ION-MICROPROBE DETERMINATIONS OF TRACE-ELEMENT CONCENTRATIONS IN GARNETS FROM ANATECTIC ASSEMBLAGES

Trace-element data for garnet, cordierite, plagioclase, alkali feldspar and tourmaline have been obtained by ion-microprobe analysis of assemblages from a pelite-derived leucogranite, a sillimanite-grade anatectic gneiss and a kyanite-grade sub-solidus schist from the High Himalaya. Results confirm that for equilibrium melting residual plagioclase will cause depletion in Sr and Eu in the melt, and residual garnet will result in strong depletion of Y and HREE, consistent with their published partition coefficients. All trace elements of interest are strongly incompatible in cordierite, implying that for equilibrium fractional melting of pelites at shallow depths ( < 5 kbar) the stability of cordierite as a restite phase will result in melts that are undepleted in Y and HREE. HREE concentrations in garnets from metasediments are independent of metamorphic grade but the metamorphic garnets are significantly less enriched in HREE than is the leucogranite garnet. This suggests either that the leucogranite is derived from a source with lower modal abundances of garnet than the analysed metasediments or that garnet from the anatectic gneiss has not reached chemical equilibrium with a granitic melt. The latter possibility is consistent with sluggish self-diffusion rates in garnets for REE at temperatures of < 800-degrees-C. Disequilibrium between pre-anatectic garnets and pelite-derived melts accounts for the observation that granites derived from crustal anatexis are often undepleted in HREE even though garnet is probably a common residual phase during crustal anatexis at pressures > 5 kbar.