Metamorphism of the Buchan type-area, NE Scotland and its relation to the adjacent Barrovian domain
Bibliographic record
Abstract
The metamorphism of the Buchan block in northeast Scotland, and its relation to the Barrovian domain to its west, have been reassessed from consideration of mineral assemblages, microstructures, phase equilibrium modelling and monazite U–Pb geochronology. Zones of increasing metamorphic grade surround a central low-grade domain (biotite zone) and define a northward-opening, U-shaped metamorphic map pattern. The eastern and southern parts of the Buchan domain show the classic Buchan-type prograde sequence of cordierite – andalusite – sillimanite – migmatite/gneiss, representing a metamorphic field gradient of gently increasing pressure between (2.5–3.0 kbar, c. 550°C) and (3.5–4.0 kbar, c. 750°C). A slightly lower pressure metamorphic field gradient (by less than c. 0.5 kbar) is interpreted for the northeastern Buchan domain. The west Buchan domain shows a prograde sequence of staurolite+andalusite – sillimanite – gneiss/migmatite, representing a metamorphic field gradient of higher pressure by c. 1 kbar or less than the classic sequence over a similar temperature range. Unique to the northwestern Buchan domain is a staurolite+cordierite+andalusite domain in which staurolite-bearing schists have been overprinted by cordierite + andalusite assemblages. Monazite U–Pb geochronology of schists and gneisses from the Buchan block, including the Cowhythe and Ellon gneisses, yields ages in the range 470 ± 5 Ma, supporting geological evidence that the gneisses are metamorphosed Dalradian strata rather than older basement gneisses. The metamorphic ages are similar to the ages of mafic igneous intrusions in the Buchan block, even though many of the exposed intrusions post-date the regional metamorphic zones. The Buchan metamorphic zones are truncated to the west by the Portsoy–Duchray Hill Lineament, a ductile shear zone that juxtaposes the Buchan rocks against higher pressure, lower temperature (kyanite-bearing) Barrovian schists to the west ( c . 5.5–7.0 kbar, c. 580–650 °C). A 2–15 km wide corridor of andalusite pseudomorphed by kyanite occurs between the Portsoy–Duchray Hill Lineament and the Keith shear zone to its west. Monazite geochronology of the Barrovian rocks west of Portsoy shows little evidence of the c. 470 Ma signature of the Buchan block, instead yielding a dominant cluster of ages at c. 450 Ma and a more poorly defined grouping at c. 500 Ma. Supplementary materials: Analytical procedures; Thermodynamic modelling; Tables S1 to S13 (mineral assemblages, whole rock compositions, mineral compositions, isotopic analyses); Figures S1 to S5 (supplementary mineral assemblage map, graphs of mineral composition vs. grade, supplementary thermodynamically calculated phase diagrams, back-scattered electron (BSE) images of monazite grains, concordia diagrams for monazite U–Pb results) are available at https://doi.org/10.6084/m9.figshare.c.5536745
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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) | 0.002 | 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".