CORUNDUM-BEARING VEINS IN CHLORITITE FROM THE ETIROL-LEVAZ AUSTROALPINE CONTINENTAL SLICE (VALTOURNENCHE, AOSTA, ITALY)

The Gressoney Shear Zone (GSZ) consists of a ca. 500 m thick, intensely deformed rock panel located at the top of the high-pressure ophiolitic rocks of the Zermatt-Saas Zone in the Western Alps. This greenschist-facies shear zone accommodated multiple non-coaxial deformation events with contrasting kinematics. In this study, detailed field mapping and structural analysis were integrated with the study of crystallographic preferred orientation (CPO) from mylonites associated with the GSZ. Quartz CPO displays a systematic variation across the shear zone: moving from the basal shear zone boundary, the c-axes pattern changes from type II cross-girdle distribution, to an asymmetric pattern characterized by clustering of c-axes on one side of the Z-direction (inclined single girdle), to a central cluster in the Y-direction. The observed CPO patterns are consistent with increasing shear strain toward the basal contact, which probably controls the transition from broad peripheral maxima indicative of basal <a> slip to an inclined single girdle with no maxima, which is indicative of prism <a> slip, and finally an elongate single maximum at the girdle centre produced by a combination of prism <a> and rhomb <a> slip. Our results further indicate that basal <a> slip is dominant in pure quartz domains, whereas with increasing proportion of second phases, prism <c> slip is activated. These features confirm that CPOs obtained from the almost pure quartzites analysed in most published studies, and generally associated with the activation of distinct slip systems controlled by temperature, cannot be straightforwardly applied to the analysis of heterogeneous shear zones and/or polymineralic mylonites. From a regional point of view, the structures observed in the field and the fabric analyses are consistent with top-to-the-SE extension post-dating subductionrelated high-pressure metamorphism and collisional nappe stacking in the studied sector of the Western Alps.

In the Western Alps, the ophiolitic Zermatt–Saas Zone (ZSZ) and the Lago di Cignana Unit (LCU) record oceanic lithosphere subduction to high (540°C, 2·3GPa) and ultra-high pressure (600°C, 3·2GPa), respectively. The top of the Zermatt–Saas Zone in contact with the Lago di Cignana Unit consists of olivine + Ti-clinohumite-bearing serpentinites (the Cignana serpentinite) hosting olivine + Ti-clinohumite veins and dykelets of olivine + Ti-chondrodite + Ti-clinohumite. The composition of this serpentinite reveals a refertilized oceanic mantle peridotite protolith that became subsequently enriched in fluid-mobile elements (FME) during oceanic serpentinization. The olivine + Ti-clinohumite veins in the Cignana serpentinite display Rare Earth Element (REE) and FME compositions quite similar to the host-rock, which suggests closed-system dehydration of this serpentinite during subduction. The Ti-chondrodite-bearing dykelets are richer in REE and FME than the host-rock and the dehydration olivine + Ti-clinohumite veins: their Nd composition points to a mafic protolith, successively overprinted by oceanic metasomatism and by subduction zone recrystallization. These dykelets are comparable in composition to eclogites within the ultra-high pressure LCU that derive from subducted oceanic mafic crust. Different from the LCU, serpentinites from the core domains of the ZSZ display REE compositions indicating a depleted mantle protolith. The oceanic serpentinization of these rocks led to an increase in FME and to seawater-like Sr isotope compositions. The serpentinites sampled at increasing distance from the ultra-high pressure LCU reveal different mantle protoliths, still preserve an oceanic geochemical imprint and contain mafic dykelets affected by oceanic metasomatism. The subduction zone history of these rocks thus occurred under relatively closed system conditions, the only possible change during subduction being an enrichment in As and Sb recorded by the serpentinites closer to the crustal LCU. The ZSZ and Cignana serpentinites thus likely evolved in a slab setting and were weakly exposed to interaction with slab-derived fluids characteristic of plate interface settings. Our data suggest two possible scenarios for the evolution of the studied ZSZ and Cignana serpentinites. They are either part of a coherent ophiolite unit whose initial lithospheric mantle was variably affected by depletion and re-fertilization processes, or they belong to separate tectonic slices derived from two different oceanic mantle sections. In the Cignana serpentinite atop the ZSZ, the presence of Ti-chondrodite dykelets similar in composition to the LCU eclogites suggests these two domains were closely associated in the oceanic lithosphere and shared the same evolution to ultra-high pressure conditions during Alpine subduction.

Multichronometric analyses were performed on samples from a transect in the French–Italian Western Alps crossing nappes derived from the Briançonnais terrane and the Piemonte–Liguria Ocean, in an endeavour to date both high-pressure (HP) metamorphism and retrogression history. Twelve samples of white mica were analysed by 39Ar–40Ar stepwise heating, complemented by two samples from the Monte Rosa nappe 100 km to the NE and also attributed to the Briançonnais terrane. One Sm–Nd and three Lu–Hf garnet ages from eclogites were also obtained. White mica ages decrease from c. 300 Ma in the westernmost samples (Zone Houillère), reaching c. 300°C during Alpine metamorphism, to <48 Ma in the internal units to the east, which reached c. 500°C during the Alpine orogeny. The spatial pattern of Eocene K–Ar ages demonstrates that Si-rich HP white mica records the age of crystallization at 47–48 Ma and retains Ar at temperatures of around 500°C. Paleocene–early Eocene Lu–Hf and Sm–Nd ages, recording prograde garnet growth before the HP peak, confirm eclogitization in Eocene times. Petrological and microstructural features reveal important mineralogical differences along the transect. All samples contain mixtures of detrital, syn-D1 and syn-D2 mica, and retrogression phases (D3) in greatly varying proportions according to local variations in the evolution of pressure–temperature–fluid activity–deformation (P–T–a–D) conditions. Samples from the Zone Houillère mostly contain detrital mica. The abundance of white mica with Si > 6·45 atoms per formula unit increases eastward. Across the whole traverse, phengitic mica grown during HP metamorphism defines the D1 foliation. Syn-D2 mica is more Si-poor and associated with nappe stacking, exhumation, and hydrous retrogression under greenschist-facies conditions. Syn-D1 phengite is very often corroded, overgrown by, or intergrown with, syn-D2 muscovite. Most importantly, syn-D2 recrystallization is not limited to S2 schistosity domains; micrometre-scale chemical fingerprinting reveals muscovite pseudomorphs after phengite crystals, which could be mistaken for syn-D1 mica based on microstructural arguments alone. The Cl/K ratio in white mica is a useful discriminator, as D2 retrogression was associated with a less saline fluid than eclogitization. As petrology exerts the main control on the isotope record, constraining the petrological and microstructural framework is necessary to correctly interpret the geochronological data, described in both the present study and the literature. Our approach, which ties geochronology to detailed geochemical, petrological and microstructural investigations, identifies 47–48 Ma as the age of HP formation of syn-D1 mica along the studied transect and in the Monte Rosa area. Cretaceous apparent mica ages, which were proposed to date eclogitization by earlier studies based on conventional ‘thermochronology’, are due to Ar inheritance in incompletely recrystallized detrital mica grains. The inferred age of the probably locally diachronous, greenschist-facies, low-Si, syn-D2 mica ranges from 39 to 43 Ma. Coexistence of D1 and D2 ages, and the constancy of non-reset D1 ages along the entire transect, provides strong evidence that the D1 white mica ages closely approximate formation ages. Volume diffusion of Ar in white mica (activation energy E = 250 kJ mol−1; pressure-adjusted diffusion coefficient D'0 < 0·03 cm2 s−1) has a subordinate effect on mineral ages compared with both prograde and retrograde recrystallization in most samples.

This work deals with ductile, brittle and brittle-ductile shear zones developed in ultramafic rocks of the Voltri Group (VG), Eastern Ligurian Alps (Italy), during the late Alpine orogeny. The late orogenic events (upper Eocenelower Miocene) are characterized by a complex superposition of syn-metamorphic (greenschist facies) and non-metamorphic structures linked to the exhumation of the subducted slabs during Alpine orogeny. In the VG these deformations are represented by low angle reverse shear zones and associated vein networks in mafic and ultramafic rocks. Many of these shear zones show multiple reactivation and thus several types of fault rocks, linked to different structural levels and stress regime, overprint in a single shear zone. In the ultramafites shear zones are associated with significant vein networks and wallrock metasomatic alteration and are characterized by a full range of fault rocks from serpentinite mylonite to fault gouge. Fault zones thickness and displacement, recrystallization of the host rocks and the brittle vs ductile style of the structures are strictly controlled by the fluid-induced reactions and are only partially linked to the structural level of development. Highest thickness of the fault zone occurs where fault rocks have been affected by huge quartz-carbonate vein networks and carbonate dominated metasomatic alteration; in this case cataclasite and fault breccia develop and brittle deformation overprint ductile ones. Superposition of ductile over brittle structures occurs along discrete low thickness shear zones, where sepiolite and phyllosilicate minerals replace serpentine minerals and pyroxene and/or in presence of very fine grained fault gouge. We investigated the complex interaction between brittle and ductilestructuresby constructing detailed structural maps, and logs of fault rock distribution and textures and studying the associated vein networks in order to understand the role of fluids in deformation. We discuss the implication for regional structural analysis and the late orogenic evolution.

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.

An eclogite-bearing continental tectonic slice in the Zermatt–Saas high-pressure ophiolites at Trockener Steg (Zermatt, Swiss Western Alps)

The Zermatt–Saas Zone is an eclogite-facies meta-ophiolite unit representing the fossil oceanic lithosphere of the Jurassic Tethys. In the Italian Northwestern Alps, the Zermatt–Saas Zone includes a chaotic rock unit, or mélange, c. 40 m thick, interposed between serpentinites and calcschists. The mélange consists of decimetre-scale ultramafic layers and boudins embedded in a serpentine + carbonate-rich matrix showing a block-in-matrix fabric. The mélange has the same Alpine tectonometamorphic evolution as the surrounding rocks, starting with a prograde path developed under high-pressure (HP) conditions followed by a retrograde path during exhumation. The kinematic and metamorphic relations between inside- and outside-boudin foliations indicate that boudinage and shearing developed during the prograde HP path. Fluid–rock interaction enhanced shearing and focused ductile and brittle–ductile deformation along lithological contacts between rigid blocks and boudins and flowing carbonaceous matrix. Despite a pervasive orogenic evolution, the primary tectonosedimentary features of the mélange are still recognizable in some outcrops and are attributed to an intra-oceanic (Jurassic) setting characterized by mass-transport processes. The present-day fabric of the studied mélange unit thus results from the superimposition of the Alpine processes, responsible for fluid-assisted stratal disruption and mixing in the subduction channel, on the original stratigraphy formed during intra-oceanic gravitational processes. Supplementary material : Supplementary figures and tables are available at https://doi.org/10.6084/m9.figshare.c.6924080 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

All in an engineer’s life

The Nikubuchi peridotite body occurs as a tectonic body in the Sanbagawa metamorphic belt. The peridotites consist of dunite, spinel wehrlite, spinel websterite and spinel-bearing metagabbro, and are characterized by the presence of aluminous pyroxenes and green spinel.

The eclogite-facies Servette metaophiolites (St. Marcel Valley, Italian Western Alps) belong to the Piedmont Nappe. They are glaucophanite, chloriteschists, talcschists and associated mineralised quartzite derived from different types of hydrated Tethyan oceanic crust affected by sea-floor and sub-sea floor hydrothermal alteration. We first describe the geology and lithology of the Servette metaophiolites and their chemical signature, and then we focus on the estimation of the Alpine subduction-related peak P-T metamorphic conditions and associated microstructures. Estimation of the peak P-T conditions was performed via calculations of pseudosections representing the equilibrium assemblages in the studied rocks using THERMOCALC. These calculations yielded relatively uniform values for the high-pressure metamorphic equilibration, with temperatures of 550 ± 60°C and pressures of 2.1 ± 0.3 Gpa, higher than the previously estimated P-T in the St. Marcel Valley (T max = 500°C and P max = 1.4 GPa), but lower than those obtained in other localities of the Zermatt-Saas zone (T up to 550-600° C and P up to 2.5-3.0 GPa). Comparison with similar rocks from the Zermatt-Saas zone helps to constrain the peculiar conditions at the P peak of these hydrated rocks within the Alpine subduction slab.

Detailed multiscale structural analyses and mapping (1:20 scale) integrated with petrological investigation were used to study a portion of the Zermatt-Saas serpentinites that crop out in upper Valtournanche (north-western Italy). Results are synthesized in a foliation trajectory map that displays the transposed original lithostratigraphy of a serpentinite body exposed at Creton. The serpentinite body comprises magnetite sheets and rare, decimetre-thick, diopsidite layers and lenses. Moreover, veins and aggregates of Ti-chondrodite and Ti-clinohumite, olivine-rich layers and lenses, veinlets of olivine, and layers of dark pyroxenite are embedded in the serpentinites. Serpentinites and associated rocks record three relative age groups of ductile structures: D1 consists of rare folds and S1 foliation; D2 is a group of isoclinal folds and a very pervasive foliation (S2), which is the dominant structure; D3 includes a crenulation and shear zones affecting S2. The detailed meso-structural and microstructural analyses allowed individuating the metamorphic environment of successive deformation stages and correlating the resulting tectono-metamorphic investigation with those already inferred in surrounding areas. In addition, metre- to submillimetre-sized pre-D2 structural, mineralogical, and textural relics have been clearly identified in spite of the strong transposition imposed during the development of S2 high pressure - ultra-high pressure foliation.

A Permo-Triassic pelite-carbonate rock series (with interacalated metabasitic rocks) in the Cordilleras Beticas, Spain, was metamorphosed during the Alpine metamorphism at high pressures (Pmin near 18 kbar). The rocks show well preserved sedimentary features of evaporites such as pseudomorphs of talc, of kyanite-phengitetalc-biotite, and of quartz after sulfate minerals, and relicts of baryte, anhydrite, NaCl, and KCl, indicating a salt-clay mixture of illite, chlorite, talc, and halite as the original rock. The evaporitic metapelites have a whole rock composition characterized by high Mg/(Mg+Ca) ratios>0.7, variable alkaline and Sr, Ba, contents, but are mostly K2O rich (<8.8 wt%). The F (<2600 ppm), Cl (<3600 ppm), and P2O5 (<0.24 wt%) contents are also high. The pelitic member of this series is a fine grained biotite rock. Kyanite-phengite-talc-biotite aggregates in pseudomorphs developed in the high pressure stage. Albite-rich plagioclase was formed when the rocks crossed the albite stability curve in the early stages of the uplift. Scapolite, rich in NaCl (Ca/(Ca+Na) mol% 24–40) and poor in SO4, with Cl/(Cl+CO3) ratios between 0.6 and 0.8, formed as porphyroblasts, sometimes replacing up to 60% of the rock in a late stage of metamorphism (between 10 and 5 kbar, near 600°C). No reaction with albite is observed, and the scapolite formed from biotite by: $$\begin{gathered} Al - biotite + CaCO_3 + NaCl + SiO_2 \hfill \\ = Al - poor biotite + scapolite + MgCO_3 + KCl \hfill \\ + MgCl_2 + H_2 O \hfill \\ \end{gathered}$$

The Gotthard massif in central Switzerland, part of the autochthonous basement of the Alps, has been metamorphosed during the Alpine orogeny. Structural studies of the metasedimentary hornblende rocks along the southern border reveal the existence of several mineral generations which were formed at different times and under different conditions. The sequence of formation can be deduced from their mutual intergrowth and orientation relative to the fabric of the rock. Steiger [1962] concluded that the Alpine orogeny in this region consisted of two distinct tectonic phases separated by a thermal phase. The main tectonic phase was the dragging along of the Gotthard massif by the overlying northward-moving nappes. Most of the pre-Alpine mineral assemblage was crushed and new minerals were formed which show preferential N-S orientation (direction of the nappe movements). The subsequent thermal metamorphism was linked with anatectic processes in the Lepontinic region south of the Gotthard massif. At this time porphyroblastic minerals of random orientation were formed along the southern margin of the massif. A weaker tectonic phase consisting of an E-W contraction caused N-S oriented small folds and wrinklings in mica-rich schists, and produced cross biotites. Hornblende was formed in pre-Alpine times as well as during both the main tectonic and the thermal phase of the Alpine orogeny. K-Ar ages were determined for 17 hornblendes. N-S oriented hornblendes show exclusively ages of 46 m.y.; random hornblendes yield ages of 23 to 30 m.y. Partially oriented hornblendes give apparent ages of 23 to 112 m.y. The hornblende ages are definitely higher than Rb-Sr ages of bioties (16 m.y.) from this region [Jager, 1962]. It is concluded that the N-S oriented hornblendes give a minimum age Rb-Sr ages of the biotites (16 m.y.) appear to indicate the time of the E-W contriction of proximately date the period (23 to 30 m.y.) of the anatexis in the Lepontinic region. The Rb-Sr ages of the biotites (16 m.y.) appear to indicate the time of the E-W constriction of the massif and the Lepontinic region. Partially oriented hornblendes cannot be related to any particular event. They may in part represent relics of pre-Alpine origin which suffered differential loss of argon during the Alpine metamorphism.

Rb-Sr age determinations on 19 samples, micas and total rocks, from the Swiss and Italian Alps were made. The special techniques for the determination of young ages by this method are described. The main contributions of our age results to the problems of Alpine geology are: The last main recrystallization phase in the central part of the Alps has been dated by age determinations on recrystallized rocks from the lowest nappe units. The age results are in the range from 16 to 21 m. y., 16 m. y. in the nappe region and the Gotthard massif farther north, 21 m. y. in the root region to the south of the 16-m. y. region. The interval of 5 m. y. is explained by a different time of uncovering after one metamorphic phase, rather than as having been caused by two different phases. This uncovering is believed to have been an uplift in the south, causing quick erosion there. Gliding of the overlying pile of nappes to the north could have been connected with this uplift. Several samples from the Alps give Rb-Sr ages older than the Tertiary Alpine metamorphism: a granite, 193±21 m. y. by total rock analysis, and a muscovite, 295±14 m. y., from a pegmatite from the Hercynian Aare and Tavetsch massifs which were metamorphosed during the Alpine orogeny. Furthermore, we found an age of 267±11 m. y. on the biotite from the pre-Alpine Habkern granite which occurs only in exotic boulders embedded in soft sediments. Two micas and a total rock from the Austroalpine Silvretta nappe gave Paleozoic ages, 306±13 m. y., 293±12 m. y., and 356±21 m. y. According to geologists the crystalline complex of the Silvretta, which overlies fossiliferous Mesozoic sediments, was thrust as a block over deeper nappe units during the Alpine orogeny, which did not greatly influence the internal structure of the Silvretta nappe. Both the young overthrust and the low grade of Alpine metamorphism were confirmed by our measurements.

Assessing the relative role of rift-inherited hyper-extension and subduction/collisional dynamics in establishing the lithostratigraphic associations and overall architecture of orogens has key implications for constraining convergent margin dynamics. Giorgio Vittorio Dal Piaz, with his seminal papers across the late '90's and early 2000's, first suggested that seemingly unrelated Jurassic ophiolites, Paleozoic continental basement and Triassic shelf sediment from the Zermatt-Saas Zone were already part of a Jurassic ocean-continent transition zone, prior to undergoing eclogite facies metamorphism during the Alpine orogeny. Subsequent studies, performed in other parts of the Western Alps and Alpine Corsica provided further evidence that a large part of the apparent complexity of the axial zone of Alpine-type orogens may be a result of rift inheritance.