Petrology and Rb-Sr age of Precambrian rhyolitic dikes, Llano County, Texas

Research Article| February 01, 1976 Petrology and Rb-Sr age of Precambrian rhyolitic dikes, Llano County, Texas STEPHEN E. DELONG; STEPHEN E. DELONG 1Department of Geological Sciences, State University of New York at Albany, Albany, New York 12222 Search for other works by this author on: GSW Google Scholar LEON E. LONG LEON E. LONG 2Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Search for other works by this author on: GSW Google Scholar Author and Article Information STEPHEN E. DELONG 1Department of Geological Sciences, State University of New York at Albany, Albany, New York 12222 LEON E. LONG 2Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1976) 87 (2): 275–280. https://doi.org/10.1130/0016-7606(1976)87<275:PARAOP>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 Email Permissions Search Site Citation STEPHEN E. DELONG, LEON E. LONG; Petrology and Rb-Sr age of Precambrian rhyolitic dikes, Llano County, Texas. GSA Bulletin 1976;; 87 (2): 275–280. doi: https://doi.org/10.1130/0016-7606(1976)87<275:PARAOP>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 Rhyolite porphyry (llanite) and iron-rich melarhyolite dikes intrude Precambrian metamorphic rocks in north-central and south-eastern Llano County, respectively. These dikes are chemically and mineralogically similar, except that rhyolite porphyry is richer in quartz and alkali feldspar phenocrysts, whereas melarhyolite contains a greater abundance of biotite and magnetite. There are corresponding enrichments of SiO2 in rhyolite porphyry and of FeO + Fe2O3 in melarhyolite. The two rock types may have originated as two fractions of a common parent magma enriched in salic and mafic phases, respectively, to produce rhyolite porphyry and melarhyolite. This proposed relation is compatible with major-element chemistry and Rb-Sr systematics. Isotopic analyses of five melarhyolite whole rocks, two rhyolite porphyry whole rocks, groundmass and feldspar phenocrysts from rhyolite porphyry, and a nearby diabase dike yield an isochron age of 1,106 ± 6 m.y. (λRb = 1.39 × 10−11yr−1 and initial 87Sr/86Sr of 0.7028 ± 0.0002. Other data, including Zartman's (1964) analyses of the microcline fraction of alkali feldspar phenocrysts from rhyolite porphyry, indicate subsequent Rb-Sr redistributions. These may be related to devitrification of melarhyolite groundmass and to unmixing of alkali feldspar phenocrysts in rhyolite porphyry. 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.