High-Pressure Melting of Eclogite and the P-T-X History of Tonalitic to Trondhjemitic Zoisite-Pegmatites, Munchberg Massif, Germany

Abstract

SUMMARY The zoisite-pegmatites provide an excellent example of theonset of metabasite melting at eclogite-facies conditions.Such conditions can be expected in a down-going oceanicslab as well as in collision zones.The newly produced meltin a metabasaltic rock of MORB composition is tonalitic(to trondhjemitic) and shares some characteristics withthe TTG series or with adakites. The amount of melt,forming at a temperature of less than 7508C, however,is small, as a result of the combined effect of relatively lowtemperature and restricted availability of water, and themelt does not separate from the rock unit. Only if themelt fraction increases will the melt segregate and mightmigrate upwards, influencing melting in the overlyingmantle wedge. Because of the poor outcrop situation ofthe Mu«nchberg area in the Variscan chain of CentralEurope, it is unfortunately impossible to give an estimateof the amount of melt produced. This will be a crucialquestion to solve: at what degree of melting, at whattemperature in a given composition andat what geological(mechanical) conditions the melt will effectively separate.Our data suggest the following model: high-pressurepartial melting and pegmatite formation occurred at con-ditions only slightly above the water-saturated solidus andcan be a fluid-conserving or fluid-recycling process.Any fluid released from hydrous minerals such as zoisite,epidote or amphibole is trapped in small fractions offully to nearly water-saturated siliceous melt (for truedehydration melting, a free fluid phase never exists).Neither the fluid nor the siliceous melt will separate andmigrate into the overlying rock units. During uplift, cool-ing and crystallization, the water, which is stored in thesiliceous melt, is again liberated and encounters a dehy-drated eclogite-facies assemblage. Here, it hydrates theeclogite-facies assemblage, leading to crystallization ofhydrous minerals such as amphibole or mica. This modelof fluid conservation or fluid recycling implies also thatthe trace element budget of the whole-rock does notchange during melting and subsequent crystallization.The pegmatites from the Hangendserie of the Mu«nchbergarea still preserve some trace element signatures of theirindividual protoliths, which means that the partial meltsdid not mix andwere not homogenized. Element transportoccurs onlyona restricted scale of probably much lessthanthe present thickness of the Hangendserie.The coarse grain size of the pegmatitic rocks is, how-ever, a good condition to preserve textural disequilibriumand to record different stages of the uplift and coolinghistory. Reheating is indicated in the pegmatites by partialresorption of zoisite and crystallization of a second andthird generation with distinctly different composition.This observation is in line with the idea that upliftoccurred during the collision event and shortly afteror simultaneously with stacking of the nappes; that is,in a short time frame, which is in many cases close to thetime resolution of available geochronometric methods.