Characteristics and origins of diverse Variscan peridotites in the Gföhl Nappe, Bohemian Massif, Czech Republic

Abstract

In the Variscan Orogen, the Gföhl terrane in the Moldanubian zone of the Bohemian Massif appears to be unusual among members of high-pressure collisional belts in containing diverse types of mantle-derived peridotite. Three groups of peridotite have been identified in the Czech part of the Gföhl Nappe, based on major and trace element compositions, pressure–temperature (P–T) conditions, and cooling rates. Type I consists of spinel and garnet peridotite, is devoid of garnet pyroxenite or eclogite layers, has depleted major element and REE compositions, yields P–T estimates that lie in a low P/T regime, and experienced very rapid cooling. Type II is distinguished by relatively high Fe contents and an abundance of garnet pyroxenite layers. Type III consists of garnet peridotite, contains garnet pyroxenite and eclogite layers, shows a range of LREE depletion to enrichment, yields P–T estimates in a medium P/T regime, and cooled more slowly than Type I. Such a diversity of peridotite types in the Gföhl Nappe is ascribed to derivation from different mantle sites between and beneath Bohemia and Moldanubia during their Early Carboniferous collision. Type I is likely suboceanic lithosphere and asthenosphere that originated in a pre-collision ocean basin between Bohemia and Moldanubia, Type II appears to be a disrupted mafic–ultramafic cumulate complex, and Type III probably represents subcontinental lithosphere that was derived from the mantle wedge beneath Bohemia.