Open AccessArticle

The Discovery of the New UHP Eclogite from the East Kunlun, Northwestern China, and Its Tectonic Significance

Feng Chang

Guibin Zhang

^* and

Lu Xiong

The Key Laboratory of Orogenic Belts and Crustal Evolution, MOE, School of Earth and Space Sciences, Peking University, Beijing 100871, China

Author to whom correspondence should be addressed.

Minerals 2024, 14(6), 582; https://doi.org/10.3390/min14060582

Submission received: 1 May 2024 / Revised: 28 May 2024 / Accepted: 29 May 2024 / Published: 31 May 2024

(This article belongs to the Special Issue Microbeam Analysis Characterization in Petrogenesis and Ore Deposit)

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Abstract

The East Kunlun Orogenic Belt (EKOB), northwestern China, recording long-term and multiple accretionary and collisional events of the Tethyan Ocean, belongs to a high-pressure to ultra-high-pressure (HP-UHP) metamorphic belt that underwent complex metamorphic overprinting in the early Paleozoic. In this contribution, we carry out an integrated study, including field investigations, petrographic observations, whole-rock analyses, zircon U-Pb dating, and P-T condition modeling using THERMOCALC in the NCKFMASHTO system for the eclogites, especially for the newly discovered UHP eclogite in the eastern part of EKOB. The eclogites exhibit geochemistry ranging from normal mid-ocean ridge basalt (N-MORB) to enriched mid-ocean ridge basalt (E-MORB). Zircons from the eclogites yield metamorphic ages of 416–413 Ma, indicating the eclogite facies metamorphism. Coesite inclusions in garnet and omphacite and quartz exsolution in omphacite and pseudosection calculation suggest that some eclogites experienced UHP eclogite facies metamorphism. The eclogites from the eastern part of EKOB record peak conditions of 29–33 kbar/705–760 °C, first retrograde conditions of 10 kbar at 9.5–12.5 kbar/610–680 °C, and second retrograde conditions at ~6 kbar/<600 °C. New evidence of the early Paleozoic UHP metamorphism in East Kunlun is identified in our study. Thus, we suggest that these eclogites were produced by the oceanic crust subducting to the depth of 100 km and exhumation. The presence of East Gouli and Gazhima eclogites in this study and other eclogites (430–414 Ma) in East Kunlun record the final closure of the local branch ocean of the Proto-Tethys and the evolution from subduction to collision.

Keywords:

ecolgite; coesite; ultra-high-pressure (UHP) metamorphism; East Kunlun

1. Introduction

Convergent plate margin, an essential subject of plate tectonics theory, corresponds to the subduction zone at the depth of the lithospheric mantle and the orogenic belt at the depth of the crust [1,2,3]. Generally, an orogenic belt records evidence of various stages during the Wilson cycle, thus, study of the typical basaltic composition capable of tracking changes in both temperature and pressure is essential for reconstructing the local orogenic processes. For instance, eclogite has undertaken high-pressure (HP) to ultra-high-pressure (UHP) metamorphism and primarily consists of omphacite and garnet. The formation of eclogite can track changes in the geodynamics of orogenic processes [4,5,6], and overprint the retrograde metamorphism during the subsequent post-collision stage. At the initial establishment of plate tectonic theory, it was believed that only mafic oceanic crust could subduct to the depth of the mantle, while the continental crust with a low density could only collide and thicken. It was not until coesite and diamond were successively identified in the continental collision zone that the deep continental subduction with UHP metamorphism was confirmed [2,7,8]. From a geodynamic perspective, HP-UHP eclogites raise two factors related to the mechanisms of their burial of protolith and exhumation to the surface [9]. In most scenarios, HP-UHP rocks are associated with the subduction of oceanic crust [10,11] or continental basalt [12,13,14]. By contrast, their exhumation is also linked to the late stages of oceanic subduction, during the transition from oceanic subduction to continental collision [15,16]

The East Tethys tectonic domain is one of the three major tectonic domains (East Tethys, Paleo-Asian Ocean, and Pacific Ocean tectonic domain) closely related to the formation and evolution of the Chinese continental crust [17,18]. The East Tethys domain has three evolution stages, including the Proto-Tethys Ocean, Paleo-Tethys Ocean, and Neo Tethys Ocean. In the northern margin of Qinghai–Xizang Plateau, two HP/UHP metamorphic belts associated with Proto-Tethys evolution have been identified: the North Qilian Orogenic Belt and northern Qaidam metamorphic, formed in oceanic and continental subduction settings, respectively [19,20,21,22,23].

Recently, another HP/UHP metamorphic belt was identified along the East Kunlun Orogenic Belt [20,24,25,26,27,28,29,30,31], in particular the UHP metamorphic records identified from not only the eclogite, but also the gneiss from Kehete area. Despite the East Kunlun metamorphic belt being a response to the final closure of the branch ocean of the Proto-Tethys Ocean in East Kunlun areas, different geologists have different views on scientific issues of eclogite, such as whether the protoliths of eclogite are continental or oceanic basalt, which orogeic stage eclogite produces, and whether the peak metamorphism is high pressure or ultra-high pressure. Furthermore, the specific closure, the orogenic stage in which eclogite is produced, and the exhumation mechanism are still controversial. Mo et al. [32] restricted oceanic spreading and subduction to 579–518 Ma and 508–450 Ma, respectively. Similarly, Fu et al. [33] suggested continental collision with the duration of 450–426 Ma. In addition, other researchers suggested that continental subduction terminated no earlier than 426–430 Ma. Thus, these different results hinder our understanding of the tectonic evolution in EKOB during the early Paleozoic. To address these questions, in this study, we conducted a detailed petrological investigation of the eclogites occurring in the Gouli–Gazhima area (Figure 1), the eastern part of the East Kunlun Orogenic belt, combined with whole-rock major, trace elements, and metamorphic P-T estimations by THERMOCALC software, zircon geochronology. Our aims are to reveal their protoliths, metamorphic P-T conditions, U–Pb ages, and tectonic significance, which, in turn, help to better understand the tectonic evolution, especially the Proto-Tethys evolution, of the East Kunlun Orogenic Belt.

2. Geological Background

2.1. Regional Geology and Tectonics

The East Kunlun Orogenic Belt (EKOB), located in the northeast of Qinghai–Xizang Plateau (Xizang: new official name used to replace Tibet), is an import component of the Central China Orogenic Belt (Figure 1a). It extends for over 1500 km, distributing between the Qaidam Basin and Bayan Har Block, separated from the West Kunlun Orogenic Belt (WKOB) and Qinling Orogenic belt by the Altun Fault and Elashan, respectively [34,35,36]. The East Kunlun orogenic belt is characterized by multi-cycle tectonic–magmatic events [37], corresponding to the formation and growth of the continental crust in the EKOB and the evolution of the Proto-Tethys, Paleo-Tethys, and Neo-Tethys in the region. In a new tectonic architecture from [38], the EKOB consists of three mélange belts: the Qimantagh–Xiangride mélange zone (QXM), the Aqikekulehu–Kunzhong mélange zone (AKM), and the Muztagh–Buqingshan–Anemaqen ophiolitic mélange zone (MBAM). The QXM and AKM mélange belts are the suture boundaries of Proto-Tethys and respond to the island arc and SSZ ophiolite, respectively. The MBAM belt recorded not only the closure of Proto-Tethys, but also the Paleo-Tethys Ocean in the East Kunlun area. The three mélange belts partition the EKOB into four fault-bound tectonic domains: the North Qimantagh Belt (North Kunlun Belt), the Central Kunlun Belt, the South Kunlun Belt, and Bayanhar Terrane (Figure 1b).

2.2. Local Geology

The study eclogites, located in the eastern part of EKOB, were identified in the Langmuri-Gazhima area [28], Reshui Township, Dulan County, andQinghai Province (Figure 1b,c). The exposed strata in this area include the Paleoproterozoic Jinshuikou Group and metamorphic basement of the Mesoproterozoic Xiaomiao Formation (Figure 1c and Figure 2a). The Jinshuikou Group, having undergone medium- and high-grade metamorphism, is regarded to be the oldest metamorphic basement rocks exposed in the EKOB [39,40], which mainly consists of gneiss and schist, along with amphibolites, granulites, and eclogites [41]. The Jinshuikou Group has Paleoproterozoic–Mesoproterozoic formation ages and underwent the Neoproterozoic and Paleozoic metamorphic events [42]. The magmatic rocks in this area mainly consist of Late Ordovician granite and Caledonian Indochinese granodiorite (Figure 1c). Eclogite blocks in this area are scattered within the granitic gneiss of the Paleoproterozoic Jinshuikou Group. Near the study area, Wenquan HP eclogite and Kehete UHP eclogite were recognized in [20,24,26,43]. These eclogites in the eastern part of EKOB are generally distributed in the E-W direction, consistent with the faults, suggesting that they are controlled by the regional tectonic framework and could have been incorporated into the matrix/wall rock during exhumation.

In this study, eclogite samples were collected from the East Gouli position (Figure 1c and Figure 2b,c) and Gazhima position (Figure 1c and Figure 2d,e), named “E-Gouli-number” and “LMG-number”, respectively. We give preference to eclogite samples with mild degeneration, a higher density, and greenish color in the field, because strongly retrograde eclogites, especially those with amphibolite facies metamorphic overprinting, exhibit a deep black color on behalf of the consumption of green omphacite and pink garnet.

3. Analytical Methods

3.1. Mineral Chemistry

Mineral major elements were analyzed using a JEOL JXA 8230 electron microprobe (EMPA, JEOL Corpiration, Tokyo, Japan) at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences (SESS), Peking University (PKU), Beijing, China. The operating conditions were set at a 15 kV acceleration voltage, 10 nA specimen current, 10–20 s measuring time, and 1–2 μm beam size. The final results were calibrated using the calibration methods and reference materials followed by [44]. Representative results of the minerals are listed in Tables S1 and S2.

3.2. Whole-Rock Major and Trace Elements

The samples were washed and trimmed to remove weathered surfaces, and then powdered in an agate mill to 200 mesh for major and trace elemental analyses. The sample powders were mixed with lithium tetraborate at a 1:10 ratio, and then fused at 1150 °C in a Pt alloy crucible. The whole-rock major element contents for metabaistes were determined by X-ray fluorescence (XRF) after fusing into glass discs at SESS, PKU. Trace elemental compositions were analyzed using an inductively coupled plasma mass spectrometer (ICP–MS; Thermo Scientific iCAP RQ, Waltham, MA, USA) at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, SESS, PKU. Chinese Geochemical National standards (GSR-1 GSR-2, GSR-11, and BCR-2) were used to calibrate the contents of the major and trace elements. The detailed analytical protocol is described by [29]. Analytical uncertainties were better than ±2% for the major elements and ±5% for the trace elements. Repeated analyses yielded consistent results for the same samples. All major and trace element data are listed in Table S3.

3.3. Zircon U-Pb Dating, Trace Element Analyses

Zircon was separated via standard density and magnetic separation techniques. The clearest, least-cracked zircon grains from East Gouli-08 and LMG-06 were hand-picked and mounted in epoxy resin under a binocular microscope and polished to expose the cores of the grains. The internal structure of the zircons in the polished mounts was examined using cathode luminescence (CL) techniques with a field emission gun environmental scanning electron microscope (QUANTA-200F) equipped with a Garton Mono CL3+ spectrometer. The CL images were acquired with a 2 min scanning time at an accelerating voltage of 15 kV and beam current of 120 nA at the School of Physics, PKU.

Zircons U-Pb dating and trace element analyses were carried out at the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, SESS, PKU, using a Thermal iCAP RQ ICP-MS equipped with a GeoLas 193 nm laser ablation system with an in-house sample cell. A 32 μm laser diameter spot method was used, and laser fluence and repetition rates were set at 8.5 J·cm⁻² and 10 Hz. The instrumental mass bias and U/Pb fractionation were corrected using the standard zircon 91,500 [45]. Data reduction was performed using the iolite software. The correction of common Pb and all concordia plots and weighted mean age calculations were obtained using IsoplotR [46]. The elemental contents were calibrated with NIST SRM 610 glass as an external standard and ²⁹Si as an internal standard. The analytical results are listed in Table S4.

3.4. Raman Spectrum

The Raman spectrum analysis for the identification of mineral (inclusion) species was conducted using a HORIBA Jobin Yvon confocal LabRAM HR Evolution micro-Raman system installed at SESS, PKU. The system was equipped with a frequency-doubled Nd: YAG green laser (532.06 nm), a 100× short-working distance objective, and a stigmatic 800 mm spectrometer with a 600 groove/mm grating. The laser power was 100 mW at the source. The confocal hole was set at 100 μm and the corresponding spectral resolution was ±0.7 cm⁻¹. During the experiments, Raman spectra between 100 and 1500 cm⁻¹ were recorded, with a continuing time of 5 s.

4. Results

4.1. Sample Petrography

The surrounding rock (gneiss of Paleoproterozoic Jinshuikou Group) is medium-coarse granular with a columnar blastic texture and gneissic-like structure (Figure 2a), which is mainly composed of garnet, biotite, muscovite, feldspar, and quartz with minor accessory minerals (rutile, sphene, and zircon) (Figure 3a–c).

The E-Gouli eclogites (Figure 2b,c) are hypidiomorphic granular and consists of garnet (35–40 vol.%; 0.5–3 mm), ompahcite (20–25 vol.%; 0.3–1.5 mm), symplectite (25–30 vol.%), quartz (<10 vol.%; 0.5–2.0 mm), and minor accessory minerals (dominated by rutile and zircon). Garnet is subhedral and fragmented, and contains some mineral inclusions, such as quartz and rutile (Figure 3d). The omphacite grains are light green and distributed in the matrix in the form of short columns or plates (Figure 3d). The symplectite in the E-Gouli eclogite occurs as (1) fingerprint-like symplectite distributed between the garnet and omphacite in a matrix (Figure 3d) and (2) narrow corona symplectite with a light green color surrounding the garnet (Figure 3e,f). In the scanning electron microscope micrograph, fingerprint-like symplectite composed of plagioclase and clinopyroxene can be seen, representing the high-pressure granulite facies metamorphic retrogression after the eclogite facies of omphacite + garnet. The corona symplectite surrounding the garnet grain is composed of amphibole + plagioclase.

The LMG eclogites (Figure 2d,e) have a granuloblastic and massive structure and mainly consist of garnet (45–50 vol.%; >1 mm), ompahcite (30–35 vol.%; 0.5–3 mm), symplectite (10–15 vol.%), and quartz (<5 vol.%; 0.5–2 mm). Garnet has an isometric granular texture with a larger grain diameter of up to 1 mm (Figure 3g). Omphacite has an idiomorphic granular texture and includes polycrystalline quartz inclusion with radial cracks (coesite pseudomorph), indicating the previous coesite-bearing eclogite facies metamorphism (Figure 3h,i). The symplectite in the LMG eclogite is composed of plagioclase and clinopyroxene without the presence of amphibole, implying that the LMG eclogite underwent finite high-pressure granulite facies overprinting and the garnet and omphacite were well preserved (BSE images in Figure 3g,h).

4.2. Mineral Chemistry and Assemblage

4.2.1. East Gouli Eclogite (E-Gouli-08)

Garnet occurs as cracked, anhedral grain, which is mainly composed of almandine (42.4–45.7 mol.%), grossular (34.4–35.7 mol.%), pyrope (19.1–21.4 mol.%), and minor spessartine (<1 mol.%). The zonation characteristics of garnet components are weak, and no obvious color zonation can be seen under a polarizing and electron microscope. Garnet grains are characterized by a gradual decrease in X_Grs [Ca/(Ca + Mg + Fe + Mn)] and X_Prp [Mg/(Ca + Mg + Fe + Mn)] from core to mantle (0.36 to 0.34 and 0.22 to 0.19, respectively) (Figure 4a,b). Nevertheless, garnet is nearly homogeneous in composition from mantle to rim. Omphacite has Al₂O₃ contents of 10.5–15.9 wt.%, FeO of 2.5–3.13 wt.%, and its Na₂O content is also relatively high with a jadeite proportion (X_Jd) of 34.8–41.1 mol% (Figure 4d). Clinopyroxene can only be observed under an electron microscope, representing high-pressure granulite-phase retrograde metamorphism after eclogite-phase metamorphism. Clinopyroxene has lower aluminum and Na contents compared to omphacite (X_Jd < 16 mol%, X_Jd = Na/(Ca + Na + Fe³⁺)), but higher Mg contents (15.1–17.52 wt.%). In the pyroxene classification diagram [47], clinopyroxene is classified as augite (Figure 4e; Table S1). Plagioclase occurs as symplectite in a matrix, composed of albite (48–90 mol%) and anorthite (10–51 mol%), as well as trace amounts of anorthite end-member composition (<1 mol%). In the feldspar classification diagram [48], except for two results falling into the labradorite region, all other samples fall into the oligoclase region (Figure 4f; Table S1).

Based on petrographic observations, it can be roughly confirmed that the three mineral assemblages of East Gouli eclogite are as follows: (1) Eclogite Facies: characterized by garnet, residual omphacite, rutile grains, and quartz inclusions in garnet; (2) Retrograde High-Granulite Facies: characterized by fingerprint-like symplectite in the matrix and clinopyroxene (classified as augite) + plagioclase (+rutile) assemblage; and (3) Retrograde Amphibolite Facies: representing further degeneration, characterized by narrow green symplectite formed around the garnet, it is a combination of amphibole and plagioclase.

4.2.2. Gazhima Eclogite (LMG-06)

The garnet of the LMG-06 eclogite is nearly homogeneous in composition, mainly consisting of almandine (39.8–41.42 mol.%) and grossular (30.2–30.5 mol.%), as well as pyrope (27.5–29.1 mol.%) with minor spessartine (<1 mol.%) (Figure 4a,c). Compared to the garnet from E-Gouli-08, it has a relatively lower proportion in almandine and a higher proportion in pyrope, which also indicates that the LMG-06 eclogite has a higher temperature and pressure record than E-Gouli-08 and underwent a homogeneous effect (Figure 4c). Omphacite occurs as the inclusion and grain in a matrix. It shows Al₂O₃ values of 10.5–12.3 wt.% and FeO values of 2.11–2.83 wt.%. Omphacite in LMG-06 generally has higher jadeite contents (38.1–43.4 mol %) than omphacite in E-Gouli-08 (Figure 4d). Clinopyroxene in symplectite has lower Na₂O contents (X_Jd = 6.0–19.0 mol%) and higher MgO and CaO contents than omphacite and belongs to diopside in a classification diagram (Figure 4e; [47]). Plagioclase occurs as symplectite in a matrix, composed of albite (70.0–91.0 mol%) and anorthite (9.0–31.0 mol%), as well as trace amounts of anorthite (≤1 mol%), except for one result with a value of An₅₅Ab₄₅. In the feldspar classification diagram [48], most of the samples fall into the oligoclase region (Figure 4f; Table S2).

Compared with the E-Gouli-08 eclogite, the LMG-06 eclogite has undergone more slight retrogression, and, thus, shows no obvious signs of amphibolite facies metamorphism. Based on the pseudomorph of coesite, it is preliminarily judged that the LMG eclogite from Gazhima has reached ultra-high-pressure metamorphism and undergone less retrogression than the E-Gouli eclogite (Figure 3h). Therefore, only two mineral assemblages can be identified: (1) conjectural ultra-high-pressure eclogite facies (UHP EC facies): garnet+ residual omphacite grain + rutile grain + coesite; and (2) high-pressure granulite facies regression: characterized by a symplectite assemblage surrounding the mineral edge, consisting of low-sodium clinopyroxene (mineralogically classified as diopside) + plagioclase (+rutile).

4.3. Whole-Rock Compositions

4.3.1. East Gouli Eclogite (E-Gouli)

The East Gouli eclogites (E-Gouli) have SiO₂ of 46.63–49.57 wt.%, TiO₂ of 1.21–2.55 wt.%, and a loss of ignition (LOI = 1.49–2.64 wt.%) (Table S3). They are low-K and sub-alkaline in composition, with slight variations in alkalis (1.33–2.45 wt.%) and notable variations in both Mg# (38–47.76) and Al₂O₃ contents (15.53–19.43 wt.%). In the classification diagrams of SiO₂ vs. (K₂O + Na₂O) and Zr/Ti vs. Nb/Y [49,50,51], the East Gouli eclogites plot in the field of basalt (Figure 5a,b). In the Ti vs. V diagram by Shervais [52], three samples plot in the MORB field, the boundary of MORB and OIB, and the boundary of MORB and OFB (ocean flood basalt), respectively (Figure 5c). All samples belong to the tholeiitic series (Figure 5d).

The East Gouli (E-Gouli) eclogites contain total rare earth elements (REE) contents of 54.49–106.79 ppm, LREE contents of 42.87–90.82 ppm, and HREE contents of 12.00–19.05 ppm (Table S3). The chondrite-normalized REE patterns are sub-parallel, show negative Eu anomalies (close to 0.9), and a slight enrichment in LREE relative to HREE ((La/Yb)_N of 1.03–3.24). This places the patterns of East Gouli above the literature values for N-MORB (Figure 6a). On a primitive mantle-normalized diagram, the East Gouli eclogites show enrichments in U, Ta, and Pb, and depletions in Ba, Sr, Nb, Zr, and Th, roughly coinciding with the eclogites with only minor plagioclase and abundant accessory minerals. The contrary tendency of Nb and Ta is controlled by the rutile. The patterns of the study samples distribute between the N-MORB and OIB and partly overlap with E-MORB, indicating the enriched metasomatic mantle source (Figure 6b).

4.3.2. Gazhima Eclogite (LMG)

The Gazhima eclogites (LMG) have no obvious major element differences compared with the Gouli samples, except for the narrow variations in LOI (0.9–2.24 wt.%). The Gazhima eclogites have SiO₂ of 44.52–48. 70 wt.%, TiO₂ of 1.22–1.73 wt.%, Al₂O₃ of 12.67–16.8 wt.%, and alkalis of 2.21–3.95 wt.% (Table S3). According to the plots in Figure 5, the Gazhima eclogites belong to the tholeiitic series basalt and MORB field. The lower LOI contents of the Gazhima eclogites represent the less-hydrated minerals, which is associated with the slight retrogression or dominant anhydrous mineral in retrogression (e.g., clinopyroxene and plagioclase), rather than the abundant hydrated minerals formed in amphibolite facies.

The total REE contents of the Gazhima eclogites (LMG) range from 33.10 to 71.04 ppm, LREE’s from 23.69 to 55.51 ppm, and HREE’s from 9.64 to 18.23 ppm (Table S3). The REE distribution patterns of the eclogites are close to a parallel distribution and have almost no Eu anomalies (δEu = 0.92–1.19), consistent with the petrographic characteristics of the Gazhima eclogite, which is generally fresh and difficult to observe plagioclase. LREE is slightly enriched compared to HREE, with a (La/Yb)_N of 0.98–2.05. The distribution ranges of the Gazhima eclogites are lower than the OIB line, and most samples have a similar distribution trend to E-MORB (Figure 6a, [53]).

On a primitive mantle-normalized diagram, the LMG eclgoties show enrichments in Ba, U, Ta, and Pb, and depletions in Nb, Zr, and Hf. The Sr element has no obvious distribution characteristics from slightly depleted to slightly enriched, which may be due to the varying degrees of feldspar contents in the selected LMG samples (Figure 6b).

4.4. Zircon Geochronology and REE Patterns

The E-Gouli-08 eclogite from the East Gouli position and the LMG-06 eclogite from the Gazhima position in the eastern part of EKOB were selected for LA-ICP-MS zircon U-Pb dating and the results are listed in Supplementary Table S4.

4.4.1. East Gouli Eclogite (E-Gouli-08)

The zircon grains from the E-Gouli-08 sample are 80–180 μm long with an aspect ratio of 1:1–3:1. Most grains are subhedral to weakly elongated in shape, with variegated or misty luminescence characteristics (Figure 7a), representing the low contents of trace elements such as U, REE, and Th [54], a few mineral inclusions, and low Th/U ratios of 0.09–0.15 (Table S4). A few grains have narrow bright rims, which may be related to later retrogression or fluid metasomatism. While the bright rims are too narrow for laser ablation, we could not confirm the later age records from the rim. These luminescence characteristics of zircon from the E-Gouli-08 sample are common in metamorphic zircons from other eclogites in East Kunlun [29,30,43]. Twenty-seven analyses of the E-Gouli-08 eclogite zircon grains yielded a concordia age of 412.9 ± 2.5 Ma (MSWD = 0.29), slightly younger than the weighted-mean age of 414.0 ± 2.3 Ma (MSWD = 2.1; Figure 8a,b; Table S4). The zircons from the E-Gouli-08 eclogite show no or a slightly negative Eu anomaly, lower LREE contents, and flat HREE patterns (Figure 8c; Table S4), representing zircon growth under the condition of the absence or a little amount of plagioclase and the coexistence of garnet [55,56].

4.4.2. Gazhima Eclogite (LMG-06)

The zircon grains from LMG-06 are short prismatic to anhedral and colorless, with an aspect ratio of less than 2:1 and a grain size from 70 to 200 μm. The zircon grains exhibit nebulous or fir leaf zonal luminescence characteristics (Figure 7b) with low Th/U ratios of 0.01–0.03 (Table S4). The zircons in LMG-06 do not have narrow bright rims due to the lower influence of later retrogression or fluid metasomatism. The zircons have less inclusions, and the off-white luminescence characteristics are associated with the lower contents of REEs, U, and Th on account of growth during the metamorphic stages. No core-rim structure is identified in the zircons, which represents a complete metamorphic reaction with a high degree of eclogite facies metamorphism. Thirty-nine analyses of LMG-06 eclogite yield a concordant or nearly concordant age of 415.0 ± 1.6 Ma (MSWD = 1.9) and an approximately equal weighted mean age of 416.1 ± 3.2 Ma (MSWD = 3.2; Figure 8d,e; Table S4). Compared with the zircons from E-Gouli-08, the zircons from the LMG-06 eclogite have flatter HREE partitioning models and no Eu anomalies (Figure 8f; Table S4), thus, the zircons grew in an environment containing garnet without plagioclase.

Additionally, we found in situ garnet inclusion within the zircon under the scale of a rock slice (Figure 9) for the first time compared with the previous identifications of eclogite facies minerals inclusions within zircon epoxy resin disc samples from EKOB.

4.5. Metamorphic P-T Calculations

In order to obtain more specific temperature and pressure calculations (P-T conditions), we selected appropriate samples and conducted phase equilibrium simulation calculations. Due to the identification of coesite pseudomorph, the largest jadeite component of omphacite, and the mildest degree of retrogression in the LMG-06 sample, this indicated that this sample was relatively fresh and closer to the original eclogite. Therefore, this sample was selected for phase equilibrium simulation to calculate the metamorphic P-T calculations. In a typical figure of phase equilibrium modelling, the rock composition with the calculated phase relations infers the P-T conditions in equilibrium. The mineral assemblages and mineral compositions reflect the evolution of the rock, which is associated with the burial and exhumation of a metamorphic rock [57].

Phase equilibrium modeling for the LMG-06 sample was performed in the NCKFMnMASHTO system (Na₂O-CaO-K₂O-FeO-MnO-MgO-Al₂O₃-SiO₂-H₂O-TiO₂-Fe₂O₃) using THERMOCALC 3.45 [58]. The internally consistent thermodynamic dataset ds62 was chosen [57]. Mixing models were used for clinopyroxene, amphibole and melt [59], garnet, orthopyroxene, mica and biotite [60], ilmenite [61], plagioclase [62], and epidote [57]. Quartz, coesite, rutile, and aqueous fluid (H₂O) all belong to pure phases. The bulk-rock compositions were taken from XRF analyses and normalized. In phase equilibrium simulations, the calculation of the effective bulk-rock composition and water content employs the mass balance method [63,64], which involves the weighted summation of the edge components in garnet and the average components of various matrix minerals. The H₂O content for the LMG-06 eclogite from Gazhima was also determined by normalized calculations in efficient bulk-rock compositions so that the final-phase assemblages were just stable above the solidus. All P₂O₅ was removed and the total CaO was adjusted proportionally to consider the chemical contribution of apatite. The bulk rock composition for the eclogite determined by XRF with the form of weight percentage for each oxide is presented in Table 1, and the effective bulk-rock compositions for constructing a pseudosection were normalized by molar proportion according to the NCKFMnMASHTO system.

The P-T pseudosection calculated for the LMG-06 sample is drawn over the window of 5–35 kbar and 500–900 °C in Figure 10a with the isopleths of X_Grs [=Ca/(Fe^{2 +} +Mn + Mg + Ca)] and x (cpx/omp) = Fe²⁺/(Fe²⁺ + Mg) in omphacite/clinopyroxene for relevant mineral assemblages and involved eclogite facies, high-pressure granulite facies, and granulite facies (Figure 10b). The fluid-absent solidus appears at temperature of ~730 °C. The conversion lines of muscovite-biotite and rutile- ilmenite may occur at 16–17 kbar and <~9 kbar, respectively.

The first mineral assemblage identified through petrography observation (Stage 1) includes garnet and omphacite, rutile, and coesite without plagioclase, therefore, the P-T conditions of this mineral assemblage are higher than the quartz–coesite transformation line. According to the changes in the composition of the garnet compositional zoning in LMG-06, it can be seen that there is an increase in grossular composition from the core to the edge of the garnet. Combined with the similar increase in adjacent UHP eclogite from Kehete in [24], this characteristic change in composition represents a prograde metamorphic stage. Therefore, the isopleth of 30.2–30.5 mol.% of the garnet composition falls into the Grt + Omp + Ms + H₂O + Rt + Qtz/Coe region, obtaining a pressure range of 25–32 kbar, which may represent the pressure range of garnet’s heating and prograde process. Constrained by the x (omp) isopleth of the ompahcite in matrix from 0.12 to 0.14 and the coesite occurrence, the temperature and pressure conditions of peak stage were obtained at 29–33 kbar/705–760 °C (Figure 10b), which are also associated with the oriented quartz exsolution needles in omphacite (Figure 11).

During the stage of high-pressure granulite facies retrogression, no quartz formed and no traces of transformation of rutile to ilmenite were found. Therefore, the pressure of this stage should be lower than the quartz disappearance line and higher than the rutile/ilmenite transformation line. By using the x (cpx) composition range of monoclinic pyroxene from 0.14 to 0.17 as a constraint, temperature and pressure conditions of approximately 9.5–12.5 kbar/610–680 °C for high-pressure granulite facies (HGR) were obtained, representing the isothermal decompressional retrograde process after the peak metamorphism of UHP eclogite (Figure 10b). Based on the metamorphic traces of the amphibolite facies identified in the eclogite (E-Gouli-08) from eastern Gouli, it is believed that the eclogite in the eastern part of EKOB underwent a clockwise metamorphic trajectory of ultra-high-pressure eclogite facies → high-pressure granulite facies → amphibolite facies.

5. Discussion

5.1. Identification of UHP Metamorphism and P-T Path

On the basis of detailed petrographic observations, it can be found that the eclogites in the eastern region of East Kunlun Orogenic Belt are generally relatively fresh, with the jadeite molar fraction reaching 40 mol.%. Although the retrograde metamorphism process changed the minerals of eclogite facies, the degree of retrogression is not significant. There are still many eclogite facies minerals remaining. The retrogression degree of the Gazhima eclogite (LMG) is lower than that of the E-Gouli sample, and it also retains more evidence of ultra-high pressure (coesite inclusion and polycrystalline quartz after metamorphism), corresponding to a subduction depth of at least 100 km.

Furthermore, abundant oriented quartz exsolution needles in omphacite (Figure 11) are also typical in UHP eclogites [11,65,66], which can be attributed to the fact that a partial silicon atom forms the hexagonal coordination of the octahedron structure in a high enough pressure stage, while decompression leads to the precipitation of Si and forms the quartz exsolution texture in clinopyroxene retrograded from super-silicic pyroxene. Hence, the proof above provides new evidence for the early Paleozoic ultra-high-pressure metamorphism in the eastern part of East Kunlun region, supplementing the eclogite facies metamorphism identified by Qi et al. (2016) with T = 650–750 °C and P = 1.8–2.0 GPa [28].

A nearly isothermal decompression of the P-T path for theLMG-06 eclogite was obtained from the phase equilibrium modeling from the peak stage (UHP eclogite facies) of 29–33 kbar/705–760 °C to the retrograde stage (HP granulite facies) of 9.5–12.5 kbar/610–680 °C. However, the further retrograde metamorphic process is still unclear due to the lack of observation of minerals in the amphibolite facies stage. Therefore, referring to the nearby sites [20,24], the metamorphic P-T conditions of the amphibolite facies were cited to simulate a metamorphic temperature below 600 °C and pressure of about 6 kbar, representing the process of cooling and decompression from high-pressure granulite facies (HP granulite facies) to amphibolite facies (AM Facies).

In summary, a clockwise P-T evolution trajectory of eclogite in this study was reconstructed. It can be concluded that its metamorphic P-T path is: a peak stage (ultra-high-pressure eclogite facies, assemblage of omphacite + garnet + coesite + rutile) of 29–33 kbar/705–760 °C → retrograde high-pressure granulite facies mainly composed of clinopyroxene + plagioclase + rutile with conditions of 9.5–12.5 kbar/610–680 °C → retrograde amphibolite facies with dominant amphibole + plagioclase + ilmenite of ~6 kbar/<600 °C (Figure 12).

5.2. Metamorphic Geochronology

We considered the zircon geochronology of the LMG and E-Gouli eclogites comprehensively based on their similar mineral combinations and the adjacent positions consistent chronological characteristics between the two. No protolith age, especially Neoproterozoic records, was identified like previous studies [26,29], and all zircons are metamorphic without relict cores. The chondrite-normalized rare earth elements (REE) patterns of the zircons have a flat heavy rare earth distribution pattern and positive or no Eu anomalies, so it can be predicted that the zircons were formed in environments lacking plagioclase and enriched garnet, while no coesite inclusions were identified in the zircons.

The identification of garnet inclusion (Figure 9) reinforced the eclogite facies age recorded by zircons from the eclogite samples. Therefore, it is speculated that their zircons represent the eclogite facies ages, but it is uncertain whether the zircons recorded the ultra-high-pressure eclogite facies age because no coesite inclusion was identified within the zircon grains. The East Gouli eclogite (E-Gouli) has a concordia age of 412.9 ± 2.5 Ma (Figure 8a), with a weighted mean age of 414.0 ± 2.3 Ma, and the concordia age of the Gazhima eclogite (LMG) is 415.0 ± 1.6 Ma (Figure 8d), with the weighted mean age of 416.1 ± 3.2 Ma (Figure 8e).

To sum up, we believe that the eclogite (LMG and E-Gouli) in the southern Langmuri research area, eastern part of East Kunlun Mountain, has recorded the eclogite facies metamorphism yielding 416–413 Ma, and the duration of the eclogite facies metamorphism should be longer than this time scale. Referring to the ages of the Langmuri eclogite (430 Ma, [28]) and Kehete eclogite [43], it is believed that the metamorphic ages of the eclogites in the southern Langmuri research area yield 430–413 Ma.

5.3. Tectonic Implication

It is generally believed that the external fluids associated with retrograde metamorphism will not alter the significant element migration of eclogites [31,67], especially for high-field-strength elements (HFSE) and some rare earth elements [67,68]. As discussed above, the low loss on ignition (LOI) and relatively uniform distribution patterns of the trace and rare earth elements in the eclogites suggest that the vast majority of elements in the eclogites only underwent a slight degree of retrogression, which is associated with the petrological characteristics of the preservation of abundant omphacite. Therefore, immobile elements (typical HFSEs and REEs, such as Nb, Ta, La, Ce, Yb, and Y) can be selected for the tectonic identification of the protoliths, because these elements are not easily migrated during the metamorphic process and can preserve the characteristics of the protoliths to the greatest extent possible.

The eastern LMG eclogite generally has relatively low Zr/Y values, ruling out the possibility of intraplate basalt. In the Zr vs. Zr/Y diagram, it can also be seen that almost all of the LMG samples fall into the MORB area, while one of the E-Gouli eclogites falls into the MORB area, and the other two fall into the WPB field (Figure 13a). In the Ta/Yb vs. Ce/Yb diagram, all samples aggregate in the MORB field (Figure 13b). In the Hf-Th-Nb and Nb-Zr-Y triangle diagrams, except for a few samples falling into the island arc basalt area (IAB), all other samples converge in the E-MORB to N-MORB domains (Figure 13c,d), which may suggest that the eclogites from this study originate from MORB-type oceanic crust basalts. Moreover, the geothermal gradient of the LMG-06 eclogite is approximately 7–7.6 °C/km, located in the range of <10 °C/km, and belongs to the “cold subducting plate type” of subduction zone metamorphism, which is characterized by oceanic and continental crustal rocks subducting into the deep mantle and experiencing UHP metamorphism [69,70]. This is consistent with the protolith properties and P-T conditions of the eastern eclogites in EKOB, indicating that the peak metamorphic stage of the eastern eclogites is related to the closure of the Proto-Tethys Ocean in the region and represents the subduction type eclogite.

Taking into account these characteristics, we believe that the protoliths of the eastern eclogites (E-Gouli and LMG eclogites) are subducted MORB-type oceanic basalts or gabbros, which are similar to the characteristics of the oceanic eclogites identified in the Kehete, eastern section of the East Kunlun region [24,43], that is, the MORB-type basic rocks of the oceanic crust are involved in the subduction process and the subduction depth has been estimated at about 100 km, reaching ultra-high-pressure (UHP) metamorphism before exhumation.

In addition, the degeneration process in the P-T path of the LMG-06 eclogite in this area is first isothermal decompression and then cooling depressurization, and no obvious overprinting of granulite facies and obvious thermal relaxation phenomena are observed, which is different from the “decompressional heating” process of continental eclogites in the north Qaidam and East-Kunlun [23,75,76,77,78,79]. It is further proved that the eclogite (E-Gouli and LMG) in this study is an oceanic-type eclogite that has undergone UHP metamorphism. The phenomena of a low geothermal gradient and no heating decompression indicate that the study eclogites have not been buried in the lower crust for a long time and influenced by heat source upwelling during exhumation. According to the characteristics of the geothermal gradient in the subduction zone [70], the eclogites have undergone the rapid exhumation near or along the subduction channel.

The metamorphic age duration of the eastern eclogite in EKOB is 430–413 Ma, which represents the subduction of the Proto-Tethys Ocean in the Kunlun region. During this period, a continuous subduction process of the oceanic crust occurred, and the oceanic basalt can reach a depth of ~100 km.

Based on the integration of petrology, geochemistry, and geochronology presented in this paper and previous studies, a tectonic model can be summarized by the two steps described below:

The branch ocean of the Proto-Tethys Ocean formed between the Kunlun Block (south) and the Qaidam block (north). With the end of the expansion of the ocean basin, the process transformed to decay and began subduction (Figure 14a). The ocean crust subducted northward, and pulled the continental crust of the South Kunlun block to subduction. The Qaidam block and the Kunlun block were approaching each other with the continued subduction.

The oceanic crust completely subducted (Figure 14b) at the latest at 430 Ma, and dragged the leading edge continental crust into the mantle. At that time, the Qaidam and Kunlun blocks merged, forming a new suture boundary between the two blocks. The subducting plate broke off, and the deep oceanic crust underwent eclogite facies metamorphism and exhumation. The eclogites were exhumated along different tracks and exhibited different retrogressions. The exhumation eclogite reached the middle–lower crust in accord with decompressional heating. The eclogite exhumated along the subduction channel with cold geothermic gradient underwent the retrograde metamorphism with the coupling of decompression with a temperature decrease, which is associated with the ultra-high-pressure eclogite in this study.

The UHP eclogite containing the coesite inclusion was exhumated from a depth of more than 100 km. Considering the age distribution of the eastern eclogites in EKOB, the metamorphic age of the eclogite facies lasted 17 Myr from 430 to 413 Ma. Subsequently, the Qaidam Block and the Kunlun Block were completely sutured together. The eclogite facies record yielding 413 Ma could represent the final time limit of compressional tectonics, and the eclogite was exhumated and underwent retrogression subsequently due to post-collision extension. In this period, the compressive tectonic background began to transform into the extensional process from the contraction.

6. Conclusions

Coesite inclusions and exsolution texture, together with maximum-pressure estimation, were discovered in eclogites from the eastern part of the East Kunlun Orogenic Belt, which provides new evidence of UHP metamorphism in this metamorphic belt.

The study eclogites underwent three metamorphic stages involving peak UHP eclogite facies metamorphism (29–33 kbar/705–760 °C) and decompressional cooling to high-pressure granulite facies metamorphism (9.5–12.5 kbar/610–680 °C), followed by retrograde amphibolite facies metamophism of ~6 kbar/<600 °C, presenting a clockwise P-T path.

The study eclogites yield the eclogite facies age of ~414 Ma and represent oceanic crust subduction with a depth over 100 km and subsequent exhumation along the subduction channel, corresponding to slight retrogression. The regional eclogite facies metamorphic ages of 430–414 Ma recorded in the eastern eclogites within the orogenic belt were in response to the final closure of the branch ocean of Proto-Tethys in East Kunlun and the evolution from subduction to collision.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14060582/s1, Table S1: Representative mineral compositions for eclogite from East Gouli area, East Kunlun Orogenic Belt; Table S2: Representative mineral compositions for eclogite from Gazhima area, East Kunlun Orogenic Belt; Table S3: Whole-rock major and trace element compositions of eclogites from East Kunlun Orogenic Belt; Table S4: Zircon U-Pb ages and trace-element compositions by LA-ICP-MS for eclogites from East Kunlun Orogenic Belt.

Author Contributions

Conceptualization, G.Z. and F.C.; methodology, F.C. and L.X.; software, F.C.; validation, L.X., G.Z.; formal analysis, F.C.; investigation, F.C., G.Z. and L.X.; resources, G.Z.; writing—original draft preparation, F.C.; writing—review and editing, G.Z.; visualization, G.Z. and F.C.; supervision, L.X. and G.Z.; project administration, G.Z.; funding acquisition, G.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (NSFC), grants 42372061, 41972056 and 91755206.

Data Availability Statement

The data from this publication have been summarized in Supplementary Tables S1–S4.

Acknowledgments

We are grateful to the editors and three reviewers for their constructive reviews and comments that substantially improved this work. We would like to thank Ying Cui, Xinjian Bao and Xiaoli Li for their help with the experimental analysis. We are grateful to Shuaiqi Liu and our driver Jinbang He for their fieldwork in Qinghai.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Geological and tectonic setting. (a) Overview map showing the location of Kunlun within the Chinese Central Orogenic Belt (modified from Dong et al. [27]). (b) Simplified tectonic map of Kunlun showing major tectonic divisions, ophiolitic mélanges, and sampling locations. HGF: Hongliuquan–Golmud hidden Fault; QXM: Qimantag–Xiangride ophiolitic mélange; AKM: Aqikekulehu–Kunzhong ophiolitic mélange; and MBAM: Muztag–Buqingshan–Animaqen ophiolitic mélange. (c) Geological map and sampling sites of our study area (modified from Qi et al. [18]).

Figure 2. Outcrop photographs of eclogite in eastern part of East Kunlun Orogenic belt (EKOB). (a) Porphyritic lamellar blastoblastic structure of gneiss; (b,c) East Gouli eclogite (E-Gouli); and (d,e,f) Gazhima eclogite (LMG).

Figure 3. Petrography of eclogites in eastern part of EKOB. Photomicrograph of gneiss (a–c), E-Gouli eclogite (d–f), and LMG eclgite from Gazhima position (g–i). The pseudomorph of coesite, radial cracks, and polycrystalline quartz included by omphacite grain from LMG eclogite samples is identified. Local enlarged Backscattered Electron (BSE) images of the symplectite are presented in Figure 3g,h.

Figure 4. (a) Triangular diagram summarizing garnet compositions. (b,c) Compositional data show profiles across garnet porphyroblasts in east Gouli (E-Gouli-06) and Gazhima (LMG-08), respectively. X_Fe = Fe²⁺/(Fe²⁺ + Mn + Mg + Ca). X_Ca, X_Mg, and X_Mn defined accordingly. (d,e) Ternary classification diagrams for pyroxene in eclogite after [47]. (f) An-Ab-Or triangular diagram [48] showing compositions of plagioclase in E-Gouli-06 and LMG-08 eclogite.

Figure 5. Classification diagrams of eclogite in the eastern section of EKOB. (a) SiO₂-Na₂O+K₂O diagram; (b) Nb/Y-Zr/Ti diagram (Pearce, 2014); (c) Ti-V diagram (Shervais, [52]); and (d) series classification diagram.

Figure 6. Chondrite-normalized REE patterns (a) and primitive mantle-normalized trace element patterns (b) for ecgolites from Gazhima (LMG) and East Gouli (E-Gouli), eastern part of EKOB. Chondrite, primitive mantle, N-MORB, E-MORB, and OIB values are from Sun and McDonough (1989).

Figure 7. Cathodoluminescence (CL) images of representative zircons from (a) East Gouli eclogite (E-Gouli-08) and (b) Gazhima eclogite (LMG-06), east part of East Kunlun Orogenic Belt, China. The circles are the ablation sites of 32 μm laser spot.

Figure 8. U-Pb concordia diagrams (a,d), weighted average of mean ages (b,e) and chondrite-normalized REE patterns (c,f) of from E-Gouli-08 eclogite (a–c) and LMG-06 eclogite (d–f). MSWD: mean square weighted deviation.

Figure 9. Observations under the scale of rock slice (a,b) and Raman spectra for (c) garnet included by in situ zircon from LMG eclogite, East Kunlun.

Figure 10. Phase relationships (a) and resulting P–T pseudosection (b) simulated in NCKFMASHTOMn system for Gazhima eclogite (LMG-06). Omphacitic pyroxene with jadeite component > 20% is marked as omphacite (omp).

Figure 11. Omphacite grain distributing in matrix (a) and quartz exsolution needles in omphacite (plane polarized image, (b)) from LMG eclogite, East Kunlun.

Figure 12. P-T paths of our sample and nearby eclogites) from the eastern part of EKOB. Published P-T paths: 1-eclogite from [43]; 2-eclogite from [24].

Figure 13. Tectonic discrimination diagrams. (a) Zr vs. Zr/Y diagram (Pearce and Norry, 1979, [71]); (b) Ce/Yb vs. Ta/Yb plot (Pearce, 1982; [72]); (c) Hf-Th-Nb plot (Cabanis and Lecole, 1989; [73]); and (d) Nb-Zr-Y plot (Meschede, 1986; [74]) of eclogites from eastern part of East Kunlun Orogenic Belt (EKOB). CA—calc-alkaline; SH—shoshonitic; TH—tholeiitic; VAB—Volcanic Arc Basalt; and WPB—Within plate basalt.

Figure 14. Schematic diagram showing the tectonic evolution of the eastern eclogites from East Kunlun Orogenic Belt (EKOB).The evolution model contains two stage of (a) oceanic subduction and (b) merging of the Qaidam and Kunlun blocks.

Table 1. Bulk-rock XRF analyses and effective bulk-rock compositions of the eclogite (LMG-06).

XRF Whole-Rock Compositions (wt.%)
SiO₂	TiO₂	Al₂O₃	TFe₂O₃	MnO	MgO	CaO	Na₂O	K₂O	P₂O₅	LOI
46.83	1.40	16.80	12.60	0.19	8.55	10.47	2.49	0.09	0.09	0.29
Normalized molar proportion used for phase equilibria modelling (mol.%)
SiO₂	TiO₂	Al₂O₃	CaO	MgO	FeO	K₂O	Na₂O	MnO	O	H₂O
49.01	1.06	10.98	12.35	12.43	9.85	0.07	2.9	0.12	0.22	1.31

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Chang, F.; Zhang, G.; Xiong, L. The Discovery of the New UHP Eclogite from the East Kunlun, Northwestern China, and Its Tectonic Significance. Minerals 2024, 14, 582. https://doi.org/10.3390/min14060582

AMA Style

Chang F, Zhang G, Xiong L. The Discovery of the New UHP Eclogite from the East Kunlun, Northwestern China, and Its Tectonic Significance. Minerals. 2024; 14(6):582. https://doi.org/10.3390/min14060582

Chicago/Turabian Style

Chang, Feng, Guibin Zhang, and Lu Xiong. 2024. "The Discovery of the New UHP Eclogite from the East Kunlun, Northwestern China, and Its Tectonic Significance" Minerals 14, no. 6: 582. https://doi.org/10.3390/min14060582

APA Style

Chang, F., Zhang, G., & Xiong, L. (2024). The Discovery of the New UHP Eclogite from the East Kunlun, Northwestern China, and Its Tectonic Significance. Minerals, 14(6), 582. https://doi.org/10.3390/min14060582

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