Ca. 780 Ma OIB-like mafic dykes in the Western Jiangnan orogenic Belt, South China: evidence for large-scale upwelling of asthenosphere beneath a post-orogenic setting
Bibliographic record
Abstract
Post-collisional alkaline mafic rocks record significant information regarding crust–mantle interaction in the subduction channel and provide an excellent opportunity to investigate the mechanism of transition from pre-collisional to post-collisional magmas. In this contribution, we present whole-rock major and trace element and radiogenic isotope data obtained from alkaline mafic dykes in the Guzhang-Anjiang areas in the southwestern Jiangnan orogenic belt (JOB), South China. LA–ICP–MS U–Pb zircon dating of four representative samples yields a consistent age of ~780 Ma. These mafic dykes are characterized by large variations in SiO2 (42.4–54.1 wt.%) and MgO (2.2–15.0 wt.%), alkalis (K2O+Na2O = 1.96–11.19 wt.%), Nb (17.55–49.62 ppm) and Ta (1.15–3.26 ppm). They exhibit ocean island basalt (OIB)-like geochemical features, which are characterized by enrichment in light rare earth elements (LREEs) and large ion lithophile elements (LILEs) relative to heavy rare earth elements (HREEs) and by positive zircon εHf(t) (+1.86 to +5.14) and whole-rock εNd(t) (+0.31 to +1.80) values. All these geochemical characteristics suggest these dykes were derived from a small degree (2–5%) of partial melting of an asthenospheric mantle source that had previously been modified by earlier subduction processes. Based on the previous reports and present study, we infer that three periods of Neoproterozoic basaltic magmatism occurred in the western Jiangnan Orogen, and constrain the time of the tectonic regime transformation at ~830, and at ~780 Ma. The source of these basaltic rocks varied in composition spatially and evolved with time. The ~780 Ma OIB-like rocks in this study serve as an important indicator of large-scale upwelling of the asthenospheric mantle and the transition from orogenic to post-orogenic tectonic setting.
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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 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.008 | 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".