Dora-Maira Massif Research Articles

Comparison between 2D and 3D microstructures and implications for metamorphic constraints using a chloritoid–garnet-bearing mica schist

Abstract. Despite the fact that rock textures depend on the 3D spatial distribution of minerals, our tectono-metamorphic reconstructions are mostly based on a 2D visualisation (i.e. thin sections). This work compares 2D and 3D investigations of petrography and microstructures, modal abundances, and local bulk rock composition and their implication for P–T estimates, showing the pros and cons and reliability of 2D analysis. For this purpose, a chloritoid–garnet-bearing mica schist from the Dora-Maira Massif in the Western Alps has been chosen. In particular, for 2D a thin section scan has been combined with chemical X-ray maps, whereas for 3D the X-ray computerised axial microtomography (µCT) has been applied. Two-dimensional investigations are readily accessible and straightforward but do not consider the entire rock volume features. Conversely, the rise of 3D techniques offers a more comprehensive and realistic representation of metamorphic features in the 3D space. However, they are computationally intensive, requiring specialised tools and expertise. The choice between these approaches should be based on the research aims, available resources, and the level of detail needed to address specific scientific questions. Nevertheless, despite differences in the modal distribution, the estimated bulk rock compositions and relative thermodynamic modelled phase fields show similarities when comparing the 2D and 3D results. Also, since different thin section cut orientations may influence the results and consequent interpretations, three different cuts from the 3D model have been extrapolated and discussed (i.e. XZ, YZ, and XY planes of the finite-strain ellipsoid). This study quantitatively corroborates the reliability of the thin section approach for tectono-metamorphic reconstructions, still emphasising that 3D visualisation can help understand rock textures.

Tectonic architecture of the northern Dora-Maira Massif (Western Alps, Italy): field and geochronological data

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.

Fluid metasomatism of subducted continental crust: Insights from Si isotope compositions of metasomatic rocks from the Western and Eastern Alps

Serpentinite-derived fluids play a crucial role in mass exchange between the Earth's crust and mantle. However, there is currently a limited understanding of how serpentinite-derived fluids influence the Si isotope composition of mantle and subducted crustal materials. Whiteschists in the Dora-Maira Massif of the Western Alps and leucophyllites in Austria of the Eastern Alps were formed through serpentinite-derived fluid metasomatism of metagranite protoliths. Thus, they can provide valuable insights into the geochemical diversity of such rocks resulting from fluid–rock interactions. A special focus is on Si isotope compositions, which were analyzed in whiteschists, leucophyllites, and their corresponding country rocks. The aim of this paper is to investigate the impact of serpentinite-derived metasomatic fluids on the Si isotope compositions of subducted continental crust. With the exception of one whiteschist sample, which contains high amounts of pyrope, and one pyrope megablast, all other whiteschists and leucophyllites exhibit Si isotope compositions similar to those of their country rocks. This observation suggests that the Si isotopes of whiteschists and leucophyllites were inherited from their respective country rocks; fluid metasomatism did not significantly modify the Si isotope compositions of whiteschists and leucophyllites. Because pyrope has much lower δ30Si values than quartz and phengite as further major mineral constituents of the studied whiteschists, the samples with high pyrope contents reveal lighter Si isotopes than those containing lower amounts of pyrope. Interestingly, the metagranites and the majority of whiteschists and leucophyllites exhibit Si isotope compositions below the igneous array, implying that their protoliths are S-type granites. These granites commonly contain sedimentary components derived from their sources. Our results document that Si behaves immobile during serpentinite-derived fluid metasomatism of the subducted continental crust. Combined with previous results, we were able to show that slab derived fluids, including serpentinite-derived fluids, produce only a minimal influence on the Si isotope composition of the overlying mantle wedge and its derived magmas. Thus, the Si isotope compositions of rocks, even if highly affected by metasomatizing fluids, can be used as significant fingerprints that refer to their unaltered precursors.

Nitrogen behavior during fluid-rock interaction in a continental subduction zone: Results from the UHP Dora-Maira Massif whiteschists and Eastern Alps leucophyllites

Deep-Earth cycling of nitrogen (N) along the subduction pathway, remains relatively poorly understood, particularly that related to deep subduction of continental crust. The ultrahigh-pressure (UHP) Dora-Maira Massif whiteschists in the Western Alps and their lower-P-T equivalent leucophyllites in the Eastern Alps show a decrease in N concentration and an increase in δ15N relative to their presumed country rock protoliths. On average, the whiteschists contain 20.7 ± 10.5 ppm N (mean ± 1σ) with δ15Nair = +2.7 ± 2.5‰ (mean ± 1σ) and the leucophyllites contain 59.8 ± 12.3 ppm N with δ15Nair = +4.3 ± 2.0‰, whereas the metagranitic country rocks from Dora-Maira contain 41.3 ± 12.5 ppm N with δ15Nair = −1.9 ± 3.2‰ and the country rocks from the Eastern Alps contain 91.1 ± 42.5 ppm N with δ15Nair = +1.7 ± 2.1‰. The loss of N and isotope shift in the whiteschists and leucophyllites from these two localities was accompanied by loss of LILE (Rb, Ba), Sr, CaO, Na2O, and FeOtotal. The other stable isotope systems applied to study both suites (Mg, Fe, O, Li and Ba) show no obvious correlation with variations in δ15N. Infiltration by and interaction with a serpentinite-derived fluid depleted in N and LILE could have led to leaching of N from the country rock protoliths and production of the +4.6‰ (Dora-Maira Massif) and +2.6‰ (Eastern Alps) isotope shifts adequately explained using a Rayleigh distillation model. The N exchange and leaching process is best explained as involving N2-NH4 or NH3-NH4 at about 550–600 °C, the latter the approximate temperatures at which the fluid infiltration is thought to have occurred. These results provide new insights regarding N behavior and isotopic fractionation during fluid-rock interactions occurring in UHP rocks and point to the importance of H2O-rich fluids from serpentinite dehydration as agents of metasomatism along deep subduction interfaces. Furthermore, the data for the metagranites indicate the potential of this lithology to retain significant amounts of N to at least 100 km and beyond sub-arc depths.

Multi-stage subduction-related metasomatism recorded in whiteschists from the Dora-Maira Massif, Western Alps

Whiteschists from the Dora-Maira massif (Western Alps, Italy) are Mg and K-rich metasomatised granites which experienced ultra-high-pressure metamorphism and fluid–rock interaction during Alpine continental subduction. The sources and timing of fluid infiltration are a source of significant debate. In this study, we present boron (B) isotopes and other fluid-mobile trace-element (FME) concentrations in various generations of phengite from whiteschists and their country rock protoliths to investigate the sources and timing of metasomatic fluid influx. Reconstructed bulk rock concentrations based on modal data and mineral compositions indicate that significant amounts B and other FME were added to the rock during prograde metamorphism, but that this fluid influx postdates the main Mg metasomatic event. High B concentrations (150–350 upmug/g) and light {delta^{text{11}}}text{B} values (− 16 to −4 ‰) recorded in phengite point to a B-rich sediment-derived fluid as the main source of B in the whiteschists. Further redistribution of FME during metamorphism was associated with breakdown of hydrous minerals, such as talc, phlogopite, and ellenbergerite. The source of the Mg-rich fluids cannot be constrained based on the B data in phengite, since its signature was overprinted by the later main B metasomatic event. Rare tourmaline-bearing whiteschists record additional information about B processes. Tourmaline {delta^{text{11}}}text{B} values (− 6 to +1 ‰) are in isotopic equilibrium with similar fluids to those recorded in most phengite, but phengites in tourmaline-bearing samples record anomalous B isotope compositions that reflect later redistribution of B. This study demonstrates the utility of in situ analyses in unravelling complex fluid–rock interaction histories, where whole-rock analyses make it difficult to distinguish between different stages of fluid–rock interaction. Polymetasomatism may result in decoupling of different isotopic systems, thus complicating their interpretation. The Dora-Maira whiteschists interacted with multiple generations of fluids during subduction and therefore may represent a long-lived fluid pathway.

From divergent to convergent plate boundary: A ca. 200 Ma Wilson cycle recorded by ultrahigh-pressure eclogites in the Dora-Maira Massif, Western Alps

Sparse eclogite exposure in accretionary-to-collisional orogens cannot only reveal the sites of ancient subduction zones and plate boundaries, but also elucidate the multi-stage tectonic evolution in the Wilson cycle. The southern part of the Dora-Maira Massif in the Western Alps is well known for its ultrahigh-pressure (UHP) rocks and contains small occurrences of petrographically distinct types of eclogite. Due to the lack of detailed geochronological and geochemical data on these rocks, the nature of their protolith and related geodynamic setting has remained unknown. In the current study, 33 samples from 13 localities were studied by whole-rock major- and trace-element analysis, and a selection of the samples were studied using Sr-Nd-Hf isotopes, as well as high-resolution elemental mapping and U-Pb geochronology of zircon. According to macroscopic appearance, petrography, chemical composition, and principal-component analysis of multi-elements, two major types are distinguished. Light-colored eclogite is phengite-rich, commonly foliated, bears kyanite and quartz/coesite, shows relatively high MgO, K2O, SiO2, εNd(t) (−2.2 to +1.4), and εHf(t) (+3.5 to +7), with a protolith geochemically similar to an enriched-type mid-oceanic-ridge basalt (E-MORB). Dark eclogite generally is massive, rutile-rich, shows higher values of Ti, Fe, P, Nb, and Zr, εNd(t) values between −2.8 and −0.8, εHf(t) values between −6.1 and −3.2, with an oceanic island basalt (OIB)-type protolith composition. Magmatic zircon cores, which are characterized by steep heavy rare earth element (HREE) patterns and negative Eu anomalies, yield consistent protolith ages of ca. 253−252 Ma in both eclogite types. Metamorphic zircon domains with flat HREE patterns and insignificant Eu anomalies yield a younger mean age of ca. 34 Ma, which is the age commonly assigned to eclogite-facies UHP metamorphism. Considering the various geochemical signatures of E-MORB- to OIB-like eclogites and the regional tectonic evolution, their protolith is best explained as rocks, which crystallized from rift-related basaltic magma, associated with the break-up of the Pangea Supercontinent that eventually resulted in the birth of the Piemonte-Liguria Ocean (Alpine Tethys Ocean) between the future Eurasia and Adria plates. Therefore, the UHP eclogites in the Dora-Maira Massif likely fingerprint a multi-stage tectonic evolution from divergent (continental extension, rifting) to convergent (subduction zone) plate boundary, corresponding to the beginning and end of a Wilson cycle. Incidentally, they reveal that mafic rocks in the Alpine basement units may not be polymetamorphic, but may actually consist of post-Variscan products that underwent only Alpine metamorphism.

H2O budget and metamorphic re-equilibration in polycyclic rocks as recorded by garnet textures and chemistry

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 (⁓ 1–2 wt% H2O) which is also responsible of the growth of metamorphic zircon at ~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 °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.

Elastic thermobarometry of ultrahigh-pressure metapelites from the Brossasco-Isasca Unit (Dora-Maira Massif, Western Alps)

Elastic thermobarometry of ultrahigh-pressure metapelites from the Brossasco-Isasca Unit (Dora-Maira Massif, Western Alps)

Fluid effect on zircon O and U-Pb isotopes during ultrahigh-pressure metamorphism: Insights from the Dora-Maira Massif of the Western Alps

Zircon geochemistry such as U-Pb and O isotopes have been widely used in dating and tracing complex geological processes. However, it still remains unclear how fluid action affects zircon geochemistry during metamorphic and metasomatic processes in subduction zones. Here a systematic study on zircon U-Pb dating, O isotopes and trace elements as well as whole-rock O isotopes was carried out for the coesite-bearing whiteschists, jadeite quartzites and granitic gneisses from the Dora-Maira Massif, Western Alps. Whole-rock and zircon geochemistry supports a common protolith, i.e., Permian S-type granites, for the above three types of rocks and an intense fluid metasomatism during the Alpine orogeny to form whiteschists and jadeite quartzites. Zircon cores in all samples have nearly identical δ18O values (9‰–11‰), whereas their apparent 206Pb/238U ages show a greater variability due to Pb loss during metamorphism. Zircon rims formed in the late Eocene to early Oligocene can be categorized into two types. Type-I rims occur in granitic gneisses and jadeite quartzites. They have high δ18O values consistent with zircon cores, but much lower contents of P and Y as well as lower Th/U ratios than the cores. Their growth can be attributed to internal metamorphic fluid action at the UHP metamorphic stage. Type-II rims occur in whiteschists and jadeite quartzites. They have remarkably lower δ18O values (5‰–8‰) and Th/U ratios (<0.01), compared with zircon cores and Type-I rims. Their growth can be ascribed to external fluids during the metasomatic process. Some zircon domains in whiteschists and jadeite quartzites show a positive correlation between δ18O values and apparent 206Pb/238U ages, which suggest the simultaneous impacts on U-Pb-O isotopes during external fluid metasomatism. This process can be attributed to the fluid-assisted dissolution and recrystallization of protolith zircons. Especially, coesite inclusions that would have been expected to occur only in young zircon rims formed during UHP metamorphism are also observed in the relict magmatic zircon cores, indicating that the fluid-related metasomatism at the UHP metamorphic conditions also affected these pre-existing protolith-related cores. Therefore, fluid action in subduction zones reveals significant impacts on both the U-Pb and O isotope systems of zircon, especially when external metasomatic fluids are involved. Therefore, a detailed study on zircon, including microstructure, mineral inclusion and geochemical data of different growth and recrystallization domains, is needed in order to unravel continental crustal evolution based on zircon U-Pb ages and O isotope compositions.

Barium isotope behavior during interaction between serpentinite-derived fluids and metamorphic rocks in the continental subduction zone

Serpentinites in subduction zones are important fluid reservoirs for crust-mantle interaction and arc magma formation. The serpentinite-derived fluids can carry fluid-mobile elements such as Ba and potentially affect the geochemical composition of the metamorphic rocks in the subduction zones. However, the Ba isotope behavior during interaction between serpentinite-derived fluids and metamorphic rocks is still poorly understood. The whiteschists of the Dora-Maira Massif in the Western Alps have experienced fluid infiltration by serpentinite-derived fluids during deep subduction. Here we report the Ba isotope data of the whiteschist and its protolith metagranite to investigate the Ba isotope behavior during fluid-rock interaction. The metagranites, which have experienced ultrahigh-pressure metamorphism, have a small variation of δ138Ba (−0.25 to 0.26‰), while the whiteschists display a large Ba isotope variation from −0.99‰ to 0.48‰. Such a large variation in whiteschists cannot be explained by chemical weathering, protolith heterogeneity, or kinetic effects. Instead, considering the negative correlations of δ138Ba with Ba and other fluid-mobile elements (e.g., Sr, Cs, Zn, and Cu) and the geochemical composition features of serpentinite-derived fluids, the large Ba isotope variation in whiteschists is inferred to be induced by the interaction between serpentinite-derived fluid and metagranite during intense fluid infiltration. This is consistent with the observation that the whiteschists with low Ba contents tend to exhibit heavier Mg-Fe isotope compositions but lighter Li isotope compositions, which indicates that the related fluids were derived from the dehydration of mantle wedge serpentinite. The fluid infiltrating process is well explained by a numerical model with high fluid-rock ratios of 4 to 12. Our modeling further suggests that serpentinite-derived fluid has a distinctly higher δ138Ba value than the depleted upper mantle. During fluid-rock interaction, heavy Ba isotopes prefer to be infiltrated into fluids with an apparent fractionation factor (Δ138Barock-fluid) of mostly −0.5‰ to −0.1‰ but can be down to −0.8‰ to −0.7‰ required for a few samples. A two-stage dehydration and metasomatism model at different depths can well explain the formation of Dora-Maira whiteschists. Whereas the Ba isotope composition of mantle wedge serpentinite is mainly dictated by crust-derived fluids from the subducting slab, an interaction between the serpentinite-derived fluids and the metagranite protolith resulted in a large Ba isotope variation of whiteschists. In case of a general involvement of serpentinites in subduction zones, serpentinite-derived fluids would significantly affect the Ba isotope compositions of subarc mantle peridotites and also arc lavas.

Archaeometric characterization of historical-contemporaneous marbles from Waldensian Valleys of Piedmont (Dora-Maira Massif, Cottian Alps, Italy)

This paper deals with mineralogical-petrographic, microstructural and geochemical analyses for the archaeometric characterization of silicate-bearing granoblastic marbles from the so-called Waldensian Valleys of Piedmont (i.e. Pellice, Germanasca and lower Chisone valleys, Cottian Alps, Italy). These stones, belonging to the Dora-Maira Massif of the Penninic domain (pre-Carboniferous), are often associated with a talc-bearing mineralization. Since the XIV century, “Waldensian” marbles have been subjected to a relatively intense extractive activity (nowadays almost completely abandoned) and have been widely used, especially during XVI-XIX century, for decoration purposes and as building materials in the Duchy of Savoy, as well as later, in Italy and abroad. Fifteen samples representing four lithofacies and commercial types (named “Statuary white”, “Bardiglio gray”, “Zebrato gray” and “Cipollino green” marbles) from four different quarries (named Cabitto, La Majera, Rocca Corba, and Caugis), were studied through a multianalytical approach. The isotopic signature, combined with the mineral chemistry, allowed tracing marbles origin uniquely. The resulting database can be employed either in the cultural heritage, or in the conservation and restoration science, to identify each “Waldensian” marble from different decorative stones with similar microstructural and/or compositional features, but different origin.

The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolution of the Briançonnais distal margin

Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and -thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the “Prepiemonte units” known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dronero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The TRSCM values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.

A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)

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.

Tourmaline boron isotopes trace metasomatism by serpentinite-derived fluid in continental subduction zone

Subduction of low-density continental crust to subarc depths is generally associated with preceding subduction of the oceanic slab. However, the original oceanic signature is often overprinted by subsequent fluid metasomatism from the subducting continental crust. As a result, it is generally difficult to trace the geochemical processes of previous oceanic subduction in collisional orogens. This issue can be potentially resolved by applying B isotopes to metamorphic rocks from continental subduction zones. Here a combined study of whole-rock geochemistry and in situ tourmaline B isotopes was carried out for coesite-bearing whiteschist and phengite schist, as well as country rock metagranitoids from the Dora-Maira Massif in the Western Alps. While all these metamorphic rocks have a similar protolith of Permian granites, whiteschist and phengite schist experienced Mg-rich fluid metasomatism during the continental subduction, evidenced by their much higher MgO contents than the metagranitoids. Tourmaline in the metagranitoid (Tur-G) is schorlitic (XMg = 15–55) with low δ11B values from −13 to −6‰, and is consistent with a magmatic origin. In contrast, tourmaline in metasomatic rocks (Tur-S) is mainly dravitic with the highest XMg worldwide (XMg = 90–98) and high δ11B values of −5 to +1‰. Integrated with whole-rock geochemistry and previous studies, Tur-S is interpreted to grow during the infiltration of external fluids that were highly enriched in MgO and relatively enriched in 11B. According to the B isotope compositions of precursor tourmalines, it is estimated that the external fluids had significantly higher δ11B values than +2.4‰. Based on B isotope compositions of both Tur-S and metasomatic fluids, our quantitative modeling suggests that the metasomatic fluids most likely originated from the mantle wedge serpentinite that was formed during the preceding oceanic subduction stage. Therefore, tourmaline B isotopes in the ultrahigh pressure metamorphic continental crust can be used to trace preceding fluid metasomatism at the interface between oceanic slab and mantle wedge. The mantle wedge serpentinite plays an important role in modifying the geochemical composition of deeply subducted supracrustal rocks and probably also the mantle sources of arc magmas.

Thermodynamic analysis of HP-UHP fluid inclusions: The solute load and chemistry of metamorphic fluids

Subduction fluids play a crucial role in regulating long-term chemical cycles. Their characterisation is essential to understand the processes responsible for metasomatism, oxidation and melting of the mantle wedge. Both direct (fluid inclusion studies) and indirect (thermodynamic modelling) approaches to study subduction fluids have reliability issues due to the complexity of the investigated processes. Post-entrapment processes (e.g., solvent loss by diffusion or decrepitation and/or chemical reactions between host mineral and trapped fluid) are likely to modify the chemical fingerprint of ultra-high pressure (UHP) fluid inclusions, while thermodynamic modelling of solute-bearing fluids at UHP conditions is still at the beginning of its application. In this work, we apply and compare data obtained by both approaches for fluid inclusions trapped within UHP clinopyroxene from a chemically simple Ol-Cpx-Dol-Cal marble (Brossasco-Isasca Unit, Dora-Maira Massif, Western Italian Alps). Classical molecular-fluid thermodynamics is adequate to qualitatively describe the post-entrapment reactions between fluid inclusions and host clinopyroxene. However, an electrolytic fluid model is necessary to describe the chemical composition of the solute-bearing aqueous fluids at the peak metamorphic condition (H2O: 96.30 mol%/88.49 wt%; solutes: 3.61 mol%/11.34 wt%/2.08 mol/kg; other volatiles: 0.09 mol%/0.17 wt%) generated by progressive rock dissolution. Comparison of the model fluid composition with that inferred from the analysis of fluid inclusions clarifies the types and the extent of post-trapping chemical modification of the UHP fluid inclusions. Our data reveal that the fluid-host reactions carry up to 42 mol% of host clinopyroxene component in the fluid inclusion bulk composition, whereas the fluid inclusion decrepitation and the water diffusion in the host clinopyroxene (through dislocations and/or micro-fractures) cause an H2O loss ranging from 18 mol% to 99 mol%. Applying these approaches, we demonstrate that the most relevant post-entrapment process is H2O loss. We also demonstrate that some fluid inclusions did not experience post-entrapment fluid-host modification and, thus, preserve the original fluid geochemistry.

Using the elastic properties of zircon-garnet host-inclusion pairs for thermobarometry of the ultrahigh-pressure Dora-Maira whiteschists: problems and perspectives

The ultrahigh-pressure (UHP) whiteschists of the Brossasco-Isasca unit (Dora-Maira Massif, Western Alps) provide a natural laboratory in which to compare results from classical pressure (P)–temperature (T) determinations through thermodynamic modelling with the emerging field of elastic thermobarometry. Phase equilibria and chemical composition of three garnet megablasts coupled with Zr-in-rutile thermometry of inclusions constrain garnet growth within a narrow P–T range at 3–3.5 GPa and 675–720 °C. On the other hand, the zircon-in-garnet host-inclusion system combined with Zr-in-rutile thermometry would suggest inclusion entrapment conditions below 1.5 GPa and 650 °C that are inconsistent with the thermodynamic modelling and the occurrence of coesite as inclusion in the garnet rims. The observed distribution of inclusion pressures cannot be explained by either zircon metamictization, or by the presence of fluids in the inclusions. Comparison of the measured inclusion strains with numerical simulations shows that post-entrapment plastic relaxation of garnet from metamorphic peak conditions down to 0.5 GPa and 600–650 °C, on the retrograde path, best explains the measured inclusion pressures and their disagreement with the results of phase equilibria modelling. This study suggests that the zircon-garnet couple is more reliable at relatively low temperatures (< 600 °C), where entrapment conditions are well preserved but chemical equilibration might be sluggish. On the other hand, thermodynamic modelling appears to be better suited for higher temperatures where rock-scale equilibrium can be achieved more easily but the local plasticity of the host-inclusion system might prevent the preservation of the signal of peak metamorphic conditions in the stress state of inclusions. Currently, we cannot define a precise threshold temperature for resetting of inclusion pressures. However, the application of both chemical and elastic thermobarometry allows a more detailed interpretation of metamorphic P–T paths.

Geology of the southern Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic unit, Western Alps)

ABSTRACT In the Valmala sector of the southern Dora Maira Massif (Western Alps), two different eclogite- and blueschist-facies units (i.e. the Rocca Solei and Dronero units, respectively), are separated by a shear zone (i.e. the Valmala Tectonic Unit), which peculiarly consists of mixed slices of ophiolitic and continental rocks. A detailed geological map at 1:10,000 scale allowed to point out that the tectonic slices within the Valmala Tectonic Unit consist of ‘native’ rock slices wrenched from the overlying Dronero Unit, and ‘exotic’ rocks likely sourced from other units of the Dora Maira and from a continental margin and an oceanic basin. On the contrary, rock slices sourced from the underlying Rocca Solei Unit are lacking. The overall tectonic stack results after an early subduction-related deformation phase (i.e. the D1), and the pervasive overprinting of two subsequent exhumation-related deformation phases (i.e. the D2 and D3). The Valmala Tectonic Unit is inferred to have played a role in decoupling the southern Dora Maira Massif during subduction, and/or in driving exhumation of the ultra-high pressure rocks occurring in the adjoining Brossasco-Isasca Unit.

A step towards unraveling the paleogeographic attribution of pre-Mesozoic basement complexes in the Western Alps based on U\u2013Pb geochronology of Permian magmatism

The Briançonnais Domain (Western Alps) represented the thinned continental margin facing the Piemonte-Liguria Ocean, later shortened during the Alpine orogeny. In the external part of the External Briançonnais Domain (Zone Houillère), the Palaeozoic basement displays microdioritic intrusions into Carboniferous sediments and andesitic volcanics resting on top of the Carboniferous sediments. These magmatic rocks are analysed at two well-known localities (Guil volcanics and Combarine sill). Geochemical data show that the two occurrences belong to the same calc-alkaline association. LA-ICP-MS U–Pb ages have been obtained for the Guil volcanics (zircon: 291.3 ± 2.0 Ma and apatite: 287.5 ± 2.6 Ma), and the Combarine sill (zircon: 295.9 ± 2.6 Ma and apatite: 288.0 ± 4.5 Ma). These ages show that the calc-alkaline magmatism is of Early Permian age. During Alpine orogeny, a low-grade metamorphism, best recorded by lawsonite-bearing veins in the Guil andesites, took place at about 0.4 GPa, 350 °C in the External Briançonnais and Alpine metamorphism was not able to reset the U–Pb system in apatite. The Late Palaeozoic history of the Zone Houillère is identical to the one recorded in the Pinerolo Unit, located further East in the Dora-Maira Massif, and having experienced a garnet-blueschist metamorphism during the Alpine orogeny. The comparison of these two units allows for a better understanding of the link between the Palaeozoic basements, mostly subducted during the Alpine convergence, and their Mesozoic covers, generally detached at an early stage of the convergence history.

Micro- and nano-size hydrogrossular-like clusters in pyrope crystals from ultra-high-pressure rocks of the Dora-Maira Massif, western Alps

The supracrustal metamorphic rocks of the Dora-Maira Massif, western Alps, have been intensively studied. Certain ultra-high-pressure lithologies contain coesite and nearly end-member composition pyrope, Mg3Al2Si3O12, making this locality petrologically and mineralogically unique. Structural OH-, loosely termed “water”, in pyrope crystals of different composition has been investigated numerous times, using different experimental techniques, by various researchers. However, it is not clear where the minor OH- is located in them. IR single-crystal spectra of two pyropes of composition {Mg2.79,Fe2+0.15,Ca0.04}Σ2.98[Al]2.02(Si)2.99O12 and {Mg2.90,Fe2+0.04,Ca0.02}Σ2.96[Al]2.03(Si)3O12 were recorded at room temperature (RT) and 80 K. The spectra show five distinct OH- bands located above 3600 cm-1 at RT and seven narrow bands at 80 K and additional fine structure. The spectra were curve fit and the OH- stretching modes analyzed and assigned. It is argued that OH- is located in microscopic- and nano-size Ca3Al2H12O12-like clusters. The basic substitution mechanism is the hydrogarnet one, where (H4O4)4- ⇔ (SiO4)4-, and various local configurations containing different numbers of (H4O4)4- groups define the cluster type. The amounts of H2O range between 5 and 100 ppm by weight, depending on the IR calibration adopted, and are variable among crystals. Hydrogrossular-like clusters are also present in a synthetic pyrope with a minor Ca content grown hydrothermally at 900 °C and 20 kbar, as based on its IR spectra at RT and 80 K. Experiment and nature are in agreement, and OH- groups are partitioned into various barely nano-size hydrogrossular-like clusters. This proposal is new and significant mineralogical, petrological, and geochemical implications result. Ca and proton ordering occur. Hypothetical “defect” and/or coupled-substitution mechanisms to account for structural OH- are not needed to interpret experimental results. OH- incorporation in pyrope of different generations at Dora-Maira is discussed and OH- could potentially be used as an indicator of changing P_{{{text{H}}_{{text{2}}} {text{O}}}} {text{(}}a_{{{text{H}}_{{text{2}}} {text{O}}}} {text{) - }}T conditions in a metamorphic cycle. Published experimental hydration, dehydroxylation, and hydrogen diffusion results on Dora-Maira pyropes can now be interpreted atomistically.

What’s in the sandwich? New P–T constraints for the (U)HP nappe stack of southern Dora-Maira Massif (Western Alps)

What’s in the sandwich? New P–T constraints for the (U)HP nappe stack of southern Dora-Maira Massif (Western Alps)