REACTION TEXTURES AND FLUID BEHAVIOR IN VERY HIGH-PRESSURE CALC-SILICATE ROCKS OF THE MUNCHBERG GNEISS COMPLEX, BAVARIA, GERMANY

Calc-silicate rocks occur as elliptical bands and boudins intimately i nterlayered with eclogites and high-pressure gneisses in the Munchberg gneiss complex of NE Bavaria. Core assemblages of the boudins consist of grossular-rich garnet, diopside, quartz, zoisite, clinozoisite, ca lcite, rutile and titanite. The polygonal granoblastic texture commonl y displays mineral relies and reaction textures such as post-kinematic grossular-rich garnet coronas. Reactions between these mineral phases have been modelled in the CaO-Al2O3-SiO2-CO2-H2O system with an inter nally consistent thermodynamic data base. High-pressure metamorphism i n the calc-silicate racks has been estimated at a minimum pressure of 31 kbar at a temperature of 630 degrees C with X(H2O)less than or equa l to 0.03. Small volumes of a CO2-N-2-rich fluid whose composition was buffered on a local scale were present at peak-metamorphic conditions . The P-T conditions for the onset of the amphibolite facies overprint are about 10 kbar at the same temperature. X(CO2) of the H2O-rich flu id phase is regarded to have been <0.03 during amphibolite facies cond itions. These P-T estimates are interpreted as representing different stages of recrystallization during isothermal decompression. The prese nce of multiple generations of mineral phases and the preservation of very high-pressure relies in single thin sections preclude pervasive p ost-peak metamorphic fluid flow as a cause of a re-equilibration withi n the calc-silicates. The preservation of eclogite facies, very high-p ressure relies as well as amphibolite facies reactions textures in the presence of a fluid phase is in agreement with fast, tectonically dri ven unroofing of these rocks.