several plutons of the northeastern Mojave Desert yielded similar dates.A similar major culmination of Mesozoic pluton emplacement in early and middle Late Cretaceous time is represented by scattered plutons in Nevada and probably by most of the plutons in the Peninsular Ranges and southern deserts of California. Earliest Tertiary dates were obtained from far-southeastern California (50 to 66 m.y.).Most dates in the Cordilleran batholith belt of the Southwest, which parallels the edge of the late Mesozoic continent, are between 160 and 75 m.y. and are essentially coeval with metamorphism in the parallel high-pressure, low-temperature Franciscan belt to the west (approximately 150 to 75 m.y.). About 75 m.y. ago, this pattern abruptly changed to a Laramide one with abundant 75 to 50 m.y. dates in a belt running through southern Arizona into southeastern California at a high angle to the edge of the continent. Only very sparse dates of this age are found in Nevada. Abundant Laramide dates are not encountered again until the Idaho, Boulder, and Coast Range batholiths. These important changes in Cordilleran tectonics may be effects of a change from dominantly dip-slip subduction during Franciscan time to dominantly north-northeast strike-slip subduction during Laramide time. Furthermore, the initiation of rifting of eastern North America in Triassic time, changes in spreading rate, and reorientation of spreading directions in the Atlantic Ocean appear to correlate with the beginning of widespread pluton emplacement in western North America, changes in the rate of magma generation in the Cordillera, and shifts of the Cordilleran tectonic pattern, respectively, during Mesozoic time. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.