Ultramafic xenoliths from the Bearpaw Mountains, Montana, USA; evidence for multiple metasomatic events in the lithospheric mantle beneath the Wyoming Craton

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doi: 10.1093/petrology/egh027
Authors:Downes, Hilary; MacDonald, Ray; Upton, Brian G. J.; Cox, Keith G.; Bodinier, Jean-Louis; Mason, Paul R. D.; James, Dodie; Hill, Peter G.; Hearn, B. Carter, Jr.
Author Affiliations:Primary:
University of London, School of Earth Sciences, London, United Kingdom
Other:
Lancaster University, United Kingdom
University of Edinburgh, United Kingdom
University of Oxford, United Kingdom
Université de Montpellier, France
University of Utrecht, Netherlands
U. S. Geological Survey, United States
Volume Title:Journal of Petrology
Source:Journal of Petrology, 45(8), p.1631-1662. Publisher: Oxford University Press, Oxford, United Kingdom. ISSN: 0022-3530
Publication Date:2004
Note:In English. 69 refs.; illus., incl. 12 tables, sketch map
Summary:Ultramafic xenoliths in Eocene minettes of the Bearpaw Mountains volcanic field (Montana, USA), derived from the lower lithosphere of the Wyoming craton, can be divided based on textural criteria into tectonite and cumulate groups. The tectonites consist of strongly depleted spinel lherzolites, harzburgites and dunites. Although their mineralogical compositions are generally similar to those of spinel peridotites in off-craton settings, some contain pyroxenes and spinels that have unusually low Al2O3 contents more akin to those found in cratonic spinel peridotites. Furthermore, the tectonite peridotites have whole-rock major element compositions that tend to be significantly more depleted than non-cratonic mantle spinel peridotites (high MgO, low CaO, Al2O3 and TiO2) and resemble those of cratonic mantle. These compositions could have been generated by up to 30% partial melting of an undepleted mantle source. Petrographic evidence suggests that the mantle beneath the Wyoming craton was re-enriched in three ways: (1) by silicate melts that formed mica websterite and clinopyroxenite veins; (2) by growth of phlogopite from K-rich hydrous fluids; (3) by interaction with aqueous fluids to form orthopyroxene porphyroblasts and orthopyroxenite veins. In contrast to their depleted major element compositions, the tectonite peridotites are mostly light rare earth element (LREE)-enriched and show enrichment in fluid-mobile elements such as Cs, Rb, U and Pb on mantle-normalized diagrams. Lack of enrichment in high field strength elements (HFSE; e.g. Nb, Ta, Zr and Hf) suggests that the tectonite peridotites have been metasomatized by a subduction-related fluid. Clinopyroxenes from the tectonite peridotites have distinct U-shaped REE patterns with strong LREE enrichment. They have 143Nd/144Nd values that range from 0.5121 (close to the host minette values) to 0.5107, similar to those of xenoliths from the nearby Highwood Mountains. Foliated mica websterites also have low 143Nd/144Nd values (0.5113) and extremely high 87Sr/86Sr ratios in their constituent phlogopite, indicating an ancient (probably mid-Proterozoic) enrichment. This enriched mantle lithosphere later contributed to the formation of the high-K Eocene host magmas.
Sections:Petrology
Subsections:Igneous petrology
Subjects:Clinopyroxenite; Geochemistry; Igneous rocks; Inclusions; Lithogeochemistry; Lithosphere; Mantle; Mantle plumes; Metals; Metasomatism; Peridotites; Plutonic rocks; Pyroxenite; Rare earths; Ultramafics; Websterite; Xenoliths; Blaine County Montana; Montana; United States; Bearpaw Mountains; Wyoming Craton
Coordinates:N474200 N490000 W1081500 W1095000
Abstract Numbers:04M/3418
Record ID:2004079987
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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