Early Stages Of Exhumation Research Articles

Deformation Pattern of Well-Preserved High-Pressure Rocks (SE Syros, Cyclades)

New, detailed geological/structural mapping and field-based structural analysis were carried out to investigate the deformation pattern of well-preserved high-pressure rocks of the Blueschist Unit exposed in SE Syros (Cyclades, Greece). Geological mapping revealed the occurrence of extensive alternations between different rock groups, as well as interfingering patterns in map-scale that are possibly the result of folding. The earlier ductile deformation phase recognized in the mapped area is associated with the development of a penetrative foliation, which was formed at eclogite/blueschist-facies conditions under peak metamorphism. The subsequent main deformation phase occurred under blueschist facies conditions synchronous with the early stages of exhumation of the high-pressure rocks. This phase is mainly associated with the formation of WNW-trending folds and a pervasive axial planar foliation linked with ESE-directed shearing. The main deformation ceased under blueschist-facies conditions, and exhumation of the rocks to greenschist-facies conditions took place under very weak and localized deformation. Greenschist retrogression observed in the southwestern part of the mapped area seems to be controlled by fluids, rather than by intense deformation and formation of major syn-greenschist shear zones.

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.

The fate of kyanite in ultrahigh-pressure metasedimentary rocks, southwestern Tianshan, NW China

Kyanite, an aluminous phase common in high- to medium-pressure metapelitic rocks, is critical in revealing complex tectonometamorphic histories of orogenic belts. So far it has not been documented in the well-established low-temperature – ultrahigh-pressure (LT-UHP) metamorphic belt of Chinese southwestern Tianshan. By detailed petrographic investigations, kyanite is recognized as minute inclusions or porphyroblasts in two different types of metasedimentary rocks. In omphacite-garnet phengite schists of the Habutengsu area, kyanite only forms needles associated with acicular phengite and lawsonite (pseudomorphed by paragonite and clinozoisite), and coesite (or its pseudomorph) in the extremely low-Ca core domain of well-zoned garnet. The acicular inclusion assemblage in garnet cores probably witnessed large overstepping of peak-stage reactions in metapelitic rocks in a cold subduction zone. The absence of matrix kyanite in this rock type may be ascribed to elevated Na and Ca activities during the growth of high-Ca garnet rim. This is supported by the common presence of sodic and calcic phases (e.g., paragonite and omphacite) in the matrix or the garnet rim domain, with absence in garnet core. In a suite of calcschists of the Muzhaerte area, kyanite occurs as a matrix and/or inclusion phase. In relatively fresh, less foliated chloritoid-ankerite calcschists, kyanite is a major matrix phase with coesite inclusions and locally surrounded by prophyroblasts or fine-grained aggregates of both paragonite and phengite. In altered chlorite-white mica calcschists, kyanite is rarely embedded in white mica aggregates but commonly enclosed in Fe-rich chloritoid porphyroblasts. The mantling of ankerite by acicular to prismatic Fe-rich chloritoid suggests locally increased Fe2+ activity and Al transport by fluids along grain boundaries. The common white mica corona around kyanite in the calcschists suggests elevated Na and/or K activities postdating kyanite formation. Outcrop-scale variations of kyanite textures and whole-rock alkali element contents support the infiltration of Na- and/or K-bearing fluids, probably a dominant mechanism responsible for the lack of kyanite in the Tianshan UHP metasedimentary rocks. P-T estimates suggest the kyanite corrosion took place at 15–21 kbar and 530–610 °C, corresponding to the early exhumation stage. This study suggests that kyanite may be a major peak phase of the Tianshan UHP metasedimentary rocks, and the complete understanding of its prograde evolution is a prerequisite to unravel the fate of subducted sediments and their chemical geodynamics in cold subduction zones.

Insights Into Episodic Exhumation of the Western Tibetan Plateau Since the Late Cretaceous From Low‐Temperature Thermochronology

AbstractThe western Tibetan Plateau (TP) is characterized by low relief with high elevation separated by deep river valleys. When and how this characteristic topography developed remains ambiguous. Here, we present apatite (U‐Th)/He and fission‐track ages from three groups of samples with different geomorphic and geological settings. Thermal histories and exhumation rates were extracted from the thermochronological data. An early stage of exhumation (ca. 0.1 km/Ma) during 70–40 Ma was recorded by all samples, followed by slow exhumation (0.03–0.06 km/Ma) since the Eocene for the sample farthest away from faults and incised valley. The second stage of faster exhumation (ca. 0.13–0.15 km/Ma) during 30–23 Ma was revealed by samples from the hanging wall of thrust faults, whereas the third stage of exhumation during 15–12 Ma (ca. 0.1 km/Ma) was identified from samples in the Dingzi Zangbo valley. Combined with the regional geological setting, we propose that (a) the Late Cretaceous‐Early Eocene exhumation in the western TP was related to regional thrust‐induced crustal thickening and led to the formation of the proto‐TP; (b) the proto‐TP was modified by local structures and river incision. The Oligocene exhumation might be caused by local thrust activity, whereas the Miocene exhumation might be related to the transition from internal to external drainage. The continuous activity of the Karakoram fault resulted in another reorganization of drainage which led to the slow exhumation since 9 Ma. Our results highlight that tectonic and drainage network reorganization play an important role in shaping the geomorphology of the western TP.

Multistage growth of garnet fingerprints the behavior and property of metamorphic fluids in a Paleotethyan oceanic subduction zone

Metamorphic fluids released from the subducting crust are of great importance to understand the recycling of volatile materials at convergent plate margins. Garnet in metamorphic rocks can retain chemical zonation and crystal inclusions produced during crustal subduction, providing the record of geochemical recycling in subduction zones. This paper presents a combined study of mineralogy and geochemistry for metamorphic garnets in oceanic-type eclogite from the Paleotethyan subduction zone in the Hong'an orogen, east-central China. The results show distinct zonation of major and trace elements, crystal inclusions, water contents, and oxygen fugacity in garnet. Five stages of garnet growth were recognized based on the garnet composition and phase equilibrium modelling. Growth of Stage I to IV garnets occurs during prograde metamorphism of the Paleotethyan oceanic crust from blueschist to eclogite facies along low geothermal gradients of 7.8–9.7 °C/km. Stage V garnet was formed through dissolution and reprecipitation during mineral-fluid reaction in the early exhumation stage. Garnet grains exhibit a decrease of water content from core to rim, indicating a gradual decrease of bulk water content in host eclogite during the garnet growth. Combined with the theoretical calculation based on the mineral modes and mineral water contents, it is suggested that the subducting oceanic crust would release reduced amounts of fluids during prograde metamorphism from greenschist to eclogite facies. The different garnet zones have f O 2 values varying from ΔFMQ+1.77 to ΔFMQ+3.26, suggesting that the fluids released from the subducting oceanic crust were oxidizing during subduction to peak eclogite facies and its early exhumation stage. These garnet f O2 values are correlated with the water contents of water-saturated garnet, implying that the water content of metamorphic garnet is a potential means to recover the redox state of its host eclogite. The increase of f O2 values during prograde metamorphism of the eclogite indicates the breakdown of chlorite and possible fluid influx from serpentinite, whereas the significant drop of f O2 values during the early exhumation stage indicates the breakdown of lawsonite to release the oxidized fluid. This study demonstrates that a combined study of water content, major and trace elements, and mineral inclusions in metamorphic garnet can potentially provide a new perspective into the behavior and property of fluids released from the subducting oceanic crust. • Garnet exhibits zoning in inclusions, major-trace elements, water content and oxygen fugacity. • Multiple stages of garnet growth were recognized along the P-T path of Hong'an eclogite. • Garnet water contents reflect decrease of fluid release from subducting oceanic crust. • Garnet water contents are correlated with redox conditions of its host eclogite. • Oxidized fluid is released during the dehydration of the subducting oceanic crust.

Favorable P–T–ƒO2 conditions for abiotic CH4 production in subducted oceanic crusts: A comparison between CH4-bearing ultrahigh- and CO2-bearing high-pressure eclogite

Subduction is a key element in the carbon cycle between the Earth’s surface and deep interior. In addition to pressure (P) and temperature (T), oxygen fugacity (ƒO2) is an intensive variable that can change the speciation, stability, and mobility of carbon-bearing phases in subduction zones. However, tracking redox evolution and associated carbon speciation in high-pressure metamorphic rocks from subduction zones remains challenging. In this study, we identified CH4-rich fluid inclusions in ultra-high-pressure (UHP; i.e., coesite-bearing) eclogites (the UHP-CH4 eclogite) and CO2-rich fluid inclusions in high-pressure (HP; i.e., quartz-bearing) eclogites (the HP-CO2 eclogite) from the Chinese southwestern (SW) Tianshan palaeosubduction zone, respectively. Combining the garnet-clinopyroxene oxybarometry and comprehensive thermodynamic calculations, we revealed that the ƒO2 of the UHP-CH4 eclogite decreases to a minimum of ∼FMQ − 3 at peak P–T conditions (2.8 GPa, 525 °C) and CH4 is expected to form in graphite-saturated CHO fluids. Whereas during the early-exhumation stage (2.5 GPa, 600 °C), the UHP-CH4 eclogite is predicted to buffer higher ƒO2 conditions of ∼FMQ − 0.9. In contrast, the HP-CO2 eclogite, which has a higher bulk CO2 content (∼10 wt%) and elevated Fe3+/ΣFe ratio (0.24) than the UHP-CH4 eclogite, suggesting a stronger oxidative seafloor alteration of the HP-CO2 eclogite protolith before subduction, stabilizes graphite and CO2-rich fluids at peak ƒO2 conditions between FMQ and FMQ + 1 (2.6 GPa, 500 °C). These data demonstrate that the thermal structure (P–T conditions) of the subduction zone and the composition of the slab are first-order controls on the redox evolution of subducting slabs and the changing speciation of carbon in slab fluids buffered by the metamorphic rock system. Therefore, we propose that the cold subduction of a weakly altered oceanic crust (e.g., UHP-CH4 eclogites from the SW Tianshan) provides ideal conditions for the formation of ultra-deep abiotic CH4.

Development and passive exhumation of high-pressure shear zones (Blueschist Unit, Syros): Insights from quartz and columnar calcite microstructures

Quartz and columnar calcite microstructures in two major high-pressure (HP) shear zones (Syros Island, Greece), which carry meta-igneous rocks over calcite marbles, were studied to investigate the deformation conditions associated with their development and exhumation. Quartz-rich bands within the hanging-wall of the shear zones record the deformation during the operation of the shear zones under HP conditions at slightly decreasing temperatures (from 500 °C to 450 °C). At HP conditions, the shear zones are associated with (E)SE-directed shearing under plane strain conditions, as well as with a temporal increase in strain rate of one order of magnitude (from ∼10−12 to ∼10−11). Well-preserved columnar calcite within marbles of the footwall indicates that ductile shearing along the shear zones should have abruptly ceased during the early stages of exhumation, passing to a period of relative quiescence within the stability field of aragonite at temperatures ∼430–450 °C. Progressive exhumation and cooling below 400 °C are associated with weak and distributed (E)NE-directed shearing with strain rate slower than ∼10−14. Our results from north(east) Syros indicate a change in transport direction from (W)NW-(E)SE at the deep subduction levels to (W)SW-(E)NE during exhumation. This change is linked to a change in deformation, from localized to weak and distributed revealing that the Blueschist Unit in north(east) Syros was exhumed as a nearly rigid body.

Pervasive detachment faults within the slow spreading oceanic crust at the poorly coupled Antilles subduction zone

Oceanic crust formed at slow-spreading ridges is currently subducted in only a few places on Earth and the tectonic and seismogenic imprint of the slow-spreading process is poorly understood. Here we present seismic and bathymetric data from the Northeastern Lesser Antilles Subduction Zone where thick sediments enable seismic imaging to greater depths than in the ocean basins. This dataset highlights a pervasive tectonic fabric characterized by closely spaced sequences of convex-up Ridgeward-Dipping Reflectors, which extend down to about 15 km depth with a 15-to-40° angle. We interpret these reflectors as discrete shear planes formed during the early stages of exhumation of magma-poor mantle rocks at an inside corner of a Mid-Atlantic Ridge fracture zone. Closer to the trench, plate bending could have reactivated this tectonic fabric and enabled deep fluid circulation and serpentinization of the basement rocks. This weak serpentinized basement likely explains the very low interplate seismic activity associated with the Barbuda-Anegada margin segment above.

Behavior of barium isotopes during high-pressure metamorphism and fluid evolution

Barium (Ba) and its stable isotopes are powerful geochemical tracers for the recycling of subducted materials into the convective mantle. However, this requires a good understanding of the behavior of Ba isotopes during subduction-zone fluid processes. Here we present the first integrated study on Ba isotopes for two eclogite-vein systems (Ganghe and Hualiangting) that formed at high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic conditions from the Dabie orogen, central China. The Ganghe system displays constant δ138/134Ba (−0.01‰ to 0.03‰) in the UHP eclogites but relatively high δ138/134Ba (0.17‰) in the HP vein. In contrast, in the Hualiangting system, δ138/134Ba of the UHP eclogites vary from −0.14‰ to 0.05‰, while δ138/134Ba of the multiple HP veins and the retrograded amphibolites range from −0.17‰ to 0.34‰ and from −0.11‰ to 0.46‰, respectively.The combined results suggest that Ba isotopes can be considerably fractionated during subduction-zone fluid processes. HP-UHP metamorphic dehydration of the eclogites during the early stages of exhumation can produce an isotopically heavy initial vein-forming fluid due to the preferential leaching of epidote enriched with heavy Ba isotopes from the host eclogites. However, such processes have limited effects on the bulk eclogites because the Ba-rich phengite exists as a residual phase. During vein formation, light Ba isotopes are preferentially enriched in minerals relative to the fluids, resulting in the residual fluids with high δ138/134Ba. In addition, the fluids released from the gneisses have negative δ138/134Ba which is different from the eclogites, indicating that Ba isotopes can be used to trace the source of the metasomatic fluids. This study demonstrates that the UHP metamorphic rocks and HP veins can record Ba isotope fractionation during subduction-zone metamorphism and fluid-rock interaction, and recycling of the metamorphic rocks could modify Ba isotopic composition of the mantle wedge and arc volcanics.

Formation process of Al-rich calcium amphibole in quartz-bearing eclogites from The Sulu Belt, China

AbstractAluminum-rich and Si-poor calcium amphibole [~3.9 Al atoms per formula unit (apfu) and ~5.5 Si apfu for 23 O] occur in the quartz-bearing eclogites from the Donghai area, Sulu ultrahigh-pressure metamorphic belt, eastern China. Most of the aluminous amphibole phases are retrograde products from the exhumation and hydration stage and are texturally divided into a mantle phase around a porphyroblastic garnet and a crack-filling (vein) phase of a garnet. Less aluminous amphibole occurs as symplectite phase with plagioclase after omphacite. The formation process of the aluminous amphibole in the quartz-bearing samples is discussed on the basis of the analytical data by EPMA, FIB-TEM, and EBSD.The mantle amphibole occurs between garnet and symplectite or quartz. A set of plagioclase and aegirinediopside/argirine-hedenbergite thin monomineralic bands forms at the boundary between the mantle amphibole and matrix quartz. However, these monomineralic bands do not occur at the mantle amphibole-symplectite boundary. These textural differences indicate that the recrystallization of the aluminous amphibole around garnet was controlled by significant local reactions, and the size of equilibrate domains was probably several tens of micrometers or less.The mantle amphibole is composed of inner (garnet-side) and outer (matrix-side) zones. The inner zone is compositionally homogeneous, and its atomic Al/Si value is ~0.63–0.66 and similar to that of garnet. Atomic Ca/Si value in the inner zone is also almost uniform and is generally identical to that of garnet. The outer zone exhibits a monotonic decrease in the Al/Si and Ca/Si values outward, and its composition at the outermost margin is similar to that of the symplectitic amphibole. The crack-filling amphibole has a composition similar to the inner zone of the mantle amphibole. The CPO pattern of the crack-filling amphibole is different from that of the adjacent mantle amphibole, showing that the crack-filling amphibole is cut by the mantle amphibole. The textural relationship between the mantle and crack-filling amphibole phases and their compositional characteristics imply that: (1) the mantle type is a slightly later stage product than the crack-filling type, and (2) the boundary between the inner and outer zones of the mantle aluminous amphibole probably corresponds to the initial surface of the porphyroblastic garnet. The inner zone is considered to have grown inward by simple substitution of garnet, using the tetrahedral and octahedral cations of the garnet as the basic framework. On the other hand, most of the outer zone of the mantle-type amphibole grew outward in the matrix from the initial surface of the garnet porphyroblast. The mantle amphibole shows a CPO similar to that of amphibole in the adjacent symplectite domain, suggesting that these two types of amphibole formed almost simultaneously, sharing crystallographic orientation with each other.The formation of crack-filling aluminous amphibole was probably promoted by the hydraulic microfrac-turing process at an early stage of exhumation and hydration. The mantle and symplectitic amphibole phases formation was promoted by the subsequent infiltration of metamorphic fluid. The aluminous amphibole in the SiO2 phase-bearing eclogites probably recrystallized with the formation of a localized SiO2-undersaturated reaction domain because of rapid exhumation and subsequent rapid cooling of the Sulu UHP metamorphic belt.

Insight Into the Early Exhumation of the Cycladic Blueschist Unit, Syros, Greece: Combined Application of Zoned Garnet Geochronology, Thermodynamic Modeling, and Quartz Elastic Barometry

AbstractConstraining conditions and mechanisms of the early stages of exhumation from within subduction zones is challenging. Although pressure, temperature, and age can be inferred from the exhumed rock record, it is generally difficult to derive each of these parameters from any single rock, thus demanding assumptions that diverse data from multiple samples can be safely combined into a single pressure‐temperature‐time (P‐T‐t) path that might then be used to infer tectonic context and mechanisms of exhumation. Here, we present new thermobarometric and geochronologic information preserved in a single sample from Syros, Greece, to deduce the conditions and rates of the earliest phase of exhumation as a part of the well‐preserved high‐pressure metamorphic rocks of the Cycladic Blueschist Unit (CBU). The sample studied here is a garnet‐bearing, quartz‐mica schist that records two distinct metamorphic events. Results from thermodynamic models and quartz‐in‐garnet elastic geobarometry show that metamorphic garnet cores formed as P‐T conditions evolved from ∼485°C and 2.2 GPa to 530°C and 2.0 GPa, and that garnet rims formed as conditions evolved from ∼560°C and 2.1 GPa to ∼550°C and 1.6 GPa. Sm‐Nd geochronology on garnet cores and rims yields ages of 45.3 ± 1.0 and 40.5 ± 1.9 Ma, respectively, thus indicating a 4.8 ± 2.1 Myr growth span. Given the decompression path calculated based on garnet core and rim P‐T estimates, we conclude that the distinct phases of garnet growth preserve evidence of the initial exhumation of portions of the CBU.

Anatexis of the deeply subducted continental crust during exhumation: A case study of Rongcheng migmatite in the Sulu orogen

Migmatite in the Sulu ultra‐highpressure (UHP) terrane is an ideal sample to provide information on anatexis during the exhumation of the deeply subducted continental crust. The studied migmatite is composed of the leucosome and melanosome with strongly ductile shear deformation. Petrographic observations clearly show an anatexis process, for example, cuspate and triangular K‐feldspars in triple junction and along grain boundaries, felsic veinlets and pockets mainly consisting of K‐feldspar + quartz + plagioclase ± biotite. The zircon grains of the migmatite have a core‐rim structure: the inherited magmatic core and the anatectic overgrowth rim. The inherited cores exhibit oscillatory zonings in the cathodoluminescence (CL) images and display strong heavy rare earth element (HREE) enrichment with clearly positive Ce and negative Eu anomalies. The inherited cores have an upper intercept U‐Pb age of 814 ± 45 Ma with εHf(t) values of −5.6 to ‐10.6 (mean = −8.5 ± 0.8, n = 16). Thus, protolith of the migmatite is a mid‐Neoproterozoic granitic rock. The overgrowth rims exhibit a typical anatexis genesis, that is, a bright homogeneous luminescence in the CL images, very low Th/U ratios (<0.02) with low U (<100 ppm) and Th (<1 ppm) contents, and lower total REE and Y contents with lower (Gd/Lu)N ratios and higher Hf/Y ratios than the inherited cores. They are Late Triassic with ages ranging from 211 to 234 Ma (mean = 223 ± 8 Ma), which, combined with previous studies on the Sulu UHP terrane, suggests that the anatexis occurred during the early exhumation stage. They have higher Ti‐in‐zircon temperatures ranging from 760 to 813°C (mean = 791 ± 16°C) than the pre‐anatexis zircon (244 ± 5 Ma and 739°C), suggesting a ‘hot’ exhumation. They have εHf(t) values of −17.6 to −14.5 (mean = −15.5 ± 0.6) which are lower than the estimated value (~12.0) on the crustal evolution array, suggesting an addition of crustal fluid during the anatexis. The anatexis also dramatically affected the zircon Lu–Hf isotopic system, that is, 176Yb/177Hf and 176Lu/177Hf isotopic ratios are markedly decreased but initial 176Hf/177Hf ratios are increased from the inherited magmatic cores to the anatectic rims. Thus, we propose that the anatexis plays a critical role in the exhumation of the UHP metamorphic rocks. The anatexis could lubricate the subducted slices and further facilitate their buoyant rise from the mantle depth to the crustal level.

Zircon U-Pb Dating and Petrogenesis of Multiple Episodes of Anatexis in the North Dabie Complex Zone, Central China

The North Dabie complex zone (NDZ), central China, is a high-T ultrahigh-pressure (UHP) metamorphic terrane. It underwent a complex evolution comprising of multistage metamorphism and multiple anatectic events during the Mesozoic continental collision, characterized by granulite-facies overprinting and a variety of migmatites with different generations of leucosomes. In this contribution, we carried out an integrated study including field investigation, petrographic observations, zircon U-Pb dating, and whole-rock element and Sr-Nd-Pb isotope analysis for the migmatites in the NDZ and their leucosomes and melanosomes. As a result, four groups of leucosomes have been recognized: Group 1 (garnet-bearing leucosome), strongly deformed leucosomes with coarse-grained peritectic garnet; Group 2 (amphibole-rich leucosome), weakly deformed to undeformed amphibole-rich leucosomes with coarse-grained peritectic amphibole and no garnet; Group 3 (amphibole-poor leucosome), weakly deformed to undeformed amphibole-poor leucosomes with minor fine-grained amphibole; Group 4 (K-feldspar-rich leucosome), K-feldspar-rich leucosomes mainly composed of coarse-grained quartz, plagioclase and K-feldspar. Zircon SHRIMP and LA-ICPMS U-Pb dating suggest that the Group 1 leucosomes formed at 209 ± 2 Ma whereas the rest of the leucosome groups (Groups 2–4) occurred between 145–110 Ma, in response to decompression under granulite-facies conditions during the early stage of exhumation, and to heating during post-orogenic collapse, respectively. Furthermore, the garnet-bearing leucosomes were resulted from fluid-absent anatexis related to biotite dehydration melting, while the other three groups of leucosomes were formed during large-scale fluid-present partial melting and coeval migmatization. This migmatization comes from heating from the mountain-root removal and asthenosphere upwelling, together with the influx of fluids derived from country rocks at mid-upper crustal levels. However, all the leucosomes and melanosomes display Pb-isotopic compositions similar to those observed for the NDZ UHP rocks (eclogites and granitic gneisses), suggesting a common source from the Triassic subducted Neoproterozoic lower-crustal rocks. In addition, the Cretaceous partial melting and migmatization began at 143 ± 2 Ma with three age-peaks at 133 ± 3 Ma, 124 ± 3 Ma and 114 ± 7 Ma, respectively.

Scapolite pegmatite from the Nordøyane ultra-high pressure domain, Western Gneiss Region, Norway: Partial melting driven by infiltration of mantle-derived fluid

Scapolite, a volatile-bearing framework silicate, is found in pegmatites and leucosomes spatially associated with (ultra)-high pressure ((U)HP) eclogites in the Nordøyane domain of the Western Gneiss Region (WGR), Norway. The field relationships, petrology, and stable isotope geochemistry of scapolite-bearing rocks offer insights into the role of fluids in partial melting of WGR gneisses during the Scandian orogeny. Three types of scapolite pegmatite are distinguished based on mineralogy, texture, and scapolite chemistry. The most abundant Type 1 pegmatites are associated with dioritic to granodioritic migmatites, including local diatexite, developed in dioritic to granitoid orthogneisses. They are zoned from scapolite+quartz cores to hornblende+plagioclase rims, and concentrated in low-strain areas including boudin necks between disaggregated eclogite bodies. They are interpreted as late-stage differentiates of the (grano)dioritic migmatites, which also contain accessory scapolite in leucosomes. Type 2 and 3 pegmatites also contain coarse-grained scapolite but lack hornblende-rich selvages; Type 2 scapolites are texturally and chemically distinct from other varities. Type 1 and 3 pegmatites and host rock migmatites contain meionitic scapolite with high S- and C– contents, whereas Type 2 pegmatites contain marialitic scapolite with high Cl-contents. The pegmatites crystallised at amphibolite-facies conditions, ca. 750–800 °C and 1 GPa. Scapolites in Type 1, 2, and 3 pegmatites and host rock leucosomes share similar δ13C (−7.9 to −2.2‰) and δ34S (−1.1 to 5.7‰) compositions, interpreted to indicate a common source of volatile components. Comparison with literature data suggests a mantle source for the S and C in the scapolite-forming fluids. We propose that infiltration of mantle-derived fluids into the Baltican continental slab during subduction or the early stages of exhumation drove partial melting in subducted Baltican crust.

Subduction Reversal in a Divergent Double Subduction Zone Drives the Exhumation of Southern Qiangtang Blueschist‐Bearing Mélange, Central Tibet

Abstract(Ultra) high‐pressure (HP) rocks can be exhumed rapidly by subduction reversal or divergent plate motion. Recent studies show that subduction reversal can in particular occur in a divergent double subduction zone when the slab pull of one slab exceeds that of the other, shorter one, which then experiences a net upward pull. This recent hypothesis, first proposed for Triassic HP‐rocks exposed in the central Qiangtang mélange belt in central Tibet, can explain the exhumation of (ultra) HP rocks through upward slab movement. However, this model lacks the support of kinematic evidence. In this study, based on the recognition of multiple deformational phases, we analyze the kinematics of the HP‐bearing mélange in central Qiangtang. Based on new 40Ar‐39Ar geochronology data and those collected from the literature, we present a temporal framework for the new observations. We recognize a switch in sense of shear between the prograde (D1) and exhumation (D2‐3) paths. The change of shear sense reflects the reversal from downward to upward movement of the oceanic slab below. Early D2 represents the early exhumation stage that caused retrograde metamorphism from eclogite to blueschist facies. No magmatism occurred during this period. Continued exhumation from blueschist facies to greenschist facies resulted in D2‐D3 structures. Voluminous igneous activity occurred during this stage. We suggest that subduction reversal in a divergent double subduction zone can best explain the kinematic evolution and temporal framework above. This exhumation model may provide a new perspective on the exhumation mechanism for other HP rocks around the world.

Subduction channel fluid-rock interaction: Indications from rutile-quartz veins within eclogite from the Yuka terrane, North Qaidam orogen

High-pressure (HP) or ultrahigh-pressure (UHP) rutile-quartz veins that form at mantle depths due to fluid-rock interaction can be used to trace the properties and behavior of natural fluids in subduction zones. To explore the fluid flow and the associated element mobility during deep subduction and exhumation of the continental crust, we investigated the major and trace elements of Ti-rich minerals. Additionally, U–Pb dating, trace element contents, and Lu–Hf isotopic composition of zircon grains in the UHP eclogite and associated rutile-quartz veins were examined in the North Qaidam UHP metamorphic belt, Yuka terrane. The zircon grains in the rutile-quartz veins have unzoned or weak oscillatory zonings, and show low Th/U ratios, steep chondrite-normalized patterns of heavy rare earth elements (HREEs), and insignificant negative Eu anomalies, indicating their growth in metamorphic fluids. These zircon grains formed in 431 ​± ​3 ​Ma, which is consistent with the 432 ​± ​2 ​Ma age of the host eclogite. As for the zircons in the rutile-quartz veins, they showed steep HREE patterns on one hand, and were different from the zircons present in the host eclogite on the other. This demonstrates that their formation might have been related to the breakdown of the early stage of garnet, which corresponds to the abundance of fluids during the early exhumation stage. The core-rim profile analyses of rutile recorded a two-stage rutile growth across a large rutile grain; the rutile core has higher Nb, Ta, W, and Zr contents and lower Nb/Ta ratios than the rim, indicating that the rutile domains grew in different metamorphic fluids from the core towards the rim. The significant enrichment of high field strength elements (HFSEs) in the rutile core suggests that the peak fluids have high solubility and transportation capacity of these HFSEs. Furthermore, variations in the Nb vs. Cr trends in rutile indicate a connection of rutile to mafic protolith. The zircon grains from both the rutile-quartz veins and the host eclogite have similar Hf isotopic compositions, indicating that the vein-forming fluids are internally derived from the host eclogite. These fluids accumulated in the subduction channel and were triggered by local dehydration of the deeply subducted eclogite during the early exhumation conditions.

Depositional age and provenance in the San Luis Formation, Sierras Pampeanas, Argentina: Evidence from detrital zircon studies

The San Luis Formation is one of the lithostratigraphic units constituting the early Paleozoic crystalline basement of the Sierras de San Luis, which are part of the Sierras Pampeanas of central Argentina. The low-grade metamorphic overprint of the San Luis Formation makes it a key unit for the modelling of its sedimentary environment, which belongs to a deep-marine environment. This formation crops out in two belts named as Eastern and Western, whose stratigraphic correlation has not been clarified. Five samples from both belts of the San Luis Formation show U-Pb age distributions of detrital zircons with dominant peaks that are characteristic of orogenic systems of the West Gondwanan landmasses. However, detrital zircon age spectra and maximum depositional ages differ substantially in each belt of the San Luis Formation. While the Western belt displays polymodal age patterns with its most prominent age peaks at ∼635 Ma, a second most important population appears at 535–575 Ma with subordinate groups at 720–775 Ma and 1.7–2.2 Ga; the Eastern belt shows an almost unimodal age pattern dominated by an age peak at ∼535 Ma, with very subordinate peaks at ∼620 Ma, ∼780 Ma and ∼1,017 Ma. Regarding the maximum depositional age, two different ages have been recognized in the Western belt, ∼555 Ma at its bottom and ∼530 Ma at its top; whereas in the Eastern belt the maximum depositional age is ∼515 Ma. The age spectra of detrital zircons from the San Luis Formation record the evolution of the nearby Pampean orogen. At an early stage of exhumation, the Pampean orogenic system chiefly liberated recycled sedimentary detritus containing zircons with Neoproterozoic (∼635 Ma), Mesoproterozoic (∼1,000 Ma) and subordinate amounts of Paleoproterozoic (∼2,200 Ma) ages. Upon advanced unroofing and active erosion, the Pampean sequences sourced first-cycle detritus with latest Neoproterozoic to early Cambrian ages (555–525 Ma). A major conclusion is that the San Luis Formation accumulated in at least two stages, filling successive deep-marine basins that were marginal to the Pampean orogen during and after its latest Neoproterozoic to Cambrian development.

A ductile extensional shear zone at the contact area between HP-LT metamorphic units in the Talea Ori, central Crete, Greece: deformation during early stages of exhumation from peak metamorphic conditions

We report on an extensional ductile shear zone in central Crete at the contact area between the high-pressure low-temperature (HP-LT) metamorphic Phyllite-Quartzite unit sensu stricto (PQ s.str.) and the Talea Ori group (Plattenkalk unit). Mapping and microscopic analysis along the 20 km long contact reveal extensional shear bands, shear band cleavages (C′-type) and associated quartz veins in both units, occurring over a width of up to a kilometer. The shear offset along the shear bands is systematically perpendicular to the contact with the hanging block (PQ s.str.) being downfaulted. Abundant discordant quartz veins are associated to shear band boundaries, asymmetric boudinage and foliation boudinage, forming m-wide vein networks. These mesoscopic deformation structures together with related microstructures such as strain shadows, growth rims of albite porphyroblasts and stylolites indicate dissolution–precipitation creep as main deformation mechanism accompanied by vein formation. Temperatures during deformation are indicated to be close to peak metamorphic temperatures (≥ 300–350 °C) by the growth rims of albite porphyroblasts and micas present in the shear band cleavages, which are consistent with quartz vein microstructures showing subgrains and sutured high angle grain boundaries. Peak metamorphic temperatures inferred by the degree of graphitization of carbonaceous material are similar in the hanging wall and the footwall of the shear zone, which is consistent with fast and nearly adiabatic exhumation. Our study demonstrates the importance of ductile shear zones with high strain rate dissolution–precipitation creep and vein formation in HP-LT metamorphic rocks for the early exhumation history.

Phengite megacryst quasi-exsolving phlogopite, from Sulu ultra-high pressure metamorphic terrane, Qinglongshan, Donghai County (eastern China): New data for P-T-X conditions during exhumation

A large crystal of trigonal phengite (Phe-3T; K0.96Na0.02Ba0.01)(Al1.35Mg0.40Fe2+0.11Fe3+0.13Ti0.02)Σ=2.02[Si3.44Al0.56]O10(OH)2), sampled from a phengite+quartz metamorphic vein in the eclogites-bearing rocks of Sulu UHP metamorphic terrane, exhibits unusual thin lamellae of ferrian-aluminian trigonal/monoclinic phlogopite polytypes (Phl-3T: (K0.78Na0.01)(Mg1.48Fe2+0.39Fe3+0.45Al0.30Ti0.06)Σ=2.68[Si2.98Al1.02]O10((OH)1.77F0.23); Phl-1M: (K0.85Ca0.01)(Mg1.43Fe2+0.45Fe3+0.54Al0.21Ti0.06)Σ=2.69[Si2.88Al1.12]O10((OH)1.99F0.01)). This assemblage is rarely observed in UHP eclogite-bearing rocks, and has never been reported before in the Sulu UHP metamorphic terrane. A detailed crystal-chemical characterisation of Phe-3T, Phl-3T and Phl-1M allowed the development of a thermodynamic model that estimates the P-T conditions of formation of such an assemblage and helps to understand the relationship between mica polytypes and a quartz dominated (silica oversaturated) system, which characterised the Sulu vein. Phe-3T varies in composition upon approaching Phl-3T/1M, showing a decrease of Si and Al and an increase of Fe and Mg contents. Observed phlogopites cannot be ascribed to bare exsolving processes, and require the involvement of a non-isochemical reaction, dominated by exsolution (quasi-exsolution process) in combination with a contribution from an “external chemical supplier” (most probably provided by circulating fluids). Mass balance calculations between the mineralogical assemblage of Sulu vein and the solute components of alkali-alumino-silicate high pressure fluids demonstrate that a silicic-type major element composition accounts for a phengite-quartz based rock. The occurrence of phlogopite micas can be explained as an effect of disproportion in combination with supply components, i.e. Mg, Al and K, from alkali-alumino-silicate high pressure fluids.On the basis of energy modelling of the reaction process from phengite to phlogopite in presence of quartz and a matter exchanger, we determined that such a transformation is weakly dependent on T over the 500–930 °C thermal range. The intersection of the calculated P-T assemblage equilibrium curve with the country rock P-T metamorphic path occurs at P ~ 2.4 ± 0.2 GPa and T ~ 700 °C, thus reflecting an early stage of exhumation, close to the metamorphic peak of the Sulu UHP terrane.

Short-lived polyphase deformation during crustal thickening and exhumation of a collisional orogen (Ribeira Belt, Brazil)

The Ribeira Belt (Brazil) is a Neoproterozoic collisional-related feature that was located in a south-central position in West Gondwana. We present quantitative data on finite strain, flow vorticity and deformation temperatures for the Curitiba Terrane, a major segment of the southern Ribeira Belt. Six deformation phases (D1-D6) related with crustal thickening and exhumation were recognized. D1 and D2-related microstructures are preserved exclusively within porphyroblasts, in part grown during stages of high-pressure (∼9–12 kbar) isobaric heating after crustal thickening. D3 phase was active from peak metamorphism attained in contrasting crustal levels (810–400 °C), to the early stage of exhumation (500–400 °C), as indicated by petrological, microstructural and quartz c-axis fabric evidence. Kinematic vorticity results indicate that the SL3 mylonitic fabric resulted from a simple shear-dominated deformation related with westward thrusting. North-verging overturned D4 folds with E-W-trending subhorizontal axes derived from a pure shear-dominated deformation. Regional D5 open folds with subvertical axes and NNE-SSW-trending traces were produced by indentation tectonics. D6 phase comprises retrograde orogen-parallel transcurrent shear zones related with scape tectonics. Geochronological data indicate that D3-D6 phases occurred between 584 and 580 Ma, suggesting a fast exhumation rate of ∼8 mm/year for the deepest rocks from the southern Ribeira Belt.