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Complex subvolcanic magma plumbing system of an alkali basaltic maar-diatreme volcano (Elie Ness, Fife, Scotland)

Complex subvolcanic magma plumbing system of an alkali basaltic maar-diatreme volcano (Elie Ness, Fife, Scotland)
Complex subvolcanic magma plumbing system of an alkali basaltic maar-diatreme volcano (Elie Ness, Fife, Scotland)
Alkali basaltic diatremes such as Elie Ness (Fife, Scotland) expose a range of volcanic lithofacies that points to a complex, multi-stage emplacement history. Here, basanites contain phenocrysts including pyrope garnet and sub-calcic augites from depths of ~60 km. Volcanic rocks from all units, pyroclastic and hypabyssal, are characterised by rare earth element (REE) patterns that show continuous enrichment from heavy REE (HREE) to light REE (LREE), and high Zr/Y that are consistent with retention of garnet in the mantle source during melting of peridotite in a garnet lherzolite facies. Erupted garnets are euhedral and unresorbed, signifying rapid ascent through the lithosphere. The magmas also transported abundant pyroxenitic clasts, cognate with the basanite host, from shallower depths (~35–40 km). These clasts exhibit wide variation in texture, mode and mineralogy, consistent with growth from a range of compositionally diverse melts. Further, clinopyroxene phenocrysts from both the hypabyssal and pyroclastic units exhibit a very wide compositional range, indicative of polybaric fractionation and magma mixing. This is attributed to stalling of earlier magmas in the lower crust—principally from ~22–28 km— as indicated by pyroxene thermobarometry. Many clinopyroxenes display chemical zoning profiles, occasionally with mantles and rims of higher magnesium number (Mg#) suggesting the magmas were mobilised by juvenile basanite magma. The tuffs also contain alkali feldspar megacrysts together with Fe-clinopyroxene, zircon and related salic xenoliths, of the ‘anorthoclasite suite’—inferred to have crystallised at upper mantle to lower crustal depths from salic magma in advance of the mafic host magmas. Despite evidence for entrainment of heterogeneous crystal mushes, the rapidly ascending melts experienced negligible crustal contamination. The complex association of phenocrysts, megacrysts and autoliths at Elie Ness indicates thorough mixing in a dynamic system immediately prior to explosive diatreme-forming eruptions.
Alkali basalt, diatreme, clinopyroxene, pyroxenite, magma, eruption
0024-4937
70-85
Gernon, T.M.
658041a0-fdd1-4516-85f4-98895a39235e
Upton, B.G.J.
161f82be-02db-4248-8cfb-49d6367a91c9
Ugra, R.
903bd8f9-cc4a-48e2-ac4f-f886661a79a7
Yücel, C.
658ee4a9-9063-43d3-8ea4-e77727cc00c3
Taylor, R.N.
094be7fd-ef61-4acd-a795-7daba2bc6183
Elliott, H.
06593dc4-19b8-46ff-8725-3fd4bfe39619
Gernon, T.M.
658041a0-fdd1-4516-85f4-98895a39235e
Upton, B.G.J.
161f82be-02db-4248-8cfb-49d6367a91c9
Ugra, R.
903bd8f9-cc4a-48e2-ac4f-f886661a79a7
Yücel, C.
658ee4a9-9063-43d3-8ea4-e77727cc00c3
Taylor, R.N.
094be7fd-ef61-4acd-a795-7daba2bc6183
Elliott, H.
06593dc4-19b8-46ff-8725-3fd4bfe39619

Gernon, T.M., Upton, B.G.J., Ugra, R., Yücel, C., Taylor, R.N. and Elliott, H. (2016) Complex subvolcanic magma plumbing system of an alkali basaltic maar-diatreme volcano (Elie Ness, Fife, Scotland). Lithos, 264, 70-85. (doi:10.1016/j.lithos.2016.08.001).

Record type: Article

Abstract

Alkali basaltic diatremes such as Elie Ness (Fife, Scotland) expose a range of volcanic lithofacies that points to a complex, multi-stage emplacement history. Here, basanites contain phenocrysts including pyrope garnet and sub-calcic augites from depths of ~60 km. Volcanic rocks from all units, pyroclastic and hypabyssal, are characterised by rare earth element (REE) patterns that show continuous enrichment from heavy REE (HREE) to light REE (LREE), and high Zr/Y that are consistent with retention of garnet in the mantle source during melting of peridotite in a garnet lherzolite facies. Erupted garnets are euhedral and unresorbed, signifying rapid ascent through the lithosphere. The magmas also transported abundant pyroxenitic clasts, cognate with the basanite host, from shallower depths (~35–40 km). These clasts exhibit wide variation in texture, mode and mineralogy, consistent with growth from a range of compositionally diverse melts. Further, clinopyroxene phenocrysts from both the hypabyssal and pyroclastic units exhibit a very wide compositional range, indicative of polybaric fractionation and magma mixing. This is attributed to stalling of earlier magmas in the lower crust—principally from ~22–28 km— as indicated by pyroxene thermobarometry. Many clinopyroxenes display chemical zoning profiles, occasionally with mantles and rims of higher magnesium number (Mg#) suggesting the magmas were mobilised by juvenile basanite magma. The tuffs also contain alkali feldspar megacrysts together with Fe-clinopyroxene, zircon and related salic xenoliths, of the ‘anorthoclasite suite’—inferred to have crystallised at upper mantle to lower crustal depths from salic magma in advance of the mafic host magmas. Despite evidence for entrainment of heterogeneous crystal mushes, the rapidly ascending melts experienced negligible crustal contamination. The complex association of phenocrysts, megacrysts and autoliths at Elie Ness indicates thorough mixing in a dynamic system immediately prior to explosive diatreme-forming eruptions.

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Accepted/In Press date: 15 August 2016
Published date: 1 November 2016
Keywords: Alkali basalt, diatreme, clinopyroxene, pyroxenite, magma, eruption
Organisations: Geology & Geophysics, Geochemistry

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Local EPrints ID: 399447
URI: https://eprints.soton.ac.uk/id/eprint/399447
ISSN: 0024-4937
PURE UUID: 6906c553-c729-45fa-a956-2a179cc1f023

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Date deposited: 16 Aug 2016 11:02
Last modified: 10 Dec 2019 06:26

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Contributors

Author: T.M. Gernon
Author: B.G.J. Upton
Author: R. Ugra
Author: C. Yücel
Author: R.N. Taylor
Author: H. Elliott

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