Evaporites as precursors of massif anorthosite

Abstract

Research Article| January 01, 1978 Evaporites as precursors of massif anorthosite Randall L. Gresens Randall L. Gresens 1Department of Geological Sciences, University of Washington, Seattle, Washington 98195 Search for other works by this author on: GSW Google Scholar Author and Article Information Randall L. Gresens 1Department of Geological Sciences, University of Washington, Seattle, Washington 98195 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1978) 6 (1): 46–50. https://doi.org/10.1130/0091-7613(1978)6<46:EAPOMA>2.0.CO;2 Article history First Online: 02 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 Randall L. Gresens; Evaporites as precursors of massif anorthosite. Geology 1978;; 6 (1): 46–50. doi: https://doi.org/10.1130/0091-7613(1978)6<46:EAPOMA>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 SocietyGeology Search Advanced Search Abstract There is evidence that evaporites were present in premetamorphic rocks associated with massif-type anorthosite. In the Grenville Series, which contains the Adirondack anorthosite massifs, anhydrite and halite are preserved in marble. Evaporites and their enclosing shales represent concentrations of Na, Ca, Al, and Si- the elements essential to formation of anorthosite. Permissive evidence for derivation of anorthosite from evaporite sequences includes the high oxidation state of some massif anorthosite, consistent with derivation from oxidized metasedimentary rock; widespread occurrence of scapolite in anorthosite and other Grenville rocks; Sr-isotope data, which yield 87 Sr/86 Sr ratios that correspond to the values for Precambrian sea water; and O-isotope data, which yield metasedimentary 18O/16O ratios for the Adirondack anorthosites.A chemical model is proposed whereby fluids generated by dissolution of evaporite under high-grade metamorphism react with pelitic rock. The initial high ratios of aNa+/aH+ and aCa2+/aH+ maintain the plagioclase stability field. As the fluid migrates and reacts, the Na and Ca concentrations are diminished, and the release of H+ to the fluid is buffered by sulfate and/or carbonate, but the concentration of K+ increases. When the ratio of aK+/aH+ is sufficiently high, both plagioclase and K-feldspar are stable, and gradations of anorthosite into syenite, quartz syenite, or charnockite are produced. The fluid may have a composition intermediate between a supercritical aqueous solution and silicate magma and may evolve toward a low-melting composition in the quartz-plagioclase-orthoclase system. Anatexis could account for potassic dikes that sometimes cut anorthosite borders.Textural features of massif anorthosite, the replacement origin of anorthosite at the borders of some massifs, widespread reports of metasomatism in Grenville rocks, and well-documented reports of other metasomatic anorthosites support the model.Massif anorthosites are restricted to an age range of 1.0 to 1.7 b.y. because (1) sea water was not sufficiently saline to produce evaporites during earlier Earth history and (2) Paleozoic and younger shelf-type evaporites are not yet metamorphosed, but remain on the stable 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.