Coincidence of gabbro and granulite formation and their implication for Variscan HT metamorphism in the Moldanubian Zone (Bohemian Massif), example from the Kutná Hora Complex
Abstrakt
Leucocratic metagabbro and amphibolite from a mafic-ultramafic body within migmatite and granulite in the Kutna Hora Complex were investigated. The mafic-ultramafic rocks show amphibolite facies metamorphism, but in the central part of the body some metagabbro preserves cumulus and intercumulus plagioclase, clinopyroxene and spinel.
Spinel forms inclusions in both clinopyroxene and plagioclase and shows various degree of embayment structure, that was probably a result of reaction with melt during magmatic crystallization. In the metagabbro, garnet forms coronae around clinopyroxene at the contacts with plagioclase.
Amphibolite contains garnet with prograde zoning and plagioclase. Phase relations of igneous and metamorphic minerals indicate that magmatic crystallization and subsequent metamorphism occurred as a result of isobaric cooling at a depth of 30-35 km.
U-Pb dating on zircon from leucogabbro yielded a Variscan age (337.7 +/- 2 Ma) that is similar or close to the age of granulite fades metamorphism (ca 340 Ma) in the Moldanubian Zone. Based on the calculated PT conditions and age data, both the mafic-ultramafic body and surrounding granulite shared the same exhumation path from their middle-lower crustal position at the end of Variscan orogeny.
The coincidence of mafic-ultramafic intrusives and granulite-amphibolite facies metamorphism is explained by lithospheric upwelling beneath the Moldanubian Zone that occurred due to slab break-off during the final stages of subduction of the Moldanubian plate beneath the Tepla Barrandian Block, The model also addresses questions about the preservation of minerals and/or their compositions from the early metamorphic history of the rocks subjected to ultradeep subduction and subsequent granulite fades metamorphism.