Trace Elements in Continental-Margin Magmatism: Part III. Alkali Granites and Their Relationship to Cratonization

Abstract

Research Article| January 01, 1981 Trace Elements in Continental-Margin Magmatism: Part III. Alkali Granites and Their Relationship to Cratonization John J. W. Rogers; John J. W. Rogers 1Department of Geology, University of North Carolina at Chapel Hill, Mitchell Hall 029A, Chapel Hill, North Carolina 27514 Search for other works by this author on: GSW Google Scholar Jeffrey K. Greenberg Jeffrey K. Greenberg 2Wisconsin Geological Survey, 1815 University Avenue, Madison, Wisconsin 53706 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1981) 92 (1_Part_II): 57–93. https://doi.org/10.1130/GSAB-P2-92-57 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation John J. W. Rogers, Jeffrey K. Greenberg; Trace Elements in Continental-Margin Magmatism: Part III. Alkali Granites and Their Relationship to Cratonization. GSA Bulletin 1981;; 92 (1_Part_II): 57–93. doi: https://doi.org/10.1130/GSAB-P2-92-57 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 Alkali granites form as the final tectonic-magmatic event in shield areas and in many Phanerozoic orogenic belts. They occur mostly as discordant, post-tectonic plutons. Plutons of various ages and geographic locations are lithologically extremely similar, with an average composition of 72.5% SiO2, 0.3% TiO2, 14.1% A12O3, 2.2% total Fe as Fe2O3, 0.4% MgO, 1.3% CaO, 3.6% Na2O, and 4.8% K2O. The alkali granite suites differ from the felsic rocks of most calcalkalic batholiths in having higher K2O/MgO ratios and few rocks with less than 65% SiO2. Alkali granite magmas may form by normal crystallization differentiation from calc-alkalic adamellites (quartz monzonites), although not all alkali granite magmas have originated by that process.Most alkali granites do not show evidence of having been derived from a crustal source. In some occurrences, pre-existing sial apparently was not present in the area at the time that the alkali granite magmas evolved. Thus, alkali granites may be part of the process that causes development of stable cratons by addition of new sialic material to thicken the continental crust. The higher concentration of K2O in younger shield areas and compensatory changes in abundances of other elements are caused by the higher abundance of alkali granites in younger cratons. 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.