Simultaneous operation of opposing reaction mechanisms: The influence of matrix heterogeneity on post‐kinematic garnet crystallization in an inverted metamorphic sequence
Bibliographic record
Abstract
Abstract Metapelites from the inverted Barrovian sequence in the Sikkim Himalaya (northeast India) are shown to be largely continuous with respect to their bulk rock compositions, microstructures and pressure–temperature–time–deformation ( P – T – t – D ) histories. However, the upper garnet–lower staurolite zone demarcates a region of microstructurally anomalous post‐kinematic garnet populations contained within strongly segregated matrices. The different microstructures within samples from this region cannot be attributed to differences in their thermobarometric histories or bulk compositions, but are instead interpreted to represent an otherwise unexposed level of the Daling Group that is now exposed along a post‐metamorphic thrust splay. These heterogeneous samples contain several discrete garnet populations that progressively crystallized with increasing P – T . Garnet populations that experienced the most protracted growth now form complex polycrystals that exhibit crystallographically controlled and morphologically irregular interfaces adjacent to micaceous and quartzofeldspathic domains respectively. Electron backscatter diffraction indicates that these polycrystalline garnet structures contain numerous coalesced porphyroblasts that are structurally uncorrelated across their grain boundaries. However, a crystallographically preferred orientation at the polycrystal scale is interpreted to derive from epitaxial crystallization of early‐formed garnet porphyroblasts on precursor mica. Later‐nucleated porphyroblasts within polycrystals preferentially concentrated towards quartzofeldspathic domains, with the overall nucleation distribution likely controlled by a complex interplay between chemical heterogeneities, strain partitioning and epitaxial crystallization. The subsequent growth of these polycrystals was equally spatially heterogeneous; it was moderated by differences in the efficiency of grain boundary transfer between quartzofeldspathic and micaceous domains that precluded thin section‐scale chemical equilibration. In contrast to samples from Sikkim containing more typical porphyroblastic populations in continuous and disseminated matrices, heterogeneous availability of garnet‐forming components within this strongly layered matrix is shown to have resulted in grain‐scale variations in growth rates and the spatial juxtapositioning of interface‐controlled microstructures and locally equilibrated chemical compositions with those that were transport controlled.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| 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 teacher head, 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".