High-pressure melting of eclogite and the P-T-X history of tonalitic to trondhjemitic zoisite-pegmatites, Münchberg Massif, Germany

Zoisite-bearing high-pressure pegmatites from the Münchberg Massif, Germany, provide an excellent example of the characteristics of the onset of metabasite melting at eclogite-facies conditions. The pegmatites were derived by partial melting of a mid-ocean ridge basalt (MORB)-like eclogite at T ≥680°C/2.3GPa to 750°C/3.1GPa, which produced small amounts of tonalitic to trondhjemitic melt. The melt concentrated locally in isolated, small melt pockets and crystallized primary zoisite as liquidus phase at P ≥2.3GPa/680°C to 2.1GPa/750°C. Compositional zoning of pegmatite zoisite records an ensuing multi-stage uplift history with successive, discrete crystallization events at 14±0.2GPa/650-700°C and 1.0±0.1GPa/620-650°C. Resorption textures indicate reheating and thermal perturbation of the whole system prior to each successive crystallization event. Final solidification of zoisitepegmatites occurred at 0.9± 0.1GPa/620-650°C.The data suggest that isolated melt + zoisite crystal mush pockets formed an integral part of the eclogite throughout uplift from melt formation at T ≥680°C/ 2.3 GPa to 750°C/3.1GPa to final solidification at ∼0.9 GPa/ 620-630°C; that is, over a depth range of 45-60 km. The entire pegmatite-forming process was probably fluid conserving: fluid present during melt formation was trapped by fully or nearly water-saturated siliceous melts, whereas fluid liberated during pegmatite crystallization interacted with dehydrated eclogite-facies assemblages to form amphibolite-facies hydrous minerals. A set of empirical D ^{melt/eclogite} values based on mean zoisite-pegmatite and eclogite composition were used to model the onset of partial highpressure melting of metabasites.