Pre-eruptive magma mixing in ash-flow deposits of the Tertiary Rum igneous centre, Scotland

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doi: 10.1007/s00410-004-0584-0
Authors:Troll, Valentin R.; Donaldson, Colin H.; Emeleus, C. Henry
Author Affiliations:Primary:
Trinity College, Department of Geology, Dublin, Ireland
Other:
University of St. Andrews, United Kingdom
University of Durham, United Kingdom
Volume Title:Contributions to Mineralogy and Petrology
Source:Contributions to Mineralogy and Petrology, 147(6), p.722-739. Publisher: Springer International, Heidelberg-New York, International. ISSN: 0010-7999
Publication Date:2004
Note:In English. 64 refs.; illus., incl. 4 tables, geol. sketch map
Summary:The N Marginal Zone of the Rum Igneous Centre is a remnant of an early caldera and its infill. It is composed of intra-caldera breccias and various small-volume pyroclastic deposits, overlain by prominent rhyodacite ash-flow sheets of up to 100 m thickness. The ash-flows were fed from a feeder system near the caldera ring-fault, and intrusive rhyodacite can locally be seen grading into extrusive deposits. A variety of features suggest that the ash-flows were erupted from a magma chamber that contemporaneously hosted felsic and mafic magmas: i) chilled basaltic inclusions in rhyodacite; ii) formerly glassy basaltic to andesitic enclaves with fluid-fluid relationships; iii) feldspars with thick reaction rims enclosed in the basaltic to andesitic inclusions, yet with cores chemically resembling those of the rhyodacite: iv) trace element compositions of the rhyodacite and the mafic enclaves form a mixing line between the end-member rhyodacite and basalt compositions; v) feldspars with resorption embay-ments cutting through internal zonation of the crystals; vi) complexly zoned crystals with sieve-textured zones that are overgrown with euhedral zones; vii) oscillatory zonation of feldspar phenocrysts in the rhyodacite, showing sharp increases in anorthite (ΔAn > 10%) followed by gradual decrease in An-content (ΔAn < 4%). This evidence points to eruption of ash-flows from a felsic magma chamber that was periodically replenished by injection of mafic magma. Diffusional mixing between the two magmas was permitted by T and compositional differences, but was slow due to the contrast in viscosities and densities. [G.L.B.]
Sections:Petrology
Subsections:Igneous petrology; Volcanology
Mineral Groups:Framework silicates
Subjects:Ash flows; Calderas; Cenozoic; Chemical composition; Eruptions; Geochemistry; ICP mass spectra; Igneous rocks; Inclusions; Isotopes; Magmas; Major elements; Mass spectra; Metals; Mineral composition; Mixing; Models; Petrography; Pyroclastics; Rare earths; Rhyodacites; Spectra; Tertiary; Thermal ionization mass spectra; Trace elements; Volcanic centers; Volcanic features; Volcanic rocks; Volcanism; X-ray fluorescence spectra; Europe; Great Britain; Hebrides; Highland region Scotland; Inner Hebrides; Inverness-shire Scotland; Rhum; Scotland; United Kingdom; Western Europe
Coordinates:N570000 N570000 W0062200 W0062200
Abstract Numbers:04M/3754
Record ID:2004084787
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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