Decoding the Carbonatite and Clinopyroxenite Association at Hogenakkal, India: Insights into U–Pb Monazite Geochronology, <i>In situ</i> Trace Element, and Sr-C-O Isotope Geochemistry
Bibliographic record
Abstract
Abstract The Paleoproterozoic Hogenakkal complex (India) consists of carbonatites and clinopyroxenite emplaced within granulite country rocks. U–Pb monazite ages (~2.49 Ga) suggests that the carbonatites were emplaced in a post-collisional setting following arc magmatism and collision events between the Western Dharwar Craton and the Biligiri Rangan block (Southern Granulite Terrane). The carbonatites are of two distinct types: silicate-rich, xenolith-bearing early carbonatite-I, and silicate-poor late carbonatite-II. The absence of alkaline silicate rocks combined with the lack of compositional evolution and the presence of xenocrystal clinopyroxene, together with differing Sr isotopic signatures between clinopyroxenite (clinopyroxene: 0.70147 ± 0.00021; 2σ, n = 19) and carbonatite (apatite: 0.70175 ± 0.00015; 2σ, n = 17) suggests that these lithologies were not derived from a common carbonate-bearing alkaline silicate magma or represent an antiskarn. Additionally, the C–O isotopic signature (δ13C = −5.56 to −8.02‰; δ18O = 9.50–9.90‰) combined with the absence of dolomite and primary Fe–Mg silicates suggests that the carbonatites originated from mantle-derived calcitic magma, possibly by partial melting of a carbonated eclogite source. Subsequent to emplacement there was extensive phlogopitization of the pre-existing clinopyroxenite. Magmatic-to-carbohydrothermal evolution is governed by fractionation of magmatic calcite (Cal-1a) and apatite (Ap-1a), as well as interactions between the carbonatite and pre-existing clinopyroxenite and K-feldspar pegmatite. These processes collectively controlled the formation of lithology-specific REE mineralization. In carbonatite-I, the magmatic stage begins with the crystallization of LREE-rich calcite (Cal-1a) [(La/Yb)Cn: 18–36] and apatite (Ap-1a) [(La/Yb)Cn: 45–98]. The transition to the late-magmatic-to-carbohydrothermal stage involved the formation of phlogopite, actinolite, and allanite-(Ce), which co-precipitated with LREE-depleted Cal-1b [(La/Yb)Cn: 0.6–5.8] and Ap-1b [(La/Yb)Cn: 11–31]. Carbonatite-II has high REE and Sr enrichment, with REE-rich Ap-2 and Sr-REE mineralization, including celestine, monazite-(Ce), and hydroxylbastnäsite-(Ce). During the carbohydrothermal stage, LREE-depleted Cal-2 [(La/Yb)Cn: 1.6–6.1] crystallizes together with intergrowths of monazite-(Ce), celestine, and hydroxylbastnäsite-(Ce), and monazite-(Ce) rims on Ap-2. Externally derived hydrothermal fluids overprint the carbohydrothermal assemblage within carbonatite-I, clinopyroxenite, and K-feldspar pegmatites, forming Sr-LREE-poor Cal-3 [(La/Yb)Cn &lt; 1], REE-Sr-bearing epidote, LREE-depleted Ap-3 [(La/Yb)Cn: 0.7–6.2], allanite-(Ce), and actinolite. Elevated δ18O (11.50‰), radiogenic Sr in calcite and some apatite (particularly Cal-3 and Ap-3), combined with prominent negative-Ce anomalies in Cal-3, indicate the involvement of low temperature, externally derived oxidizing hydrothermal fluids.
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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 | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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".