Late Cretaceous Transtension in the Eastern Tibetan Plateau: Evidence From Postcollisional A‐Type Granite and Syenite in the Changdu Area, China
Bibliographic record
Abstract
Abstract The Late Cretaceous is an important geological time interval for the Tibetan Plateau because it corresponds to the period when the tectonic regime changed from Lhasa‐Qiangtang collision to Indo‐Asian assembly. However, the nature of and controls on the change in tectonic regime are poorly constrained. In this paper, we report results of a study of two intrusions in the Changdu area of the Eastern Tibetan Plateau. Zircon U‐Pb dating shows that both intrusions formed at ca. 77.6–74.3 Ma. The Bangda intrusion has A‐type granite affinity and a peraluminous character, whereas the Ruduo intrusion is a metaluminous syenite. Both intrusions have very similar trace element compositions, slightly enriched zircon ε Hf (t) values (−9.3 and −1.7), and EM‐2‐like Sr‐Nd‐Pb isotope ratios. These features of the two intrusions indicate that their magmas were derived from partial melting of an alkali‐rich basaltic lower crust and a small proportion of mantle melt. The occurrence of alkaline intrusions is consistent with Late Cretaceous extension in the Eastern Tibetan Plateau. Based on the results of this study and previous data, we propose an intraplate extensional tectonic model, in which there was NS‐NNW‐directed Late Cretaceous transtension in the Eastern Tibetan Plateau following the Lhasa‐Qiangtang collision. This extension is interpreted to have been triggered by the Bangong‐Nujiang slab break‐off at around 110 Ma and driven by the far‐field subduction of the Neo‐Tethys oceanic crust.
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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 | 0.003 | 0.003 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.001 | 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".