Garnet pyroxenite in the biskupice peridotite, Bohemian Massif: Anatomy of a variscan high-pressure cumulate

Abstract

A strongly layered garnet pyroxenite, consisting of cm-scale layers of garnetite, garnet orthopyroxenite, garnet clinopyroxenite, and websterite, occurs in the Biskupice garnet peridotite, which is located in migmatitic gneiss of the Gföhl Assemblage in the Vysočina District, western Moravia. Major- and trace-element compositions and REE patterns of minerals and layers in the pyroxenite are consistent with origin of the layers by HT-HP crystallization and accumulation of variable proportions of garnet, orthopyroxene, and clinopyroxene. The present compositions of the minerals (pyroperich garnet, low-Al, low-Ca enstatite and low-Al, high-Ca diopside) are the result of extensive subsolidus re-equilibration at ~900 oC and ~37 kbar, during the Variscan (332Ma) Orogeny. Neodymium and strontium isotopic values (ε335/Nd, +1.8 to +2.1; (87Sr/86Sr)335, 0.7049) and negative HFSE anomalies indicate a crustal component in the melt from which the layers crystallized. Most garnet pyroxenites and eclogites in Gföhl garnet peridotites are thought to have formed by HT-HP crystallization from silicate melts containing crustal components, and the Biskupice garnet pyroxenite is an excellent example of such a petrogenetic process.