Origin of Variscan Garnet and Spinel Pyroxenites from the Bohemian Massif (Moldanubian Zone, Czech Republic and Austria)

Abstract:

In the Czech and Austrian parts of the Bohemian Massif, garnet and spinel pyroxenites and eclogites form layers and lenses in mantle–derived peridotites that are enclosed by migmatitic gneiss and granulite. We have analyzed major and trace elements, Sr and Nd isotopes for a suite of pyroxenites, which vary in composition and origin, from nine localities in the Gföhl Unit and Kutná Hora Complex of the Moldanubian Zone. Based on conventional thermobarometry, the pyroxenites fall into three P–T regimes: (1) HP–HT at c. 3.3 GPa and 950 °C; (2) MP–HT at 1.5–2.5 GPa and 875–975°C; and (3) HP–MT at 3.5–3.7 GPa and c. 810 °C.

The pyroxenite suite exhibits large variations in elemental and isotopic compositions, reflecting their complex origin and evolution. Based on REE compositions of clinopyroxene (cpx), four different types (Type I-IV) of pyroxenites can be distinguished: (Type I) LREE–depleted; (Type II) convex–upward REE pattern, combined with significant enrichment in La; (Type III) variable, LREE–enriched patterns, and (Type IV) convex–upward REE pattern. Such different REE patterns in cpx reflect derivation from different compositions of mantle source melts, ranging from primitive (Type I) to enriched (Types II, III, and IV). For the pyroxenite suite as a whole, a positive correlation between Sr/Nd and Eu/Eu* argues for the presence of a crustal component (oceanic crust), which underwent significant plagioclase fractionation before incorporation in the parental pyroxenite melts. Variations in compatible elements (Ni, Sc, and Co) indicate that parental melts underwent combined fractional crystallization and assimilation of host peridotite at some localities.