TRACE-ELEMENT MODELING OF PELITE-DERIVED GRANITES

The presence or absence of a vapour phase during incongruent-melt reac tions of muscovite and biotite together with the composition of the pr otolith determines the trace-element characteristics of the resulting melt, provided that equilibrium melting occurs for those phases that h ost the trace elements of interest. For granitic melts, Rb, Sr and Ba provide critical constraints on the conditions that prevailed during m elting, whereas REE are primarily controlled by accessory phase behavi our. Mass-balance constraints for eutectic granites that are formed by the incongruent melting of muscovite in pelites indicate that melting in the presence of a vapour phase will result in a large melt fractio n, and deplete the restite in feldspar. Hence the melt will be charact erized by low Rb/Sr and high Sr/Ba ratios. In contrast, vapour-absent melting will result in a smaller melt fraction, and an increase in the restitic feldspar. Consequently high Rb/Sr and low Sr/Ba ratios are p redicted. Vapour-absent melting will also enhance the negative Eu anom aly in the melt. Granites that result from the incongruent melting of biotite in the source will be characterized by higher Rb concentration s than those that result from the incongruent melting of muscovite. Th e Himalayan leucogranites provide an example of unfractionated, crusta lly derived eutectic melts that are enriched in Rb but depleted in Sr and Ba relative to their metasedimentary protoliths. These composition s may be generated by the incongruent melting of muscovite as a low me lt fraction (F approximately 0.1) from a pelitic source under vapour-a bsent conditions.