GEOCHEMICAL SIGNATURES OF RAPID MELT SEGREGATION IN THE CRUST

Several examples of rapid melt segregation have been suggested from geochemical studies of crustal anatectic zones where rates of segregation have been estimated by magma geochemistry. Trace element data from the anatectic High Himalaya leucogranites suggest that the magmas are derived from muscovite-bearing metasedimentary protoliths and have been extracted quickly, on the timescale of less than 10 my years (Harris et al., Chem. Geol. v.162, 155-167, 2000). As part of an experimental investigation of melt generation and segregation in crustal source rocks, we have performed partial melting deformation experiments on solid cores from a two-mica pelite at 0.7 GPa confining pressure and at temperatures ranging from the solidus, at 725^oC, up to 920^oC. Electron microprobe and SIMS analyses of the experimental cores have been performed. Major element data from muscovite-derived glasses show melt to be rich in SiO2 and Al2O3, peraluminous, with molecular Al2O3/CaO + Na2O + K2O ranging between 1.37 and 1.46. K2O is greater than Na2O with Na/K ratios ranging from 0.46 to 0.59. The glasses are similar to those analyzed in experimental phase equilibria studies and plot in the granite senso-stricto field in Ab-An-Or ternary plots. Trace elements however show evidence of disequilibrium. SIMS analyses on experiments at high temperature show muscovite-derived glass to have LREE (>10 x CI) enriched compared to HREE (between 1-10 x CI) with a positive Eu anomaly. Additional analyses on biotite-derived glass show similar patterns but with overall higher REE (between 100 - 1000 x CI) and Nb is enriched at least 10 times that of the muscovite-derived melt. The data show that at high melt fraction, even under deformation, melt derived by hydrous phase breakdown remains heterogeneous in trace elements. The melts appear to retain their geochemical signatures from the specific dehydration melting reaction, at least on the experimental time-frame, and their REE patterns suggest incomplete dissolution of accessory phases. Partial melt extracted rapidly may record these kinds of REE patterns.