High-pressure Ultrahigh-pressure Research Articles

Petrology of UHP eclogite-facies felsic schist in the Western Tianshan subduction zone, China

Although quartzo-feldspathic metasedimentary rocks are widespread in high pressure-ultrahigh pressure (HP-UHP) metamorphic belts worldwide, their petrogenesis and metamorphic evolution is poorly understood. We discovered an UHP eclogite-facies felsic schist in the Western Tianshan metamorphic belt, China. Petrological observations and phase equilibria modeling both indicate the felsic schist experienced UHP metamorphism in the coesite stability field. In particular, it experienced prograde metamorphism at 21–24 kbar, 445°C–470°C, a pressure peak at 25–28 kbar and 490°C–525°C, and eventually heating with decompression to 20 kbar and 560°C. The obtained clockwise P-T path was consistent with those of other lithologies (eclogite and pelitic schist) from the same belt, which provides new evidence for the coherent exhumation of the UHP unit of the Western Tianshan metamorphic belt. The final uplift of the Western Tianshan oceanic crust to the surface is attributed to fluid activity and late tectonic deformation.

Tracking the multi-stage metamorphism and exhumation history of felsic gneisses in the South Altyn ultra-high pressure metamorphic belt, Western China

Felsic gneisses are usually the dominant rock types in most high-pressure (HP) and ultrahigh-pressure (UHP) terranes, although their peak metamorphic conditions and complete P–T paths are rarely known. This lack of knowledge is attributed to the high variance of mineral assemblages at HP/UHP conditions of the felsic gneisses, or to the effects of retrograde-related overprinting. Thus, it is difficult to distinguish whether felsic gneisses underwent HP/UHP metamorphism. In this paper, we focus on the felsic gneiss that has been previously documented as having formed under HP granulite-facies metamorphism with peak P–T of > 1.15 GPa and > 860 °C from the Danshuiquan locality in the South Altyn HP–UHP belt. The Danshuiquan felsic gneiss is strongly deformed and is spatially associated with weakly deformed garnet-rich felsic gneiss. On the basis of detailed microstructure observations on mineral assemblages and phase equilibria modelling, three stages of metamorphic evolution for the weakly deformed garnet-rich felsic gneiss are identified. The first stage is represented by garnet cores, revealing a prograde P–T path from 1.5 GPa/750 °C to 2.1 GPa/900 °C. The second stage is characterized by garnet mantles with multiphase inclusions, defining HP eclogite-facies metamorphic conditions of 2.2–2.6 GPa/950–1100 °C. The third stage is represented by the composition of garnet rims, biotite, and plagioclase, suggesting an HP granulite to amphibolite facies decompression cooling process to 1.1–1.3 GPa/710–770 °C. In contrast, the strongly deformed felsic gneiss just records a deformation-related decompression process from 1.1 to 1.2 GPa/750–760 °C to 0.87–0.98 GPa/760–770 °C, and finally to 0.83 GPa/740 °C. Zircon U–Pb dating yields c. 920 Ma protolith ages, c. 500 Ma eclogite-facies ages, and c. 480 Ma granulite to amphibolite facies retrograde ages for the Danshuiquan felsic gneisses. Our results show that the Danshuiquan felsic gneisses underwent a similar metamorphic evolution process to that of Yinggelisayi HP–UHP rocks in the eastern segment of the South Altyn Tagh. Most HP mineralogical records tend to be eliminated by the strong deformation at crustal depths (∼30 km), meaning that the peak metamorphic conditions of the studied felsic rocks in UHP metamorphic terranes are previously underestimated.

Discovery of Early Paleozoic eclogite-facies metamorphic rocks in the western part of North Qinling Orogen and its geological significance

西秦岭地处中国中央造山系东西转换衔接部位，随着东秦岭以及祁连、柴北缘和东昆仑早古生代高压-超高压变质岩石的陆续发现和深入研究，西秦岭造山带变质作用研究吸引了大家的普遍关注。本文在大范围野外地质调查基础上，在北秦岭造山带西段天水南部的秦岭岩群长英质片麻岩中发现了一套石榴子石斜长角闪岩（榴闪岩），并开展了详细的锆石形貌和内部结构、微区微量元素和U-Th-Pb同位素研究。CL图像显示榴闪岩锆石普遍具有核-幔-边或核-边结构，部分存在原岩残留锆石。定年结果得到榴闪岩原岩残留锆石年龄为710±52Ma，同时得到497±3Ma、452±3Ma和423±7Ma三期变质年龄。其中497±3Ma变质年龄来自锆石核部并显示出轻稀土亏损、重稀土平坦且没有明显负Eu异常的稀土配分曲线特征，表明该时期的矿物组合中有石榴子石但没有斜长石，与榴辉岩相变质锆石特征一致；452±3Ma变质年龄来自锆石幔部或边部，对应测点重稀土元素的分异加大，并出现弱的负Eu异常，说明此时石榴子石被消耗且出现少量斜长石；423±7Ma变质年龄来自锆石的最边部，对应稀土配分曲线表现出更明显的负Eu异常和更大的重稀土元素分异特征，指示此时岩石中石榴子石含量更少，斜长石含量更多。上述结果表明该榴闪岩可能经历了~500Ma的榴辉岩相变质作用，并在~450Ma和~420Ma叠加了两期退变质改造。天水地区榴闪岩无论是其野外产状，还是变质锆石的形貌和内部结构、稀土配分曲线特征及其所记录的原岩和三期变质年龄都与北秦岭造山带东段大陆俯冲型高压-超高压变质榴辉岩近乎一致，指示早古生代时期，北秦岭造山带西段与东段经历了相似的大陆（深）俯冲和折返过程，它们共同构成一条统一的早古生代高压-超高压变质岩带。;The western part of North Qinling Orogen (NQO) is located at the conjunction area of several Paleozoic high pressure-ultrahigh pressure (HP-UHP) belts in the central China continent, the North Qinling UHP belt in the east and the North Qaidam and the East Kunlun UHP belts in the west, thus its metamorphism has drawn broad scientific interest. We document here an integrated study of zircon morphology, microstructure, in-situ trace element compositions and U-Th-Pb dating on a newly discovered garnet amphibolite in southern Tianshui, the western part of NQO. The garnet amphibolite occurs as a lensoid block in the felsic gneiss of the Qinling Complex, and mainly consists of amphibolite (50%~60%), garnet (10%~15%), quartz (~10%), plagioclase (5%~10%), muscovite (~5%) and ilmenite (~3%). Zircons from the garnet amphibolite display obvious core-mantle-rim or core-rim structures with a few protolith zircon relicts. LA-ICP-MS U-Pb dating yielded a protolith age of 710±52Ma and three metamorphic ages of 497±3Ma, 452±3Ma and 423±7Ma. The 497±3Ma metamorphic cores display flat heavy rare earth elements (HREE) patterns without obvious Eu anomalies, indicating they were formed in presence of garnet but absence of plagioclase under eclogite-facies condition; The 452±3Ma metamorphic mantles or rims show shallow HREE slopes with moderately negative Eu anomalies, indicating that these zircons grew accompanied by initial appearance of plagioclase and decomposition of garnet; The 423±7Ma metamorphic rims exhibit steep HREE patterns and marked negative Eu anomalies, indicating massive crystallization of plagioclase and decomposition of garnet during their formation. These results suggest that the garnet amphibolite probably experienced eclogite-facies metamorphism at~500Ma and two stages retrograde overprint at ~450Ma and ~420Ma. The occurrence, zircon microstructures, protolith and multiple metamorphic ages of the garnet amphibolite from Tianshui area are basically consistent with the continental-type HP-UHP rocks of the eastern part of NQO. Therefore, we suggest that both western and eastern part of NQO experienced similar continental deep subduction and exhumation processes and thus they form a united Early Paleozoic HP-UHP metamorphic belt.

Multi-stage metamorphism and deformation of the North Qinling Orogenic Belt: Constraints from petrology, geochronology, and structural analysis of the Qinling Complex

The Qinling Complex is the only continental nucleus that outcrops in the North Qinling Orogenic Belt (NQOB) and preserves valuable evolutionary information of NQOB during the early Paleozoic. There are many high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic rocks distributed in the northern and southern parts of the Qinling Complex. Whether the central part (namely the Qihe tectonic slab) underwent a similar HP–UHP metamorphism and what the relationship among different tectonic slabs of Qinling Complex is, are the key to revealing the formation and exhumation process of these HP–UHP rocks. Four episodes of deformation (D1–4) have been recognized in the Qinling Complex. The metamorphic pressure‐temperature conditions of syn-D3 amphibolite in the Qihe tectonic slab are limited to 3.3–6.4 Kbar and 479–538 ℃ based on equilibrium modeling, corresponding to an extensional tectonic setting. The U-Pb dating of the amphibolite yielded a protolith age of 714 ± 46 Ma, an eclogite-facies metamorphic age of 514 ± 4 Ma, and two stages of retrograde metamorphism at 462 ± 3 Ma and 418 ± 5 Ma. The parametamorphic rocks of different tectonic slabs have similar detrital zircon age spectra, which is a very different pattern from adjacent areas of the Qinling Complex. The consistency in the protolith ages, peak metamorphic ages, two stages of retrograde metamorphism, and detrital zircon age from different tectonic slabs in the Qinling Complex indicate that HP–UHP metamorphic rocks from Qinling Complex formed during the same tectonic event. Therefore, the Qinling Complex evolved in deep subduction and underwent eclogite facies metamorphism, followed by superimposed deformation structures and retrograde metamorphism during the two stages of exhumation.

Differential exhumation of ultrahigh-pressure metamorphic terranes: A case study from South Altyn Tagh, western China

South Altyn Tagh contains ultrahigh-pressure (UHP) terranes that have been exhumed from ~300 km mantle depth. Previous zircon U–Pb geochronology has yielded an eclogite-facies age of ca. 500 Ma and a high-pressure (HP) granulite-facies retrograde age of ca. 450 Ma in the Jianggalesayi area in the western segment of South Altyn Tagh. However, in the eastern segment (Yinggelisayi and Danshuiquan localities), an age range of 500–480 Ma has been determined, and it remains uncertain as to whether this age range represents the timing of the peak metamorphic stage or the retrograde overprint. Our study of newly discovered retrograde eclogite in the Danshuiquan locality shows that it underwent three stages of metamorphism, under eclogite-facies, HP granulite-facies and amphibolite-facies P–T conditions of 2.5–4.0 GPa and 870–1050 °C, 2.0–1.4 GPa and 830–940 °C, and 0.7–1.3 GPa and 704–880 °C, respectively. The decompression-dominated P–T path evolved mainly after crossing the solidus, indicating marked retrograde modification under melt-bearing conditions. LA–ICP–MS and SIMS zircon U–Pb dating yielded ca. 500 Ma eclogite-facies and ca. 484 Ma granulite-facies retrograde ages and a later retrograde age of ca. 452 Ma. The clockwise P–T–t path indicates rapid exhumation from eclogite-facies to granulite-facies within around 16 Myr, which is faster than that of the UHP rocks in the western segment. Thus, the HP–UHP rocks in South Altyn Tagh suggest a differential exhumation process for the eastern and western segments. The distinct HP–UHT metamorphism and rapid exhumation of (U)HP rocks in the eastern segment likely resulted from local mantle heating. The continuing P–T evolution of the (U)HP rocks under UHT conditions during exhumation led to a pervasive granulite-facies overprint in the eastern segment of the South Altyn Tagh. The rapid exhumation recorded in the eastern segment provides valuable insights into the exhumation mechanism of ultra-deep subducted UHP terranes.

Evolution of fluids and melts in deeply subducted continental crust: Insights from an UHP eclogite–vein system in the Dabie terrane, China

Both slab-derived high-pressure (HP) or ultrahigh-pressure (UHP) aqueous fluids and hydrous melts are important agents for material cycling in subduction zones. Although numerous studies have investigated the genesis of both fluid phases, the temporal–spatial relationship and the chemical difference between aqueous fluids and hydrous melts during subduction-zone metamorphism remain mysterious. In this study, we present detailed petrological and geochemical studies on the UHP eclogites and enclosed epidote-quartz veins in the Dabie terrane (eastern China) to understand the compositions and behaviors of fluids/melts in subduction zones. The eclogites consist mainly of garnet, omphacite, and hydrous minerals (epidote, phengite, and paragonite). Two stages of epidote, including epidote inclusions in omphacite (Ep-in) and porphyroblastic epidote (Ep-p), are recognized in the eclogites. The obviously higher contents of strontium (Sr) and light rare earth elements (LREEs) in Ep-p than in Ep-in demonstrate the previous coexistence of lawsonite and Ep-in. The combination of petrographic analyses, epidote Sr isotopes, and phase equilibria modeling indicates lawsonite in the eclogite breakdown at ~2.2–2.7 GPa and 620–750 °C during the initial exhumation, accompanied by the significant release of H2O-rich fluids. This process led to the growth of Ep-p in eclogite and the precipitation of vein omphacite, quartz, and epidote (Ep-v1). On the other hand, many lines of evidence for partial melting in the eclogites are found, including (1) multiphase solid inclusions (MSIs) of Ky + Kfs ± other phases, which represent the in situ products of phengite partial melting; (2) MSIs of Kfs ± Pl ± Qz, which represent the crystallized products of hydrous melts; and (3) veinlets of Kfs ± Di that crosscut the omphacite. Importantly, the MSIs in texture (1) are exclusively hosted in the Ep-p, which formed by lawsonite breakdown. This distinctive phenomenon indicates that the partial melting of phengite was most likely triggered by the injection of H2O from the lawsonite breakdown (i.e., water-fluxed melting). Phase equilibria modeling suggests that the partial melting of phengite occurred at ~1.5–2.0 GPa and 700–750 °C during the further exhumation stage of the eclogites. The occurrence of Ep (Ep-v2) + Pl + Kfs in the veins indicates that melts derived from the eclogites infiltrated the vein system. The compositions of the fluids and melts are quantitatively constrained using the epidote compositions (Ep-v1 and Ep-v2, respectively) and epidote–fluid/melt partition coefficients. The results indicate that the fluids have trace element patterns similar to those of the melts with high contents of large ion lithophile elements (LILEs), Th, U, Pb, and LREEs. Thus, aqueous fluids have the ability to transfer significant amounts of mobile elements from the slab like the hydrous melts, if the appropriate P–T conditions and chemical characteristics of wallrocks can be satisified.Our study reveals multistage metamorphic-fluid/melt evolution in the UHP eclogites, involving metamorphic dehydration and subsequent partial melting during the exhumation of subducted continental crust. We provide petrological evidence linking fluid liberation to the occurrence of partial melting in eclogites. Moreover, this study indicates that the development of vein systems in HP–UHP rocks may provide high-permeability channels that allow the flow and escape of melts formed during later-stage process. We propose that the occurrence of early-stage dehydration events in deeply subducted slabs facilitates the generation of hydrous melts and their transport in subduction zones.

HP–UHP eclogites in the East Kunlun Orogen, China: P–T evidence for asymmetric suturing of the Proto-Tethys Ocean

The Early Paleozoic eclogite belt extends discontinuously for ~500 km within the East Kunlun Orogen (EKO) and records the subduction history of the Proto-Tethys Ocean. This eclogite belt, therefore, is an excellent site for studying the first-order transition in convergent-margin tectonic systems, and the tectonic events associated with subduction and collision can be determined by reconstructing the high-pressure (HP)–ultrahigh-pressure (UHP) P–T–time path. In this contribution, we present P–T results from phase equilibrium modeling and Zr-in-rutile/titanite thermometry for eclogites located on the east and west sides of the EKO. In the eastern EKO, MT–UHP eclogite records peak conditions of ~30 kbar/650–720 °C, retrograde conditions of 10 kbar at ~650–700 °C, and then further retrograde conditions at 3–6 kbar and 500–580 °C. In the western EKO, LT–HP eclogite records peak conditions of ~27 kbar at 570 °C and then retrograde conditions at 10 ± 2 kbar/470–560 °C. These two types of eclogite record oceanic subduction (west) and continental collision (east), respectively. The EKO, therefore, contains two types of eclogites that indicate two end-member subduction complexes within the orogenic belt, further indicating that there are differences in the subduction-depth and collision-time from east to west. This finding is consistent with asymmetric suturing of the subducted Proto-Tethys Ocean within the EKO. Together, the P–T conditions and time-scales of the HP–UHP metamorphism can be applied to understand the process from subduction to collision between the Qaidam Block and the South Kunlun Block, as a consequence of asymmetric closure of the Proto-Tethys Ocean.

A New HP–UHP Eclogite Belt Identified in the Southeastern Tibetan Plateau: Tracing the Extension of the Main Palaeo-Tethys Suture Zone

Abstract The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is an important link between the Longmu Co–Shuanghu suture (LCSS) in the northern Tibetan Plateau and the Chiang Mai–Inthanon and Bentong–Raub sutures in Thailand and Peninsular Malaysia. These belts and sutures are generally regarded as containing the remnants of the oceanic crust of the Palaeo-Tethys that formed by seafloor spreading as a result of the separation of Gondwana- and Eurasia-derived blocks during the Middle Cambrian. In this paper we report the first discovery of abundant unaltered and retrograde eclogites that occur as irregular lenses and blocks in metasedimentary rocks of the CMOB, and these eclogites form an elongate and almost north–south-trending high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt that is ∼200 km long and ∼50 km wide. The newly discovered phengite/talc/epidote–glaucophane eclogites, lawsonite–talc–phengite eclogites, dolomite/magnesite–kyanite eclogites and phengite–kyanite-bearing retrograde eclogites have enriched (E-) and normal mid-ocean ridge basalt (N-MORB)-like affinities and mainly positive as well as some negative whole-rock εNd values (–4·34 to +7·89), which suggest an enriched and depleted oceanic lithosphere source for their protoliths. Magmatic zircons separated from the epidote–glaucophane, magnesite–kyanite and (phengite–kyanite-bearing) retrograde eclogites gave protolith ages of 317–250 Ma, which fit well within the time frame of the opening of the Palaeo-Tethys during the Middle Cambrian and its closure during the Triassic. Abundant metamorphic zircons in the eclogites indicate a Triassic metamorphic event related to the subduction of the Palaeo-Tethys oceanic crust from 235 to 227 Ma. Taking into account previous isotopic age data, we now establish the periods of Early–Middle Triassic (246–227 Ma) and Late Triassic (222–209 Ma) as the ages of subduction and exhumation of the Palaeo-Tethyan oceanic crust, respectively. Thermodynamic modelling revealed that the eclogites record distinct HP–UHP peak metamorphic conditions of 23·0–25·5 kbar and 582–610 °C for the phengite–glaucophane eclogites, 24·0–25·5 kbar and 570–586 °C for the talc–glaucophane eclogites, 29·0–31·0 kbar and 675–712 °C for the dolomite–kyanite eclogites, and 30·0–32·0 kbar and 717–754 °C for the magnesite–kyanite eclogites. These P–T estimates and geochronological data indicate that the Palaeo-Tethys oceanic slab was subducted to different mantle depths from 75 km down to 95 km, forming distinct types of eclogite with a variety of peak eclogite-facies mineral assemblages. The eclogites consistently record clockwise metamorphic P–T–t paths characterized by a heating–compression prograde loop under a low geothermal gradient of 5–10 °C km–1, indicating the rapid subduction of cold oceanic crust at a rate of 4·5–6·0 km Ma–1, followed by isothermal or cooling–decompressive retrogression and exhumation at an average rate of 3·2–4·2 km Ma–1. The newly discovered eclogites of the CMOB with their signatures of ocean-crust subduction are petrologically, geochemically and geochronologically comparable with those of the LCSS, providing powerful support for the idea that a nearly 2000 km long HP–UHP eclogite belt extends from the northern Tibetan Plateau to the southeastern Tibetan Plateau, and that it represents the main boundary suture of the Palaeo-Tethyan domain. These results have far-reaching implications for the tectonic framework and complex metamorphic evolution of the Palaeo-Tethyan domain.

Decompressional equilibration of the Midsund granulite from Otrøy, Western Gneiss Region, Norway

Abstract The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure (HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no, or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otrøy, a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsund granulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmenite and titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT system resulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths of XMg (Mg/Mg+Fe) in garnet, albite in plagioclase and XNa (Na/Na+Ca) in clinopyroxene in the stability field of melt + plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of 1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effect of equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HT at lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previously thought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR and other HP–UHP terranes worldwide.

UHP metamorphism recorded by coesite-bearing metapelite in the East Kunlun Orogen (NW China)

AbstractThe East Kunlun Orogen (EKO) is the NW part of the Central China Orogenic Belt, which records the evolutionary history of the Proto- and Palaeo-Tethys Oceans from the Cambrian to the Triassic. An Early Palaeozoic eclogite belt has been recognized in recent years, which extends discontinuously for ∼500 km as three eclogite-bearing terranes. In this study, we report an integrated study of zircon grains from mica-schists accompanying the eclogites, in terms of mineral inclusions, U–Pb age systematics andP–Tconditions. The presence of coesite is identified, as inclusions within the metamorphic domain of zircons, which provides unambiguous evidence for subducted terrigenous clastic rocks of the Proto-Tethys Ocean exhumed from coesite-forming depths. U–Pb dating of the metamorphic zircons yields a concordia age of 426.5 ± 0.88 Ma, which is likely to be the time of ultrahigh-pressure metamorphism in the Kehete terrane.P–Tcalculations suggest that metapelite may have experienced a clockwiseP–Tpath with peakP/Tconditions of 685 ± 41 °C and >28 kbar, and equilibrated at 482–566 °C and 5.6–8.9 kbar during subsequent exhumation. The high-pressure – ultrahigh-pressure (HP-UHP) metamorphic belt within the EKO may have formed by collision between the Qaidam Block and the South Kunlun Block, as a consequence of the closure of the Proto-Tethys Ocean.

About this title - HP–UHP Metamorphism and Tectonic Evolution of Orogenic Belts

High pressure (HP) and ultrahigh pressure (UHP) metamorphic rocks play a key role in understanding the tectonic evolution of orogenic belts. They have typically experienced complex changes during subduction and exhumation processes arising from recrystallization, deformation, fluid–rock interactions and even partial melting, and may therefore carry valuable records of evolving geodynamic systems in an orogenic belt. This special publication addresses the current work on HP–UHP metamorphism and its relation to the tectonic evolution of orogenic belts. This special publication contains fifteen papers covering the important orogenic belts of the Himalaya, Dabie–Sulu, Tian Shan, North Qaidam and others that have been grouped into three parts: (I) new developments in the determination of metamorphic pressure–temperature (PT) conditions and their timing, (II) overview papers of well-known HP–UHP metamorphic belts and (III) research papers for some newly discovered HP–UHP belts.

Late Triassic Dabie–Sulu Orocline: New exhumation model of the HP–UHP rocks

The Dabie–Sulu orogen is located in the eastern segment of the Central China Orogen between the North China Block (NCB) and the South China Block (SCB). The complex processes of break‐up and assemblage among the continental blocks of East Asia caused them to undergo multiple‐stage continental collisions. It is extremely important for the final kinematics and dynamics of convergence and collision among continental blocks to explore the exhumation mechanism of the HP (high pressure)‐UHP (ultra‐high pressure) metamorphic rocks. Therefore, this paper focuses on the final collisional process and tries to find a reasonable exhumation mechanism of the HP–UHP rocks. On the basis of the structural geology, petrology, seismic tomography, palaeomagnetism, and mathematical calculation, we propose a south‐eastward subduction of the NCB under the SCB in the Indosinian, and the Sulu Orogen actually became an orocline. This was caused by continuous south‐eastward subduction and the subsequent westward‐retreating delamination of the NCB slab. Coevally, it produced a two‐stage exhumation process, a vertical extrusion during the first stage (240–220 Ma) and an eastward extrusion controlled by an oroclinal contraction at the second stage (220–200 Ma).

A relic slice of archean–early Paleoproterozoic basement of Jiaobei Terrane identified within the Sulu UHP belt: Evidence from protolith and metamorphic ages from meta-mafic rocks, TTG–granitic gneisses, and metasedimentary rocks in the Haiyangsuo region

A great quantity of metamorphic rocks (Haiyangsuo Complex), including high-pressure (HP) mafic granulites with ‘red-eye-circle’ textures, retrogressive products of Grt-, Qtz-, Ep-, and Cpx-bearing amphibolites, TTG–granitic gneisses, and metasedimentary rocks, is distributed extensively within the Haiyangsuo region, northeastern Sulu ultrahigh-pressure (UHP) belt. The Haiyangsuo Complex crops out over an area of at least 70km2, rather than the 15-km2 area mentioned in previous studies. Based on integrated studies of petrography, mineral phase transformations, mineral inclusions, cathodoluminescence (CL) images of zircons, and zircon U–Pb LA–ICP–MS and amphibole Ar–Ar age dates, together with previous P–T condition estimates, abundant Neoarchean to Early Paleoproterozoic igneous protoliths, including ca. 2.6–2.5Ga meta-mafic rocks and ca. 2.7Ga, ca. 2.55–2.5Ga, and ca. 2.4Ga TTG–granitic gneisses, were identified in the Haiyangsuo region, whereas the Late Paleoproterozoic (∼1.7Ga) and Neoproterozoic (ca. 800–650 Ma) protolith ages common in the Sulu HP–UHP rocks have not been observed. The metasedimentary rocks in the region studied have a deposition age of ca. 2.1–1.9Ga, showing protolith ages similar to those of the voluminous Khondalite series of the Jingshan and Fenzishan groups from the Jiaobei Terrane, southwestern Paleoproterozoic Jiao–Liao–Ji orogenic belt.Two discrete episodic HP granulite–facies metamorphic events were identified for the first time in the meta-mafic rocks of the region and their country rocks. The first episodic HP granulite-facies metamorphism (M1) is characterized by the relict coarse-grained mineral assemblage of garnet (Grt)+clinopyroxene (Cpx)+plagioclase (Pl)±quartz (Qtz), which formed under P–T conditions of 740–850°C and 1.0–1.26GPa. In contrast, the second HP granulite-facies metamorphism (M21) is characterized by ‘red-eye-circle’ textures, with a typical fine-grained mineral assemblage of Grt+Cpx+Pl±Qtz surrounding the relict coarse-grained HP granulite-facies minerals of M1. It formed under P–T conditions of 765–845°C and 1.14–1.75GPa. Both of the HP granulite-facies metamorphic events were overprinted and replaced by a late amphibolite-facies retrogression (M22), which is typified by a fine-grained mineral assemblage of Amp+Pl+Grt and formed under P–T conditions of 580–660°C and 0.6–0.8GPa. Abundant metamorphic zircon U–Pb dating revealed that the two discrete episodic HP granulite-facies metamorphic events in the Haiyangsuo Complex occurred and recorded in metamorphic zircon domains with HP granulite-facies mineral inclusions at Late Paleoproterozoic (ca. 1.87–1.86Ga) and Middle Triassic (ca. 245–225Ma), respectively, and that each was overprinted by a subsequent amphibolite-facies retrogression at∼1.84Ga and 220–214Ma, respectively. Various metamorphic rocks (Haiyangsuo Complex) in the Haiyangsuo region have rock association, petrographical, geochronological, and P–T evolution affinities to the Paleoproterozoic Jiaobei Terrane, SW Jiao–Liao–Ji Belt, but are distinguished from the Sulu UHP rocks. Hence, the Haiyangsuo Complex is exotic to the Sulu UHP belt and can be regarded as an inseparable part of the Jiaobei Terrane during the Paleoproterozoic. The unique Middle Triassic HP granulite-facies metamorphic event first identified within the Haiyangsuo Complex coincides with that (245–225Ma) of the adjacent Jiaodong Terrane, NE Sulu UHP belt, indicating that the Haiyangsuo Complex was also involved in Triassic collision–subduction and experienced HP granulite-facies metamorphism at a lower crustal level. The ultimate formation of the Haiyangsuo Complex along the southeastern margin of the Jiaobei Terrane, SW Jiao−Liao−Ji orogenic belt, was genetically related to the collision between the North China and South China cratons. Thus, the juxtaposition of the Haiyangsuo Complex to the Sulu UHP metamorphic belt took place in the Middle–Late Triassic rather than in the Jurassic.

Zircon U–Pb dating of eclogite from the Qiangtang terrane, north-central Tibet: a case of metamorphic zircon with magmatic geochemical features

Zircon is probably the most important mineral used in the dating formation of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks. The origin of zircon, i.e., magmatic or metamorphic, is commonly assessed by its external morphology, internal structure, mineral inclusions, Th/U ratios and trace element composition. In this study, we present an unusual case of metamorphic zircon from the Qiangtang eclogite, north-central Tibet. The zircon grains contain numerous eclogite-facies mineral inclusions, including omphacite, phengite, garnet and rutile; hence, they are clearly of metamorphic origin. However, they display features similar to common magmatic zircon, including euhedral crystal habit, high Th/U ratios and enriched heavy rare earth elements pattern. We suggest that these zircon grains formed from a different reservoir from that for garnet where no trace elements was present and trace element equilibrium between zircon and garnet was achieved. U–Pb dating of zircon gave an age of 232–237 Ma for the eclogite, and that of rutile yielded a slightly younger age of ca. 217 Ma. These ages are consistent with the reported Lu–Hf mineral isochron and phengite Ar–Ar ages. The zircon U–Pb and mineral Lu–Hf isochron ages are interpreted as the time of the peak eclogite-facies metamorphism, whereas the rutile U–Pb and phengite Ar–Ar ages represent the time of exhumation to the middle crust. Thus, the distinction between metamorphic and magmatic zircons cannot be made using only Th/U ratios and heavy REE compositions for HP–UHP metamorphic rocks of oceanic derivation.

Prograde Metamorphic History of UHP Granulites from the Moldanubian Zone (Bohemian Massif) Revealed by Major Element and Y + REE Zoning in Garnets

Major and trace element distribution in garnet crystals from felsic granulites of the Kutná Hora Complex provides evidence of two metamorphic events, the first related to continental subduction at ultrahigh-pressure (UHP) conditions and the second to a granulite-facies overprint that occurred at mid- to lower crustal levels. To reconstruct P–T paths of both metamorphic events, pseudosection modeling was combined with major and trace element zoning in garnet. The granulites contain lenses and boudins of mantle-derived garnet peridotite and eclogite. UHP conditions are confirmed by the presence of inclusions of micro-diamond and coesite in garnet and zircon in the granulites. Compositional zoning in garnet provides evidence for a pre-granulite-facies high-pressure (HP)–UHP metamorphic history of the rocks. The two distinct metamorphic events are documented by high Y + HREE concentrations in the garnet cores and annuli in the mantle part of the garnet grains. The cores of large garnets with relatively high Ca contents and bell-shaped Mn profiles suggest formation during a prograde low- to medium-temperature metamorphic event that was coeval with HP–UHP metamorphism. Decompression and cooling during exhumation of the rocks led to partial resorption of garnet and release of trace elements into the matrix. The new garnet with high Y + HREE in the annuli was formed during the granulite-facies event at crustal levels.

Olivine‐bearing symplectites in fractured garnet from eclogite, Moldanubian Zone (Bohemian Massif) – a short‐lived, granulite facies event

AbstractRecent petrological studies on high‐pressure (HP)–ultrahigh‐pressure (UHP) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P–T loop for the HP–UHP metamorphism and amphibolite–granulite facies overprint in the Moldanubian Zone. Here further evidence is presented that the granulite facies metamorphism occurred after the HP–UHP rocks had been exhumed to different levels of the middle or upper crust. A medium‐temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well‐preserved eclogite facies assemblage with omphacite and prograde zoned garnet. Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of ~600 °C at 2.0–2.2 GPa. The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca‐rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel. Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine‐spinel equilibria and pseudosection calculations indicate temperatures of ~850–900 °C at pressure below 0.7 GPa. The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short‐lived process. The new results point to a geodynamic model where HP–UHP rocks are exhumed to amphibolite facies conditions with subsequent granulite facies heating by mantle‐derived magma in the middle and upper crust.

Coexistence of garnet blueschist and eclogite in South Tianshan, NW China: dependence of P–T evolution and bulk‐rock composition

AbstractCoexisting garnet blueschist and eclogite from the Chinese South Tianshan high‐pressure (HP)–ultrahigh‐pressure (UHP) belt consist of similar mineral assemblages involving garnet, omphacite, glaucophane, epidote, phengite, rutile/sphene, quartz and hornblendic amphibole with or without paragonite. Eclogite assemblages generally contain omphacite >50 vol.% and a small amount of glaucophane (<5 vol.%), whereas blueschist assemblages have glaucophane over 30 vol.% with a small amount of omphacite which is even absent in the matrix. The coexisting blueschist and eclogite show dramatic differences in the bulk‐rock compositions with higher X(CaO) [=CaO/(CaO + MgO + FeOtotal + MnO + Na2O)] (0.33–0.48) and lower A/CNK [=Al2O3/(CaO + Na2O + K2O)] (0.35–0.56) in eclogite, but with lower X(CaO) (0.09–0.30) and higher A/CNK (0.65–1.28) in garnet blueschist. Garnet in both types of rocks has similar compositions and exhibits core–rim zoning with increasing grossular and pyrope contents. Petrographic observations and phase equilibria modelling with pseudosections calculated using thermocalc in the NCKMnFMASHO system for the coexisting garnet blueschist and eclogite samples suggest that the two rock types share similar P–T evolutional histories involving a decompression with heating from the Pmax to the Tmax stage and a post‐Tmax decompression with slightly cooling stage, and similar P–T conditions at the Tmax stage. The post‐Tmax decompression is responsible for lawsonite decomposition, which results in epidote growth, glaucophane increase and omphacite decrease in the blueschist, or in an overprinting of the eclogitic assemblage by a blueschist assemblage. Calculated P–X(CaO), P–A/CNK and P–X(CO2) pseudosections indicate that blueschist assemblages are favoured in rocks with lower X(CaO) (<0.28) and higher A/CNK (>0.75) or fluid composition with higher X(CO2) (>0.15), but eclogite assemblages preferentially occur in rocks with higher X(CaO) and lower A/CNK or fluid composition with lower X(CO2). Moreover, phase modelling suggests that the coexistence of blueschist and eclogite depends substantially on P–T conditions, which would commonly occur in medium temperatures of 500–590 °C under pressures of ~17–22 kbar. The modelling results are in good accordance with the measured bulk‐rock compositions and modelled temperature results of the coexisting garnet blueschist and eclogite from the South Tianshan HP–UHP belt.

In situ Sr isotopic analyses of epidote: tracing the sources of multi-stage fluids in ultrahigh-pressure eclogite (Ganghe, Dabie terrane)

This study presents in situ strontium (Sr) isotope and Sr content data on multi-stage epidote crystals from ultrahigh-pressure (UHP) eclogites and omphacite–epidote veins therein at Ganghe (Dabie terrane, China), determined using LA-MC-ICP-MS. The Ganghe eclogites occur as lenses in mainly leucocratic UHP gneisses, and therefore, our data provide insights into the origin, composition, and transport scale of the discrete multi-stage fluids in UHP eclogites during the subduction and exhumation of a continental crust. Four textural types of epidote that record compositional and isotopic signatures of fluid at various metamorphic P–T conditions have been distinguished based on petrographic observations and compositional analyses. They are (1) fine-grained high-pressure (HP) epidote inclusions (Ep-In) in omphacite that define the earliest stage of epidote formation in the eclogite; (2) coarse-grained UHP epidote porphyroblasts (Ep-P) that contain omphacite with Ep-In inclusions in the eclogite; (3) fine-grained HP epidote in omphacite–epidote veins (Ep-V) as well as (4) the latest-stage epidote in disseminated amphibolite-facies veinlets (Ep-A), which crosscut the Ep-P or matrix minerals in the eclogite and HP vein. Both Ep-P and Ep-V crystals exhibit significant and complex chemical zonations with respect to the XFe (= Fe/(Fe + Al)) ratio and Sr content. In contrast to the varying Sr contents, Ep-In, Ep-P, and Ep-V have similar and narrow ranges of initial 87Sr/86Sr ratios (from 0.70692 to 0.70720 for Ep-In, from 0.70698 to 0.70721 for Ep-P, and from 0.70668 to 0.70723 for Ep-V), which are significantly different from those in Ep-A (from 0.70894 to 0.71172). The initial 87Sr/86Sr ratio of Ep-A is closer in value to the initial Sr isotopic composition of the gneisses (from 0.710790 to 0.712069) which enclose the UHP eclogite. These data indicate different sources of the eclogite-facies fluids and retrograde amphibolite-facies fluid in the Ganghe eclogites. The HP–UHP fluids responsible for the large amounts of hydrous minerals in the eclogites were internally derived and buffered. The omphacite–epidote veins were precipitated from the channelized solute-rich HP–UHP fluids released from the host eclogite. However, hydrated amphibolite-facies metamorphism during exhumation was mainly initiated by the low-Sr and high-87Sr/86Sr external fluid, which infiltrated into the eclogite from the surrounding gneisses. The eclogite-facies fluids in the Ganghe eclogites were locally derived, whereas the infiltration of the retrograde amphibolite-facies fluid from the gneisses required a long transport, most likely longer than 80 m. This study highlights that the in situ Sr isotopic analysis of multi-stage epidote can be employed as a powerful geochemical tracer to provide key information regarding the origin and behavior of various-stage subduction-zone metamorphic fluids.

The architecture of the HP–UHP Dabie massif: New insights from geothermobarometry of eclogites, and implication for the continental exhumation processes

The establishment of a refined architecture of the South Dabie Domain, which covers more than 90% eclogites in Dabie Massif, is a key to understand the subduction and exhumation processes of high-pressure (HP) and ultra-high-pressure (UHP) rocks. According to field and laboratory works, a large area of HP rocks has been recognized in the central and western part of South Dabie Domain, which was previously considered to be part of the entire UHP unit. Petrological analysis, the determination of the peak P–T conditions for all the eclogites of the South Dabie Domain has been carried out. Three litho-tectonic slices, Units-I, Units-II and Units-III have been defined. The spatial distribution of eclogites throughout the South Dabie Domain allows us to construct a structural–temperature–pressure profile, in which the three temperature and pressure units are well correlated to three lithological-tectonic units. Along this profile, two pressure gaps separate these three units with 0.69 and 0.78GPa in average, respectively. Compared to the previous work in which the South Dabie Domain has a N–S gradual P–T trend, an abrupt P–T variation observed in the Huanggang-Niutuling area in the central region, and in the Malong-Shima area in the south-eastern part of the South Dabie Domain have been identified. Based on the petrological investigation and structural analysis, a Huanggang klippe with HP metamorphism, and the Malong tectonic window with UHP metamorphism have been revealed in the South Dabie Domain in detail. With these contributions, we suggested that the present P–T pattern in the South Dabie Domain was resulted from the subduction and exhumation of several lithological-tectonic slices in Triassic. The Cretaceous exhumation and succedent uplift of a migmatite dome are the possible cause for the present architecture of the South Dabie Domain.

Fluid–rock interaction and element mobilization in UHP metabasalt: Constraints from an omphacite–epidote vein and host eclogites in the Dabie orogen

A petrological study is carried out for a high-pressure (HP) omphacite–epidote vein and its host ultrahigh-pressure (UHP) eclogites at Ganghe in the Dabie orogen. The UHP eclogite shows systematic changes toward the HP vein in mineral assemblage, mineral proportion and bulk-rock composition. The eclogite far away from the vein (>20cm) contains abundant lawsonite pseudomorphs indicated by the aggregates of epidote, kyanite and quartz porphyroblasts. The abundance and grain size of these porphyroblasts decrease gradually toward the vein in the nearby eclogite (<20cm), and they are almost absent in the eclogite in direct contact with the vein (<2.5cm). The omphacite–epidote vein contains high amounts of the components hosted in the minerals that were lost in the host eclogite, suggesting significant fluid–rock interaction and mass transfer in the eclogite–vein system during the fluid flow and vein formation. A detailed UHP fluid–eclogite interaction with various element mobilities is inferred from a combined study of metamorphic petrology, geochemistry and mass-balance calculations. The HP vein was formed by fluid liberation from the lawsonite breakdown in the host UHP eclogite at 2.8–3.0GPa and 650–680°C. The released UHP aqueous fluid reacted with the eclogite and dissolved kyanite, coesite, epidote, omphacite, rutile, zircon and apatite. This resulted in significant transfers of SiO2, Al2O3, CaO, Fe2O3, Na2O, TiO2, P2O5, LILE (Sr and Pb), REE and HFSE (Nb, Ta, Zr, Hf, U and Th) into the UHP vein-forming fluid. The solubilities of these minerals and relevant element mobilities gradually increase toward the fluid due to the increase of fluid/eclogite ratios. Fluid-mobile elements (e.g., Cs, Rb, K and Ba) were gained in the eclogite by the formation of late-stage phengite and amphibole, which consumed the interstitial fluid after the precipitation of vein minerals.Therefore, it is inferred that the UHP fluid is an efficient medium to transport not only fluid-mobile elements such as LILE and LREE but also nominally fluid-immobile elements such as HFSE and HREE. Local fluid–rock ratios and solubilities of the host minerals have largely controlled the mobilities of elements. In particular, the dissolution of minerals is a key mechanism to transfer the elements into the HP–UHP fluids released from the cold subducting slab.