T11

The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of prime importance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in the southern Dora-Maira massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now in the other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif (Western Alps), named Chasteiran Unit (Manzotti et al. 2022). It is only a few tens of metres thick and consists of garnet-chloritoid micaschists. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P–T evolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusions in the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record of growth during the prograde increase in P and T (25–27 kbar 470–500 °C) (stage 1), up to the coesite stability field (27–28 kbar 520–530 °C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to 15 kbar 500–515 °C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, began to develop during this stage, and was then overprinted by chlorite-ilmenite (stage 4). The Chasteiran Unit is discontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from, and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, much colder, P–T evolution, showing that different slices were detached from the downgoing subduction slab. The Chasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P–T conditions are comparable to the ones of the Tian Shan coesite-chloritoid-bearing rocks.

Manzotti, P., Schiavi, F., Nosenzo, F., Pitra, P., Ballèvre, M. (2022). A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps). Contributions to Mineralogy and Petrology, in press.

How to cite: Manzotti, P., Schiavi, F., Nosenzo, F., Pitra, P., and Ballèvre, M.: A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps), 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-6, https://doi.org/10.5194/egusphere-alpshop2022-6, 2022.

Extensive rock recrystallization and element redistribution during retrogression often hampers our understanding of the early stages of metamorphism. Garnet is the mineral that best preserves information about its growth during the prograde history of the rock as compositional zoning. In most metamorphic rocks, garnet zoning varies between almandine, grossular, and pyrope end-members with minor spessartine content. This variability and the diffuse presence of mineral inclusions in garnet enables the coupling of thermodynamic tools (e.g., pseudosections) with classical element exchange and elastic geothermobarometry to gather information on their pressures and temperatures of equilibration. Such studies give their best results when applied to metapelites due to their relatively large mineral variability over the typical PT range of metamorphic rocks. However, monomineralic lithotypes, such as impure quartzite or marble, consist of minerals stable over a wide PT range and therefore lack mineralogic change. Furthermore, currently available solution models are not calibrated for use on unconventional bulk rock compositions and therefore do not guarantee reliable geothermobarometric results.

In this contribution, we use elastic geobarometry to track prograde garnet growth from low- to ultrahigh-pressure conditions in three Mn-rich garnets (up to 50% sps) from an impure marble from the Lago di Cignana Unit (Italy). The rock consists of mainly quartz and calcite with garnet porphyroblasts. The three garnets show a very large core-to-rim compositional zoning with Mn-rich cores, Fe-rich mantles, and rims with a slight increase in Mg. Mineral inclusions in garnet cores and mantles are mainly quartz, with minor titanite, calcite, and apatite. Coesite, aragonite, zircon, and rutile are instead present within garnet rims. The three investigated garnets vary in shape, zonation, inclusions type and size while having a comparable core-to-rim composition. In two garnets, quartz inclusions are tiny (20-30 μm) and spread evenly within the garnets. The third garnet has larger quartz inclusions (50-100 μm) in the core and smaller in the mantle, decreasing progressively in size from the inner to the outer mantle (50-10 μm). Elastic geobarometry on these quartz inclusions in garnet allowed the tracking of the pressures at which garnet cores and mantles formed. We can show that these garnets formed during multiple distinct growth stages along the prograde path from 1.2 GPa and 430°C to 1.8 GPa and 500°C and finally at UHP conditions, as testified by the coesite-bearing garnet rims. This difference in pressure and temperature of garnet growth might be due to local (cm-to-mm-sized) changes in chemical composition at the scale of the thin section and/or to reaction overstepping.

How to cite: Gilio, M., van Schrojenstein Lantman, H. W., Girani, A., Angel, R. J., Scambelluri, M., and Alvaro, M.: The prograde history of three Mn-rich garnets from the UHP Lago di Cignana Unit (Italy), 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-18, https://doi.org/10.5194/egusphere-alpshop2022-18, 2022.

The first occurrence of ultra-high-pressure (UHP) metamorphism in the Western Alps was documented by Chopin in 1984 (Chopin, 1984) with the discovery of coesite in the southern Dora Maira massif. Since then, just one additional UHP terrain was discovered until the end of the 90’s. In recent times, new occurrences of coesite have been reported in different units of the Western Alpine belt, widening the distribution of UHP terrains, with important tectonic implications. Here, we report the first discovery of coesite in the meta-ophiolitic suite of the Monviso Massif, in the northern Lago Superiore Unit (LSU). Previous petrographic studies and thermodynamic modelling in the area suggested that these alpine units may have experienced UHP metamorphism, but no direct evidences (i.e., coesite occurrence) have been reported to date.  The presence of coesite is demonstrated by µ-Raman analyses. The Raman spectra show the typical peaks of coesite, slightly shifted towards higher wavenumbers. The main peak is located at 522 cm^–1, and the secondary peaks at 426, 270 and 178 cm^–1. Coesite inclusions consist of intact single crystals (10-60 µm) hosted by garnet, without evidence for re-equilibration features. Typical coesite-related features such as radial cracks in garnet host mineral and palisade texture are present in large polycrystalline quartz inclusions (>80 µm). Peak metamorphic conditions have been constrained through different techniques (detailed garnet inclusion analysis, elastic geobarometry and thermodynamic modelling).

The occurrence of UHP terrains along the Western Alps is becoming more common than expected. Our results, alongside with the novel evidence for UHP in the Western Alps, will lead to new tectonic models for the subduction and exhumation of UHP terrains, constraining the evolution of subduction-accretionary systems.

Chopin, C., 1984, Coesite and pure pyrope in high-grade blueschists of the western Alps: a first record and some consequences: Contributions to Mineralogy and Petrology, v. 86, p. 107–118, https://doi.org/10.1007/BF00381838.

How to cite: Ghignone, S., Scaramuzzo, E., Gilio, M., Bruno, M., Livio, F., and Alvaro, M.: First evidence of UHP in the Lago Superiore Unit (Monviso, Western Alps), 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-47, https://doi.org/10.5194/egusphere-alpshop2022-47, 2022.

The Adula nappe in the Swiss-Italian Central Alps is a continental basement nappe from the former European margin that was subducted to depths indicating (ultra)-high-pressure conditions. Many studies were performed to understand the pressure-temperature-time evolution of the Adula nappe. However, the pressure data derived from classical thermobarometry from eclogite and garnet peridotite lenses cannot be correlated with the tectonic record without several difficulties. The pressure gradient is very high, the structural record for the often suggested extrusion model is missing and the directly surrounding nappes show consistently lower pressures. Furthermore, it was discovered that at least parts of the Adula nappe underwent eclogite-facies metamorphism during the Variscan and the Alpine orogenic cycles. These two cycles were distinguished by age dating and the chemical zonation patterns of garnet, although in some cases it can be ambiguous. Otherwise, the Variscan and Alpine parageneses are hardly, if at all, possible to tell apart. Therefore, existing pressure and temperature data that were obtained using classical geobarometers rely on mineral equilibria, which may not have yielded true Alpine metamorphic conditions.

For this study, around fifty felsic and metabasic samples were collected from different lithologies on a N-S transect through the Adula nappe parallel to the direction of subduction. Raman spectroscopy on quartz inclusions (RSQI) in garnet was used as a geobarometer to measure minimum peak pressures. The advantages of this method are its independence of a chemical equilibrium and the ability to yield reliable pressure constraints even if the high-pressure mineral assemblage has been retrogressed. The Variscan and Alpine garnet domains were carefully identified using the Electron Microprobe (EMP) and the Scanning Electron Microscopy (SEM). Temperatures were determined by means of Zr-in-rutile thermometry by measuring the Zr content with EMP.

As a result, the obtained temperatures exhibit a gradient increasing from the north at ca. 500-550 ^oC to the south at around 700 ^oC. The minimum peak pressures in the northern and central Adula nappe range between 2.09 GPa and 2.17 GPa for metasediments and 1.41 GPa and 2.02 GPa for metabasites. 1.53 GPa were determined for an orthogneiss from the central part of the nappe. Lower pressures between 1.14 GPa and 1.31 GPa in the southern Adula nappe were potentially caused by viscous relaxation of the quartz inclusions during the high-temperature Lepontine metamorphism. Our new pressure data imply a very weak pressure gradient. Therefore, it is in contrast to the results of previous works, in which barometers based on a chemical equilibrium were applied. Additionally, no systematic difference in minimum peak pressures is observable for the different lithologies.

How to cite: Brunsmann, O., Germer, M., Pohl, A., Kohn, V., Könemann, V., Zhong, X., Pleuger, J., and John, T.: Barometric studies on different rock types from the Adula Nappe (Central Alps) by Raman spectroscopy of quartz inclusions in garnet, 15th Emile Argand Conference on Alpine Geological Studies, Ljubljana, Slovenia, 12–14 Sep 2022, alpshop2022-33, https://doi.org/10.5194/egusphere-alpshop2022-33, 2022.