Differentiation of the Tumatumari-Kopinang Dolerite Intrusion, British Guiana

Abstract

Research Article| October 01, 1966 Differentiation of the Tumatumari-Kopinang Dolerite Intrusion, British Guiana D. D HAWKES D. D HAWKES Dept. Geology, University College of Sierra Leone, Freetown, Sierra Leone, West Africa Search for other works by this author on: GSW Google Scholar Author and Article Information D. D HAWKES Dept. Geology, University College of Sierra Leone, Freetown, Sierra Leone, West Africa Publisher: Geological Society of America Received: 21 Dec 1964 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1966, 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 (1966) 77 (10): 1131–1158. https://doi.org/10.1130/0016-7606(1966)77[1131:DOTTDI]2.0.CO;2 Article history Received: 21 Dec 1964 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 D. D HAWKES; Differentiation of the Tumatumari-Kopinang Dolerite Intrusion, British Guiana. GSA Bulletin 1966;; 77 (10): 1131–1158. doi: https://doi.org/10.1130/0016-7606(1966)77[1131:DOTTDI]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 Guiana shield is a metamorphic, crystalline basement unconformably overlain by horizontal sediments of the Roraima Formation. During the Precambrian era, the basement and overlying sediments were intruded by sills, inclined sheets, and dykes of dolerite. The magma was tholeiitic.The Tumatumari-Kopinang intrusion consists of a dyke which rises steeply through the basement and then passes into a sill intruded along the plane of unconformity beneath the Roraima Formation. This unconformity sill, in turn, ascends into a higher sill within the Roraima Formation.In the highest part of the intrusion hornblende-granophyre occurs, but silica-poor ferrodolerite is found in the dyke. Field, petrological, and chemical evidence demonstrates that the granophyre and the ferrodolerite are late differentiates of the same parent magma. Despite contrasting mineral and chemical compositions, the trend of differentiation in both cases was toward absolute enrichment in iron. The granophyre represents moderate, and the ferrodolerite strong, fractionation of the parent magma. Fractionation was accomplished by the gravitational settling of the early-formed minerals and strengthened by the circulation of the magma by convection.Evidence from the Tumatumari-Kopinang intrusion suggests that differentiation of tholeiitic magma always results in iron enrichment and is unlikely to be affected by variations in oxygen partial pressure or in the initial composition of the magma. The presence of water may influence the nature of the mineral phases that crystallize, but does not alter the trend toward iron enrichment. 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.