HP/LT metamorphism in the Volparone Breccia (Northern Corsica, France): evidence for involvement of the Europe/Corsica continental margin in the Alpine subdution zone

The Alpine belt in Corsica (France) is characterized by the occurrence of stacked tectonic slices derived from the Corsica/Europe continental margin, which outcrop between two weakly or non‐metamorphic tectonic domains: the ‘autochthonous’ domain of the Hercynian basement to the west and the Balagne Nappe (ophiolitic unit belonging to the ‘Nappes supérieures’) to the east. These slices, including basement rocks (Permian granitoids and their Palaeozoic host rocks), Late Carboniferous–Permian volcano‐sedimentary deposits, coarse‐grained polymict breccias (Volparone Breccia) and Middle Eocene siliciclastic turbidite deposits, were affected by a polyphase deformation history of Alpine age, associated with a well‐developed metamorphic recrystallization. This study provides new quantitative data about the peak of metamorphism and the retrograde P–T path in the Alpine Corsica: the tectonic slices of Volparone Breccia from the Balagne region (previously regarded as unmetamorphosed) were affected by peak metamorphism characterized by the phengite + chlorite + quartz ± albite assemblage. Using the chlorite‐phengite local equilibria method, peak metamorphic P–T conditions coherent with the low‐grade blueschist facies are estimated as 0.60 ± 0.15 GPa and 325 ± 20 °C. Moreover, the retrograde P–T path, characterized by a decrease of pressure and temperature, is evidence of the first stage of the exhumation path from the peak metamorphic conditions to greenschist facies conditions (0.35 ± 0.06 GPa and 315 ± 20 °C). The occurrence of metamorphic peak at high‐pressure/low‐temperature (HP/LT) conditions is evidence of the fact that these tectonic slices, derived from the Corsica/Europe continental margin, were deformed and metamorphosed in the Alpine subduction zone during their underplating at ～20 km of depth into the accretionary wedge and were subsequently juxtaposed against the metamorphic and non‐metamorphic oceanic units during a complex exhumation history.     

Influence of tectonic overpressure on P–T paths of HP–UHP rocks in continental collision zones: thermomechanical modelling

The principle of lithostatic pressure is habitually used in metamorphic geology to calculate burial/exhumation depth from pressure given by geobarometry. However, pressure deviation from lithostatic, i.e. tectonic overpressure/underpressure due to deviatoric stress and deformation, is an intrinsic property of flow and fracture in all materials, including rocks under geological conditions. In order to investigate the influences of tectonic overpressure on metamorphic P–T paths, 2D numerical simulations of continental subduction/collision zones were conducted with variable brittle and ductile rheologies of the crust and mantle. The experiments suggest that several regions of significant tectonic overpressure and underpressure may develop inside the slab, in the subduction channel and within the overriding plate during continental collision. The main overpressure region that may influence the P–T paths of HP–UHP rocks is located in the bottom corner of the wedge‐like confined channel with the characteristic magnitude of pressure deviation on the order of ～0.3 GPa and 10–20% from the lithostatic values. The degree of confinement of the subduction channel is the key factor controlling this magnitude. Our models also suggest that subducted crustal rocks, which may not necessarily be exhumed, can be classified into three different groups: (i) UHP‐rocks subjected to significant (≥0.3 GPa) overpressure at intermediate subduction depth (50–70 km, P = 1.5–2.5 GPa) then underpressured at depth ≥100 km (P ≥ 3 GPa); (ii) HP‐rocks subjected to ≥0.3 GPa overpressure at peak P–T conditions reached at 50–70 km depth in the bottom corner of the wedge‐like confined subduction channel (P = 1.5–2.5 GPa); (iii) lower‐pressure rocks formed at shallower depths (≤40 km depth, P ≤ 1 GPa), which are not subjected to significant overpressure and/or underpressure.     

Evolution of Alpine eclogites in the Eastern Alps: Implications for Alpine geodynamics

In the Eastern Alps Alpine eclogites are generally associated with rocks of continental lithosphere, while eclogites that are associated with oceanic assemblages are restricted to minor exposures. Such eclogites are exposed both in the Penninic unit of the Tauern Window and in the Austroalpine nappe complex. (1) In the central southern part of the Tauern Window (Eclogite Zone) eclogites and associated high pressure metasediments of a distal continental margin are intercalated between Penninic basement units. A mylonitic eclogitic foliation and stretching lineation are contemporaneous to the high pressure metamorphism and are related to the subduction of distal Penninic continental margin sequences. Continuous subduction of cool lithosphere resulted in blueschist facies overprint of the whole Penninic nappe pile. (2) Within the Middle-AustroAlpine Koralm/Saualm region most eclogites are eclogitic mylonites documenting plastic deformation of omphacite and garnet. The meso- and macroscale structures indicate an overall extensional regime possibly related to a large-scale SE-directed ductile low-angle normal shear zone. The eclogites are associated with migmatite-like structures and are intruded by pegmatites. This indicates decreasing pressure, but isothermal or even increasing temperature conditions during exhumation.These relationships argue for the subduction of Penninic continental lithosphere in the foot-wall of the Austroalpine unit at the time of exhumation of the Koralm/Saualm eclogites. Formation of the Austroalpine eclogites is explained by subduction of continental lithosphere, and subsequent, rapid exhumation in an upper plate tectonic position within an extensional regime.     

Scales of deformation partitioning during exhumation in a continental subduction channel: A petrofabric study of eclogites and gneisses from NW Spain (Malpica‐Tui Allochthonous Complex)

Deformation partitioning is identified as the fingerprint of late Palaeozoic continental subduction that affected various lithologies whose field relationship, thermobarometric and petrofabric features are closely related. Different modes of deformation partitioning can be identified within medium temperature, high‐P eclogite lenses, between them and the host gneisses, and within the latter. Development of foliations and lineations with a coherent attitude in all these rocks and their related structural petrology demonstrate that eclogite enclosures and their country rocks underwent a common, pervasive deformational event. The published P–T stability fields of the eclogite phases that define the microscopic fabric are used to define the metamorphic conditions prevailing during the deformation event and relate it to the subduction process. The mineral equilibria of the gneisses (ortho‐ and paragneisses) fail to record the full range of those P–T conditions, but the field relationships show that eclogites were originally basic dykes emplaced in acid igneous rocks and demonstrate that the eclogites and gneisses shared a common tectonometamorphic evolution. Deformation partitioning within the latter occurred at variable scales and involved (1) meso macroscale preservation of virtually undeformed metagranite bodies, surrounded by (2) pervasively foliated and lineated gneisses, and (3) the simultaneous microscale operation in the latter of ductile and brittle–ductile mechanisms at conditions above 500°C and below 1.5 GPa. A subduction channel tectonic setting is proposed to explain the subduction of upper to mid‐crustal igneous rocks and exhumation subsequent to high‐P metamorphism. Its currently accessible dimensions, and its organization into several lithotectonic units mapped as nappes support tectonic amalgamation of units several km³ in volume. Maximum burial in the subduction channel likely reached depths shallower than the lithostatic pressure implied by geobarometric calculations, possibly conditioned by a sudden pressure drop during the initial retrogression stages accompanying exhumation.     

Crustal-scale extension in a convergent orogen : the Sterzing-Steinach mylomte zone in the Eastern Alps

Abstract

The western margin of the Tauera Window (Eastern Alps) is defined by a low angle westward dipping fault zone of potently We disp lacement. Ductile deformation of the fault rocks results in a carpet of mylonites up to 400 metres thick. Evidence from shear criteria and the excision of part of the Cretaceous-Tertiary metamorphic edifice both indicate normal displacements, and relative movement of Austroalpine nappe complex towards the west. The Sterzing-Steinach mylonite zone overprints the Alpine nappe edifice. Movements occurred on the cooling path of the Tauern metamorphism, and may be as recent as Middle Miocene.

The Kinematics and geometry of the mylonite zone constrain two likely t ectonic explanations that are both compatible with secondary thining of a thick orogenic wedge. (1) Ute the Austroalpine nappe pile due to tectonic unroofing of the Tauern window. (2) Continental escape by east-west stretching of the Alpine orogenic wedge in response to continental collision.     

Metamorphic patterns and SHRIMP zircon ages of medium‐to‐high grade rocks from the Tongbai orogen,central China: implications for multiple accretion/collision processes prior to terminal continental collision

The Qinling‐Tongbai‐Dabie‐Sulu orogenic belt comprises a Palaeozoic accretion‐dominated system in the north and a Mesozoic collision‐dominated system in the south. A combined petrological and geochronological study of the medium‐to‐high grade metamorphic rocks from the diverse Palaeozoic tectonic units in the Tongbai orogen was undertaken to help elucidate the origins of Triassic ultrahigh‐pressure metamorphism and collision dynamics between the Sino‐Korean and Yangtze cratons. Peak metamorphic conditions are 570–610 °C and 9.3–11.2 kbar for the lower unit of the Kuanping Group, 630–650 °C and 6.6–8.9 kbar for the upper unit of the Kuanping Group, 550–600 °C and 6.3–7.7 kbar for the Erlangping Group, 770–830 °C and 6.9–8.5 kbar for the Qinling Group and 660–720 °C and 9.1–11.5 kbar for the Guishan complex. Reaction textures and garnet compositions indicate clockwise P–T paths for the amphibolite facies rocks of the Kuanping Group and Guishan complex, and an anticlockwise P–T path for the granulite facies rocks of the Qinling Group. Sensitive high‐resolution ion microprobe U–Pb zircon dating on metamorphic rocks and deformed granite/pegmatites revealed two major Palaeozoic tectonometamorphic events. (i) During the Silurian‐Devonian (c. 440–400 Ma), the Qinling continental arc and Erlangping intra‐oceanic arc collided with the Sino‐Korean craton. The emplacement of the Huanggang diorite complex resulted in an inverted thermal gradient in the underlying Kuanping Group and subsequent thermal relaxation during the exhumation. Meanwhile, the oceanic subduction beneath the Qinling continental arc produced magmatic underplating and intrusion, leading to granulite facies metamorphism followed by a near‐isobaric cooling path. (ii) During the Carboniferous (c. 340–310 Ma), the northward subduction of the Palaeo‐Tethyan ocean generated a medium P/T Guishan complex in the hangingwall and a high P/T Xiongdian eclogite belt in the footwall. The Guishan complex and Xiongdian eclogite belt are therefore considered to be paired metamorphic belts. Subsequent separation of the paired belts is inferred to be related to the juxtaposition of the Carboniferous eclogites with the Triassic HP metamorphic complex during continental subduction and exhumation.     

High pressure metamorphism in the nappes of the Valle dell'Orco traverse (Western Alps collisional belt) *

Abstract A detailed field and petrological study of rocks from nappes cut by the Valle dell'Orco (Italian Western Alps), in particular the Sesia–Lanzo composite unit, has revealed geological and metamorphic histories which started in pre-alpine times and lasted up to the alpine subduction-collisional processes. During these processes the nappes sustained an early high P–low T stage and a later low P greenschist facies stage, but followed partly distinctive P–T–time trajectories. This paper discusses the kinematic evolution and the thermal history of the alpine belt from the early subduction/underthrust to the later exhumation stage. The metamorphic crystallization is often governed by incomplete and/or local equilibrium, and the pervasive syn-metamorphic deformation and the composition of the syn-metamorphic fluid phase (if present) have exerted an effective local control on reaction kinetics.     

Eclogitization of the Monviso ophiolite (W. Alps) and implications on subduction dynamics

To constrain deep (40–100 km) subduction dynamics, extensive P–T data are provided on the eclogitic Monviso ophiolite derived from the subducted Liguro‐Piemontese oceanic lithosphere (which was exhumed, together with associated continental units, before the Alpine collision). The Monviso ophiolite has so far been interpreted either as a fossilized subduction channel, with tectonic blocks detached from the slab at different depths and gathered in a weak serpentinized matrix, or as a more or less continuous portion of oceanic lithosphere. To evaluate potential heterogeneities within and between the various subunits, extensive sampling was undertaken on metasedimentary rocks and Fe–Ti metagabbros. The results indicate that the Monviso ophiolite comprises two main coherent tectonic subunits (the Monviso and Lago Superiore Units) detached during subduction at different depths and later juxtaposed at epidote–blueschist facies during exhumation along the subduction interface. Raman spectroscopy of carbonaceous material suggests (i) a difference in peak temperature of ～50 °C between these two subunits and (ii) a good temperature homogeneity within each subunit. Pseudosections and average P–T estimates using thermocalc in the Lago Superiore Unit suggest for the first time homogeneous HP to UHP conditions (～550 °C, 26–27 kbar). Parageneses, peak conditions and tectonic setting are very similar to those of the Zermatt‐Saas ophiolite, 200 km northwards, thus suggesting a common detachment mechanism for the whole Western Alpine belt.     

An (in‐)coherent metamorphic evolution of high‐P eclogites and their host rocks in the Chinese southwest Tianshan?

In the (ultra‐)high‐P–low‐T metamorphic terrane of the Chinese South Tianshan, discontinuous mafic blocks and boudins (former upper oceanic crust) are now embedded in voluminous (mainly metasedimentary) host rocks. Two different models were proposed and relate the occurrence of both high‐P and ultra‐high‐P mafic and metasedimentary rocks to either (i) a tectonic mélange style exhumation, with no exhumation of coherent units, but different lithologies derived from different depths juxtaposed and intermingled during exhumation in the subduction channel, or (ii) the evolution of two coherent metamorphic belts: one with high‐P and the other with ultra‐high‐P conditions. In contrast to most previous studies in the Chinese South Tianshan which focused either on single eclogites or metasedimentary rocks (assumed as representative), this study concentrates on the systematic investigation of both mafic boudins and their immediate sedimentary host rocks, because the investigation of both lithologies and the comparison of their metamorphic evolution is crucial to reconstruct the geodynamical context of the whole (ultra‐)high‐P–low‐T metamorphic complex. Several sample pairs consisting of both lithologies were geochemically investigated and their respective metamorphic evolution was reconstructed using geothermobarometry and thermodynamic modelling. The latter approach considers changes in the mineral assemblage during the metamorphic evolution, as well as changes in mineral composition, which may help to determine the metamorphic history of a rock despite the preservation of critical mineral assemblages. All samples experienced a clockwise P–T path with overall maximum P–T conditions of 540–550 °C and 1.9–2.25 GPa for the host rocks, and 555–575 °C and 2.2–2.5 GPa for the eclogites. Peak‐metamorphic temperatures of ~525–540 °C of the metasedimentary host rocks were also confirmed by Raman spectroscopy of carbonaceous material. Results from thermobarometry and thermodynamic modelling are consistent with the observation that none of the samples contains mineral relicts indicating UHP conditions (like coesite in garnet) and neither conventional thermobarometry, nor thermodynamical modelling resulted in P–T conditions in the stability field of coesite. Thus, no evidence of ultra‐high‐P conditions was found. Given that the whole sampled river valley lies within the proposed ‘ultra‐high‐P sub‐belt’ and considering former studies, which showed that at other places within this ‘unit’ both ultra‐high‐P and high‐P rocks are now juxtaposed on a small scale, the formation of the whole (ultra‐)high‐P–low‐T metamorphic belt in the Chinese South Tianshan as a tectonic mélange style exhumation is more convincing than the formation and juxtaposition of two coherent metamorphic units with high‐P and ultra‐high‐P conditions respectively.     

Exhumation mechanisms of melt‐bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates

A series of 2D petrological–thermomechanical numerical experiments was conducted to: (i) characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic (UHPM) rocks during collision of spontaneously moving plates and (ii) study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high‐temperature–ultrahigh pressure metamorphic (HT–UHPM) rocks. To this end, the models include fluid‐ and melt‐induced weakening of rocks. Five distinct modes of exhumation of (U)HPM rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows: vertical crustal extrusion, large‐scale crustal stacking, shallow crustal delamination, trans‐lithospheric diapirism, and channel flow. The variation in exhumation mechanisms for (U)HPM rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and/or did not include fluid‐ and melt‐induced weakening of rocks. This difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid‐ and melt‐related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments, such as slab breakoff, vertical crustal extrusion, large‐scale stacking, shallow crustal delamination and relamination, and eduction of the continental plate. Consequently, the significance of channel flow for the exhumation of UHPM rocks may have been overstated based on the results of the earlier numerical experiments. In addition, the results from this study extend over a larger proportion of the high‐temperature range of P–T conditions documented from UHPM rocks, including those retrieved from HT–UHPM rocks, than the results of experiments from previous numerical models. In particular, the highest peak metamorphic temperatures (up to 1000 °C) are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab. Nonetheless, some extreme temperature conditions which have been suggested for the Kokchetav and Bohemian massifs, perhaps up to 1100–1200 °C, are still to be achieved in experiments using numerical models.     

Exhumation of eclogite and blueschist (Cyclades,Greece): Pressure–temperature evolution determined by thermobarometry and garnet equilibrium modelling

High‐P rocks such as eclogite and blueschist are metamorphic markers of palaeo‐subduction zones, and their formation at high‐P and low‐T (HP–LT) conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P–T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering the comprehension of exhumation processes. In this study, we present an extensive petrological data set that permits refinement of the shape of the P–T trajectory for different subunits of the CBU on Syros. High‐resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi‐empirical thermobarometry, garnet equilibrium modelling and P–T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results are also combined with Raman spectrometry of carbonaceous material (RSCM) to retrieve the metamorphic peak temperature distribution at the scale of the island. A major result of this study is the good agreement between all the independent thermobarometric methods, permitting reconstruction of the prograde and retrograde P–T trajectories. Garnet compositional zoning was used to retrieve prograde, peak and retrograde growth stages in line with the results of the P–T isochemical phase diagrams, RSCM temperature and peak‐pressure crystallization of the garnet–omphacite–phengite assemblage. Our results are consistent with previous thermobarometric estimates from other occurrences of CBU rocks (Tinos, Andros), suggesting a multistage exhumation process with (1) early syn‐orogenic exhumation within the subduction channel, (2) isobaric heating at mid‐crustal depths (~10–12 kbar) following thermal re‐equilibration of the lithosphere from a cold syn‐orogenic regime in the subduction zone to a warmer post‐orogenic regime in the back‐arc domain and (3) exhumation and cooling related to a post‐orogenic phase of extension following slab retreat. Expanding to the general aspects of subduction zones, we suggest that such metamorphic evolution of HP–LT units should be regarded as a characteristic feature of exhumation driven by slab rollback.     

The metahyaloclastitic matrix of a unique metavolcanic block reveals subduction in the Somozas Mélange (Cabo Ortegal Complex,NW Iberia): tectonic implications for the assembly of Pangea

The allochthonous Cabo Ortegal Complex (NW Iberian Massif) contains a ~500 m thick serpentinite‐matrix mélange located in the lowest structural position, the Somozas Mélange. The mélange occurs at the leading edge of a thick nappe pile constituted by a variety of terranes transported to the East (present‐day coordinates; NW Iberian allochthonous complexes), with continental and oceanic affinities, and represents a Variscan suture. Among other types of metaigneous (calcalkaline suite dated at 527–499 Ma) and metasedimentary blocks, it contains close‐packed pillow‐lavas and broken pillow‐breccias with a metahyaloclastitic matrix formed by muscovite–paragonite–margarite–garnet–chlorite–kyanite–hematite–epidote–quartz–rutile. Pseudosection modelling in the MnCNTKFMASHO system indicates metamorphic peak conditions of ~17.5–18 kbar and ~550 °C followed by near‐isothermal decompression. This P–T evolution indicates subduction/accretion of an arc‐derived section of peri‐Gondwanan transitional crust. Subduction below the Variscan orogenic wedge evolved to continental collision with important dextral component. Closure of the remaining oceanic peri‐Gondwanan domain and associated release of fluid led to hydration of the overlying mantle wedge and the formation of a low‐viscosity subduction channel, where return flow formed the mélange. The submarine metavolcanic rocks were deformed and detached from the subducting transitional crust and eventually incorporated into the subduction channel, where they experienced fast exhumation. Due to the cryptic nature of the high‐P metamorphism preserved in its tectonic blocks, the significance of the Somozas Mélange had remained elusive, but it is made clear here for the first time as an important tectonic boundary within the Variscan Orogen formed during the late stages of the continental convergence leading to the assembly of Pangea.     

Subduction initiation in the Neo‐Tethys: constraints from counterclockwise P–T paths in amphibolite rocks of the Nagaland Ophiolite Complex,India

High‐P metamorphic rocks that are formed at the onset of oceanic subduction usually record a single cycle of subduction and exhumation along counterclockwise (CCW) P–T paths. Conceptual and thermo‐mechanical models, however, predict multiple burial–exhumation cycles, but direct observations of these from natural rocks are rare. In this study, we provide a new insight into this complexity of subduction channel dynamics from a fragment of Middle‐Late Jurassic Neo‐Tethys in the Nagaland Ophiolite Complex, northeastern India. Based on integrated textural, mineral compositional, metamorphic reaction history and geothermobarometric studies of a medium‐grade amphibolite tectonic unit within a serpentinite mélange, we establish two overprinting metamorphic cycles (M₁–M₂). These cycles with CCW P–T trajectories are part of a single tectonothermal event. We relate the M₁ metamorphic sequence to prograde burial and heating through greenschist and epidote blueschist facies to peak metamorphism, transitional between amphibolite and hornblende‐eclogite facies at 13.8 ± 2.6 kbar, 625 ± 45 °C (error 2σ values) and subsequent cooling and partial exhumation to greenschist facies. The M₂ metamorphic cycle reflects epidote blueschist facies prograde re‐burial of the partially exhumed M₁ cycle rocks to peak metamorphism at 14.4 ± 2 kbar, 540 ± 35 °C and their final exhumation to greenschist facies along a relatively cooler exhumation path. We interpret the M₁ metamorphism as the first evidence for initiation of subduction of the Neo‐Tethys from the eastern segment of the Indus‐Tsangpo suture zone. Reburial and final exhumation during M₂ are explained in terms of material transport in a large‐scale convective circulation system in the subduction channel as the latter evolves from a warm nascent to a cold and more mature stage of subduction. This Neo‐Tethys example suggests that multiple burial and exhumation cycles involving the first subducted oceanic crust may be more common than presently known.     

The P–T path of the ultra-high pressure Lago Di Cignana and adjoining high-pressure meta-ophiolitic units: insights into the evolution of the subducting Tethyan slab

The Lago di Cignana ultra‐high‐pressure unit (LCU), which consists of coesite–eclogite facies metabasics and metasediments, preserves the most deeply subducted oceanic rocks worldwide. New constraints on the prograde and early retrograde evolution of this ultra‐high pressure unit and adjoining units provide important insights into the evolution of the Piemontese–Ligurian palaeo‐subduction zone, active in Paleocene–Eocene times. In the LCU, a first prograde metamorphic assemblage, consisting of omphacite + Ca‐amphibole + epidote + rare biotite + ilmenite, formed during burial at estimated P < 1.7 GPa and 350 < T < 480 °C. Similar metamorphic conditions of 400 < T < 650 °C and 1.0 < P < 1.7 GPa have been estimated for the meta‐ophiolitic rocks juxtaposed to the LCU. The prograde assemblage is partially re‐equilibrated into the peak assemblage garnet + omphacite + Na‐amphibole + lawsonite + coesite + rutile, whose conditions were estimated at 590 < T < 605 °C and P > 3.2 GPa. The prograde path was characterized by a gradual decrease in the thermal gradient from ～9–10 to ～5–6 °C km^?1. This variation is interpreted as the evidence of an increase in the rate of subduction of the Piemonte–Ligurian oceanic slab in the Eocene. Accretion of the Piemontese oceanic rocks to the Alpine orogen and thermal relaxation were probably related to the arrival of more buoyant continental crust at the subduction zone. Subsequent deformation of the orogenic wedge is responsible for the present position of the LCU, sandwiched between two tectonic slices of meta‐ophiolites, named the Lower and Upper Units, which experienced peak pressures of 2.7–2.8 and <2.4 GPa respectively.     

A counter-clockwise P–T path for the Voltri Massif eclogites (Ligurian Alps, Italy)

Integrated petrological and structural investigations of eclogites from the eclogite zone of the Voltri Massif (Ligurian Alps) have been used to reconstruct a complete Alpine P–T deformation path from burial by subduction to subsequent exhumation. The early metamorphic evolution of the eclogites has been unravelled by correlating garnet zonation trends with the chemical variations in inclusions found in the different garnet domains. Garnet in massive eclogites displays typical growth zoning, whereas garnet in foliated eclogites shows rim‐ward resorption, likely related to re‐equilibration during retrogressive evolution. Garnet inclusions are distinctly different from core to rim, consisting primarily of Ca‐, Na/Ca‐amphibole, epidote, paragonite and talc in garnet cores and of clinopyroxene ± talc in the outer garnet domains. Quantitative thermobarometry on the inclusion assemblages in the garnet cores defines an initial greenschist‐to‐amphibolite facies metamorphic stage (M1 stage) at c. 450–500 °C and 5–8 kbar. Coexistence of omphacite + talc + katophorite inclusion assemblage in the outer garnet domains indicate c. 550 °C and 20 kbar, conditions which were considered as minimum P–T estimates for the M2 eclogitic stage. The early phase of retrograde reactions is polyphase and equilibrated under epidote–blueschist facies (M3 stage), characterized by the development of composite reaction textures (garnet necklaces and fluid‐assisted Na‐amphibole‐bearing symplectites) produced at the expense of the primary M2 garnet‐clinopyroxene assemblage. The blueschist retrogression is contemporaneous with the development of a penetrative deformation (D3) that resulted in a non‐coaxial fabric, with dominant top‐to‐the‐N sense of shear during rock exhumation. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblages (M4 & M5 stages), which are not associated with a penetrative structural fabric. The combined P–T deformation data are consistent with an overall counter‐clockwise path, from the greenschist/amphibolite, through the eclogite, the blueschist to the greenschist facies. These new results provide insights into the dynamic evolution of the Tertiary oceanic subduction processes leading to the building up of the Alpine orogen and the mechanisms involved in the exhumation of its high‐pressure roots.     

Exhumation of oceanic blueschists and eclogites in subduction zones: Timing and mechanisms

High-pressure low-temperature (HP–LT) metamorphic rocks provide invaluable constraints on the evolution of convergent zones. Based on a worldwide compilation of key information pertaining to fossil subduction zones (shape of exhumation P–T–t paths, exhumation velocities, timing of exhumation with respect to the convergence process, convergence velocities, volume of exhumed rocks,…), this contribution reappraises the burial and exhumation of oceanic blueschists and eclogites, which have received much less attention than continental ones during the last two decades.Whereas the buoyancy-driven exhumation of continental rocks proceeds at relatively fast rates at mantle depths (≥ cm/yr), oceanic exhumation velocities for HP–LT oceanic rocks, whether sedimentary or crustal, are usually on the order of the mm/yr. For the sediments, characterized by the continuity of the P–T conditions and the importance of accretionary processes, the driving exhumation mechanisms are underthrusting, detachment faulting and erosion. In contrast, blueschist and eclogite mafic bodies are systematically associated with serpentinites and/or a mechanically weak matrix and crop out in an internal position in the orogen.Oceanic crust rarely records P conditions > 2.0–2.3 GPa, which suggests the existence of maximum depths for the sampling of slab-derived oceanic crust. On the basis of natural observations and calculations of the net buoyancy of the oceanic crust, we conclude that beyond depths around 70 km there are either not enough serpentinites and/or they are not light enough to compensate the negative buoyancy of the crust.Most importantly, this survey demonstrates that short-lived (< ～ 15 My), discontinuous exhumation is the rule for the oceanic crust and associated mantle rocks: exhumation takes place either early (group 1: Franciscan, Chile), late (group 2: New Caledonia, W. Alps) or incidentally (group 3: SE Zagros, Himalayas, Andes, N. Cuba) during the subduction history. This discontinuous exhumation is likely permitted by the specific thermal regime following the onset of a young, warm subduction (group 1), by continental subduction (group 2) or by a major, geodynamic modification of convergence across the subduction zone (group 3; change of kinematics, subduction of asperities, etc).Understanding what controls this short-lived exhumation and the detachment and migration of oceanic crustal slices along the subduction channel will provide useful insights into the interplate mechanical coupling in subduction zones.     

Sm-Nd Isotope Tracer Study of UHP Metamorphic Rocks: Implications for Continental Subduction and Collisional Tectonics

Sm-Nd isotope tracer techniques are powerful tools in identification of the protolith nature of UHP and HP rocks and can be used to constrain modeling of tectonic processes of continental collision. UHP rocks may have diverse origins, and not all of them carry the same significance for subduction of continental blocks. In this paper, Sm-Nd isotopic data are compiled for UHP and HP rocks, mostly represented by eclogites and garnet peridotites, from the Alpine, Hercynian (Variscan), and Caledonian belts of western Europe; the Pan-African belts of northern Africa; and the Ross belt of Antarctica. These data then are compared with the isotopic characteristics of the UHP rocks from the Dabie orogen of central China. Except for the coesite-bearing quartzitic metasediments of Dora-Maira (Western Alps), which are clearly of continental origin, all HP and UHP rocks (eclogites and ultramafic rocks) from the Alpine, Hercynian, and Pan-African belts have oceanic affinities with the characteristic positive ε_Nd(T) values (= metamorphic initial ¹⁴³Nd/¹⁴⁴Nd ratios). They represent segments of oceanic lithosphere that were subducted, underwent eclogite-facies metamorphism, and later were tectonically transported into orogenic zones during continental collisions. By contrast, the majority of UHP rocks from the European Caledonide and the Dabie orogen have negative ε_ND(T) values, indicating continental affinity. This suggests that these mafic and ultramafic rocks have had a long crustal residence time and that their UHP metamorphism is indicative of subduction of ancient and cold continental blocks, as represented by some Precambrian gneiss terranes containing mafic components including greenschists, amphibolites, or basic granulites.

In the Dabie orogen, none of the UHP eclogites analyzed thus far have shown oceanic affinity; thus they do not represent subducted Tethys Ocean crust. The preservation of ultrahigh ε_ND(0) values (+170 to +260) in eclogites of very low Nd concentrations (average 0.5 ppm) from the Weihai region and of the extraordinarily low δ¹⁸O in many eclogites and gneisses, the general absence of syntectonic granites in the Dabie Shan, and the available age data obtained by different techniques all point to a rapid rate of exhumation and the absence of a pervasive aqueous fluid phase during the entire process of subduction and exhumation of the Dabie UHP terrane.     

Tectonometamorphic evolution of the Atbashi high‐P units (Kyrgyz CAOB,Tien Shan): Implications for the closure of the Turkestan Ocean and continental subduction–exhumation of the South Kazakh continental margin

The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. ⁴⁰Ar/³⁹Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during T_max conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the T_max (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma.     

Interaction of deformation and metamorphism during subduction and exhumation of hydrated oceanic mantle: Insights from the Western Alps

Petrological investigations supported by multi‐scale structural analysis of eclogitized serpentinite in the Zermatt–Saas Zone of the Western Alps allows for the determination of mineral assemblages related to successive fabrics, upon which the P–T–d–t path of these hydrated mantle rocks can be inferred. Serpentinites of the upper Valtournanche, with lenses and dykes of metagabbro and meta‐rodingite, display an Alpine polyphase metamorphic evolution from eclogite to epidote‐amphibolite facies conditions associated with three successive foliations having different parageneses in these rocks. Serpentinite mainly consists of serpentine with minor magnetite; however, where S1 and S2 foliations are pervasive, metamorphic olivine, together with Ti‐clinohumite and clinopyroxene, are also found. The mineral assemblage associated with D1 includes serpentine1, clinopyroxene1, opaque minerals, titanite ± olivine1, Ti‐clinohumite1 and ilmenite; the D2 assemblage is the same (±chlorite) but minerals have different compositions. The assemblage associated with D3 comprises serpentine3, opaque minerals, ±chlorite3, ilmenite and amphibole3. Ti‐clinohumite is associated with veins that are older than D2 and pre‐date D3. Veins that post‐date D3 are characterized by amphibole + chlorite or by serpentine. P–T conditions for S2 parageneses evaluated using two pseudosections for different bulk compositions suggest that these rocks experienced pressures >2.5 ± 0.3 GPa at temperatures slightly higher than 600 °C. The late epidote–amphibolite facies re‐equilibration associated with D3 and D4 developed during late syn‐exhumation deformation related to folding and testifies to a small temperature decrease. These results, which were integrated in the regional framework, suggest that different portions of the Zermatt–Saas Zone registered different P–T peak conditions and underwent different exhumation paths. In addition, the inferred P–T–d–t path suggests that the Valtournanche serpentinites re‐equilibrated close to the UHP conditions registered by the Cignana meta‐cherts. These results imply that tectonic slices exhumed after UHP metamorphism might be wider than previously reported or that small‐size UHP units, tectonically sampled during the Alpine convergence, are more abundant than those that have been detected to date.     

Geology and petrology of the Austroalpine Châtillon slice,Aosta valley,western Alps

Slices of continental crust pertinent to the lower Austroalpine domain of the western Alps, crop out within the ophiolitic Piemonte Zone. Among them, the Châtillon slice was studied in detail. The slice consists of orthogneiss with subordinate metabasics and very minor paraschist. The garnet-phengite-epidote-albite orthogneiss is characterised by polyphase garnet porphyroclasts. Metabasics consist of prasinite lenses and eclogite relics. Phengite-clinozoisite eclogite is characterised by small garnet idioblasts with prograde zoning; jadeite content in omphacite increases towards the rim; Si content in phengite decreases towards the rim. Garnet-glaucophane-phengiteparagonite micaschist is characterised by polymetamorphic garnet porphyroclasts, and small Alpine garnet idioblasts. A pre-Alpine amphibolite-facies metamorphism is inferred for the polymetamorphic rocks of the Châtillon slice. Paragneiss and micaschist probably derive from pre-Alpine “kinzigites”; the orthogneiss protolith was a late-Variscan porphyritic granitoid. Thermobarometry in the eclogite constrains the metamorphic peak at T ≤ 560 °C and P = 16 kbar. The HP minerals were partly retrogressed to greenschist-facies assemblages during the late Alpine tectono-metamorphic recrystallisation. The inferred Alpine P-T conditions are consistent with those for other Penninic and Austro-Alpine nappes of the northwestern internal Alps. The Châtillon slice is very similar to the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone and to the other eclogite-facies Austroalpine slices of the Dent Blanche Nappe, but it could also represent a portion of the Sesia-Lanzo Zone basement, which experienced a somewhat different subduction depth. The tectonic position of the Châtillon slice within the Piemonte Zone is essential to reconstruct the geometric relationships in the Austroalpine-Piemonte nappe stack of the northwestern internal Alps.