Oldman C J, Warren C J, Harris N B W, Kunz B E, Spencer C J, Argles T W, Roberts N M W, Hammond S J, Degli-Alessandrini G
School of Environment, Earth and Ecosystems, Faculty of Science, Technology, Engineering and Mathematics, The Open University, Walton Hall, Milton Keynes, MK6 6AA UK.
Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON KL7 3N6 Canada.
Contrib Mineral Petrol. 2025;180(9):62. doi: 10.1007/s00410-025-02247-z. Epub 2025 Aug 22.
The nature, location, longevity and pressure-temperature conditions of different crustal melt reactions during orogenesis provide constraints on the structure, mechanical strength and exhumation of orogenic middle crust as well as element mobilisation and crustal differentiation. The Himalayan orogen offers a natural laboratory for studying crustal melting by exposing both migmatites and leucogranites in its structurally highest levels. We combine previous frameworks that link petrography or bulk geochemistry to melt reaction with in-situ trace-element analyses of large-ion lithophile elements in feldspar, mica, and garnet, U-Th-Pb isotopes in monazite and zircon and thermometry calculations in samples from the Badrinath region of the Garhwal Himalaya. Our samples naturally fall into three groups that we interpret as having formed by fluid-present melting of muscovite (Group 1, all migmatites; 650-750 °C), muscovite dehydration melting (Group 2, migmatites, leucosomes and leucogranites; 730-800 °C) and biotite dehydration melting (exemplified by a single leucogranite that contained zoned and inclusion-rich garnet and porpyroblastic K-feldspar). Geochronological data suggest that melting occurred over 20 Ma, with different samples experiencing different reactions and capturing different parts of the record at different times. Despite experiencing the same thermal history, individual outcrops typically only record one melting reaction instead of a progression through fluid-present melting followed by muscovite-dehydration melting. We interpreted this as being due to local compositional variations and availability of fluids. Our results show that petrographic observations and the mineral chemistry record are similar between (source) migmatites and (product) granites, but that fluid-present reactions are only documented in migmatites.
The online version contains supplementary material available at 10.1007/s00410-025-02247-z.
造山作用期间不同地壳熔体反应的性质、位置、持续时间以及压力-温度条件，对造山带中地壳的结构、机械强度和折返，以及元素迁移和地壳分异施加了限制。喜马拉雅造山带通过在其构造最高层位出露混合岩和淡色花岗岩，为研究地壳熔融提供了一个天然实验室。我们将先前把岩石学或全岩地球化学与熔体反应联系起来的框架，与来自加瓦尔喜马拉雅山脉巴德里纳特地区样品中长石、云母和石榴石中大离子亲石元素的原位微量元素分析、独居石和锆石中的U-Th-Pb同位素以及测温计算相结合。我们的样品自然地分为三组，我们将其解释为通过白云母的流体存在熔融（第1组，所有混合岩；650 - 750°C）、白云母脱水熔融（第2组，混合岩、浅色体和淡色花岗岩；730 - 800°C）和黑云母脱水熔融（以一个含有分带且富含包裹体的石榴石和斑状变晶钾长石的单一淡色花岗岩为例）形成的。地质年代学数据表明，熔融作用持续了超过20 Ma，不同的样品经历了不同的反应，并在不同时间捕捉到了记录的不同部分。尽管经历了相同的热历史，但单个露头通常只记录一种熔融反应，而不是从流体存在熔融到白云母脱水熔融的连续过程。我们将此解释为是由于局部成分变化和流体的可用性。我们的结果表明，（源区）混合岩和（产物）花岗岩之间的岩相学观察和矿物化学记录相似，但流体存在反应仅记录在混合岩中。
在线版本包含可在10.1007/s00410-025-02247-z获取的补充材料。
