Geochemical Constraints on Leucogranite Magmatism in the Langtang Valley, Nepal Himalaya |
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| Authors: | INGER SIMON; HARRIS NIGEL |
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| Institution: | 1Department of Earth Sciences, University of Leeds Leeds LS2 9JT, UK
2Department of Earth Sciences, The Open University Milton Keynes MK7 6AA, UK |
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| Abstract: | A complex of crustally derived leucogranitic sills emplacedinto sillimanite-grade psammites in the upper Langtang Valleyof northern Nepal forms part of the Miocene High Himalayan graniteassociation. A series of post-tectonic, subvertical leucograniticdykes intrude the underlying migmatites, providing possiblefeeders to the main granite sills. The leucogranite is peraluminous and alkali-rich, and can besubdivided into a muscovite–biotite and a tourmaline–muscovitefacies. Phase relations suggest that the tourmaline leucogranitescrystallized from a water-undersaturated magma of minimum-meltcomposition at pressures around 3–4 kbar. Potential metasedimentaryprotoliths include a substantial anatectic migmatite complexand a lower-grade mica schist sequence. Isotopic constraintspreclude the migmatites as a source of the granitic melts, whereastrace-element modelling of LILEs (Rb, Sr, and Ba), togetherwith the Nd and Sr isotopic signatures of potential protoliths,strongly suggest that the tourmaline-bearing leucogranites havebeen generated by fluid-absent partial melting of the muscovite-richschists. However, REE and HFSE distributions cannot be reconciledwith equilibrium melting from such a source. Systematic covariationsbetween Rb, Sr, and Ba can be explained by variations in protolithmineralogy and P–T–aH2O. Tourmaline leucogranites with high Rb/Sr ratios represent low-fraction-melts(F{small tilde} 12%) efficiently extracted from their protolithsunder conditions of low water activity, whereas the heterogeneoustwo-mica granites may result from melting under somewhat higheraH2O conditions. The segregation of low-degree melts from sourcewas probably by deformation-enhanced intergranular flow andmagma fracturing, with the mechanisms of migration and emplacementcontrolled by variations in the uppercrustal stress regime duringlate–orogenic extensional collapse of the thickened crust. |
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