This study explores the growth of garnet across the quartz–coesite transition in metapelite. Garnet chemistry and texture were investigated in several metapelite samples (garnet-chloritoid micaschist) collected across different units in the northern Dora-Maira Massif (Western Alps), and thermodynamic modelling was used to constrain the P–T (pressure and temperature) conditions of garnet growth. Two groups of garnet-chloritoid micaschist were identified.

The first one displays evidence for a single garnet generation (Alpine in age) and occurs both in the basement (Muret Unit and Chasteiran Unit) and cover (Serre Unit) of the northern Dora-Maira Massif. In these rocks, garnet crystals display similar texture, chemistry, and P–T conditions of nucleation and growth. Coesite is found as tiny inclusions in the garnet outer cores. Thermodynamic modelling indicates that garnet cores (alm₇₆prp₆grs₅₋₆) nucleated in the quartz stability field or at the quartz–coesite transition at 2.5–2.7 GPa and 470–530 °C. Its growth (alm₇₈₋₈₀prp₈₋₁₀grs₅₋₆) culminated at 2.9–3.0 GPa and 530–550 °C in the coesite–chloritoid stability field. Peak burial conditions are very similar from sample to sample, irrespective of the unit where they occur, and samples were modelled in P–T conditions of 0.1 GPa and 20 °C. These P–T values are considered to be within the uncertainties accepted for thermodynamic modelling, suggesting that overstepping did not play a major role during garnet formation. A second stage of garnet growth (garnet rim), characterized by a sharp increase in grossular (up to 26 mol %) and decrease in pyrope (up to 3 mol %) is commonly observed: it developed during decompression in the quartz stability field (from 2.4–1.5 GPa) and slight cooling (540–500 °C).

The second group of metapelite was identified only in the basement (Muret Unit). In these samples, coesite is absent and polycyclic garnet crystals are present. A narrow Alpine garnet rim overgrew pre-Alpine garnet relicts at much lower P (∼ 2.1–2.2 GPa) than the quartz–coesite equilibrium. The absence of garnet growth at ultrahigh pressure (UHP) conditions in the second group of metapelite may be related to the details of the reaction mechanisms, especially the timing and amount of fluid access inside the system, as well as the consequent changing scale of the effective bulk compositions during garnet growth. In the polycyclic rocks, kinetics plays a major role.

We performed a review of garnet compositions in metapelites from a variety of terrains and P–T conditions, and we modelled the bulk-rock composition of an average metapelite. This shows (i) systematic variations of garnet composition with P–T conditions (decreasing grossular content with increasing P and increasing pyrope content with increasing T) and (ii) that the garnet compositions reported from the studied area are consistent with those described in other terrains equilibrated at the quartz–coesite transition.

The Chasteiran Unit in the northern Dora-Maira Massif reached ultra-high-pressure conditions in the chloritoid-coesite stability field. The chemical and isotopic behaviours of zircon, garnet, and rutile from a metapelite were explored to reconstruct the metamorphic evolution of this Unit.

Zircon crystals display detrital cores and thin (< 5 μm) metamorphic rims which cannot be dated. 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 pressure and temperature 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 cores and rims 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 also indicate stability of rutile during burial at a minimum pressure of 2.0 GPa. Inclusions of rutile in garnet are commonly surrounded by fractures and some crystals display ilmenite exsolution lamellae, suggesting that despite their mode of occurrence, they might have behaved as an open system during retrograde metamorphism. 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 suggest a different timing of burial and exhumation for the northern and southern Dora-Maira Massif and imply that the peak ultra-high-pressure episode in the northern Dora-Maira Massif is older than the peak eclogite-facies in the overlying oceanic units. Therefore, different extensional allochthons derived from the Briançonnais hyperextended palaeomargin may have been buried and/or detached from the downgoing slab at different times.

This chapter reviews the available data - a huge amount of data indeed, the Alps being one of the most intensively studied belts in the world. It first characterizes the type of crustal material that was involved in the subduction process, in order to understand its previous history (lower crust, upper crust or both) and its location before the subduction (tilted blocks along the distal part of a palaeomargin, extensional allochthons). The chapter then reviews the basic knowledge about the high pressure-ultra-high pressure rocks in terms of P-T paths and timing. Finally, a discussion of the present geometry of the subducted continental units leads to some considerations about the kinematic evolution of the mountain belt. The Alpine belt displays numerous examples of subducted continental material. Structural considerations and timing data show that two distinct, unrelated, episodes of continental subduction have taken place.

Garnet is an exceptionally useful mineral for reconstructing the evolution of metamorphic rocks that have experienced multiple tectonic or thermal events. Understanding how garnet crystallizes and its mechanical behaviour is important for establishing a petrological and temporal record of metamorphism and deformation and for recognizing multiple geologic stages within the growth history of an individual crystal. Here, we integrate fine-scale microstructural (electron backscatter diffraction [EBSD]) and microchemical (Laser Ablation Inductively Coupled Plasma Mass Spectrometry [LA-ICP-MS] mapping) data obtained on a polycyclic garnet-bearing micaschist from the Alpine belt. Results suggest that fragmentation of pre-Alpine garnet porphyroblasts occurred during the late pre-Alpine exhumation and/or the onset of the Alpine burial, such that the older pre-Alpine garnet fragments were transported/redistributed during Alpine deformation and acted as nucleation sites for Alpine garnet growth. These processes produced a bimodal garnet size distribution (millimetre- and micrometre-sized grains). Thermodynamic modelling indicates that Alpine garnet grew during the final stage of burial (from 1.9 GPa 480°C to 2.0 GPa 520°C) and early exhumation (down to 1.6 GPa 540°C) forming continuous idioblastic rims on and sealing fractures in pre-Alpine garnet grains. We propose that fragmentation–overgrowth processes in polycyclic rocks, coupled with ductile deformation, may produce a bimodal garnet size distribution in response to fragmentation and re-distribution of pre-existing grains; these clasts can act as new nucleation sites during a subsequent orogenic cycle.

This Special Issue of the Swiss Journal of Geosciences entitled “Evolution of collisional orogens in space and time: the Alpine-Himalayan system in 4 dimensions”, was proposed during the joint meeting “Geosciences for a sustainable future” organized by the Società Geologica Italiana and Società Italiana di Mineralogia e Petrografia held in Turin (Italy) in September 2022.

The issue focuses on the evolution of collisional orogens through a multidisciplinary approach. As a matter of fact, continental plate collisions give rise to collisional-related orogenic belts that are some of the most spectacular and dominant features on our planet.

During collision of continental plates, considerable deformation occurs with large scale overthrusting, burial and metamorphism of continental lithosphere portions. The final anatomy and the shape of collisional belts are highly diverse, due to the interactions of several controlling factors, including the pre-collisional tectonic history, the rate and the angle of convergence, the mechanical strength and thermal state of the involved colliding plates.

High-pressure and ultra-high-pressure metamorphic terrains display an internal architecture consisting of a pile (or stack) of several coherent tectonic thrust sheets or units. Their identification is fundamental for understanding the scale and mechanisms active during subduction and exhumation of these crustal slices. This study investigates the geometry of the northern Dora-Maira Massif and the kinematics of the major tectonic boundaries, combining field and geochronological data. The tectonic stack of the northern Dora-Maira Massif comprises the following units. The lowermost unit (the Pinerolo Unit) is mainly characterized by Upper Carboniferous fluvio-lacustrine (meta-)sediments. The Pinerolo unit is overthrust by a pre-Carboniferous basement. The latter is subdivided in two tectonic units (the Chasteiran and Muret Units) with different Alpine metamorphism (ultra-high-pressure and high-pressure, respectively). The pre-Carboniferous basement of the Muret Unit is thicker than previously thought for two main reasons. Firstly, some paragneisses, traditionally assumed to be Carboniferous and/or Permian in age, display detrital zircon ages indicating a main source at about 600 Ma. Secondly, three samples of the Granero Orthogneiss, previously assumed to be a Permian intrusive body, have provided zircon U-Pb ages of 447 +/- 1 Ma, 456 +/- 2 Ma and 440 +/- 2 Ma, indicating a late Ordovician or early Silurian age for the protoliths. The uppermost unit (the Serre Unit) comprises porphyritic (meta-) volcanic and volcaniclastic rocks dated to the Permian (271 +/- 2 Ma), on top of which remnants of the Mesozoic cover is preserved. Detailed mapping of an area about 140 km2 shows that (i) the ultra-high pressure Chasteiran Unit is localized at the boundary between the Pinerolo and Muret Units, (ii) the Granero Orthogneiss may be considered as the mylonitic sole of the Muret Unit, characterized by a top-to-W sense of shear, and (iii) the contact between the Muret and Serre Units displays ductile-to brittle structures (La Fracho Shear Zone), indicating a top-to-the-NW displacement of the hangingwall with respect to the footwall. A final episode of brittle faulting, cutting across the nappe stack (the Trossieri Fault), indicates an extensional stage in the core of the Alpine belt, as previously documented in more external zones. This work provides a necessary and robust basis before an accurate discussion of processes acting during continental subduction of the Dora-Maira Massif may be understood.

Florencite is a hydrous light rare-earth elements (LREE) aluminium phosphate [REEAl₃(PO₄)₂(OH)₆], that amongst the REE-rich minerals is quite common. The main end-members are Ce-, La- and Nd-rich terms that were found in several genetic environments. Despite the large occurrence worldwide, to the authors' knowledge, florencite has attracted very few studies, particularly concerning the characterization of its Raman spectrum. We present a detailed study of the Raman spectrum of florencite, combining experimental measurements and theoretical calculations. Experimental Raman spectra (in the 100–1300 cm⁻¹ spectral range) are measured on four florencite samples characterized by different chemical composition, that is, different REE abundance. The results highlight a remarkable coincidence between different Raman spectra measured on each sample, despite the significantly different chemical compositions in terms of their REE content. The same similarities were also observed in the computed spectra at the ab initio level; moreover, the calculations allowed the attributions of the different Raman signals to specific vibrational modes. 

Massive dehydration is expected to occur during oceanic subduction. The situation is quite different during continental subduction, where often a large amount of crustal material has experienced a first orogenic cycle before the burial during the second subduction/collision cycle (rocks are therefore polycyclic). The amount and timing of dehydration and/or hydration episodes in polycyclic rocks strongly controls the extent of metamorphic re-equilibration during the second orogenic cycle. This study aims at estimating the fluid budget in polycyclic metapelites from the Muret Unit (Dora-Maira Massif, Western Alps). The excellent preservation, in a kilometre-scale low-strain domain, of pre-Alpine minerals and structures allows the comparison with the textures and structures observed in the pervasively foliated adjacent rocks. In the low-strain domain, the main foliation is pre-Alpine and defined by high-temperature minerals whereas the Alpine high-pressure overprint is static. Pre-Alpine garnet porphyroblasts were fractured and partially dissolved before the growth of the Alpine garnet over pre-Alpine garnet fragments. The preservation of the overall shape of the original pre-Alpine porphyroblasts suggests that pre-Alpine garnet crystals were pseudomorphically replaced by chlorite during the late Variscan retrogression. This process was likely triggered by an episode of fluid-rock interaction and moderate hydration (similar to 1-2 wt% H2O) which is also responsible of the growth of metamorphic zircon at similar to 304 Ma. In the high-strain domain, the dominant fabric is Alpine and developed at high-pressure conditions. Despite the difference in the strain intensity, metapelites from both low- and high-strain domains developed the same peak Alpine assemblage garnet-chloritoid-glaucophane-muscovite-rutile in the presence of a free H2O phase at 21-22 kbar and 530-560 degrees C. Thermodynamic modelling indicates that after the peak pre-Alpine metamorphism, a minimum re-hydration of at least 1 wt% was needed in order to reach H2O-saturation during the Alpine cycle. Alpine garnet mainly sealed fractures and formed thin discontinuous overgrowth on the partially dissolved pre-Alpine garnet. Its growth occurred during the Alpine prograde to peak evolution, involving progressive consumption of chlorite and lawsonite and resulting in a prograde growth zoning. The preservation of the Alpine garnet growth zoning and the absence of complex compositional modifications suggest that the rock did not record massive pulses of fluid infiltration during subduction. Instead, the main fluid-rock interaction episode was limited and occurred before the Alpine re-equilibration during the late Variscan evolution.

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. It is only a few tens of metres thick and consists of graphite-rich, garnet–chloritoid micaschists, whose protoliths may be black shales of Silurian age. 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 510–530 °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. 

Deciphering the pre-orogenic evolution of subducted continental basement is challenging due to pervasive reworking of crust during subduction and exhumation. Survival of such polycyclic basement may occur locally in low strain domains bounded by intensely overprinted rocks. The Palaeozoic history of basement involved in Alpine continental subduction is investigated in the northern Dora-Maira Massif where a kilometre-scale domain of low strain preserves a pre-Alpine amphibolite-facies foliation in garnet-biotite orthogneiss and garnet-staurolite micaschist. By contrast, a first generation garnet is the only pre-Alpine relict in pervasively reworked domains surrounding the low-strain domain. Thermodynamic modelling based on garnet isopleths in micaschist constrains the pre-Alpine pressure–temperature (P–T) evolution from 4 to 5 kbar and ~500°C to 6–7 kbar and ~650°C, which is consistent with Barrovian metamorphism up to the staurolite zone. In this micaschist, monazite included in garnet rims provide an age of 324 ± 6 Ma (95% confidence interval; c.i.). On the basis of textural and chemical data, this is interpreted as recording peak Barrovian metamorphic conditions. Low Th/U metamorphic zircon overgrowths and crystals yield an age of 304 ± 2 Ma (95% confidence interval). On the basis of the trace element concentrations and rare earth element (REE) patterns measured in garnet and metamorphic zircon, the latter is tentatively interpreted as having grown during early exhumation or cooling, involving garnet consumption and fluid infiltration. The reconstructed Variscan Barrovian metamorphism of the northern Dora-Maira basement is consistent with that documented in the External Crystalline Massifs and in the Austroalpine domain of the Alps. The Palaeozoic basement of the Dora-Maira Massif likely represents upper crustal material, later involved in Alpine continental subduction under high- to ultra-high-pressure conditions.