Contemporaneous eruption of Nb-enriched basalts – K-adakites – Na-adakites from the 2.7 Ga Penakacherla terrane: implications for subduction zone processes and crustal growth in the eastern Dharwar craton, India
Bibliographic record
Abstract
An association of Nb-enriched basalts (NEB), high-MgO andesites (HMA), and flows with adakitic characteristics are interlayered with tholeiitic pillow basalts in the 2.7 Ga Penakacherla greenstone belt of eastern Dharwar craton. Two populations of basalt are present, a high-Mg# Ni (0.65–0.56, 106–52 ppm) and low-Mg# Ni (0.45–0.34, 32–13 ppm) counterpart; Nb spans 6.3–18 ppm relative to “normal” arc tholeiitic basalts, where Nb ∼3 ppm, and hence qualify as NEB. Basalts plot on the low-Ce/Yb trend of intraoceanic arcs, and have fractionated heavy rare-earth elements (HREE) indicative of melting with residual garnet at >90 km. Ratios of Nb/Ta (7.6 ± 0.7), Zr/Hf (44 ± 0.8), and Zr/Sm (27 ± 2.4) are systematically low, high, and similar to respective primitive mantle ratios of 17, 36, and 25, consistent with a mid-ocean ridge basalt-like mantle source in the sub-arc mantle wedge. Intermediate compositions are divided into high-K but low-Na (K2O 1.8–5.3; Na2O 0.5–2.1 wt.%) and low-K but high-Na (K2O 0.10–1.5; Na2O 4.1–5.6 wt.%) populations defining distinct magma series; accordingly, these are termed K-adakitic and Na-adakitic rocks, respectively. The Na-type has SiO2 ≥56 wt.%, MgO <3 wt.%, Mg# ∼0.5, Na2O ≥3.5 wt.%, K2O ≤3 wt.%, Yb ≤1.9 ppm, Cr ≥30 ppm, with slightly lower limits of Al2O3 ≥15 wt.% and La/Yb 7.5–8.2 versus ≥20, thus conforming to most criteria for Na-adakites. NEB are interpreted as melts of mantle wedge hybridized by adakitic melts having residual garnet; and Na-adakites are slab melts of low-Mg basalt in the garnet–amphibolite facies. K-adakitic flows are melts of mafic lower crust, or melts of lower crust delaminated into mantle wedge asthenosphere.
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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 | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 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 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".