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The Weaklaw Vent, SE Scotland: Metasomatism of eruptive products by carbo-hydro-fluids of probable mantle origin

Published online by Cambridge University Press:  21 October 2019

Brian G. J. Upton*
Affiliation:
School of GeoSciences, University of Edinburgh, West Mains Rd., EdinburghEH9 3FE, UK
Nic Odling
Affiliation:
School of GeoSciences, University of Edinburgh, West Mains Rd., EdinburghEH9 3FE, UK
Linda A. Kirstein
Affiliation:
School of GeoSciences, University of Edinburgh, West Mains Rd., EdinburghEH9 3FE, UK
John R. Underhill
Affiliation:
Centre for Exploration Geoscience, Applied Geosciences Unit, Institute of Petroleum Engineering, School of Energy, Geoscience, Infrastructure & Society, Heriot-Watt University, EdinburghEH14 4AS, UK
Jacek Puziewicz
Affiliation:
Institute of Geological Sciences, University of Wrocław, Pl. M. Borna, 50-204Wroclaw, Poland
Theodore Ntaflos
Affiliation:
Department für Lithosphärenforschung, Universität Wien, Althanstrasse 14, 1090Vienna, Austria
Bogusław Bagiński
Affiliation:
IGMiP Faculty of Geology, University of Warsaw, al. Żwirky i Wigury 93, 02-089, Warsaw, Poland
Steve Hillier
Affiliation:
The James Hutton Institute, Aberdeen, AB15 8QH. UK Department of Soil and Environment, Swedish University of Agricultural Sciences (SLU), SE-75007, Uppsala, Sweden
Jens C. Andersen
Affiliation:
Camborne School of Mines, University of Exeter, Penryn Campus, Tremough, PenrynTR10 9FE, UK
Gavin K. Rollinson
Affiliation:
Camborne School of Mines, University of Exeter, Penryn Campus, Tremough, PenrynTR10 9FE, UK
Jean-Philippe Perrillat
Affiliation:
Université de Lyon 1, CNRS, UMR 5276, Laboratoire de Lyon, Villeurbanne, F-69622, France
*
*Author for correspondence: Brian G. J. Upton, Email: [email protected]

Abstract

The Weaklaw vent in SE Scotland (East Lothian coast), inferred to be Namurian, produced lava spatter and volcanic bombs. The latter commonly contained ultramafic xenoliths. All were metasomatised by carbonic fluids rich in incompatible elements. The lavas and xenoliths are inferred to have been basanites and lherzolites prior to metasomatism. The abundance and size of (carbonated) peridotite xenoliths at Weaklaw denotes unusual rapidity of magma ascent and high-energy eruption making Weaklaw exceptional in the British Isles. The lavas and xenoliths were altered subsequently by low-temperature (<200°C) carbo-hydrous fluids to carbonate, clay and quartz assemblages. A small irregular tuffisite ‘dyke’ that transects the ejecta is also composed dominantly of carbonates and clays. The peridotitic xenoliths are typically foliated, interpreted as originating as pre-entrainment mantle shear-planes.

Analyses of the relic spinels shows them to be compositionally similar to spinels in local unaltered lherzolites from near-by basanitic occurrences. Chromium showed neither significant loss nor gain but was concentrated in a di-octahedral smectite allied to volkonskoite. It is in the complex association of smectite with other clays, chlorite and possibly fuchsite that the diverse incompatible elements are concentrated.

We conclude that late Palaeozoic trans-tensional fault movement caused mantle shearing. Rapid ascent of basanite magma entrained large quantities of sheared lithospheric mantle. This was followed by ascent of an aggressive carbonate-/ hydroxyl-rich fluid causing pervasive metasomatism. The vent is unique in several ways: in its remarkable clay mineralogy and in displaying such high Cr-clays in a continental intra-plate setting; in being more productive in terms of its ‘cargo’ of peridotite xenoliths; in presenting an essentially un-eroded sequence of Namurian extrusives; and, not least, for giving evidence for post-eruptive, surface release of small-melt, deep-source fluids.

Type
Article
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2019

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Footnotes

Associate Editor: Leone Melluso

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