Genesis and source affinities of heterogeneous ultramafic rocks in the North Kongling Complex, Yangtze craton: Architecture of a Paleoproterozoic accretionary orogen
Bibliographic record
Abstract
Ultramafic massifs are minor but meaningful components of orogens because they record the mantle properties and the processes of plate generation, subduction, accretion, and collision, thus providing key information about crust-mantle interaction processes and the evolution of orogens. Here, we present an integrated study of field observations, whole-rock and mineral geochemistry, zircon U-Pb ages, and whole-rock Sm-Nd and Re-Os isotopes for serpentinized harzburgites and olivine pyroxenites from the Paleoproterozoic Shuiyuesi mélange belt, North Kongling Complex, to constrain the tectonic evolution of the Yangtze craton, South China. The serpentinized harzburgites are characterized by enrichment of light rare earth elements (LREEs) and negative εNd(t) (−4.3 to −3.9) values, but they have high olivine Mg# (91.7−93.1) and very low Re contents (<0.09 ppb) and negative γOs(t) (−5.1 to −4.0) values. The Archean Re-Os model ages (TRD = 2.82 Ga) of the harzburgites that have the most depleted Re-Os isotope composition represent estimates for the age of the initial melt extraction. Metasomatic zircons from the serpentinized harzburgites yield a weighted average age of ca. 2.04 Ga, which is interpreted to represent the age of metasomatism during a subduction event. These geochemical and isotopic features and geochronologic results suggest that the protolith of the serpentinized harzburgites in the North Kongling Complex originated as fragments of late Mesoarchean (ca. 2.82 Ga) subcontinental lithospheric mantle and experienced melt/fluid metasomatism in a subarc mantle wedge at 2.04 Ga. The olivine pyroxenites are cumulates, and they are characterized by high MgO, Ni, and Cr contents and depleted to slightly enriched LREEs but depletion in high field strength elements (HFSEs). They have positive εNd(t) (+2.0 to +6.6) values and Paleoproterozoic Nd model ages (2.41−2.05 Ga), suggesting that the olivine pyroxenites were derived from a Paleoproterozoic depleted mantle source. Metamorphic zircons from the olivine pyroxenites indicate subsequent metamorphism at 2.0 Ga, in response to a collision. The olivine pyroxenites together with the metabasites in the Shuiyuesi mélange belt represent the dismembered fragments of oceanic lithosphere that formed in a suprasubduction zone during the Paleoproterozoic. Combined with the Archean and Paleoproterozoic rocks in the North Kongling Complex, it appears that the Mesoarchean subcontinental lithospheric mantle peridotites and Paleoproterozoic ophiolitic rocks were simultaneously accreted during a Paleoproterozoic orogenic event. We further propose that a subduction channel may have played an important role in the architecture of the accretionary orogen, in which both oceanic lithospheric materials and subcontinental lithospheric mantle wedge materials would have been simultaneously scraped off and accreted during slab subduction-accretion.
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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 | 0.001 |
| Scholarly communication | 0.000 | 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".