Relationship between deformation, equilibration temperatures, REE and radiogenic isotopes in mantle xenoliths (Ray Pic, Massif Central, France): an example of plume-lithosphere interaction? |
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Authors: | N A Zangana H Downes M F Thirlwall E Hegner |
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Institution: | (1) Birkbeck/UCL Research School of Geological and Geophysical Sciences, Birkbeck College, Malet St. London, WC1E 7HX, UK, GB;(2) Department of Geology, Royal Holloway University of London, Egham, TW20 0EX, UK, GB;(3) Institut für Mineralogie, Petrologie und Geochemie, University of Tübingen, Wilhelmstrasse 56, D-72074 Tübingen, Germany, DE |
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Abstract: | Anhydrous spinel peridotite xenoliths from the Ray Pic Quaternary alkali basalt volcano (French Massif Central) show a wide
range of mineralogical and geochemical compositions, reflecting significant heterogeneities in the shallow sub-continental
lithospheric mantle. Variations in modal mineralogy, mineral chem istry, REE patterns and radiogenic isotope data suggest
that depletion by partial melting and enrichment by cryptic metasomatism were the major mantle processes which caused the
heterogeneity. The lithospheric mantle beneath Ray Pic contains two contrasting types of peridotite: (i) lherzolites with
LREE-depleted compositions, high 143Nd/144Nd, low 87Sr/86Sr and unradiogenic Pb isotope ratios; (ii) lherzolites, harzburgites and a wehrlite with LREE-enriched patterns, low 143Nd/144Nd, high 87Sr/86Sr and radiogenic Pb isotope ratios. The former closely resemble depleted MORB-source mantle. The latter are related to enrichment
by recent infiltration of small degree partial melts or fluids from the asthenospheric mantle, possibly related to the “low
velocity component” observed by Hoernle et al. (1995) in European Neogene alkaline magmas. Thus, the Ray Pic peridotite xenoliths
represent interaction between asthenospheric mantle-derived melts/fluids and depleted lithospheric mantle. This is probably
linked to the upwelling mantle plume imaged beneath the Massif Central (Granet et al. 1995). A relationship between textural
deformation, equilibration temperature and geochemistry of the xenoliths suggests that the hotter (> 900 °C) undeformed regions
are LREE-enriched and tend to have more enriched isotope ratios, whereas the cooler (< 900 °C) regions have undergone more
deformation and are more depleted both in LREE and in isotope compositions.
Received: 27 July 1996 / Accepted: 25 November 1996 |
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