Emplacement of the Butler Hill Granite, a shallow pluton within the St. Francois Mountains batholith, southeastern Missouri

Abstract

Research Article| September 01, 1980 Emplacement of the Butler Hill Granite, a shallow pluton within the St. Francois Mountains batholith, southeastern Missouri J. RONALD SIDES J. RONALD SIDES 1Department of Geology, University of Texas at Arlington, Arlington, Texas 76019 Search for other works by this author on: GSW Google Scholar Author and Article Information J. RONALD SIDES 1Department of Geology, University of Texas at Arlington, Arlington, Texas 76019 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1980) 91 (9): 535–540. https://doi.org/10.1130/0016-7606(1980)91<535:EOTBHG>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 J. RONALD SIDES; Emplacement of the Butler Hill Granite, a shallow pluton within the St. Francois Mountains batholith, southeastern Missouri. GSA Bulletin 1980;; 91 (9): 535–540. doi: https://doi.org/10.1130/0016-7606(1980)91<535:EOTBHG>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 St. Francois Mountains batholith of southeastern Missouri appears to be a shallow composite batholith that has intruded a roof of its own volcanic ejecta. The complex is composed mostly of silicic intrusive units and rhyolitic pyroclastic rocks that are mostly about 1,500 m.y. old. Field, petrographic, and chemical data suggest that the batholith has been tilted to the southwest and beveled by erosion, thus exposing a 6-km stratigraphic section of the volcanic roof and a 4-km cross section of the underlying intrusive units.The Butler Hill Granite is the largest single intrusive unit in the St. Francois Mountains batholith and probably controlled the development of most of the batholith. The Butler Hill Granite is characterized by small but systematic chemical variations, with SiO2 and K2O most abundant in the southwest and Fe2O3, TiO2, MgO, and CaO most abundant in the northeast. Moreover, the pluton is characterized by striking gradational changes in mineralogy, with plagioclase and perthitic orthoclase most abundant in the northeast and southwest, respectively. These modal and chemical changes support the tilted-batholith hypothesis, which predicts that the Butler Hill Granite should be structurally highest and most differentiated to the southwest and deeper and less differentiated to the northeast. The large increase in plagioclase to the northeast is due to increased fluid pressure resulting from the greater original depth of this magma. High fluid pressure (about 3 kb) has resulted in partial elimination of the one-feldspar field, so that albite to sodic oligoclase formed rather than a mixed feldspar. These modal data, along with volcanic stratigraphic evidence and chemical relationships expressed in the quartz-albite-orthoclase-water system, suggest that the depth of crystallization of Butler Hill Granite now exposed in the northeasternmost part of the pluton was about 10 km. 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.