The Kellerjoch Gneiss (Tyrol, Eastern Alps): An Ordovician pluton with A-type affinity in the crystalline basement nappes north of the Tauern Window
Bibliographic record
Abstract
The Kellerjoch Gneiss (KG, Schwazer Augengneis) is part of the Austroalpine basement nappes north of the Tauern Window and extends from Schwaz in the West to Wörgl in the East.It is tectonically intercalated between the Innsbruck Quartzphyllite Nappe in the footwall and the Greywacke Zone in the hanging wall.Microscopical observations imply that the KG is a shallow intruded porphyroitic metagranite.Embayed phenocrysts of quartz and simple twins of K-feldspar phenocrysts with diameters up to more than 1.5 cm support this interpretation.Remnants of the magmatic paragenesis are K-feldspar + albitic plagioclase + Ti-rich biotite.The latter show recrystallization (Variscan or eo-Alpine?) to Ti-poor biotite and a Ti phase (rutile, ilmenite, titanite).The eo-Alpine metamorphic paragenesis consists of muscovite + biotite + albite + chlorite ± stilpnomelane and mineral compositions indicate eo-Alpine metamorphic conditions around 300-400 °C and 5-7 kbar.TIMS single zircon U-Pb dating yielded concordia ages of 468 ± 1 Ma and 469 ± 1 Ma (± 2 SD) for two samples of the KG, respectively, interpreted as the igneous formation age.Zircons extracted from a meta-pegmatite with the mineral assemblage garnet 1 (alm-rich) + garnet 2 (grs-rich) + chlorite + stilpnomelane + albite + quartz, crosscutting the Kellerjoch Gneiss, are slightly younger with a U-Pb concordia age of 462 ± 1 Ma.A strongly mylonitizised orthogneiss (Stengelgneis) which occurs adjacent to the KG gave a slightly higher zircon U-Pb concordia age of 475 ± 1 Ma.The most common accessory minerals in the KG are monazite, allanite, apatite, zircon and xenotime.Based on textural investigations, two generations of monazite can be distinguished.Primary monazite form crystals with diameters of 40 μm.Some of these monazites are replaced by a corona of apatite and allanite, which indicates a (Variscan?)metamorphic overprint.Secondary monazite occurs as small-scale grains with a diameter of 5-10 μm mostly within the cleavage of muscovite and biotite and intergrown with rutile or ilmenite.Occasionally, secondary monazite is intergrown with xenotime, in which case formation temperatures of <400°C can be inferred from monazite-xenotime miscibility gap thermometry.Electron microprobe-based U-Th-Pb dating of primary monazite yielded CHIME (chemical Th-U-total Pb isochron method) ages of 465 ± 22 Ma and 469 ± 34 Ma, which are in good agreement with the zircon ages.Due to low Th, U, Pb-contents it was not possible to date the small secondary monazite grains.However, based on textural evidence their growth during the eo-Alpine event is likely.Additionally, the low yttrium contents correlate well with low-grade P-T conditions.With regard to the age of its protolith, the KG is another important example of the prominent Lower Ordovician magmatic event, which is found in many places throughout the Austroalpine.While most of these Lower Ordovician magmas are I-type or S-type, the KG is special in that it shows geochemically an A-type affinity.Der Kellerjochgneis (KG, Schwazer Augengneis) ist Teil der austroalpinen Basementdecken nördlich des Tauern Fensters und erstreckt sich von Schwaz im Westen bis nach Wörgl im Osten.Tektonisch liegt er zwischen dem Innsbrucker Quarzphyllit im Liegenden und der Grauwackenzone im Hangenden.Es handelt sich um einen seicht intrudierten porphyrischen Metagranit.Die magmatischen Relikte sind K-Feldspar + albitreicher Plagioklas + Ti-reicher Biotit.Letzterer rekristallisierte (variszisch oder eoalpidisch?)zu Ti-armen Biotit und einer Ti-Phase (Rutil, Ilmenit, Titanit).Die eoalpine Mineralparagenese besteht aus Muskovit + Biotit + Chlorit ± Stilpnomelan und die eoalpinen P-T Bedingungen waren ca.300-400 °C und 5-7 kbar.TIMS U-Pb Datierung von Einzelzirkonen ergab Concordia-Alter von 468 ± 1 Ma und 469 ± 1 Ma (± 2 SD) welche als Intrusionsalter interpretiert werden.Zirkone aus einem diskordantem Metapegmatit mit der Mineralparagenese Granat1 (alm-reich) + Granat2 (grs-reich) + Chlorit + Stilpnomelan + Albite + Quarz ergaben ein jüngeres U-Pb Alter von 462 ± 1 Ma.Der starker mylonitisierte Stengelgneis ergab ein U-Pb Alter von 475 ± 1 Ma.Die häufigsten akzessorischen Mineralphasen im KG sind Monazit, Allanit, Apatit, Zirkon und Xenotim.Basierend auf texturellen Beobachtungen konnten zwei Generationen von Monazit unterschieden werden: Primärer Monazit bildet ca.40 μm grosse Körner und wird oft von Apatit und Allanit im Zuge einer späteren (variszischen?) Metamorphose umgewandelt.Sekundäre Monazite treten als kleine Körner mit einem Durchmesser von 5-10 μm in den Spaltflächen von Muskovit und Biotit oft auch mit Rutil oder Ilmenit verwachsen auf.Manchmal sind sie auch mit Xenotim verwachsen was auf Bildungstemperaturen von ca.<400°C mittels Monazit-Xenotim Solvusgeothermometrie hinweist.U-Th-Pb Elektronenstrahlmikrosondendatierung von primären Monaziten er-gab sog.CHIME (chemical Th-U-total Pb isochron method) Alter von 465 ± 22 Ma und 469 ± 34 Ma, welche mit den U-Pb Altern aus den Zirkonen sehr gut übereinstimmen.Wegen der geringen Th, U, Pb-Gehalte war es aber nicht möglich die sekundären Monazite zu datieren.Ihr texturelles Auftreten in der eoalpinen Hauptschieferung weist aber auf ein eoalpines Wachstum hin.Die niedrigen Y-Gehalte weisen ebenfalls auf ein niedriggradiges P-T Ereignis hin.Basierend auf den geochronologischen Daten stellt der KG ebenfalls ein weiteres Mitglied der Unter-Ordovizischen Magmatite dar, die im Austroalpin weit verbreitet sind.Im Gegensatz zu den meisten Unter-Ordovizischen I-Typ oder S-Typ Graniten weist der KG geochemisch eine ausgeprägte A-Typ Affinität auf.
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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 | 0.000 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.003 | 0.001 |
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".