Lower crustal and possible shallow mantle samples from beneath the Hebrides; evidence from a xenolithic dyke at Gribun, western Mull

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doi: 10.1144/gsjgs.155.5.0813
Authors:Upton, B. G. J.; Aspen, P.; Rex, D. C.; Melcher, F.; Kinny, P.
Author Affiliations:Primary:
University of Edinburgh, Department of Geology and Geophysics, Edinburgh, United Kingdom
University of Leeds, United Kingdom
Montan-Universität, Austria
Curtin University of Technology, Australia
Volume Title:Journal of the Geological Society of London
Source:Journal of the Geological Society of London, Vol.155(Part 5), p.813-828. Publisher: Geological Society of London, London, United Kingdom. ISSN: 0016-7649
Publication Date:1998
Note:In English. 41 refs.; illus., incl. 7 tables, sketch map
Summary:A Permian (268 ± 2 m.y.) olivine nephelinite dyke cutting Proterozoic (Moinian) psammites on the W coast of Mull contains abundant xenoliths inferred to be from the lower crust and possibly uppermost mantle. Most are pargasite pyroxenites grading to hornblendites, with pyroxene granulite orthogneisses next in abundance, followed by high-grade meta-arenites, and scarce xenoliths and megacrysts composed of anorthoclase, ferrosalite, apatite, magnetite and ilmenite. The pargasite pyroxenites are derived from ultramafic protoliths that have undergone recrystallization, modal metasomatism (with introduction of amphibole) and deformation prior to entrainment. The mesocratic xenoliths are granulite-facies orthogneisses with plagioclase (An38-22), augite, magnetite, ilmenite and apatite; Pb-U SHRIMP dating of zircons indicates a crystallization age of 1850 ± 50 m.y. The most evolved of the orthogneisses are two-feldspar quartz diorites with subordinate sanidine (Or64-74). The magmas parental to the postulated underplating intrusion are thought to have been mildly alkaline (transitional) basalts. The metasomatic introduction of amphibole to the ultramafic rocks is attributed to pervasive influx of Fe, Ti, K, LREE-rich partial melts from the asthenosphere. The megacryst suite, however, may represent a younger event involving intrusion of a geochemically evolved vein system within the amphibole pyroxenites. Estimates of T of equilibrium gave ∼ 800°C, suggesting a geothermal gradient of ∼ 27uC/km. [R.A.H.]
Subsections:Igneous petrology
Subjects:Absolute age; Archean; Chain silicates; Crust; Dates; Dikes; Facies; Granulite facies; Inclusions; Intrusions; Ion probe data; Lower crust; Mantle; Mass spectra; Mineral assemblages; Minerals; Nesosilicates; Orthosilicates; Paleoproterozoic; Paleozoic; Permian; Petrography; Precambrian; Proterozoic; Pyroxene group; SHRIMP data; Silicates; Spectra; Temperature; U/Pb; Underplating; Upper mantle; Upper Precambrian; Xenoliths; Zircon; Zircon group; Argyllshire Scotland; Europe; Great Britain; Hebrides; Inner Hebrides; Mull Island; Scotland; United Kingdom; Western Europe; Gribun Scotland; Strathclyde region Scotland
Coordinates:N561500 N564000 W0054000 W0063000
Abstract Numbers:99M/838
Record ID:1998055224
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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