Direct dating of mylonite evolution; a multi-disciplinary geochronological study from the Moine thrust zone, NW Scotland

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doi: 10.1144/gsjgs.155.5.0745
Authors:Freeman, S. R.; Butler, R. W. H.; Cliff, R. A.; Rex, D. C.
Author Affiliations:Primary:
University of Leeds, School of Earth Sciences, Leeds, United Kingdom
Volume Title:Journal of the Geological Society of London
Source:Journal of the Geological Society of London, Vol.155(Part 5), p.745-758. Publisher: Geological Society of London, London, United Kingdom. ISSN: 0016-7649
Publication Date:1998
Note:In English. 45 refs.; illus., incl. sects., 4 tables, geol. sketch map
Summary:Pb-Sr dating of sub-closure T, syndeformationally crystallized white micas from the Moine mylonites at Knockan and Dundonnell has given ages of 437-408 m.y. From morphological and geochemical evidence (EPMA results for 14 phengites), all these micas were (re)crystallized during the Moine Thrust shearing, and these ages are therefore interpreted as marking the end of the crystal plastic deformation associated with this shearing. The Pb-Sr ages are internally consistent and broadly compatible with existing data. The general cessation of ductile deformation at ∼ 430 m.y. probably dates the general initiation of imbrication within the footwall to the Moine Thrust; the age of 430 m.y is therefore likely to be a time of significant brittle displacement on the underlying Ben More Thrust. K-Ar dating of these Moine mylonite micas gave anomalously old ages in comparison, as a result of incorporation of excess Ar during shearing; this effect persists into the hanging wall and becomes indiscernible at ∼ 1.5 km structurally above the base of the shear zone, so this 1.5 km distance may represent the distance that fluid can have flowed out of the shear zone during deformation. [R.A.H.]
Sections:Age determination
Subjects:Absolute age; Chemical composition; Dates; Deformation; Devonian; Faults; K/Ar; Major elements; Metamorphic rocks; Mica group; Muscovite; Mylonites; Ordovician; Paleozoic; Rb/Sr; Shear zones; Sheet silicates; Silicates; Silurian; Strain; Tectonics; Caledonides; Europe; Great Britain; Moine thrust zone; Scotland; United Kingdom; Western Europe; Ben More Thrust
Abstract Numbers:99M/14
Record ID:1998055219
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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