T2
Continental subduction is hereafter defined as continental, dominantly crustal, material recording high-pressure metamorphism (in most cases at blueschist- and eclogite-facies P-T conditions), without any prejudice upon pre-orogenic position and syn-orogenic burial mechanism. The main questions related to continental subduction in the Western Alps may be summarised as follows:
- In most models, continental subduction is succeeding oceanic subduction, the buoyant continental crust being dragged down by the subducting slab of oceanic lithosphere. Subduction erosion has also been proposed as a major mechanism (e.g. for the Sesia-Dent Blanche nappes). Do these models apply in the Western Alps, in the lack of a ‘true’ subduction zone?
- Which parts of the continental crust are subducted? Is it the entire palaeomargin, or just parts of it, for example extensional allochthons? What is the role of the inherited structures associated with the rifting history of the palaeomargin?
- What is the age of the HP/UHP metamorphism? Is it the same along as well as across the belt, or is this metamorphism diachronous? In the latter case, does this reflect the progressive burial of a single continental plate, or are there several continental domains separated by oceanic domains?
- What are the mechanisms for the exhumation of the HP-UHP rocks? Is erosion the driving force for exhumation? What is the record of erosion in the nearby sedimentary basins? Erosion may have been combined to other tectonic processes, like buoyant uprise of the HP-UHP bodies, associated or not with plate divergence, slab roll-back, … Collision may have reworked most of the evidence, but a careful analysis of the field evidence provides major clues.
How to cite: Ballèvre, M.: Continental subduction in the Western Alps: major issues., 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-77, https://doi.org/10.5194/egusphere-alpshop2024-77, 2024.
Metamorphic rocks cropping out north of the Periadriatic Line are objects of our study to better understand the Cenozoic subduction process during the collision of microcontinent Adria with the European plate forming the Central and Eastern Alps. For this purpose, we collected rocks along the Malenco valley that are suitable to allow us to decipher their pressure-temperature (P-T) evolution. About 1.5 km northwest of the village of San Giuseppe, calcareous schists rich in phengite were sampled. Their bulk-rock compositions were determined. The minerals in three samples were carefully chemically characterized with the electron microprobe. Thermodynamic modelling followed for two garnet-bearing samples using the program package PERPLE_X.
The protoliths of the samples were probably carbonate-bearing psammopelites. The two modelled rocks are characterized by alternating, a few mm thick layers either enriched in phengite or quartz representing the main foliation. The phengite-rich layers host most of the mafic minerals, whereas quartz-rich layers also contain plagioclase, which is nearly pure albite. The mafic minerals are hornblende, idiomorphic garnet with diameters between 50 to 200 µm, and minor biotite and chlorite. One sample also contains some epidote, the other one some titanite. Accessories are zircon, apatite, and opaque phases. Carbonate is lacking.
Modelling of the peak metamorphism is based on phengite with Si contents between 3.25 (rim) and 3.35 per formula unit (pfu) and zoned garnet. The zonation is characterized by significantly decreasing Mn, slightly decreasing Mg (0.05 to 0.04 Xpyrope), and clearly increasing Ca contents (e.g., 0.25 to 0.36 Xgrossular in one sample) from core to rim. The modelling yielded, consistently for both samples, a pressure decrease from 13.5 kbar at 570°C to 11.5 kbar at 550°C. In spite of the temperature decrease, a growth of garnet occurred because the modelling predicts 2-2.5 vol% garnet coexisting with about 20-25 vol% phengite and 30-35 vol% as well as significant quantities of omphacite (20-25 vol%), biotite (7-10 vol%), and paragonite (7-10 vol%) at the pressure peak, but 4.5 vol% garnet at 11.5 kbar due to breakdown of omphacite, biotite, and paragonite. The decomposition of these minerals also led to increasing contents of phengite with Si contents of 3.25 to 3.30 pfu as well as significant quantities of hornblende and albite during further pressure release. The observed chlorite seems to be a late retrogression product.
We suggest that the studied samples are monocyclic metamorphic rocks. They were located at (or near) the surface of the downgoing European plate and subducted to Earth’s depths of about 50 km in the Cenozoic. Hydrous fluids were present during the subduction process and early exhumation evidenced by the aforementioned mineral reaction (paragonite breakdown). Major deformation occurred at the pressure peak and during early exhumation. The corresponding tectonic movements led to nappe stacking, so that the contact to the Malenco Unit, which represents Permian lowermost crust and underlying mantle, in the south was established.
How to cite: Massonne, H.-J., Li, B., Iaccarino, S., and Zhang, J.: Metamorphic evolution of calcareous schists of the Margna Nappe at Valmalenco, Central Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-29, https://doi.org/10.5194/egusphere-alpshop2024-29, 2024.
The Chasteiran Unit in the northern Dora-Maira Massif reached UHP conditions in the chloritoid-coesite stability field. The chemical and isotopic behaviour of zircon, garnet, and rutile was explored in a metapelite in order to reconstruct a timeline for the metamorphic evolution of this Unit.
Zircon crystals display detrital cores and thin (< 5 mm) undatable metamorphic rims. The dominant zircon population consists of Late Neoproterozoic (⁓600 Ma) magmatic grains whereas the youngest zircon cluster is Ordovician in age (∼470 Ma).
Garnet records three main growth stages: initial growth during a prograde P and T increase in the quartz stability field (2.5‒2.7 GPa at 470‒500 °C, inner core ‒ stage 1), peak growth in the coesite stability field (2.7‒2.8 GPa at 510‒530 °C, outer core ‒ stage 2), and final growth of the garnet rim between 2.3 GPa 520 °C and 1.5 GPa 510 °C (stage 3), contemporaneously with lawsonite consumption coupled with fluid production. LA-ICP-MS U-Pb dating of garnet indicates two distinct stages of growth for garnet cores and rims at ∼61 Ma and ∼43 Ma, respectively. The time interval separating the growth of garnet core and rim is consistent with our thermodynamic modelling, which indicates the absence of garnet growth during the initial stage of exhumation, between 2.7. GPa and 2.3 GPa.
Rutile is found both as inclusions in garnet and in the matrix. Rare inclusions of jadeite and Si-rich muscovite constrain rutile growth during burial at a minimum P of 2.0 GPa. Inclusions of rutile in garnet are commonly surrounded by fracture and some crystals display ilmenite exsolution lamellae, suggesting that despite their mode of occurrence, they might have behaved as an open system during later events. Rutile consumption took place during exhumation, as suggested by the increase in Ti content in garnet and muscovite rims and thermodynamic modelling. Rutile in the matrix is partially replaced by ilmenite corona, which developed at P < 1.5 GPa, after garnet growth. SIMS U-Pb dating of rutile, irrespective of its petrographic mode of occurrence, yields a date of ∼37 Ma.
Our geochronological data puts new constraints on the metamorphic evolution of the Chasteiran Unit, which will be discussed in the context of published chronological data and P‒T estimates from the Dora-Maira Massif.
How to cite: Manzotti, P., Whitehouse, M. J., Jeon, H., Millonig, L. J., Gerdes, A., Poujol, M., and Ballèvre, M.: Petrochronology of the UHP Chasteiran Unit (northern Dora-Maira Massif), 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-35, https://doi.org/10.5194/egusphere-alpshop2024-35, 2024.
For a better process understanding of the subduction of the Ligurian Ocean and the adherent European plate under microcontinent Adria including the exhumation of deeply subducted rocks, we have investigated a micaschist from the ultrahigh-pressure (UHP) terrane of the southern Dora Maira Massif. This micaschist crops out about 1 km north of the hamlet of Masueria and contains quartz (40-45 vol%), phengite (almost 30 vol%), garnet (17 vol%), which is strongly variable in size (diameter of 300 µm to almost 1 cm), kyanite (7 vol%), pseudomorphs after jadeite (4 vol%) and different accessory minerals. The compositions and textural relations of the minerals were carefully studied with an electron microprobe. After determination of the bulk-rock composition of the micaschist, which points to a pelitic protolith, thermodynamic modelling with PERPLE_X was undertaken to reconstruct the metamorphic evolution of this rock.
The early mineral assemblage found as inclusions in extended cores of large garnet grains being chemically fairly homogeneous consists of quartz, chloritoid, staurolite, paragonite and kyanite. This assemblage formed at pressure-temperature (P-T) around 12.5 kbar and 600 °C, before relatively large volumes of garnet, after significant overstepping of its P-T limits, overgrew these minerals under release of considerable amounts of water. A relatively narrow rim developed around the inclusion-rich garnet core as the result of early subduction of the rock to depths corresponding to pressures of 20 kbar accompanied by slight heating. Only paragonite reacted to jadeite + kyanite at pressures of 26 kbar before UHP conditions were reached. This reaction resulted in another pulse of water released. Nevertheless, the subsequent burial at UHP to P-T conditions of 34±2 kbar and 715±35 °C, at which phengite with Si contents of 3.47 per formula unit (pfu) equilibrated, occurred under water-absent conditions so that possibly no coesite formed from quartz as the result of overstepping the coesite-quartz transition. The retrograde path is only characterized by the formation of phengite with Si contents lower than 3.4 pfu around UHP phengite and the replacement of jadeite mainly by albite rods. The latter reaction occurred at pressures below 18 kbar, a retrograde path provided that is characterized by slight cooling down to pressures of about 15 kbar as suggested by previous researchers of the Dora Maira UHP terrane. The described retrogression occurred in absence of free H2O, but defornation caused the partial recrystallization of UHP phengite by phengite with lower Si contents.
The studied polymetamorphic micaschist does not indicate a polycyclic metamorphism. A flat subduction to 45 km (~12.5 kbar) was followed by steep subduction to 110 km. During subduction, pulses of hydrous fluid changed the rock during prograde metamorphism in the pressure range of about 11 to 26 kbar clearly. At UHP and during early retrogression (down to ~15 kbar), changes took place only by deformation as virtually no hydrous fluids were released in the rock or infiltrated it.
How to cite: Li, B., Massonne, H.-J., Iaccarino, S., and Zhang, J.: Polymetamorphic evolution of a micaschist from the ultrahigh-pressure terrane of the southern Dora Maira Massif, Western Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-30, https://doi.org/10.5194/egusphere-alpshop2024-30, 2024.
This contribution aims to present to the Alpine scientific community some of the latest studies we performed in the Central Alps, with a focus on the origin of the Barrovian metamorphism in the Lepontine dome.
The Lepontine dome is a metamorphic and structural dome formed by crystalline basement nappes of the Penninic domain. The area is characterized by a Barrovian metamorphic imprint, which is testified by peak-temperature mineral isograds and by a pervasive mineral and stretching lineation in amphibolite-facies. Isograds locally intersect the tectonic nappe boundaries, an observation that was frequently considered as evidence of post-nappe emplacement heating. Nonetheless, the NW-SE directed lineation suggests that peak metamorphic conditions developed coeval to the emplacement of Lepontine nappes.
We addressed this inconsistency through a multidisciplinary approach. We combined extensive geological and structural mapping with U-Pb zircon dating, which permitted to identify a belt of syn-kinematic migmatites dated at 31-33 Ma. The discovery of these Alpine migmatites defines a major crustal-scale shear zone which we named “Maggia-Adula shear zone”. This thrust divides the Adula and Maggia nappes on top from the Simano below, with the Cima Lunga unit pinched and sheared between them.
We modelled nappe emplacement along this main shear zone with a simple thermo-kinematic numerical model, which revealed that heat was mainly advected from depth during overthrusting. Heat conduction also contributed to the final configuration of the peak-temperature isotherms and shear heating played a role in shaping the inverted metamorphism observed around the thrust.
Finally we investigated the cooling history of Lepontine garnet-paragneisses sampled at different tectonic levels within the nappe pile. We applied multicomponent diffusion modelling to Alpine garnet rims which experienced re-equilibration at close-to-peak conditions. The rocks within the main shear zone experienced post-peak conditions of ca. 635 °C and 0.8 GPa, with a subsequent very fast cooling of 100-400 °C/Myr. These high cooling rates can’t be explained with regional exhumation, and confirm the hypothesis of shear heating acting within the shear zone.
In conclusion, our results indicate that the origin of Barrovian metamorphism in the Lepontine dome can be ascribed to the emplacement of a hot Alpine nappe, which we refer to as the “Maggia-Adula nappe”. This event produced Barrovian isograds, amphibolite-facies lineation and migmatites at ca. 31 Ma. In specific locations within the Lepontine dome, our results also suggest that we should re-consider the distribution of peak-temperature isotherms. The subsequent cooling of the Lepontine area was spatially and temporally heterogeneous and could have determined a later re-heating in the northern units.
Assessing the extent of Barrovian metamorphism and its finite imprint in the field at different tectonic levels is challenging and requires an interdisciplinary study. Geometrical interpretation alone is insufficient to understand the origin of Barrovian metamorphism without considering the physical forces leading to deformation and metamorphism in mountain-building processes.
How to cite: Tagliaferri, A., Schenker, F. L., Schmalholz, S. M., and Moulas, E.: A multidisciplinary study of the Barrovian metamorphism in the Lepontine dome gives new insights into the heating history of the Central Alps, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-36, https://doi.org/10.5194/egusphere-alpshop2024-36, 2024.
The Austroalpine Unit is a nappe stack that formed by accretion of Adria-derived material in Late Jurassic to middle Late Cretaceous times. Its history is mostly recorded by upper crustal non-metamorphic rocks and lower crustal upper greenschist to eclogite facies metamorphic rocks. Data from the ubiquitous mid-crustal, low-grade metamorphic units are, however, either missing or difficult to interpret, complicating the link between the shallow and deep orogenic levels. We present new pressure-temperature-time-deformation data for the Permian Präbichl Formation, sampled in the Tirolic-Noric Nappe System (TNNS) below the overlying Juvavic Nappe System (JNS) at two localities. This formation consists of lower greenschist facies clastic sediments and corresponds to the Permian cover of the pre-Variscan basement. The metamorphic assemblage of the Präbichl Formation contains chloritoid + muscovite ± pyrophyllite + hematite + rutile + quartz. Phase equilibrium calculations and Raman spectroscopy on carbonaceous material indicate peak P-T conditions of ~350°C and 0.4-0.5 GPa. In both samples, 10 to 30 µm xenotime show systematic chemical zoning with a heterogeneous core and a distinct MREEs-rich rim. We targeted each chemical domain by in-situ LA-ICP-MS U-Pb dating. The concordant U-Pb ages from cores range between 632 Ma and 250 Ma, and likely reflect an inherited component. Younger dates were measured in the xenotime rims. In the eastern sample (Noric Nappe), a concordant cluster yields a weighted mean age of 133.6 ± 2.8 Ma (MSWD: 1.7, n: 14). Host-inclusion relationships of chloritoid and xenotime suggest coeval growth of the xenotime rim and chloritoid porphyroblasts, linking the U-Pb age to the growth of the main metamorphic assemblage. An additional set of discordant analyses yield an anchored discordia age of 91.5 ± 3.6 Ma (MSWD: 1.2, n: 7). In the western sample (Staufen-Höllengebirge Nappe), a set of concordant and discordant analyses yield an anchored age of 90.1 ± 1.4 Ma (MSWD: 1.8, n: 16). Xenotime and chloritoid are not observed in direct contact, and this sample is characterized by a pervasive crenulation cleavage, which postdates chloritoid growth. From the distribution and morphology of xenotime we conclude that post-peak dissolution-precipitation related to crenulation cleavage formation facilitated growth of the rim. These results have two key implications. Firstly, the 133.6 ± 2.8 Ma date coincides with the age of the latest syn-orogenic sediments overthrusted by the Juvavic Dachstein Nappe. It is therefore interpreted as the age of peak metamorphism after thrusting of the JNS over the TNNS ceased. The peak pressure of 0.4-0.5 GPa at that time corresponds to an overburden of ~17 km, which cannot be solely explained by the thickness of the JNS, which has a present day thickness of 5-10 km, suggesting the existence of a missing unit. Secondly, the 90-92 Ma dates correspond to the timing of the onset of post-orogenic sedimentation in the Gosau basins overlying both the TNNS and JNS and the exhumation of the Austroalpine eclogites. This implies a major change of dynamics at all levels of the orogen at that time.
How to cite: Hollinetz, M. S., Huet, B., Grasemann, B., Schneider, D. A., McFarlane, C. R. M., Bryda, G., and Mandl, G. W.: P-T-t-d history of low-grade Permian metasediments in the Austroalpine Unit, 16th Emile Argand Conference on Alpine Geological Studies, Siena, Italy, 16–18 Sep 2024, alpshop2024-40, https://doi.org/10.5194/egusphere-alpshop2024-40, 2024.
