Oxygen and hydrogen isotope studies of a Precambrian granite-rhyolite terrane, St. Francois Mountains, southeastern Missouri

Research Article| November 01, 1976 Oxygen and hydrogen isotope studies of a Precambrian granite-rhyolite terrane, St. Francois Mountains, southeastern Missouri DAVID B. WENNER; DAVID B. WENNER 1Department of Geology, University of Georgia, Athens, Georgia 30602 Search for other works by this author on: GSW Google Scholar HUGH P. TAYLOR, JR. HUGH P. TAYLOR, JR. 2Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1976) 87 (11): 1587–1598. https://doi.org/10.1130/0016-7606(1976)87<1587:OAHISO>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 DAVID B. WENNER, HUGH P. TAYLOR; Oxygen and hydrogen isotope studies of a Precambrian granite-rhyolite terrane, St. Francois Mountains, southeastern Missouri. GSA Bulletin 1976;; 87 (11): 1587–1598. doi: https://doi.org/10.1130/0016-7606(1976)87<1587:OAHISO>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 Isotopic analyses were made on whole-rock samples and minerals from 25 granites, 21 rhyolites, and 10 basaltic dikes and sills from the l,500-m.y.-old St. Francois Mountains terrane. The δD of chlorite is relatively uniform at −44 to −65, but the whole-rock and feldspar δ18O values systematically increase from +7 in the northeast to +14 in the southwest, correlating with an increasing intensity of “brick-red” alteration of the K-feldspars. Coexisting quartz and feldspar are typically not in isotopic equilibrium, except in the northeast. The coarser grained (>1 mm) quartz (δ18O = 8.8 to 10.6) is isotopically similar to “normal” igneous quartz, but the δ18O of the finer grained quartz correlates with the whole-rock δ18O pattern. Although the St. Francois terrane is geologically similar to many low-18O Tertiary volcanic-plutonic complexes that have interacted with meteoric-hydrothermal fluids at high temperatures, only a single locality (Stono Mountain) contains low-18O quartz and feldspar (δ18O quartz = 4.6 to 5.4). Even at this locality, the feldspars were later enriched in 18O by the same alteration event that affected the rest of the region. This second stage of hydrothermal activity apparently involved aqueous fluids having δ18O ≈ 0 to −6 and δD = 0 to −25. The temperature of alteration may have been as low as 50° to 100°C in the southwest (upper part of the volcanic section). This event affected some of the basaltic dikes and sills, but others were not altered and must have been intruded afterward. Rb-Sr and K-Ar age data suggest that the regional alteration occurred as late as 1,100 to 1,200 m.y. ago, long after the age of primary igneous activity. The hydrothermal fluids apparently originated as meteoric surface waters, indicating that during late Precambrian time such waters were isotopically similar to present-day meteoric waters in warm climates; this interpretation implies that the late Precambrian oceans must have been isotopically similar to present-day ocean. 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.