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TitleMuscovite dehydration melting; reaction mechanisms, microstructures, and implications for anatexis
AuthorDyck, B; Waters, D J; St-Onge, M R; Searle, M P
SourceJournal of Metamorphic Geology 2019 p. 1-24, Open Access logo Open Access
Alt SeriesEarth Sciences Sector, Contribution Series 20160157
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html
AreaHimalayas; Langtang; Everest; Tibet; Nepal
Lat/Long WENS 85.2500 87.0000 28.3833 27.7833
Subjectsigneous and metamorphic petrology; mineralogy; Nature and Environment; Science and Technology; anatexis; muscovite; feldspar; quartz; crystallization; bedrock geology; lithology; metamorphic rocks; gneisses; augen gneisses; metasedimentary rocks; psammites; pelites; semipelites; quartzites; igneous rocks; intrusive rocks; granites; monzo-granites; tonalites; leucogranites; granitic rocks; structural features; faults; tectonic setting; orogenies; metamorphism; sillimanite; silicates; tourmaline; apatite; biotite; plagioclase; thermal analyses; modelling; pressure-temperature conditions; Greater Himalayan Series; Everest Series
Illustrationsphase diagrams; cross-sections; tables; geoscientific sketch maps; location maps; photomicrographs; equal-area stereonet projections
ProgramGEM2: Geo-mapping for Energy and Minerals Baffin Bedrock Mapping
Released2019 09 30
AbstractDehydration melting of muscovite in metasedimentary sequences is the initially dominant mechanism of granitic melt generation in orogenic hinterlands. In dry (vapour-absent) crust, muscovite reacts with quartz to produce K-feldspar, sillimanite, and monzogranitic melt. When water vapour is present in excess, sillimanite and melt are the primary products of muscovite breakdown, and any K-feldspar produced is due to melt crystallization. Here we document the reaction mechanisms that control nucleation and growth of K-feldspar, sillimanite, and silicate melt in the metamorphic core of the Himalaya, and outline the microstructural criteria used to distinguish peritectic K-feldspar from K-feldspar grains formed during melt crystallization. We have characterized four stages of microstructural evolution in selected psammitic and pelitic samples from the Langtang and Everest regions: (a) K-feldspar nucleates epitaxially on plagioclase while intergrowths of fibrolitic sillimanite and the remaining hydrous melt components replace muscovite. (b) In quartzofeldspathic domains, K-feldspar replaces plagioclase by K+-Na+ cation exchange, while melt and intergrowths of sillimanite+quartz form in the aluminous domains. (c) At 7-8 vol.% melt generation, the system evolves from a closed to open system and all phases coarsen by up to two orders of magnitude, resulting in large K-feldspar porphyroblasts. (d) Preferential crystallization of residual melt on K-feldspar porphyroblasts and coarsened quartz forms an augen gneiss texture with a monzogranitic-tonalitic matrix that contains intergrowths of sillimanite+tourmaline+muscovite+apatite. Initial poikiloblasts of peritectic K-feldspar trap fine-grained inclusions of quartz and biotite by replacement growth of matrix plagioclase. During subsequent coarsening, peritectic K-feldspar grains overgrow and trap fabric-aligned biotite, resulting in a core to rim coarsening of inclusion size. These microstructural criteria enable a mass balance of peritectic K-feldspar and sillimanite to constrain the amount of free H2O present during muscovite dehydration. The resulting modal proportion of K-feldspar in the Himalayan metamorphic core requires vapour-absent conditions during muscovite dehydration melting and leucogranite formation, indicating that the generation of large volumes of granitic melts in orogenic belts is not necessarily contingent on an external source of fluids.
Summary(Plain Language Summary, not published)
The formation of granite by dehydration and melting of white mica is studied and 4 textural stages are recognized and documented. This enables metamorphic reaction mechanisms and volume of external water required (or not) to be determined in orogenic belts of any age.

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