Orbicular Rocks of the Lonesome Mountain Area, Beartooth Mountains, Montana and Wyoming

Abstract

Research Article| August 01, 1963 Orbicular Rocks of the Lonesome Mountain Area, Beartooth Mountains, Montana and Wyoming DAVID JEFFREY LEVESON DAVID JEFFREY LEVESON Dept. Geology, Brooklyn College, Brooklyn, N. Y Search for other works by this author on: GSW Google Scholar Author and Article Information DAVID JEFFREY LEVESON Dept. Geology, Brooklyn College, Brooklyn, N. Y Publisher: Geological Society of America Received: 26 Jan 1961 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1963, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1963) 74 (8): 1015–1040. https://doi.org/10.1130/0016-7606(1963)74[1015:OROTLM]2.0.CO;2 Article history Received: 26 Jan 1961 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation DAVID JEFFREY LEVESON; Orbicular Rocks of the Lonesome Mountain Area, Beartooth Mountains, Montana and Wyoming. GSA Bulletin 1963;; 74 (8): 1015–1040. doi: https://doi.org/10.1130/0016-7606(1963)74[1015:OROTLM]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 The Lonesome Mountain area in the Beartooth Mountains, Montana and Wyoming, includes large outcrops of orbicular gneiss in a terrain of amphibolites, migmatites, and granitic gneisses. The orbicules have cores of chiefly plagioclase, biotite, or hornblende-biotite, surrounded by alternating plagioclase-rich and biotite- or hornblende-rich shells. Radially oriented biotite and plagioclase are common in the cores. Tangentially oriented biotite forms the biotite-rich shells. Field and laboratory evidence suggests that orbicules formed as abnormal by-products of granitization of amphibolites. Orbicule formation involves feldspathization without complementary quartz enrichment.Metamorphic orbicules and Liesegang phenomena are compared. Both result from diffusion processes that produce discontinuous, periodic precipitation or crystallization.Dimensional analyses of shell spacings aid in determining the mechanism of orbicule formation. Rhythmically banded phenomena fall into two groups according to spacing of their bands: (1) spacing in exponential progression—metamorphic orbicules, spherulites, Liesegang rings, some agates; (2) spacing in arithmetic progression—igneous orbicules, concretions, oölites, oscillatory-zoned plagioclase, stalactites, varved argillites.Objects of group 1 form by internally controlled rhythmic processes, which are always disequilibrium phenomena. Those of group 2 form by externally controlled rhythmic processes and may be equilibrium or disequilibrium phenomena. Orbicules of the Lonesome Mountain area have shells exponentially spaced and result from rhythmic diffusion processes. Laboratory evidence suggests that orbicule formation involved migration of Ca+2 ions outwards from the cores of orbicules and simultaneous migration of Na+1 and Si+4 ions inwards toward the centers of orbicules. This content is PDF only. Please click on the PDF icon to access. 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.