Metamorphic structure and P–T conditions of the metapelitic constituent of the ZEV

Abstract

Bounded by mylonite zones and imbrications the Zone of Erbendorf-Vohenstrauss is built up by medium- and low-pressure amphibolite-facies metapelites associated with amphibolites and biotite–hornblende gneisses, and intruded by the Leuchtenberg granite. Pelitic index assemblages define the staurolite– kyanite-, the garnet–kyanite-, the garnet–sillimanite- and the garnet–kyanite–sillimanite zone of medium pressure metamorphism, which is estimated at P=5–9.6 kbar and T=580–730 °C, and the cordierite zone of subsequent low-pressure metamorphism, which is estimated at P<4.5 kbar and T≤700 °C. Both facies series probably are tectonically juxtaposed. Staurolite–kyanite- and garnet zones are related by the reaction: staurolite+muscovite+quartz=garnet+ aluminium-silicate+biotite+H2O. This relation defines the only observable trace of a field gradient in the ZEV. An early sillimanite generation (sillimanite I) is found in all mineral zones as mineral inclusions in garnet rims. Post-P–Tmax evolution is indicated by the inversion kyanite⇒sillimanite (sillimanite II), the reaction: garnet+muscovite⇒sillimanite (sillimanite III)+biotite+quartz and by XFe decreasing in garnet rims. Garnet is resorbed under the conditions of stable kyanite+sillimanite+muscovite+quartz. Some staurolite–kyanite zone garnets show two stages separated by resorption which may be ascribed to published Ordovician and Late Devonian radiometric ages. The second garnet generation formed near 9.7 kbar/ 620–660 °C. No evidence for high-pressure metamorphism was found. Comparison of the medium-pressure part of the ZEV with the Zone of Tepla-Domazlice (Czech Republic) and with garnet–kyanite rocks at the northern edge of the Black Forest seems possible. The low-pressure cordierite rocks are similar to metapelites framing the Winklarn eclogites towards southeast of the ZEV, but are in contrast to the Moldanubian-type cordierite gneisses.