Melt reactions and timescales of melting in pelitic rocks—a case study from the Garhwal Himalaya
Bibliographic record
Abstract
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. Supplementary Information: The online version contains supplementary material available at 10.1007/s00410-025-02247-z.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 0.003 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".