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High Himalaya |
Department of Earth Sciences, Open University, Milton Keynes MK7 6AA
1 Department of Earth Sciences, University of Leeds, Leeds LS2 9JT, UK
During fractional melting of the crust, the availability of fluids is a critical variable that influences the melt fraction obtained, the capacity of the melt to migrate from its source and the relationship between melting and uplift. For the High Himalayan leucogranites, isotope systematics and metamorphic phase equilibria have identified micaceous metasediments as the most likely source, and the incongruent melting of muscovite as the appropriate melting reaction. High-field strength trace elements are controlled by dissolution kinetics and incomplete crystal/liquid segregation of accessory phases during crustal anatexis. In contrast, Rb and Sr are controlled by major phases contributing to the melting reaction. Hence the Rb/Sr ratios in primary melts can be correlated with the availability of fluids during partial melting.
Geochemical modelling of melts resulting from anatexis of a wide range of metasedimentary compositions (plagioclase 5–30%, muscovite 5–30%, biotite 5–20%, quartz 30–85%) indicates that Rb/Sr ratios will vary between 4 and 10 during equilibrium melting and vapour-absent conditions, but are less than 3.5 during vapour-present melting. In either case, Rb/Sr ratios in melts are reduced under disequilibrium conditions. In general Rb/Sr ratios of Himalayan leucogranites vary between 3 and 6. Such high ratios could not have formed under vapour-present conditions unless feldspar fractionation has increased the Rb/Sr ratios of primary melts. Trace-element systematics and petrographic evidence do not support significant fractionation of feldspar. Hence for most, if not all, Himalayan granites, vapour-absent conditions prevailed during fractional melting, irrespective of either source composition or the kinetics of the melting reaction. This is consistent with the observation that the melts have migrated from their source, and allows decompression melting to play a role in their genesis.
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