Granophyre Norites and Diorites of the Jarva-Varaka Massif (Monchegorsk Ore Area, Kola Region, Russia): Geology, Petrography, Geochemistry, Geochronology and Origin
Bibliographic record
Abstract
Abstract The results of geological, geochemical, and geochronological studies of granophyre rocks from the Jarva-Varaka Massif (Kola region) are presented. The 2-km section of the massif is composed of mafic and felsic norites, hypersthene diorite, pigeonite-augitic diorite, quartz diorite, and granodiorite. All these rocks contain a variable amount of granophyre (micropegmatite), from ~10% in norites that compose the lower part of the massif, to ~45% in quartz diorite and granodiorite of the upper part. The Sudbury Igneous Complex (SIC) is the only known case of a similar 2-km thick section of granophyre rocks whose composition varies from mafic at the bottom to felsic at the top. The SIC has an impact origin, which suggests a similar formation mechanism for the Jarva-Varaka Massif. The norites contain micro-xenoliths of the host high-alumina gneisses, which were transformed to rocks of the high-grade hornfels facies. Such gneisses are absent among the country rocks of other mafic intrusions in the area, and it suggests that the parental melt for mafic norites could have assimilated the host rocks. Given the relatively small size of the massif, the formation of hornfelses could have occurred if the xenoliths would be entrained by a melt under near-surface conditions, and the melt was hot enough to cause the formation of hornfelses. Spinifex-like structures in the diorites of the very marginal contact zone of the Jarva-Varaka Massif indicate very rapid cooling of a high-temperature melt, which is typical of near-surface conditions. A pseudotachylitic breccia, planar deformations in quartz, kink-bands in biotite and clinozoisite, zircon and inclusions of sillimanite and plagioclase all transformed into diaplectic glasses detected in the host rocks of the Jarva-Varaka Massif are interpreted as distinct signs of shock metamorphism. Geophysical data show no signs that beneath the Jarva-Varaka Massif there exists a large mafic intrusion that could have contaminated large volumes of crustal material, and magmatic differentiation of which could have produced a large volume of enriched melt whose crystallization could resulted in the formation of granophyre rocks from the bottom to the top of the magma chamber. All of the aforementioned data indicate that the impact origin of the 2.5 Ga Jarva-Varaka Massif is very likely.
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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 | 0.000 | 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 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 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".