Fluid influence on mineral reactions in ultrahigh-pressure granulites: a case study in the Snieznik Mts. (West Sudetes, Poland)

Small tectonic slices of undeformed eclogites and ultrahigh-pressure granul ites occur in three tectonic units of the Snieznik Mts. (SW Poland). Ultrah igh-pressure granulite/eclogite transitions with peak metamorphic condition s between 21 and 28 kbar at 800 to 1000 degrees C occur only in the Zlote u nit. Conventional U-Pb multigrain analyses of zircons from a mafic granulit e provided (207)pb/(206)pb ages between 360 to 369 Ma which are interpreted to approximate timing of original crystallisation from a melt. Diffusion k inetics and the restricted availability of a fluid phase mainly controlled the conversion from granulite to eclogite: although some bulk-chemical diff erences were also recognised. The ultrahigh-pressure granulites from the Zl ote unit exclusively contain H2O-rich inclusions with variable salinities w hich distinguishes them from high-temperature (HT)-granulites world-wide. T his is also in contrast to the fluid regime (H2O-N-2-CO2) recognised in the lower-temperature eclogites (600-800 degrees C) from the closely associate d Miedzygorze and Snieinik units. The variation in fluid composition betwee n the lower-temperature eclogites and ultrahigh-pressure granulites on the one hand and ultrahigh-pressure granulites and HT-granulites on the other h and probably indicates contrasting P-T-t paths as a result of different tec tonic environments.