Coupling pressure-temperature and time constraints in greenschist- and amphibolite-facies polymetamorphic rocks: a case study from the Austroalpine Unit (Eastern Alps, Austria)
Bibliographic record
Abstract
In low-grade metamorphic units, precise thermobarometric and geochronologic data are often ambiguous or entirely lacking, thus complicating the temporal interpretation of metamorphism and hampering the identification of complex polymetamorphic histories. We present new P-T-t-D data from samples collected in two Austroalpine nappes exposed in the Eastern Alps, Austria: the structurally upper greenschist-facies Schöckel Nappe (“Graz Paleozoic,” Drauzug-Gurktal Nappe System) and the structurally lower amphibolite-facies Waxenegg Nappe (Koralpe-Wölz Nappe System). Although polymetamorphism was previously inferred from garnet zonation indicating multiphase growth in the Waxenegg Nappe, the timing of metamorphism is poorly resolved and only limited geochronology exists in the Schöckel Nappe. Detailed petrographic investigations revealed that the chloritoid-bearing phyllite and micaschist of the Schöckel Nappe contain allanite that occasionally show partial replacement by small (<10 µm) monazite and thorite. Large (up to 500 µm) monazite exhibiting distinct core-rim chemical zoning were observed in the garnet-bearing micaschist of the Waxenegg Nappe. Careful documentation of the microstructural phase relations, thermodynamic modeling in the MnCNKFMASHT system, Raman spectroscopy of carbonaceous matter and in-situ LA-ICPMS U-(Th)-Pb dating of the accessory phases allow us to reconstruct a first metamorphic imprint at ~560°C and 4 kbar in the Waxenegg Nappe at c. 270 Ma (Permian event). Overprinting occurred at ~540°C and 8-10 kbar at c. 90 Ma (Eo-Alpine event). In the Schöckel Nappe, peak metamorphic conditions of ~470°C and 3-4 kbar existed during the Permian event at c. 260 Ma and the Eo-Alpine event in the upper part of the nappe did not exceed lower to middle greenschist-facies conditions. Our results provide unequivocal evidence for Permian metamorphism in the Schöckel Nappe, which was hitherto unknown in this part of the Austroalpine Unit. Moreover, it demonstrates that the main metamorphic signature in this unit occurred during the Permian event and that the Eo-Alpine overprint is relatively lower grade than previously proposed. Combined with the data from the Waxenegg Nappe, there is an obvious marked increase in the Eo-Alpine peak conditions of ~130°C and 5 kbar across the nappe contact with higher grade in the footwall compared to the hanging wall. This is consistent with the existence of a major normal fault between the Drauzug-Gurktal Nappe System and the Koralpe-Wölz Nappe System in the easternmost part of the Austroalpine Unit, as already identified in its central and western parts. Modern thermobarometric analytical approaches coupled with high spatial resolution geochronology on accessory minerals is allowing a more thorough assessment of the subtle metamorphic histories recorded in the fundamentally important low-grade units of orogens.
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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 | 0.002 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.001 | 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".