Migmatite-Gneiss-Granite Basement of Southwestern Nigeria:Mineralogical and Geochemical Evidence for a Possible CommonAncient Protolith
Bibliographic record
Abstract
Nigeria occupies southern section of the Pan-African orogenic belt and migmatite-gneiss-granite terrain forms a major component of its basement complex. Field geology, optical mineralogy and geochemical study of this rock unit in Idanre area, SW Nigeria were undertaken with a view to characterize them, elucidate their tectonic significance, and determine their origin. Major, trace and Rare Earth Elements (REE) composition were determined by XRF and ICP-MS techniques respectively in Bureau Veritas, Vancouver, Canada. Field geology reveals the granite which occur in three textural types (fine-grained (OGf ), porphyritic (OGp), and coarse-grained undifferentiated OGu) intrudes the low-lying migmatite-gneiss complex. The country rock comprises of aggregation of migmatite, biotite-hornblende gneiss and banded gneiss sub-units which bear pervasive tectonic signatures of polymetamorphic and orogenic activities. Optical results reveal the migmatite-gneiss country rock comprise of quartz, feldspar and biotite with granoblastic to porphyroblastic texture. The granitoids contain interlocks of quartz, K-feldspar, microcline, biotite, hornblende and accessory magnetite, and sphene. Large hornblende crystals in the coarse-grained granite are poikilitic while the porphyritic type contains myrmekite. The decreasing trends of Harker diagrams with respect to SiO2 indicates crystal fractionation. FeOt/(FeOt+MgO) versus SiO2 plot classifies the gneiss as ferroan type, Al2O3 versus MgO plot classifies it as orthogneiss, total alkali (Na2O+K2O) versus SiO2 (TAS) classified the rocks as granite. K2O versus SiO2 diagram classifies the rocks as continental granophyre. AFM [(Na2O+K2O)-Fe2O3-MgO] ternary diagram suggests the rocks are calc-alkaline, while ternary plot of Ba-Rb-Sr classifies the rock as anomalous granite. Diagram of Al2O3+Na2O+K2O versus Alumina Saturation Index (ANK vs ASI) indicates ASI<1.0 showing the granite is metaluminous. R1 versus R2 diagram indicate the rocks are largely late orogenic, La/Sm versus Sm diagram suggest that partial melting play a significant role in the magma source. A consistently low ACNK (<1.1) value, a decreasing trend on ACNK versus SiO2 diagram indicates the rocks originated from anatectic melt of igneous protoliths. The similar shape of the chondrite normalized REE distribution pattern for both the migmatite-gneiss and granite indicate similar geochemical trends which possibly connote common ancient igneous protolith.
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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 | 0.000 | 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".