Exhumation Of Eclogites Research Articles

New P-T path of Huwan high-pressure unit from Hong'an orogen, Central China Orogenic Belt: Implications for oceanic to continental subduction and exhumation in the subduction zones

Oceanic to continental subduction is a pivotal process in plate tectonics, yet characterizing this tectonic transition and subsequent exhumation remains a challenging task. This study integrates our novel pressure-temperature (P-T) estimates on a metabasite lens and its hosting metasedimentary rock in the Xiongdian area of the Huwan high-pressure unit of the Hong'an orogen, Central China Orogenic Belt, with existing geochronological data from the same and neighboring outcrops. Our preliminary but comprehensive findings reveal the evolution of this subduction transition process. After the opening at ca. 430–400 Ma the Paleotethyan ocean subducted northward and formed a warm oceanic subduction zone at ca. 400–385 Ma, marked by relatively low P and high T conditions (∼610–740 °C and ∼1.5–1.8 GPa; M1 stage). Subsequently, the Paleotethyan oceanic plate continued its subduction and cooled the overlying mantle until its final closure during ca. 385–300 Ma. This phase led to the subduction of South China Block (SCB) and the formation of a cold continental subduction zone at ca. 300–270 Ma, featured by relatively high P and low T conditions (∼570–610 °C and ∼2.1–2.7 GPa; M2 stage). Following the delamination from the subducted SCB continental lithospheric mantle, the detached high-pressure rocks experienced a two-stage accelerated exhumation process, i.e., early slow exhumation (∼0.7 ± 0.4 mm/a) to shallower mantle depth (∼610–730 °C and ∼1.5–2.1 GPa; M3 stage) at ca. 270–243 Ma and later rapid exhumation (>2.5 ± 0.9 mm/a) to approximately the Moho depth (∼550–600 °C and ∼0.8–1.3 GPa; M4 stage) at ca. 243–233? Ma. To facilitate the buoyancy-driven exhumation of eclogite blocks from their maximum depth (M2 stage), a minimum of ∼16–25 vol% of low-density metasedimentary rock is required. The increasing buoyancy caused by decreasing densities of the high-pressure rocks with exhumation (from M2 to M4 stage), slab break-off induced rebound forces, and other tectonic factors may jointly explain the accelerated exhumation pattern. Our density results also emphasize that, partly owing to a stronger contribution from a buoyancy-driven mechanism, the high-pressure rocks would be preferentially exhumed in warm to extremely warm oceanic subduction zones compared to their intermediate to cold counterparts.

Thermal expansion of talc at high temperature and implications for the exhumation of eclogites in subduction zones

Thermal expansion of talc at high temperature and implications for the exhumation of eclogites in subduction zones

Omphacite breakdown: nucleation and deformation of clinopyroxene-plagioclase symplectites

The breakdown of omphacite plays an important role in the exhumation and retrogression of eclogites. Additionally, metamorphic reactions associated with grain size reduction have the potential to significantly impact deformation mechanisms and the rheology of crustal rocks. We analyze the breakdown reaction omphacite → diopsidic clinopyroxene + plagioclase ± amphibole and associated microstructures by electron backscatter diffraction. The reaction results in the formation of (diopsidic) clinopyroxene-plagioclase symplectites. Samples were chosen from localities on Holsnøy (western Norway) and Lofoten (northern Norway), that are representative of vermicular symplectites, partly recrystallized symplectites, and deformed symplectites. Interphase misorientation analysis based on the electron backscatter diffraction results reveals that the nucleation of (diopsidic) clinopyroxene-plagioclase symplectites was crystallographically controlled, with the diopside copying the lattice orientation of the omphacite, and the plagioclase growing along diopside planes with favorable, i.e., similar, interplanar spacing. Deformation of the (diopsidic) clinopyroxene-plagioclase symplectites occurred by fracturing, transitioning into grain boundary sliding accommodated by diffusion creep. The results indicate that the formation of vermicular symplectites is not associated with enhanced permeability and fluid flow. Subsequent recrystallisation and grain-size sensitive deformation of the symplectites facilitates fluid redistribution and weakening of the retrogressed eclogites.

Garnet and zircon petrochronology reveal coherent subduction and exhumation of eclogites and associated schists in the Changning–Menglian orogenic belt, southeast Tibet

Determining whether high-grade metamorphic eclogites and surrounding metasediments underwent a shared subduction-exhumation cycle or distinct metamorphic paths before their juxtaposition at shallow levels is crucial for deciphering metamorphic processes in orogenic belts. Constructing pressure-temperature-time (P–T-t) paths for both rocks is the most straightforward approach. However, current time constraints overwhelmingly rely on dating accessory mineral like zircon, rutile, and monazite, which may only record a portion of the entire metamorphic history. Here we present an integrated petrochronological analysis using garnet Lu–Hf and Sm–Nd, and zircon U–Pb on two eclogites and their surrounding micaschists from the Changning–Menglian orogenic belt in southeast Tibet. Pseudosection modelling and conventional thermobarometers suggest a clockwise P–T path for the eclogites, from ∼1.4 GPa/∼520 °C to ∼2.5 GPa/∼600 °C, followed by decompression to <∼0.7 GPa, and peak conditions of 2.0–2.2 GPa/570–620 °C for the micaschists. Lu–Hf dating for eclogites yields ages of ca. 238–241 Ma, indistinguishable from the Sm–Nd dates of ca. 239 Ma. However, the Lu–Hf ages for micaschists (ca. 249–251 Ma) are older than the corresponding Sm–Nd dates (ca. 240 Ma). The well-preserved growth zoning in garnet indicates a short growth period in eclogites and at least ∼10 Myr in micaschists, bracketing the timing of protracted the two distinct zircon growth phases at around 238 Ma and 250 Ma. The similarity in peak metamorphic conditions and the consistent high-pressure metamorphic ages suggest a coherent subduction and exhumation process for both eclogites and micaschists. The Lu–Hf ages of the micaschists constrain the subduction of the Lancang Group dates back to ca. 250 Ma. The narrow temporal gap between the initiation of subduction and the age of the protoliths implies a rapid shift from the formation of the protoliths to the subduction of oceanic slabs and the scraped-off sediments to depths exceeding 80 km during the Triassic—a hypothesis that warrants further exploration.

Tracking Garnet Dissolution Kinetics in 3D Using Deep Learning Grain Shape Classification

Abstract The kinetics of fluid-driven metamorphic reactions are challenging to study in nature because of the tendency of metamorphic systems to converge towards chemical equilibrium. However, in cases where mineral textures that reflect incomplete reactions are preserved, kinetic processes may be investigated. Atoll garnet, a texture formed by the dissolution of a garnet’s core, has been described in 2D from thin sections of rocks worldwide. Quantifying the extent of this dissolution reaction requires a sample-wide examination of hundreds of individual grains in 3D. In this study, we quantified the distribution of atoll garnet using micro-computed tomography and grain shape analysis. A convolutional neural network was trained on human-labeled garnet grains for automated garnet classification. This approach was applied to a retrogressed mafic eclogite from the Zermatt–Saas Zone (Western Alps). Pervasive atoll-like resorption preferentially affected the larger porphyroblasts, suggesting that compositional zoning patterns exert a first-order control on dissolution rates. A kinetic model shows that the reactivity of metastable garnet to form atolls is favored at pressure–temperature conditions of 560 ± 30°C and 1.6 ± 0.2 GPa. These conditions coincide with the release of water when lawsonite breaks down during the exhumation of mafic eclogites. The model predicts dissolution rates that are three to five times faster for the garnet core than for the rim. This study shows that deep learning algorithms can perform automated textural analysis of crystal shapes in 3D and that these datasets have the potential to elucidate petrological processes, such as the kinetics of fluid-driven metamorphic reactions.

Exhumation of ultrahigh-pressure eclogite and metarodingite within UHP serpentinites from the Chinese southwestern Tianshan

Extensive research over the past two decades suggests that serpentinite loses its ability to effectively facilitate the exhumation of high-density high-pressure (HP) eclogites at depths of 60–80 km under average subduction zone geotherms. However, at Changawuzi, the western termination of Chinese southwestern Tianshan, eclogite and eclogitic metarodingite are commonly enclosed within ultrahigh-pressure Ti-chondrodite serpentinites (510–530 °C, 37 ± 7 kbar). This metamorphic mafic–ultramafic association, which has undergone a complex evolution from oceanic alteration to deep subduction and subsequent exhumation, presents a unique opportunity to study the incompletely understood mechanisms leading to the exhumation of ultrahigh-pressure (UHP) eclogite in UHP serpentinite.The Changawuzi serpentinite wraps three different rock types: retrograded eclogite, Rodingite I and Rodingite II. Field observations and petrological analyses indicate that the precursor of Rodingite I intruded as vein into the serpentinite protolith prior to subduction, sharing the evolution towards ultrahigh-pressure conditions. According to its composition, the serpentinite originated from a depleted oceanic mantle peridotite, with only minor enrichment in fluid-mobile elements (FME) during oceanic serpentinization. The eclogites have N-MORB like bulk compositions, with lower Si but higher Mg content compared to other eclogites in the Tianshan area, and appear to have been relatively unaffected by oceanic alteration processes. The Changawuzi serpentinite and eclogites likely evolved within a slab setting with limited interaction with seawater. Petrological studies and phase equilibria modeling suggest that the eclogites experienced peak UHP metamorphism at 28–34 kbar and around 460–465 °C. Density measurements yielded relatively low densities of 2.66–2.97 g/cm3 for the host serpentinites, but 3.27–3.63 g/cm3 for Rodingite I. The similarity in the low temperature/ultrahigh pressure conditions, as well as the geochemical characteristics shared by the serpentinite and eclogites, further support the connection between the Changawuzi serpentinites and the interior of the subducting slab. According to our data, two types of UHP eclogitic mafic rocks exhumated with UHP serpentinite. (1) High density Rodingite I veins are embedded within serpentinite and share a common evolution and exhumation history with the host serpentinite. (2) The eclogite and serpentinite tectonically detached from the interior of the subducting slab at different depths and ascended to the plate interface above the slab (subduction channel). These two rock types experienced separate exhumation processes within the subduction channel during the early stages of exhumation. Subsequently, the buoyant serpentinite captured the denser retrograded eclogite at a depth of approximately 80 km, and together they underwent further retrograde metamorphism as they ascended towards the surface. During a later exhumation stage, external fluid induced a second serpentinization process, leading to the formation of Rodingite II within the subduction channel. This study presents a novel exhumation mechanism specific to the Tianshan eclogites and highlights the significant role of deeply subducted serpentinite in the exhumation of eclogites and eclogitic metarodingites from sub-arc depths within an oceanic subduction zone.

UHP eclogite from western Dabie records evidence of polycyclic burial during continental subduction

Abstract Understanding the behavior of continental crust during subduction is important for investigating dynamic processes at convergent plate margins. Although simulations have predicted continental crust may experience multiple burial-partial exhumation cycles during subduction, petrological evidence of these cycles is scarce. In this study at Sidaohe, western Dabie, we combine microstructural observations and mineral chemistry with phase equilibrium modeling, Amp-Pl thermobarometry and Zr-in-rutile thermometry to constrain the P-T evolution for three eclogite samples. All samples have a similar mineral assemblage of garnet + omphacite + symplectite (amphibole + plagioclase ± clinopyroxene) + quartz, with accessory rutile/ilmenite. Element mapping and analytical traverses across large garnets from two samples show obviously systematic variations in Ca and, less strongly, Mg, Fe, and XMg [Mg/(Mg+Fe2+)]. Based on phase equilibrium modeling and calculated isopleths for grossular, pyrope and XMg in garnet, we show that P first increased from 23.0 to 28.5 kbar, then decreased to 24.0 kbar, before increasing again to a maximum of 30.5 kbar (±1.0 kbar, 2 sigma error) concomitant with a small increase in T from 580 to 605 °C (±20 °C, 2 sigma error) at the late prograde stage. These data are interpreted to indicate multiple burial cycles and partial exhumation of eclogite during ongoing continental subduction. After the Pmax stage, T first increased to a maximum of 664–644 °C at 25.0–20.0 kbar, then decreased to 581–561 °C (±30 °C, 2 sigma error) at 15.0–10.0 kbar based on results of Zr-in-rutile thermometry. Further decompression and cooling occurred across P-T fields of 590–567 °C at 12.0–10.0 kbar and 520–504 °C (±40 °C, 2 sigma error) at 8.0 kbar. Fine-grained symplectite (clinopyroxene + plagioclase ± amphibole) in the matrix is interpreted to have formed after omphacite due to dehydroxylation of nominally anhydrous minerals during decompression from the Pmax stage. By contrast, formation of coarse-grained symplectite (amphibole + plagioclase) and a veinlet of rutile + quartz that crosscuts one sample may be related to influx of externally sourced H2O. This study shows that: (1) evidence of cyclic burial and partial exhumation may be retained in low-T eclogite during continental subduction, and (2) fluid contributing to widespread retrogression of eclogite during exhumation may be internally and/or externally sourced.

Origin of the Miocene Adakitic Rocks and Implication for Tectonic Transition in the Himalayan Orogen: Constraints from Kuday Granitoid Porphyry in Southern Tibet

Abstract Postcollisional adakitic magmatism in the Himalayan Orogen provides a probe into the evolution of the collisional orogen. During the Miocene, the Himalayan Orogen underwent a tectonic transition, which was characterized by a series of tectonic events, including the activity of the North-South Trending Rift, exhumation of eclogite, rapid uplift of the orogen, and the extensive adakitic rocks. In this study, we reported the geochemistry and geochronological data of the Kuday dikes intruding into the Tethys Himalayan Sequence near the Sakya Dome of southern Tibet. The Kuday dikes are granitoid porphyries with zircon U-Pb ages of ca. 11 Ma. The Kuday granitoid porphyry dikes have high SiO2 (63.01–68.41 wt.%) and Al2O3 (17.31–19.87 wt.%) but low Mg (0.88–1.41 wt.%), Mg# (36–50), Ni (2.8–19.3 ppm), and Cr (2.9–26.4 ppm), indicating no input of mantle material. They have high Sr (934–1881 ppm), (La/Yb)N (18.84–113.13), and Sr/Y ratios (89.25–305.85) but low K2O/Na2O ratios (0.17–0.79), indicating that they are adakitic affinity. They display initial 87Sr/86Sr ratios of 0.707–0.711 and εNdt values of -3.7–-6.7. These geochemical signatures indicate that the Kuday granitoid porphyrite dikes were derived from the partial melting of the thickened lower crust of the Himalayan Orogen. Partial melting of the thickened lower crust requires additional heat, so the delamination model with lithospheric mantle thinning and asthenospheric upwelling is proposed to explain the formation of the Kuday adakitic rock. The delamination model can also provide a reasonable explanation for the tectonic events during the Miocene in the Himalayan Orogen.

Carboniferous high‐P metamorphism and deformation in the Belledonne Massif (Western Alps)

AbstractThe age and P–T conditions of Variscan high‐P(HP) metamorphism in the Palaeozoic basement of the western Alps remain poorly constrained, but is nevertheless crucial to build a consistent tectonic scenario for the southeastern domain of the Variscan Belt. We report here the results of a structural, petrological, thermobarometric, and geochronological investigation of an eclogite‐bearing unit exposed in the northeastern part of the Belledonne Massif (France). This unit is mostly composed of metasediments that are locally migmatized and contain decametre‐ to hectometre‐scale lenses of orthogneiss and amphibolites. SIMS U–Pb dating of magmatic zircon cores in two retrogressed ecogites yields ages at 456 ± 4 Ma and 448 ± 6 Ma, which are interpreted to date the emplacement of the magmatic protoliths. The peak pressure stage in the retrogressed eclogites is estimated to be &gt;1.4 GPa at 690–740°C, and was followed by decompression from 1.4 to ca. 1.0 GPa at 700–800°C. By contrast, the investigated migmatitic metasediment does not present any trace of HP metamorphism, but instead preserves prograde evolution from sub‐solidus conditions (ca. 0.8–1.1 GPa and 600–700°C) to supra‐solidus conditions (1.1–1.4 GPa and 700–780°C). A later stage of retrogression below ca. 0.5–0.8 GPa and 570–610°C is recorded in both lithologies, and is taken to indicate cooling and exhumation to upper crustal levels. Metamorphism was roughly coeval in the retrogressed eclogites and in the migmatitic metasediment. Metamorphic zircon rims yield U–Pb dates scattering between 340 and 310 Ma in both lithologies. In the migmatitic metasediment, a distinct younger age at 306 ± 3 Ma is interpreted to represent late stages of melt crystallization. In the retrogressed eclogites, zircon zoning and chemical composition (Th/U and REE) indicate initial crystallization of the rims during the HP stage followed by protracted growth during decompression to granulitic/amphibolitic conditions. Rutile U–Pb dating in one eclogite sample yields an age of 340 ± 11 Ma similar to the oldest zircon ages and is interpreted to approximate the age of the peak pressure metamorphism. Retrogression in the amphibolite facies is correlated with the development of a penetrative, N30° subvertical mylonitic S2 foliation. Regionally, this deformation occurs in a dextral transpressive corridor interpreted to represent a crustal‐scale shear zone active during the mid‐late Carboniferous. We therefore suggest that this structure has driven the exhumation of eclogites from the lower crust and their mixing with mid‐crustal felsic lithologies devoid of HP assemblages.

Compressional origin of the Aegean Orogeny, Greece

The Aegean Sea area is thought to be an actively extending back-arc region, north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean. The area shows extensive normal faulting, ductile ‘extensional’ shear zones and extensional S-C fabrics throughout the islands that have previously been related to regional Aegean extension associated with slab rollback on the Hellenic Subduction Zone. In this paper, we question this interpretation, and suggest the Cenozoic geodynamic evolution of the Aegean region is associated with a Late Cretaceous–Eocene NE-dipping subduction zone that was responsible for continent-continent collision between Eurasia and Adria-Apulia/Cyclades. Exhumation of eclogite and blueschist facies rocks in the Cyclades and kyanite-sillimanite grade gneisses in the Naxos core complex have pressures that are far greater than could be accounted for purely by lithospheric extension and isostatic uplift. We identify four stages of crustal shortening that affected the region prior to regional lithospheric extension, herein called the Aegean Orogeny. This orogeny followed a classic Wilson cycle from early ophiolite obduction (ca. 74 Ma) onto a previously passive continental margin, to attempted crustal subduction with HP eclogite and blueschist facies metamorphism (ca. 54–45 ​Ma), through crustal thickening and regional kyanite – sillimanite grade Barrovian-type metamorphism (ca. 22–14 ​Ma), to orogenic collapse (<14 ​Ma). At least three periods of ‘extensional’ fabrics relate to: (1) Exhumation of blueschists and eclogite facies rocks showing tight-isoclinal folds and top-NE, base-SW fabrics, recording return flow along a subduction channel in a compressional tectonic setting (ca. 50–35 ​Ma). (2) Extensional fabrics within the core complexes formed by exhumation of kyanite- and sillimanite gneisses showing thrust-related fabrics at the base and ‘extensional’ fabrics along the top (ca. 18.5–14 ​Ma). (3) Regional ductile-brittle ‘extensional’ fabrics and low-angle normal faulting related to the North Cycladic Detachment (NCD) and the South(West) Cycladic Detachment (WCD) during regional extension along the flanks of a major NW–SE anticlinal fold along the middle of the Cyclades. Major low-angle normal faults and ductile shear zones show symmetry about the area, with the NE chain of islands (Andros, Tinos, Mykonos, Ikaria) exposing the NE-dipping NCD with consistent top-NE ductile fabrics along 200 ​km of strike. In contrast, from the Greek mainland (Attica) along the SE chain of islands (Kea, Kythnos, Serifos) a SW-dipping low-angle normal fault and ductile shear zone, the WCD is inferred for at least 100 ​km along strike. Islands in the middle of the Cyclades show deeper structural levels including kyanite- and sillimanite-grade metamorphic core complexes (Naxos, Paros) as well as Variscan basement rocks (Naxos, Ios). The overall structure is an ~100 ​km wavelength NW–SE trending dome with low-angle extensional faults along each flank, dipping away from the anticline axis to the NE and SW. Many individual islands show post-extensional large-scale folding of the low-angle normal faults around the domes (Naxos, Paros, Ios, Sifnos) indicating a post-Miocene late phase of E–W shortening.

Subduction and exhumation of Luliangshan eclogite in the North Qaidam, northern Tibet: Constraints from petrology, geochemistry and phase equilibrium modelling

Eclogites in the high‐pressure (HP) and ultrahigh‐pressure (UHP) belt record important information about the subduction process and evolution history of the orogenic belt. The Luliangshan eclogites surrounded by granitic gneiss or paragneiss as lenses are exposed in the western segment of the North Qaidam UHP metamorphic belt, northwestern China. Petrology, mineral chemistry, and P–T pseudosection modelling show that the eclogites have experienced a multi‐stage metamorphic process. The peak eclogite‐facies metamorphic stage, is characterized by omphacite in matrix and as inclusion in garnet, with the peak mineral assemblages of garnet + omphacite + rutile + quartz atT &gt; 790°C andP &gt; 25.5 kbar. The initial HP granulite‐facies retrogression is characterized by the symplectite of diopside + plagioclase around omphacite, with P–T conditions of 911°C and 16.9 kbar. The subsequent amphibolite‐facies stage is characterized by amphibole + plagioclase symplectite around the clinopyroxene, with the metamorphic conditions of 674–686°C and 6.4–6.9 kbar. Zircon U–Pb analyses yielded two metamorphic age clusters: (a) HP granulite‐facies metamorphic age of 422–425 Ma, and (b) amphibolites‐facies retrograde age of 397–420 Ma. The protolith of eclogite have geochemical characteristics similar to those of normal mid‐ocean ridge basalt (N‐MORB); and the varyingεNd(t) values (−6.3 to 2.1) indicate that the Luliangshan eclogites were derived from a mantle source with rare crustal contamination. Combining these data with previous studies, a multi‐stage tectonic model can be proposed: In the Early Neoproterozoic, the protolith of the Luliangshan eclogites were emplaced into ancient continental crust; during 460–430 Ma, following the oceanic subduction, the subduction of continental crust was dragged by the oceanic slab and continue to subduct towards the Qilian Block, and metamorphosed at the depth of at least 75 km. After a long and slow exhumation process, it returned to the shallow crust.

From Decompression Melting to Mantle-Wedge Refertilization and Metamorphism: Insights from Peridotites of the Alag Khadny Accretionary Complex (SW Mongolia)

This study reports on mineral and bulk rock compositions of metaperidotites from the Alag Khadny accretionary complex in SW Mongolia, to reveal their nature and relationships with associated eclogites. The peridotites preserved original porphyroclastic textures and are composed of olivine, orthopyroxene relics, Cr-spinel, interstitial (not residual) clinopyroxene, and secondary chlorite, tremolite, olivine, Cr-magnetite, clinopyroxene, and antigorite. Cr-spinel has Cr# of 0.3–0.5, and primary olivine shows Mg# of 0.90–0.92. The pyroxenes are high-magnesian with low Al2O3 and Cr2O3. The bulk rocks have U-shaped normalized trace-element patterns with enrichment in LILE, L-MREE relative to HREE, and weak Pb–Sr peaks and Nb–Zr–Hf minima. Interstitial clinopyroxene exhibits V- and U-shaped normalized REE patterns with (La/Yb)N &gt; 1 (Yb = 1.2–3 of chondritic values) and enrichment in fluid-mobile elements and Zr. HREE abundances of clinopyroxene can be simulated by 23–26% partial melting of depleted mantle starting at garnet-facies (6–8%) depths, followed by hydrous or anhydrous melting at spinel-facies depths L-MREE characteristics of clinopyroxenes can be simulated by further interaction of harzburgites with an island-arc basaltic melt in a supra-subduction environment. The association of hydrous secondary minerals in the Alag Khadny peridotites suggests their retrograde metamorphism at 1.6–2.0 GPa and 640–720 °C, similar to P–T conditions reported earlier for the spatially associated eclogites. This supports metamorphism of the Alag Khadny peridotites in a mantle wedge, followed by joint exhumation of peridotites and eclogites. Given the findings above and implying the regional geological background, we advocate for a sequential Neoproterozoic evolution the Alag Khadny harzburgites from (1) their formation by decompression partial melting in an Early Neoproterozoic or older spreading center of a mid-ocean or back-arc setting, and (2) refertilization by supra-subduction melts, followed by (3) Late Neoproterozoic–Early Cambrian hydrous-fluid metamorphism and juxtaposition with eclogites.

Subduction–collision and exhumation of eclogites in the Lhasa terrane, Tibet Plateau

The Tibet Plateau is a key region to understand the evolution of the Tethys Oceans. The finding of Permian–Triassic eclogites and Carboniferous–Triassic ophiolites indicated the existence of a Paleo–Tethys Ocean in the Lhasa terrane, although the mechanism of subduction and exhumation mechanism of the high–pressure rocks remains equivocal. Here we focus on deformation, metamorphism, and geochronology of key rocks in the Sumdo area with a view to constrain the tectonic evolution of the Tethys Ocean and the geodynamic mechanism of eclogite exhumation. Structural analysis indicates two main stages of deformation (D1 and D2) in the study area. The D1 deformation (264–235 Ma) is dominated by penetrative Sn+1 foliation and solid rheological folds, resulted from the first type eclogites (~260 Ma), with exhumation at ~240 Ma along the subduction channel. The D2 deformation (213–180 Ma) is characterized by the tight and isoclinal overturned folds and thrust faults, related with the collisional exhumation (~200 Ma) of the second type eclogite (~230 Ma). Spreading of the Bangong–Nujiang and Indus–Yarlung Zangbo Tethys Oceans triggered the closure of Sumdo Tethys Ocean, as well as their initial subduction.

High-resolution 3-D crustal shear-wave velocity model reveals structural and seismicity segmentation of the central-southern Tanlu Fault zone, eastern China

The Early-Mesozoic collision between the North and South China Cratons resulted in different tectonic and geologic structures including the Tanlu Fault zone. One way to understand the geodynamic processes beneath these structures is by studying the velocity structures and spatial distribution of earthquakes in the region. We performed ambient noise tomography for the region around the central-southern Tanlu Fault zone using the vertical component of continuous ambient noise data obtained in central East China. Empirical Green's functions were retrieved from ambient noise cross-correlation functions, from which we extracted the Rayleigh wave phase velocity dispersion curves and performed quality control on the dispersion data. The direct surface wave tomography method was used to invert all phase velocity dispersion data for the 3-D shear-wave velocity model in the crust and uppermost mantle. Our results show velocity segmentations with high-velocity structures existing beneath orogenic belts and uplifts within the first 10 km depths which correspond to the High-Pressure/Ultra-High Pressure Metamorphic rocks in the area. The existence of high-velocity structures beneath orogenic belts at shallow depths implies that the exhumation of eclogites is confined to the uppermost crust. Generally, fault zones in this area are characterized by a low velocity from 0 to 5 km and relatively high velocity in the lower part of the upper crust and the middle crust. The spatial distribution of earthquakes along Tanlu Fault reveals two seismicity zones: the southern segment and the Suqian-Tancheng segment with a low-velocity transitional zone at Suqian, which suggests that the two segments respond differently to stresses. Regions of dense earthquake distribution correspond to the transition zones of high to low shear-wave velocities, which could be as a result of instability in the upper mantle and variation in compositions of crustal materials.

Origin of Devonian mafic magmatism in the East Kunlun orogenic belt, northern Tibetan Plateau: implications for continental exhumation

AbstractThis paper presents a comprehensive study of the zircon geochronology, geochemistry and Sr–Nd isotope geology of Devonian mafic rocks developed in the East Kunlun orogenic belt, northern Tibetan Plateau, and reveals their mantle sources, petrogenesis and geodynamic implications for continental exhumation. The zircon geochronology of typical samples indicates that these mafic rocks crystallized at 406∼408 Ma. They can be classified into two different groups based on petrographic observations and geochemical compositions. Group 1 rocks exhibit low TiO2 and FeOt contents and Nb/Y ratios and have enriched mid-ocean ridge basalt (E-MORB)-like compositions with slight negative Nb and Ta anomalies. However, Group 2 rocks have distinctly high TiO2 and FeOt contents and Nb/Y ratios, comparable to typical Fe–Ti-rich mafic rocks worldwide. All the samples exhibit weak enrichments in light rare earth elements, Nb and Ta relative to the primitive mantle. Based on geochemical and isotopic studies, Group 1 rocks are suggested to be derived from depleted asthenospheric mantle that was metasomatized by c. 3–5 % continental crustal components, while Group 2 rocks originated from partial melting of enriched lithospheric mantle. The high contents of Fe, Ti and Nb for Group 2 rocks could be attributed to a high degree of olivine crystallization under low fO2 conditions with delayed nucleation of Fe–Ti oxides. Combining those results with other geological data, we conclude that slab break-off was the key factor causing exhumation of eclogites and triggering flare-up of the Devonian magmatism, and that continental collision or continental subduction may have initiated at 431∼436 Ma.

The Exhumation of Subducted Oceanic‐Derived Eclogites: Insights From Phase Equilibrium and Thermomechanical Modeling

AbstractThe dynamical evolution and exhumation mechanisms of oceanic‐derived eclogites are controversial conundrums of oceanic subduction zones. The previous studies indicated that density is the primary factor controlling the exhumation of oceanic rocks. To explore their density evolution, we systematically investigate the phase relations and densities of different rock types in oceanic crust, including mid ocean ridge basalt (MORB), serpentinite, and global subducting sediments (GLOSS). According to the density of eclogites, these currently exposed natural eclogites can be classified into two categories: the self‐exhumation of eclogites (ρMORB &lt; ρMantle) and the carried exhumation of eclogites (ρMORB &gt; ρMantle). The depth limit for an exhumation of oceanic‐derived eclogites solely driven by their own buoyancies is 100–110 km, and it increases with the lithospheric thickness of the overriding plate. The parameters of carried‐exhumation, that is, KGLOSS and KSerp, are defined in order to quantitatively evaluate the assistance ability of GLOSS and serpentinites for carrying the denser eclogites. KGLOSS is mainly controlled by pressure, whereas KSerp is dominantly affected by temperature. Using 2‐D thermomechanical models, we demonstrate that the presences of low‐density, low‐viscosity GLOSS and seafloor serpentinites are the prerequisites for the exhumation of oceanic‐derived eclogites. Our results show that oceanic‐derived eclogites should be stalled and exhumed slowly at the Moho and Conrad discontinuities (named Moho/Conrad stagnation). We propose that oceanic‐derived eclogites should undergo a two‐stage exhumation generally, that is, early fast exhumation driven by buoyancy at mantle levels, and final exposure to surface actuated by tectonic exhumation facilitated by divergence between upper plate and accretionary wedge or by rollback of lower plate.

The East Anatolia–Lesser Caucasus ophiolite: An exceptional case of large-scale obduction, synthesis of data and numerical modelling

We present new, geological, metamorphic, geochemical and geochronological data on the East Anatolian–Lesser Caucasus ophiolites. These data are used in combination with a synthesis of previous data and numerical modelling to unravel the tectonic emplacement of ophiolites in this region. All these data allow the reconstruction of a large obducted ophiolite nappe, thrusted for >100 km and up to 250 km on the Anatolian-Armenian block. The ophiolite petrology shows three distinct magmatic series, highlighted by new isotopic and trace element data: (1) The main Early Jurassic Tholeiites (ophiolite s.s.) bear LILE-enriched, subduction-modified, MORB chemical composition. Geology and petrology of the Tholeiite series substantiates a slow-spreading oceanic environment in a time spanning from the Late Triassic to the Middle‒Late Jurassic. Serpentinites, gabbros and plagiogranites were exhumed by normal faults, and covered by radiolarites, while minor volumes of pillow-lava flows infilled the rift grabens. Tendency towards a subduction-modified geochemical signature suggests emplacement in a marginal basin above a subduction zone. (2) Late Early Cretaceous alkaline lavas conformably emplaced on top of the ophiolite. They have an OIB affinity. These lavas are featured by large pillow lavas interbedded a carbonate matrix. They show evidence for a large-scale OIB plume activity, which occurred prior to ophiolite obduction. (3) Early‒Late Cretaceous calc-alkaline lavas and dykes. These magmatic rocks are found on top of the obducted nappe, above the post-obduction erosion level. This series shows similar Sr-Nd isotopic features as the Alkaline series, though having a clear supra-subduction affinity. They are thus interpreted to be the remelting product of a mantle previously contaminated by the OIB plume. Correlation of data from the Lesser Caucasus to western Anatolia shows a progression from back-arc to arc and fore-arc, which highlight a dissymmetry in the obducted oceanic lithosphere from East to West. The metamorphic P-T-t paths of the obduction sole lithologies define a southward propagation of the ophiolite: (1) P-T-t data from the northern Sevan-Akera suture zone (Armenia) highlight the presence and exhumation of eclogites (1.85 ± 0.02 GPa and 590 ± 5 °C) and blueschists below the ophiolite, which are dated at ca. 94 Ma by Ar-Ar on phengite. (2) Neighbouring Amasia (Armenia) garnet amphibolites indicate metamorphic peak conditions of 0.65 ± 0.05 GPa and 600 ± 20 °C with a U-Pb on rutile age of 90.2 ± 5.2 Ma and Ar-Ar on amphibole and phengite ages of 90.8 ± 3.0 Ma and 90.8 ± 1.2 Ma, respectively. These data are consistent with palaeontological dating of sediment deposits directly under (Cenomanian, i.e. ≥ 93.9 Ma) or sealing (Coniacian‒Santonian, i.e., ≤89.8 Ma), the obduction. (3) At Hınıs (NE Turkey) P-T-t conditions on amphibolites (0.66 ± 0.06 GPa and 660 ± 20 °C, with a U-Pb titanite age of 80.0 ± 3.2 Ma) agree with previous P-T-t data on granulites, and highlight a rapid exhumation below a top-to-the-North detachment sealed by the Early Maastrichtian unconformity (ca. 70.6 Ma). Amphibolites are cross-cut by monzonites dated by U-Pb on titanite at 78.3 ± 3.7 Ma. We propose that the HT-MP metamorphism was coeval with the monzonites, about 10 Ma after the obduction, and was triggered by the onset of subduction South of the Anatolides and by reactivation or acceleration of the subduction below the Pontides-Eurasian margin. Numerical modelling accounts for the obduction of an “old” ∼80 Myr oceanic lithosphere due to a significant heating of oceanic lithosphere through mantle upwelling, which increased the oceanic lithosphere buoyancy. The long-distance transport of a currently thin section of ophiolites (<1 km) onto the Anatolian continental margin is ascribed to a combination of northward mantle extensional thinning of the obducted oceanic lithosphere by the Hınıs detachment at ca. 80 Ma, and southward gravitational propagation of the ophiolite nappe onto its foreland basin.

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

AbstractHigh‐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.

High‐Pressure Granulite Facies Overprinting During the Exhumation of Eclogites in the Bangong‐Nujiang Suture Zone, Central Tibet: Link to Flat‐Slab Subduction

AbstractThe geometric transformation of a descending plate, such as from steep to flat subduction in response to a change from normal to overthickened oceanic crust during subduction, is a common and important geological process at modern or fossil convergent margins. However, the links between this process and the metamorphic evolution of the exhumation of oceanic (ultra)high‐pressure eclogites are poorly understood. Here we report detailed petrological, mineralogical, phase equilibria, and secondary ion mass spectrometry zircon and rutile U‐Pb age data for the Dong Co eclogites at the western segment of the Bangong‐Nujiang suture zone, central Tibet. Our data reveal that the Dong Co eclogites experienced peak eclogite‐facies metamorphism (T = 610–630°C, P = 2.4–2.6 GPa) and underwent multiple stages of retrograde metamorphism. P‐T pseudosections and compositional isopleths of garnet define a complex clockwise P‐T‐t path (including two stages of decompression‐dominated P‐T path and one of isobaric heating), suggesting varying exhumation velocities. Combining previous studies with our new results, we suggest that the transformation from rapid to slow exhumation is dominated by the transition from steep to flat subduction. The flat‐slab segment, caused by subduction of buoyant oceanic plateau, led to an extremely slow exhumation and a strong overprinting of HP granulite facies at a depth of ~50 km at ~177 Ma. The slab rollback that followed in response to a substantial density increase of the eclogitized oceanic plateau resulted in another rapid exhumation process at ~168 Ma and triggered the formation of abundant near‐simultaneous or later magmatic rocks.

Tectonic Stacking of HP/LT Metamorphic Rocks in Accretionary Wedges and the Role of Shallowing Slab‐Mantle Decoupling

AbstractHigh‐pressure/low‐temperature (HP/LT) chloritoid‐bearing micaschists crop out widely in the central part of northern Turkey and represent deep‐seated subduction‐accretionary complexes. Three peak metamorphic assemblages are identified in the area studied: (1) garnet‐chloritoid‐glaucophane with pseudomorphs after lawsonite; (2) chloritoid with pseudomorphs after glaucophane; and (3) chloritoid with pseudomorphs after jadeite in addition to phengite, paragonite, quartz, chlorite, rutile, and apatite. The latter is interpreted as transformation of a chloritoid + glaucophane assemblage to chloritoid + jadeite with increasing pressure; PT modeling indicates ~17 and 22–25 kbars for the two peak parageneses. The diversity of peak metamorphic assemblages and the PT estimates suggest that basal accretion occurred at different depths within the wedge. The depth of the basal accretion is possibly controlled by the slab‐mantle decoupling depth. Stretching and thinning of the lithospheric fore arc induced by the slab rollback possibly caused shallowing of the slab‐mantle decoupling depth which limited depth of the basal accretion from 70–80 km to ~55 km within the subduction channel. A slab‐mantle coupling depth‐controlled basal accretion may also explain the scarcity of eclogite and high‐grade blueschist facies metamorphic rocks in active intraoceanic subduction zones. Because the overriding plate is young and hot in intraoceanic subductions, the slab and mantle are coupled at a relatively shallow depth before eclogitization of the oceanic crust. This prevents accretion and exhumation of eclogite along the subduction channel.