Garnet pyroxenite and eclogite in the Bohemian Massif: geochemical evidence for Variscan recycling of subducted lithosphere

Abstract

High-temperature, high-pressure eclogite and garnet pyroxenite occur as lenses in garnet peridotite bodies of the Gfohl 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 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 [eNd, +5.4 to −6.0; (87Sr/86Sr)i, 0.70314–0.71445; δ18OSMOW, 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 Dobesovice eclogite (338 Ma), Uhrov eclogite (344 Ma) and Nove Dvory garnet pyroxenite (343 Ma). Gfohl 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.