HP Rocks Research Articles

Location and polarity of Variscan sutures based on petrological and seismological data from the Bohemian Massif and the implications for the European Variscides

The high- to ultrahigh-pressure ((U)HP) metamorphic rocks are present within the European Variscan belt between the Bohemian and Iberian massifs (the Galicia-Moldanubian zone) and they are partly incorporated into the Alpine orogenic system. Due to their involvement in various allochthonous units, the affiliation of the (U)HP rocks to the suture zones that were the sites of their initial exhumation, is not always clear. The Bohemian Massif preserves the best evidence of Variscan sutures with clear relationships to the exposed (U)HP rocks. They are the Moldanubian and the Saxo-Thuringian sutures bounding the Teplá-Barrandian block from the SSE and NNE, respectively. The distribution of (U)HP rocks coincides with the boundaries of mantle lithosphere domains, delimited from large-scale seismic anisotropy, and reveals the NW-ward inclination of the Moldanubian mantle lithosphere domain beneath the Teplá-Barrandian block and thus a subduction polarity to the NW. The eastern margin of the Teplá-Barrandian block contains a magmatic arc, which is in direct contact with the Moldanubian orogenic wedge, and both are penetrated by lamprophyre dykes (∼340 Ma), which dates the cessation of the collision-related shortening and crustal consolidation. The overall crustal geometry of the Saxo-Thuringian suture implies the SE-ward polarity of subduction during its formation. However, based on seismic tomography and anisotropy model, the suture at mantle depths appears as a sub-vertical boundary between the Saxo-Thuringian and the Teplá-Barrandian lithosphere domains. The Saxo-Thuringian zone bears evidence of blueschist facies metamorphism in the (para)autochthonous units, which are strongly retrogressed. Compared to the Moldanubian zone, (U)HP rocks are less common in the Saxo-Thuringian zone and occur as nappes and klippes, some of which are exposed near the Moldanubian suture. The similarities of the Saxo-Thuringian (U)HP rocks to those in the Moldanubian zone and their allochthonous positions favour formation of some of the (U)HP rocks along the Moldanubian suture and their subsequent emplacement into the Saxothuringian zone. The Moldanubian suture appears to control the distribution of most of the (U)HP rocks exposed along the European Variscan Belt. They all show similarities regarding lithology, mainly fragments of mantle rocks included in felsic materials, and their granulite-amphibolite facies thermal overprint.

Petrology and phase equilibria of HP/LT eclogite at Gaoqiao, western Dabie and implications for lawsonite development in continental subduction zones

Lawsonite is of great significance for understanding fluid activity, element migration and crust–mantle interactions in subduction zones. Though studies have predicted lawsonite to be present under theP–Tregime during continental subduction, no lawsonite has been documented from natural (U) HP rocks in continental orogenic belt. In this study, we work on HP–LT eclogite (GQ–1 and GQ–2) at Gaoqiao, western Dabie to explore lawsonite formation and preservation during continental subduction and exhumation. Both samples have ubiquitous polymineralic aggregates of epidote/clinozoisite + paragonite/albite ± other minerals showing distinct rectangular or rhombic shapes developed as inclusions in garnet or in the matrix. Combined with recalculated bulk compositions similar to that of ideal lawsonite, we interpret these polymineralic aggregates to be pseudomorphs after lawsonite. Phase equilibrium modelling combined with compositional isopleth thermobarometry have constrained a segment of the prograde to peak stages to evolve from 19.0 to 19.5 kbar, ∼470°C to ∼20.0 kbar, 500C–505°C, then to ∼25 kbar, 530C–555°C in lawsonite stability fields. The progradeP–Tpath shows a two-stageP–Tevolution, with the first stage following a geothermal gradient of ∼7°C/km and the second stage decreasing to ∼6°C/km. Initial exhumation was inferred to follow an isothermal decompression process leading to lawsonite breakdown to form epidote/clinozosite ± paragoniteviathe reaction lawsonite + omphacite→ epidote + glaucophane ± paragonite + H2O at ∼19 kbar, 550°C. ModeledP/T–Xpseudosections calculated atT= 550°C andp= 25 kbar show that, when H2O content in bulk composition is more than 1.1 wt%, a certain amount of lawsonite (>13 mode%) should be present in eclogite. On the other hand, in the compositional range of natural intracontinental plate basalts, variations on O (Fe3+),XMgO[MgO/(MgO+FeO)],XCaO[CaO/(CaO+MgO+FeO+MnO+Na2O)],XNa2O[(Na2O/(CaO+Na2O)] andXAl2O3[Al2O3/(Al2O3+CaO+Na2O)] in bulk compositions have little influence on lawsonite development. In combination with previous studies, we conclude that during continent subduction along low geothermal gradient (<8°C/km), lawsonite could be formed under H2O present conditions. The absence of lawsonite in natural eclogite might be ascribed to retrograde overprint during exhumation.

Stretched Thin: Oligocene Extrusion and Ductile Thinning of the Basal Unit Along the Evia Shear Zone, NW Cyclades, Greece

AbstractWe present field‐based structural, geochronological, and petrological evidence for a major new Oligocene extensional shear zone, the Evia Shear Zone (ESZ), on the island of Evia in the NW Aegean Sea. Strain related to this structure occurs in a diffuse zone structurally underlying the nappe contact between the Cycladic Blueschist Unit and Basal Unit, within rock previously attributed to the Tsakei Unit or Basal Unit metaflysch. Structures within the ESZ record stretching in both the X and Y directions under ductile and brittle‐ductile conditions, indicative of a component of oblate coaxial strain during overall top‐to‐E general shear. New white mica 40Ar/39Ar geochronology from the ESZ shows that the structure was accommodating ductile deformation in the late Oligocene. Thermodynamic modeling shows that rock within the ESZ records greenschist facies retrogressive conditions of 310 ± 15°C and 7 ± 1 kbar. Zircon (U‐Th)/He thermochronology demonstrates that both the CBU and Basal Unit had exhumed into the brittle crust by the middle Miocene, slightly earlier than indicated by low‐temperature thermochronology for the HP‐LT units throughout the Aegean. We interpret these data as evidence that the Basal Unit underwent rapid exhumation via syn‐orogenic processes persisting until at least the early Miocene, facilitated in part by normal‐sense displacement along the ESZ. The occurrence of structures representative of coaxial deformation in the shear zone alludes to a major role for ductile thinning in controlling exhumation of HP rocks in the Cyclades, perhaps influencing the relative rates of exhumation observed across the region.

Along-strike variations in the fossil subduction zone of the Western Alps revealed by the CIFALPS seismic experiments and their implications for exhumation of (ultra-) high-pressure rocks

In complex plate-boundary settings, a reliable 3-D geophysical characterization of the deep tectonic structure is a fundamental starting point for a breakthrough in the analysis of processes controlling plate subduction and (U)HP rock exhumation. The Western Alps host one of the best-studied fossil subduction zones worldwide, with a well-defined deep structure in 2-D based on recent geophysical experiments. However, a full 3-D characterization of its deep tectonic structure is still lacking. Here we present a series of new receiver function cross-sections across the northern and southern Western Alps, validated and complemented by a S-wave velocity model from ambient-noise tomography that provides additional constraints between the profiles. We document a marked change in Moho attitude from the northern Western Alps, where the eastward-dipping European Moho reaches ∼45 km depth beneath the Gran Paradiso dome, to the southern Western Alps, where the European Moho reaches ∼70 km depth beneath the equivalent Dora-Maira dome. This change in Moho attitude takes place over a few tens of kilometers and was likely emphasized by deformation of the slab during subduction. The Western Alps subduction wedge is much thicker in the south than in the north, and the mantle-wedge rocks are deeply involved in orogeny exclusively in the south, where coesite is found in continental (U)HP rocks at several locations. Our detailed information on the 3-D structure of the subduction wedge provides first-order constraints for the next-generation of thermo-mechanical numerical models and may help explain the lateral variations in exhumation style revealed by the geologic record.

Neoarchean high-pressure granulite-facies anatexis of continental rocks in the Belomorian Eclogite Province, Russia

Eclogites exposed along the northeastern boundary of the Belomorian orogen in the eastern Fennoscandian Shield formed as a result of Mesoarchean–Neoarchean subduction. Highly-deformed banded TTG gneisses with eclogitized mafic pods, lenses and dykes in the Gridino association of metamorphic rocks have been modified locally to have typical migmatite structures that formed as HP rocks decompressed through HP granulite-facies metamorphic conditions (16–12 kbar, 800–850 °C) to amphibolite-facies conditions (10–9 kbar, 600–700 °C). The migmatites are located with the boundaries between felsic and mafic lithologies, which are the most suitable place for partial melting, fluid migration, and component diffusion. Symplectic intergrowths of hydrous minerals (mica, epidote) together with quartz in the leucosome are important markers of arrested reaction textures that formed by reversed hydration crystallization of residual melts or by fluids where their infiltrated through migmatite areas. There are two potassic granitic leucosomes with contrasting geochemical signatures. Garnet and phengite-bearing leucosome replaces the host TTG gneiss and percolates mafic rocks. This leucosome is distinguished by a high Ba content, striking positive Eu and Sr anomalies and corresponding low concentrations of all other trace elements. The compositions of the leucosome record initial segregation and migration of melt away from the residual source and subsequent crystal fractionation, dominated by feldspars, of the escaped melt. Migration of anatectic melts led to the formation of small bodies of leucogranite that are characterized by high contents of trace elements and negative Eu and Sr anomalies. Leucogranites formed from portions of fractionated melt that percolated the migmatites and solidified. Anatexis occurred during the Neoarchean time (∼2.7 Ga). U-rich zircon domains were partly or completely affected by radiation damage that yield discordant scattered dates between 2.7 and 1.9 Ga, which are interpreted as reflecting a thermal and fluid overprint during evolution of Belomorian province that produced recrystallization and Pb loss in Neoarchean zircons.

P–T–t–D records of Early Palaeozoic Andean‐type shortening of a hot active margin: The Dunhuang block in NW China

AbstractHigh‐pressure (HP) granulites form either in the domain of the subducted plate during continental collision or in supra‐subduction systems where the thermally softened upper plate is shortened and thickened. Such a discrepancy in tectonic setting can be evaluated by metamorphic pressure–temperature–time‐deformation (P–T–t–D) paths. In the current study, P–T–t–D paths of Early Palaeozoic HP granulite facies rocks, in the form of metabasic lenses enclosed in migmatitic metapelite, from the Dunhuang block, NW China, are investigated in order to constrain the nature of the HP rocks and shed light on the geodynamic evolution of a modern hot orogenic system in an active margin setting. The rocks show a polyphase evolution characterized by (1) relics of horizontal or gently dipping fabric (S1) preserved in cores of granulite lenses and in garnet porphyroblasts, (2) a N‐S trending sub‐vertical fabric (S2) preserved in low‐strain domains and (3) upright folds (F3) associated with a ubiquitous steep E‐W striking axial planar foliation (S3). Garnet in the granulites preserves relics of a prograde mineral assemblage M1a equilibrated at ~11.5 kbar and ~770–780°C, whereas the matrix granulite assemblage (M1b) from the S1 fabric attained peak pressure at ~13.5 kbar and ~850°C. The granulites were overprinted at ~8–11 kbar and ~850–900°C during crustal melting (M2) followed by partial re‐equilibration (M3) at ~8 kbar and ~625°C. A garnet Lu–Hf age of 421.6 ± 1.2 Ma dates metamorphism M1, while a garnet Sm–Nd age of 385.3 ± 4.0 Ma reflects M3 cooling of the granulites. The mineral assemblage, M1, of the host migmatitic metapelite formed at ~9–12.5 kbar and ~760–810°C, partial melting and migmatization (M2) occurred at ~7 kbar and ~760°C and re‐equilibration (M3) at ~5–6 kbar and ~675°C. A garnet Lu–Hf age of 409.7 ± 2.3 Ma dates thermal climax (M2) and a garnet Sm–Nd age of 356 ± 11 Ma constrains M3 for the migmatitic metapelites. The timing of this late phase is also bracketed by an emplacement age of syntectonic granite dated at c. 360 Ma. Decoupling of M1 and M2 P–T evolutions between the mafic granulites and migmatitic metapelites indicates their different positions in the crustal column, while the shared pressure–temperature (P–T) evolution M3 suggests formation of a mélange‐like association during the late stages of orogeny. The high‐pressure event D1‐M1 is interpreted as a result of Late Silurian–Early Devonian moderate crustal thickening of a thermally softened and thinned pre‐orogenic crust. The high‐temperature (HT) re‐equilibration D2‐M2 is interpreted as a result of Mid‐Devonian shortening of the previously thickened crust, possibly due to ‘Andean‐type’ underthrusting. The D3‐M3 event reflects Late Devonian supra‐subduction shortening and continuous erosion of the sub‐crustal lithosphere. This tectono‐metamorphic sequence of events is explained by polyphased Andean‐type deformation of a ‘Cascadia‐type’ active margin, which corresponds to a supra‐subduction tectonic switching paradigm.

Constraining the P–T path of (U)HP rocks with reaction overstepping during subduction; example from the Western Gneiss Region (Norway)

AbstractThis paper presents the results of petrological observations and diffusion modelling on garnet from high‐pressure to ultrahigh‐pressure ((U)HP) metamorphic rocks of the Western Gneiss Region in the Nordfjord. Garnet from kyanite‐bearing micaschist preserves two generations of garnet growth that are related to the Pre‐Caledonian granulite facies and Caledonian eclogite facies metamorphic events. Mafic eclogite, forming lenses in the micaschist, contains only eclogite facies assemblages with partial recrystallization under amphibolite facies conditions. Caledonian garnet in both the micaschist and hosting eclogite indicates reaction overstepping and nucleation near or above 550°C/2.0 GPa. Maximum pressure and temperature, calculated using pseudosection modelling for the eclogite facies event, were ~2.6 GPa and 650°C. The interface between the Pre‐Caledonian and Caledonian garnet in the micaschist shows a strong compositional gradient or possibly a compositional jump. The preservation of such a gradient together with the hummocky‐shaped composition profiles in the Caledonian garnet from the eclogite indicates either no relaxation or a short‐time of relaxation of the rocks at their peak temperature conditions, as well as their exhumation by cooling. Possibly, heating or exhuming of the rocks by isothermal decompression could have easily modified such compositional irregularities along the garnet profiles. A cooling rate of ~187°C/Ma and exhumation rate in the vertical direction of ~2.5 cm/year for the HP rocks were obtained by considering that the temperature and transport distance changes from their maximum depth and peak temperature to the surface were proportional to the time (3.5 Ma) calculated by modelling for the garnet.

Neoproterozoic Eclogite-to Granulite-Facies Transition in the Ubendian Belt, Tanzania, and the Timescale of Continental Collision

AbstractIn collision-type orogens, where high-pressure and ultrahigh-pressure (HP–UHP) metamorphism usually occurs, deeply subducted continental slabs with eclogitized mafic rocks often undergo recrystallization/overprinting with various geothermal gradients after the peak conditions at lower-to-middle-crustal levels. During the crustal stabilization, the transition from eclogite-to granulite-facies is common. We conducted metamorphic petrology and zircon geochronology on (1) bimineralic and (2) partially granulitized eclogites from the Neoproterozoic Ufipa Terrane (Southwestern Tanzania). Microtextural relationships and mineral chemistry define three metamorphic stages: eclogite metamorphism (M1), HP granulite-facies overprinting (M2), and amphibolite-facies retrogression (M3). The bimineralic eclogite has a basaltic composition and lacks M2 minerals. In contrast, the kyanite eclogite is characterized by a gabbro-dioritic whole-rock composition and contains inherited magmatic zircon. Although the matrix is highly granulitized, garnet and kyanite contain eclogite-facies mineral inclusions. Phase equilibria modeling revealed P–T conditions of 2.1–2.6 GPa and 650–860°C for the M1 stage and 1.4–1.6 GPa and 750–940°C for the M2 stage. Zircon with eclogite-facies mineral inclusions from the bimineralic eclogite lacks Eu anomaly in the REE patterns and yielded the M1 eclogite metamorphic age of 588 ± 3 Ma. Zircon overgrowths surrounding the inherited Paleoproterozoic magmatic cores in kyanite eclogite yielded 562 ± 3 Ma. A weak negative Eu anomaly in the REE patterns and the absence of eclogitic mineral inclusions suggest the zircon growths at the M2 HP granulite-facies metamorphic stage. These new data indicate an eclogite-to granulite-facies transition time of 26 ± 4 million years (Myr), suggesting a rate of HP rock exhumation toward a lower crustal level of 0.7–1.5 mm/year. Furthermore, the density evolution model indicates that buoyant host orthogneiss with low-density gabbro-dioritic eclogite plays an important role in carrying high-density basaltic eclogite. Our 2D thermomechanical modeling also suggests that a slab break-off with a lower angle subduction of <20° triggers the exhumation of the HP slab sliver with 20–30 Myr eclogite-to granulite transition time of large HP–UHP terranes in major collision zones.

Highly variable petrophysical properties in felsic high-pressure rocks of the continental crust

Seismic wave velocities and anisotropy can potentially discern lithologies and their structural configuration at depth. An increasing amount of data exists that indicates seismic anisotropy can be a valuable marker to identify internal deformation within deeply buried crust. Here, we present petrophysical properties of variably deformed felsic rocks that have been metamorphosed at blueschist and eclogite-facies conditions, representative of the bulk composition of the mid continental crust. The samples were collected from the Monte Mucrone area (Sesia Zone, Western Alps) and the Serra di Pigno, Farinole, and Tenda areas (Corsica). The mineral assemblages of all samples indicate that fluids were abundant during re-equilibration at HP conditions. Petrophysical properties were derived from ultrasonic measurements as well as calculated from crystallographic preferred orientations obtained by neutron texture goniometry. The results show that crystallographic preferred orientations are weak and yet the magnitude of seismic anisotropy produced by metamorphism of the felsic continental crust is highly variable, ranging from 1 to 12% for P waves and 1–14% for S waves depending on the strength of deformation fabrics. At the same time, absolute wave velocities remain low compared to more mafic rocks that have equilibrated at HP conditions. These results demonstrate that, other than their mafic equivalents, felsic HP rocks cannot be distinguished from their protoliths by wave velocities because their increase during HP metamorphism is too small. However, fluid availability during metamorphism and deformation causes strong variations in the seismic anisotropy of such lithologies, that might be distinguishable at depth.

New constraints on the tectonometamorphic evolution of the Erzgebirge orogenic wedge (Saxothuringian Domain, Bohemian Massif)

AbstractIn contrast to well‐documented (U)HP rocks of the Erzgebirge crystalline complex (Saxothuringian Domain, Bohemian Massif), the surrounding medium‐ to low‐grade metasediments are scarcely studied, although they provide an important link between deep subduction and mid‐crustal processes. To constrain the Variscan evolution from continental subduction to orogenic wedge, the transition from the low‐grade phyllites to the footwall medium‐grade micaschists was investigated. We used thermodynamic modelling to constrain the P‐T conditions of four deformation fabrics (D1–D4) identified by the structural analysis. The studied area was first affected by the M1‐D1 event characterized by HP‐LT minerals (garnet, chloritoid, phengite, paragonite, and rutile) defining the S1 foliation. The calculated peak P‐T conditions for M1 increase from 13 kbar and 520°C in phyllites to 25 kbar and 560°C in micaschists. The corresponding apparent geothermal gradient 6–11°C/km is typical for a subduction environment. The M2‐D2 event corresponds to deformation and metamorphic overprint of the previous fabric during partial decompression. The M3‐D3 event is mainly developed in micaschists, and the M3 overprint intensifies towards the footwall. It is accompanied by a subhorizontal S3 cleavage characterized by MP‐MT assemblage containing biotite, staurolite and ilmenite. The inferred peak P‐T conditions for M3 are 5–9 kbar and 595°C representing a barrovian‐type geothermal gradient from 17–30°C/km. Finally, all metamorphic fabrics were heterogeneously affected by the low‐grade M4‐D4 upright folding. The D1‐D2 events are interpreted to record the growth and evolution of the Saxothuringian orogenic wedge while its present‐day architecture resulted from significant vertical shortening D3 associated with barrovian type M3‐metamorphism. This stage corresponds to the ductile thinning of the wedge. A new model is proposed, in which the Erzgebirge part of the Saxothuringian Domain reveals a spectacular example of active margin evolution characterized by (i) formation of accretionary prism, (ii) building of the orogenic wedge by accretion of subducted continental crust, and (iii) its extensional collapse.

Timing of exhumation of meta-ophiolite units in the Western Alps: New tectonic implications from 40Ar/39Ar white mica ages from Piedmont Zone (Susa Valley)

A multidisciplinary approach to the study of collisional orogenic belts can improve our knowledge of their geodynamic evolution and may suggest new tectonic models, especially for (U)HP rocks inside the accretionary wedge. In the Western Alps, wherein nappes of different origin are stacked, having recorded different metamorphic peaks at different stages of the orogenic evolution. This study focuses on the External (EPZ) and Internal (IPZ) ophiolitic units of the Piedmont Zone (Susa Valley, Western Alps), which were deformed throughout four tectonometamorphic phases (D1 to D4), developing different foliations and cleavages (S1 to S4) at different metamorphic conditions. The IPZ and EPZ are separated by a shear zone (i.e. the Susa Shear Zone (SSZ)) during which a related mylonitic foliation (SM) developed. S1 developed at high pressure conditions (Epidote-eclogite vs. Lawsonite-blueschist facies conditions for IPZ and EPZ, respectively), as suggested by the composition of white mica (i.e. phengite), whereas S2 developed at low pressure conditions (Epidote-greenschist facies conditions in both IPZ and EPZ) and is defined by muscovite. White mica defining the SM mylonitic foliation (T1) is mostly defined by phengite, while the T2-related disjunctive cleavage is defined by fine-grained muscovite. The relative chronology inferred from meso- and micro-structural observations suggests that T1 was near-coeval with respect to the D2, while T2 developed during D4.A new set of radiometric ages of the main metamorphic foliations were obtained by in situ Ar/Ar dating on white mica. Different generations of white mica defining S1 and S2 foliations in both the IPZ and EPZ and SM in the SSZ, were dated and two main groups of ages were obtained. In both IPZ and EPZ, S1 foliation developed at ~46–41 Ma, while S2 foliation developed at ~40–36 Ma and was nearly coeval with the SM mylonitic foliation (~39–36 Ma).Comparison between structural, petrological and geochronological data allows to define time of coupling of the different units and consequently to infer new tectonic implications for the exhumation of meta-ophiolites of the Piedmont Zone within axial sector of the Western Alps.

Comment on “Pressure‐to‐Depth Conversion Models for Metamorphic Rocks: Derivation and Applications” by Bauville and Yamato

AbstractBauville and Yamato (2021, https://doi.org/10.1029/2020gc009280) (G‐cubed) propose model‐based methods to convert metamorphic pressures to depths based on the claim that pressure data from global (ultra)high‐pressure([U]HP) rocks challenge the lithostatic assumption and support their model which invokes excessive overpressures. It is argued here that the opposite is true: Natural pressure data are fully consistent with the lithostatic assumption. They reflect selection of (U)HP rocks by accessibility and preservation. The data are however inconsistent with the model predictions of Yamato and Brun (2017, https://doi.org/10.1038/ngeo2852) and Bauville and Yamato (2021, https://doi.org/10.1029/2020gc009280). Furthermore, their model requires critical assumptions that are not justified by the principles of rock mechanics and unsupported by microstructures from (U)HP rocks.

Local variations of metamorphic record from compositionally heterogeneous rocks (Cima di Gagnone, Central Alps): Inferences on exhumation processes of (U)HP–HT rocks

The record of metamorphic conditions is commonly highly heterogeneous in spatially close rocks of different composition and rheology. The well-studied Cima di Gagnone area in the Central Alps represents such an example, as relatively small, 1– to 100 s-meter scale, ultrahigh–pressure and high–temperature ultramafic lenses are enveloped within amphibolite–facies metasediments. We present new field observations, microstructural and petrological analyses, and thermodynamic modelling results on these metasediments, showing that these rocks generally experienced medium pressure and medium temperature conditions of 1.0–1.2 GPa and 640–700 °C, followed by a retrograde stage around 0.6–0.8 GPa and 600–675 °C. However, a few samples from the immediate proximity of the ultramafic lenses record significantly higher P–T conditions of 1.3–1.7 GPa and 750–850 °C, approaching the high pressure and high temperature conditions of the ultramafic bodies (1.5–3.1 GPa, 650–850 °C). Mineral/bulk chemistry changes during growth of new mineral phases hint to local melt/fluid interaction (i.e., metasomatism) between metasediments and ultramafics during the high temperature deformation. Preliminary U-Pb LA–ICP–MS dating suggests that zircon grains from the metasomatic reaction zone have been fully re–equilibrated during the early stage of Alpine exhumation (~36 Ma), while the large part of the metasediments records only pre–Alpine ages. We finally recast these new data into the regional P–T–t–D paths and discuss the consequences of these findings for understanding the exhumation processes of HP rocks. We found different local equilibria (ΔP up to 2 GPa; ΔT up to 160 °C) that cannot be explained simply by retrograde metamorphism or by tectonic amalgamation. Rather, our work suggests a significant role of deformation in producing variable pressure and temperature record within the rocks.

High‐P metamorphism of garnet–epidote–amphibole schists from the Yuli Belt, Eastern Taiwan: Evidence related to warm subduction

AbstractHigh‐P (HP) rocks exhumed from ancient subduction zones have been controversially argued to record much warmer geotherms than numerical modelling results. The Yuli belt in Eastern Taiwan was regarded as a mélange consisting of HP rock‐bearing tectonic blocks within metasediments (or in situ schists), witnessing the subduction of the South China Sea Plate beneath the Philippine Sea Plate and the subsequent collision during the period of 15–5 Ma. Two garnet–epidote–amphibole schist samples selected from the Tamayen block in the Yuli belt are investigated for their metamorphic evolution. The rocks comprise garnet and paragonite porphyroblasts in a foliated matrix consisting of amphibole, epidote, chlorite, quartz, albite and rutile. Large paragonite flakes crosscut clearly the main foliation, and in places develop narrow coronae of muscovite, albite or chlorite. Fine‐grained pargasite (Amp1) is the dominant variety of amphibole that defines the foliation, whereas a few coarser grains (Amp2) penetrate the foliation and both are further rimmed by a narrow light‐coloured amphibole (Amp3). Petrographic observation and phase equilibria modelling using thermocalc suggest three stages of metamorphic evolution for these rocks. The pre‐peak prograde to the peak‐stage evolution can be revealed from garnet zoning to occur in the mineral assemblages containing lawsonite and glaucophane which cannot be observed petrographically due to the post‐peak decompressional evolution. The peak conditions are constrained to be ~22 kbar/545℃ using the isopleths of the maximum pyrope and the corresponding grossular contents in P–T pseudosections. The post‐peak decompression includes three substages with different fluid behaviours: the first substage is dominated by lawsonite dehydration with the growth of paragonite; the second substage is marked by the decompressional path proceeding in fluid‐absent fields; and the third substage occurs when mineral assemblages become again saturated with water at 10–9 kbar or 12–10 kbar where glaucophane is metastably transformed into Amp1, followed by the growth of Amp2. The late‐retrograde stage is defined to be cooling with decompression on the basis of the growth of Amp3, the coronae around paragonite and the matrix albite. The peak P–T estimates for garnet–epidote–amphibole schists suggest a geotherm of ~7℃/km, much cooler than those in the previous researches in the Yuli belt, and well consistent with the results from numerical thermal modelling. We argue that HP rocks from ancient orogenic belts can robustly constrain the thermal structure of subduction zones if their peak P–T conditions as well as the post‐peak decompressional evolution are well recovered.

The Deep Structure of the Alps Based on the CIFALPS Seismic Experiment: A Synthesis

AbstractThe European Alps are the site where classic geologic concepts such as nappe theory, continental subduction and slab breakoff have been first proposed. However, the deep tectonic structure of the Alps has long been poorly constrained by independent geophysical evidence. This review paper summarizes the main results of the CIFALPS passive seismic experiment, that was launched by Chinese, French and Italian scientists in the 2010s to provide new insights on the deep tectonic structure of the Alpine region. The application of a wide range of tomographic methods to the analysis of a single fossil subduction zone makes the CIFALPS experiment a potential reference case for the analysis of other orogenic belts. Major results include: (i) the first seismic evidence of European continental crust subducted into the Adriatic upper mantle, beneath the place where coesite was first recognized in continental (U)HP rocks in the Alps; (ii) evidence of a major involvement of the mantle wedge during (U)HP rock exhumation; (iii) evidence of a serpentinized plate interface favoring continental subduction; (iv) evidence of a continuous slab beneath the Western and Central Alps, ruling out the classic model of slab breakoff magmatism; (v) evidence of a polyphase development of anisotropic fabrics in the Alpine mantle, either representing active mantle flows or fossil fabrics inherited from previous rifting stages. Detection of these major tectonic features allows to propose interpretive geologic cross‐sections at the scale of the lithosphere and upper mantle, providing a baseline for future analyses of active continental margins.

Comment to “Deep subduction of felsic rocks hosting UHP lenses in the central Saxonian Erzgebirge: Implications for UHP terrane exhumation” by

• Coesite-bearing garnet found in tributaries have led to overestimation of UHP rocks. • Geodynamic models exist alternative to exhumation of a whole UHP unit. • UHP terrane in the Erzgebirge in Saxony consists of UHP rocks embedded in HP rocks.

Subduction zone sulfur mobilization and redistribution by intraslab fluid–rock interaction

Subduction zones mediate the sulfur exchange between Earth’s interior and surface, and play a critical role in the long-term global sulfur cycle. Dehydration of the subducted slab releases aqueous fluids, which in turn interact with the surrounding wall-rocks during reactive flow through the slab. Fluids are regarded as the key agent for sulfur transfer within the slab and subsequently into the mantle wedge. However, the mechanisms underlying sulfur mobilization and redistribution during fluid–rock interaction are not fully understood. Here, we investigate three high-pressure blueschist/eclogite–selvage–vein systems from the Southwestern Tianshan metamorphic belt in northwestern China to explore sulfur behavior during fluid–rock interaction along channelized fluid pathways and within their alteration halos. Petrologic observations on mineral parageneses and δ34S compositions of sulfides reveal two different scenarios of sulfur mobilization and redistribution during fluid–rock interaction at HP conditions. Fe-rich and medium-fS2 (near the pyrite–pyrrhotite buffer) fluids favor pyrrhotite stability over pyrite and can trigger the pyrite–to–pyrrhotite transition in the reaction halo. In this case the bulk-rock sulfur budget may not vary but the sulfur isotope exchange is still effective, producing a δ34S mixing trend in the metasomatic zones. In contrast, Fe-poor and high-fS2 fluids cause conversion of pyrrhotite to pyrite, significant sulfur addition, and sulfur isotope exchange in alteration selvages. Medium-fS2 fluids can transport sulfur and thus are effective in transferring sulfur out of the slab, whereas high-fS2 fluids cause sulfur sequestration along fluid pathways and considerably reduce long-distance sulfur transfer. Multiple fluid infiltration events with different fluid sources result in several pyrite overgrowth sequences, recorded by contrasting Co–Ni element distributions and in-situ δ34S zoning of pyrite. The heaviest δ34S value (+25‰) thus far reported in HP rocks has been found in vein pyrite, suggesting that a seawater sulfate-derived δ34S signature can be transported to great depths in a subduction zone, even though the timing and mechanism of sulfate–to–sulfide reduction in the subduction zone remain unconstrained. Our study provides natural evidence for fluid-mediated sulfide transformation, sulfur transport/storage, and sulfur isotope exchange during fluid–rock interaction, and illuminates the role these processes play in sulfur release from the subducting slab and sulfur subduction into the deep mantle.

Tracking a continental deep subduction and exhumation from granulitized kyanite eclogites in the South Altyn Tagh, northern Qinghai-Tibet Plateau, China

Kyanite eclogites can provide insights into global orogenic processes within subduction-collision zones. Here we present an integrated study of the petrography, mineral chemistry, zircon U-Pb geochronology, and whole-rock geochemistry of granulitized kyanite eclogites from South Altyn Tagh, located at the northern margin of the Qinghai-Tibet plateau, China. Our results suggest that the rocks experienced eclogite-facies metamorphism (27–37 kbar, 1030–1068 °C), followed by a high pressure-ultra-high temperature (HP-UHT) stage (17–20 kbar, 945–1033 °C), isothermal decompression, and a medium pressure-(ultra)-high temperature (MP-(U)HT) (10–12 kbar, 810–950 °C) granulite-facies overprint.Zircon U-Pb dating yielded a weighted mean U-Pb age of ca. 500 Ma for the metamorphic domains, which is interpreted as the timing of HP-UHT metamorphism. In contrast, the age of ca. 800 Ma recorded by the inherited cores of zircon grains is considered to represent the protolith formation age. High Al2O3 and CaO and relatively low MgO and FeO contents, LREE enrichment and nearly flat HREE patterns, and enriched Sr-Nd isotopic compositions suggest that the protoliths of the granulitized kyanite eclogites were likely derived from an enriched mantle source affected by crustal contamination. These results, together with previous research on these associations suggested that the protoliths of granulitized kyanite eclogites and associated garnet peridotite in the Bashiwake area are derived from mafic-ultramafic intrusions composed of plagioclase-rich gabbroic rocks and olivine-rich cumulates. These mafic-ultramafic intrusions were possibly generated by plume-induced melting related to the rifting of the Rodinia supercontinent during the Neoproterozoic, and were later subjected to early Paleozoic metamorphism. The geodynamic model proposed envisages continental subduction followed by relamination of felsic and mafic-ultramafic associations at the base of the lower crust, which were subsequently exhumed, forming diapir-like bodies. Our results, combined with those from previous studies, not only provide insights into the processes of continental collision to exhumation, but also provide significant constraints on the geotectonic processes leading to the formation of HP rocks within subduction-collision orogens.

Strain Partitioning and Exhumation in Oblique Taiwan Collision: Role of Rift Architecture and Plate Kinematics

AbstractTaiwan is an archetypal example of continental accretionary wedges. Yet the generally poor knowledge of three‐dimensional strain distribution over time and role of architecture of the rifted margin shed doubt on the cylindrical two‐dimensional kinematic models of Taiwan collision. Here we provide new field‐based constraints on strain distribution, new Raman Spectroscopy on Carbonaceous Materials temperatures and apply mica‐chlorite multiequilibrium approach to determine pressure‐temperature in the Central Range of Taiwan. We identify three distinct structural domains that define zones of orthogonal shortening in the western Backbone Range and left‐lateral ductile shearing overprinted by left‐lateral transtensional brittle deformation in eastern Central Range. Field surveys show the lack of nappe stacking in the Backbone Range. Combining new temperature estimates with existing thermochronological constraints we emphasize that western Taiwan mostly inherited preorogenic thermal history. We show that metamorphic peak conditions of 5–6 kbar and 330–400 °C in the eastern Backbone Range and HP rocks of the Yuli Belt exhumed along the P‐T paths related to transcurrent deformation. We propose a three‐dimensional kinematic model of Taiwan accounting for the oblique motion of the Philippine Sea Plate relative to the plate boundary and the reactivation of a NS striking transform fault in the South China Sea rifted margin. Recent and ongoing strain partitioning in the Taiwan accretionary wedge is reflected by the coexistence of brittle left‐lateral shear, oblique extension, and contraction. Our results have impact on orogen‐based plate kinematic reconstructions that consider two‐dimensional kinematic evolution of orogens.

Plume-modified collision orogeny: The Tarim–western Tianshan example in Central Asia

AbstractPlume-modified orogeny involves the interaction between a mantle plume and subducting oceanic lithosphere at accretionary margins. We propose that a plume can also be involved in collisional orogeny and accounts for the late Paleozoic geological relations in Central Asia. Continental collision between the Tarim and Central Tianshan–Yili blocks at the end Carboniferous resulted in an orogeny lacking continental-type (ultra)high-pressure [(U)HP] rocks and significant syncollision surface erosion and uplift, features normally characteristic of continent-continent interactions. Their absence from the Tianshan region corresponded with the arrival of a mantle plume beneath the northern Tarim. Elemental and isotopic data reveal an increasing influence of the mantle plume on magmatic petrogenesis from ca. 300 to 280 Ma, immediately after collision at 310–300 Ma. The rising mantle plume interrupted the normal succession of collisional orogenic events, destroying the deeply subducted continental crust and hence preventing slab break-off–induced continental rebound. Plume-modified continental collision thus limited continental (U)HP rock exhumation and associated surface uplift.