Geochemical evolution and unusual pyroxene chemistry of the MD tholeiite dyke swarm from the Archaean craton of southern West Greenland

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Authors:Hall, R. P.; Hughes, D. J.; Friend, C. R. L.
Author Affiliations:Primary:
Portsmouth Polytech., Dep. Geol., Portsmouth, United Kingdom
Oxford Polytech., United Kingdom
Volume Title:Journal of Petrology
Source:Journal of Petrology, 26(2), p.253-282. Publisher: Clarendon Press, Oxford, United Kingdom. ISSN: 0022-3530
Publication Date:1985
Note:In English. 71 refs.; illus. incl. 6 tables
Summary:This swarm is composed of four generations of large basic tholeiite dykes which cut the entire Archaean craton. The four successive generations (MD1, MD2, MD3a, MD3b) are characterized by their orientation of cross-cutting relationships and their mineralogy, texture and progressively evolved tholeiitic chemistry. The suite varies from early (MD1) heteradcumulate norites to ophitic and sub-ophitic gabbroic and doleritic rocks (MD2 and MD3), whilst the youngest generation (MD3b) comprises plagioclase-phyric dolerites. The pyroxene chemistry generally follows in the geochemical evolution of the dykes but the clinopyroxenes are chemically highly variable and Ca-poor, many plotting in the metastable field; individual grains are often complex and show regular or irregular zoning. The MD1 dykes also contain pyroxene dendrites poikilitically enclosed by plagioclase, the dendrites varying from hypersthene bases to branches of pigeonite and sub-calcic augite terminating in augite tips. The pyroxenes of MD2 and MD3a dykes are even more complex and are mostly sub-ophitic grains with successive zones of orthopyroxene, pigeonite, sub-calcic augite, augite and ferroaugite, but opposing Ca and/or Fe depletion of enrichment trends can be seen in neighbouring grains. The presence of such a wide variety of pyroxenes of apparently both stable and metastable compositions suggests that these dykes have undergone a complex and unusual cooling history. The principal genetic factors influencing their crystallization could have been 1) supercooling, 2) the evolution of discrete interstitial liquid cells, 3) augite-pigeonite peritectic reactions and 4) plagioclase growth and delay of pyroxene nucleation during supercooling of liquid to below the basalt liquidus. [R.A.H.]
Subjects:Augite; Basalts; Chain silicates; Clinopyroxene; Crystallization; Differentiation; Dike swarms; Dikes; Feldspar group; Framework silicates; Geochemistry; Igneous rocks; Intrusions; Magmas; Metals; Petrography; Petrology; Plagioclase; Pyroxene group; Rare earths; Silicates; Textures; Tholeiitic composition; Volcanic rocks; Arctic region; Greenland; West Greenland; Intrusive rocks; MD dike swarm
Abstract Numbers:85M/4703
Record ID:1987068103
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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