Metamorphism and tectonics of a continental arc root: Insights from garnet amphibolite in the North Qaidam tectonic belt, northern Tibet
Bibliographic record
Abstract
Abstract The architecture of continental arc crust from its surface to base is a crucial aspect of understanding long-term crustal growth and reworking. However, the timing and nature of metamorphism in the middle to lower crust are some of the least-understood components of continental arc systems. In this contribution, we integrate phase equilibrium modeling and zircon geochronology to determine the metamorphic history of garnet amphibolite in the North Wulan metamorphic complex, North Qaidam tectonic belt, northern Tibet, and then provide key insights into the metamorphic nature of the middle to lower crust of an early Paleozoic continental arc, and into orogenic geodynamic processes associated with the closure of the Proto-Tethyan Ocean. Garnet amphibolite contains a peak metamorphic assemblage of garnet + orthopyroxene + hornblende + plagioclase + ilmenite + quartz ± biotite + melt, which defines a peak/near-peak pressure-temperature (P-T) range of 800–860 °C at 5–8.5 kbar, based on phase equilibrium modeling. Rounded inclusions of hornblende, plagioclase, and quartz in garnet porphyroblasts suggest prograde garnet growth at the expense of amphibole. Partial replacement of garnet and hornblende by orthopyroxene suggests high-T decompression after peak pressure. Post-peak partial replacement of garnet by plagioclase–quartz–hornblende or hornblende corona suggests decompression cooling after peak temperature. These results suggest that the garnet amphibolite records a clockwise P-T path with high-T decompression. These currently exposed granulite-facies rocks from the southern part of the North Wulan metamorphic complex represent a middle to lower crustal exposure (root) of an early Paleozoic continental arc–backarc setting (depths of >16–28 km) during the subduction of the Proto-Tethyan Ocean. Zircon petrochronology in garnet amphibolite reveals that protracted metamorphism occurred at ca. 515–449 Ma. These new insights into the early Paleozoic tectonometamorphic evolution of the North Wulan metamorphic complex constrain the northward subduction polarity of the South Qilian Ocean and the tectonic evolution of the North Qaidam tectonic belt. Clockwise P-T paths in continental arc settings are related to the thinning of the thickened arc crust triggered by the steepening of subducted oceanic slab, which is accompanied by slab retreat and rollback and could reflect the migration of continental arc magmatic axes. Our new results tie together high-resolution thermal, magmatic, and metamorphic records to explain the complex evolution of the roots of continental arc systems.
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.002 | 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) | 0.002 | 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".