THE FORMATION OF MEGACRYSTS IN MIGMATITIC AMPHIBOLITES FROM HALLAND PROVINCE, SOUTHWEST SWEDEN
Bibliographic record
Abstract
The growth of large mineral grains (megacrysts) can be promoted by a fluid phase that allows for rapid diffusion. Hence, the presence of megacrysts in metamorphic rocks can be one indicator of the presence of a silicate melt or other fluid phase during metamorphism. We studied megacrysts of hornblende (Hbl), garnet (Grt), clinopyroxene (Cpx) and orthopyroxene (Opx) associated with both tonalitic leucosomes and melanosomes in mafic migmatites from the Stensjӧ Nature Preserve and the area around Bjӧrkasjӧ Lake. Both localities are in the Eastern Belt of the Sveconorwegion Orogen, Halland Province, Sweden. We mapped both localities and examined samples with a polarizing microscope and SEM-EDS. We did quantitative mineral analyses by WDS on an electron microprobe. Whole rock analyses were done by X-ray florescence at Actlabs, Ontario. When the host rock contains Hbl+Pl (plagioclase) +/- Cpx without Grt or Bt (biotite) the megacrysts are either Hbl or Hbl+Cpx. The compositions of the megacrysts are similar to compositions of the same minerals in the host rock. Plagioclase in inclusions within and immediately surrounding Cpx megacrysts can contain up to 46 % more Ab (albite) than Pl away from the megacrysts. When the host rock contains Hbl+Pl+Grt+Cpx without biotite the megacrysts are either Grt or Cpx. When the host contains Hbl+Pl+Grt+Bt the megacrysts are Opx. These megacrysts are surrounded by a reaction rim of Hbl+Qtz+/-Cpx+/-Grt, and Hbl in the rims has distinctly lower Ti and K2O+Na2O concentrations than in the host. The anhydrous megacrysts, (Cpx, Opx, Grt), probably formed by vaporabsent (dehydration) melting. The different kinds of anhydrous megacrysts formed by different melting reactions controlled by the chemistry and mineralogy of the original rock. Hornblende megacrysts may have formed by the retrogression of Cpx during crystallization of the melt. Alternatively they may represent water-assisted melting, without partial melting, by a concentrated brine.
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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 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.001 |
| Open science | 0.002 | 0.001 |
| 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".