Garnet pyroxenite and eclogite in the Bohemian Massif: geochemical evidence for Variscan recycling of subducted lithosphere |
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| Authors: | L G Medaris Jr B L Beard C M Johnson J W Valley M J Spicuzza E Jelínek Z Mísâr |
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| Institution: | (1) Department of Geology and Geophysics, University of Wisconsin, 53706 Madison, WI, USA;(2) Department of Geochemistry, Mineralogy & Crystallography, Charles University, 128 43 Prague 2, Czech Republic;(3) Department of Geology, Charles University, 128 43 Praha 2, Czech Republic |
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| Abstract: | High-temperature, high-pressure eclogite and garnet pyroxenite occur as lenses in garnet peridotite bodies of the Gföhl nappe in the Bohemian Massif. The high-pressure assemblages formed in the mantle and are important for allowing investigations of mantle compositions and processes. Eclogite is distinguished from garnet pyroxenite on the basis of elemental composition, with mg number <80, Na2O > 0.75 wt.%, Cr2O3 < 0.15 wt.% and Ni < 400 ppm. Considerable scatter in two-element variation diagrams and the common modal layering of some eclogite bodies indicate the importance of crystal accumulation in eclogite and garnet pyroxenite petrogenesis. A wide range in isotopic composition of clinopyroxene separates  Nd, +5.4 to –6.0; (87Sr/86Sr)i, 0.70314–0.71445;  18OSMOW, 3.8–5.8%o] requires that subducted oceanic crust is a component in some melts from which eclogite and garnet pyroxenite crystallized. Variscan Sm-Nd ages were obtained for garnet-clinopyroxene pairs from Dobe ovice eclogite (338 Ma), Úhrov eclogite (344 Ma) and Nové Dvory garnet pyroxenite (343 Ma). Gföhl eclogite and garnet pyroxenite formed by high-pressure crystal accumulation (±trapped melt) from transient melts in the lithosphere, and the source of such melts was subducted, hydrothermally altered oceanic crust, including subducted sediments. Much of the chemical variation in the eclogites can be explained by simple fractional crystallization, whereas variation in the pyroxenites indicates fractional crystallization accompanied by some assimilation of the peridotite host. |
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| Keywords: | Bohemian Massif Variscan Geochemistry Garnet pyroxenite Eclogite Lithosphere |
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