中国大陆科学钻探主孔0～4500米变质岩石锆石中保存的超高压矿物包体

中国大陆科学钻探主孔0-4500米的岩心主要由榴辉岩、斜长角闪岩、副片麻岩、正片麻岩以及少量的超基性岩所组成。岩相学研究结果表明,榴辉岩的围岩普遍经历了强烈角闪岩相退变质作用的改造,峰期超高压变质的矿物组合已完全被后期退变质过程中角闪岩相矿物组合所替代。采用激光拉曼技术,配备电子探针和阴极发光测试,发现主孔224件岩心中有121件(包括榴辉岩、斜长角闪岩、副片麻岩和正片麻岩)样品的锆石中普遍隐藏以柯石英为代表的超高压矿物包体,且不同岩石类型锆石中所保存的超高压矿物包体组合存在明显差异。(含多硅白云母)金红石石英榴辉岩锆石中保存的典型超高压包体矿物组合为柯石英 石榴石、柯石英 石榴石 绿辉石 金红石和柯石英 多硅白云母 磷灰石。黑云绿帘斜长角闪岩锆石中保存的超高压矿物组合为柯石英 石榴石 绿辉石、柯石英 石榴石 多硅白云母和柯石英 绿辉石 金红石,与榴辉岩所保存的超高压矿物组合十分相似,表明该类斜长角闪岩是由超高压榴辉岩在构造折返过程中退变质而成。在副片麻岩类岩石,如石榴绿帘黑云二长片麻岩锆石中,代表性的超高压包体矿物组合为柯石英 多硅白云母和柯石英 石榴石等;而在石榴黑云角闪钠长片麻岩锆石中,则保存柯石英 硬玉 石榴石 磷灰石、柯石英 硬玉 多硅白云母 磷灰石和柯石英 石榴石 磷灰石等超高压矿物包体。在正片麻岩锆石中,标志性的超高压矿物包体为柯石英、柯石英 多硅白云母、柯石英 蓝晶石 磷灰石和柯石英 蓝晶石 榍石等。此外,在南苏鲁东海至临沭一带的地表露头以及一系列卫星孔岩心的锆石中,也普遍发现以柯石英为代表的标志性超高压矿物包体,表明在南苏鲁地区由榴辉岩及其围岩的原岩所组成的巨量陆壳物质(方圆>5000km2,厚度超过4.5km)曾整体发生深俯冲,并经历了超高压变质作用。该项研究对于重塑苏鲁-大别超高压变质带俯冲-折返的动力学模式有着重要的科学意义。     

Eclogitic metagabbro from the Lanzada Window,eastern Central Alps: confirmation of subduction beneath the Malenco Unit

Epidote-rich eclogitic metagabbro forms a small body within the Lanzada Window, upper Val Malenco, where it is associated with serpentinites and supracrustal rocks of the Lanzada–Santa Anna Zone (LSZ), which lies structurally beneath the Malenco unit. Conventional garnet–clinopyroxene geothermometry and garnet–clinopyroxene–phengite geobarometry indicate that the peak-metamorphic mineral assemblage (garnet + omphacite + epidote + phengite + titanite + apatite + pyrite) equilibrated at ca. 2.0 GPa and 525 °C. The bulk composition is Ca-rich (wollastonite-normative), suggesting that the rock underwent Ca-metasomatism prior to high-P metamorphism. The presence of eclogite within the LSZ strengthens the correlation of the LSZ with the blueschist-bearing Avers Bündnerschiefer, and confirms the former existence of a southerly-dipping subduction zone beneath the Malenco unit.     

Partial melting due to breakdown of an epidote‐group mineral during exhumation of ultrahigh‐pressure eclogite: An example from the North‐East Greenland Caledonides

In the North‐East Greenland Caledonides, P–T conditions and textures are consistent with partial melting of ultrahigh‐pressure (UHP) eclogite during exhumation. The eclogite contains a peak assemblage of garnet, omphacite, kyanite, coesite, rutile, and clinozoisite; in addition, phengite is inferred to have been present at peak conditions. An isochemical phase equilibrium diagram, along with garnet isopleths, constrains peak P–T conditions to be subsolidus at 3.4 GPa and 940°C. Zr‐in‐rutile thermometry on inclusions in garnet yields values of ~820°C at 3.4 GPa. In the eclogite, plagioclase may exhibit cuspate textures against surrounding omphacite and has low dihedral angles in plagioclase–clinopyroxene–garnet aggregates, features that are consistent with former melt–solid–solid boundaries and crystallized melt pockets. Graphic intergrowths of plagioclase and amphibole are present in the matrix. Small euhedral neoblasts of garnet against plagioclase are interpreted as formed from a peritectic reaction during partial melting. Polymineralic inclusions of albite+K‐feldspar and clinopyroxene+quartz±kyanite±plagioclase in large anhedral garnet display plagioclase cusps pointing into the host, which are interpreted as crystallized melt pockets. These textures, along with the mineral composition, suggest partial melting of the eclogite by reactions involving phengite and, to a large extent, an epidote‐group mineral. Calculated and experimentally determined phase relations from the literature reveal that partial melting occurred on the exhumation path, at pressures below the coesite to quartz transition. A calculated P–T phase diagram for a former melt‐bearing domain shows that the formation of the peritectic garnet rim occurred at 1.4 GPa and 900°C, with an assemblage of clinopyroxene, amphibole, and plagioclase equilibrated at 1.3 GPa and 720°C. Isochemical phase equilibrium modelling of a symplectite of clinopyroxene, plagioclase, and amphibole after omphacite, combined with the mineral composition, yields a P–T range at 1.0–1. 6 GPa, 680–1,000°C. The assemblage of amphibole and plagioclase is estimated to reach equilibrium at 717–732°C, calculated by amphibole–plagioclase thermometry for the former melt‐bearing domain and symplectite respectively. The results of this study demonstrate that partial melt formed in the UHP eclogite through breakdown of an epidote‐group mineral with minor involvement of phengite during exhumation from peak pressure; melt was subsequently crystallized on the cooling path.     

Current mercury ore formation on Mendeleyev

The Maksyutov metamorphic complex is the first locality where coesite pseudomorphs in garnet were described. The importance of this discovery was not understood until ultrahigh-pressure (UHP) metamorphism was independently recognized in the Dora Maira Massif of the western Alps and the Western Gneiss Region of Norway. The coesite pseudomorphs are significant because they suggest that the lower unit of the Maksyutov complex probably underwent UHP metamorphism at depths greater than 80 km in a paleosubduction zone.

The Maksyutov complex, situated in the southern Ural Mountains of Russia, forms an elongate N-S belt along the boundary between the European and Russian plates. The complex contains two superimposed tectonic unitsa lower eclogite-bearing schist unit that underwent high-pressure (HP) to UHP metamorphism and an upper meta-ophiolite unit subjected to blueschist/greenschist-facies metamorphism. The lower unit lithologies range from quartzofeldspathic, to graphite-rich, to mafic-ultramafic compositions. Mineral assemblages of the metamorphosed mafic rocks include: (1) coesite (as pseudomorphs) + garnet + omphacite + rutile + zoisite; (2) jadeite + quartz (coesite) + garnet + kyanite ± paragonite; (3) garnet + omphacite + barroisite + rutile; and (4) garnet + glaucophane + lawsonite. The upper unit is characterized by sheets of serpentinite that contain lawsonite-bearing metarodingite and rare calcium-rich eclogite. A metamorphosed melange containing blocks of ultramafic, eclogite, and quartz-jadeite rocks is situated between the two units.

The UHP metamorphic event that affected the lower unit is characterized by recumbent folding and shear zones. Subsequent large-scale, left-lateral strike-slip movements deformed both tectonic units. These deep-crustal metamorphic structures are oriented at high angles relative to the younger, N-S-trending Main Uralian thrust and the left-lateral strike-slip movement that displaced the Maksyutov block.     

苏鲁地体超高压矿物的三维空间分布

采用激光拉曼技术,配备电子探针和阴极发光测试,确认苏鲁地体大多数花岗质片麻岩,所有类型片麻岩、斜长角闪岩、蓝晶石英岩和大理岩的锆石中均隐藏以柯石英为代表的超高压包体矿物组合。其中花岗质片麻岩典型超高压包体矿物为柯石英±多硅白云母;副片麻岩为柯石英+石榴子石+绿辉石、柯石英±石榴子石+硬玉+多硅白云母+磷灰石、柯石英+多硅白云母±磷灰石;斜长角闪岩为柯石英+石榴子石+绿辉石±金红石;蓝晶石英岩为柯石英+蓝晶石+金红石+磷灰石、柯石英+蓝晶石+多硅白云母+金红石;大理岩为柯石英+透辉石、柯石英+橄榄石。表明苏鲁地体由榴辉岩及其围岩所组成的巨量陆壳物质曾普遍发生深俯冲,并经历了超高压变质作用。锆石的矿物包体分布特征及相应的阴极发光图像研究表明,在同一样品中,锆石的成因特征存在明显差异。有的锆石显示继承性(碎屑)锆石的核(core)、超高压变质的幔(mantle)和退变质的边(rim);有的锆石则具有超高压的核、幔和退变质的边;而有的锆石却记录了深俯冲的核、超高压的幔和退变质的边。标志着苏鲁超高压变质带各类岩石副矿物锆石均具有十分复杂的结晶生长演化历史。因此,在充分研究锆石中矿物包体性质、分布特征以及相应阴极发光图像的基础上,采用SHRIMP离子探针技术,在锆石晶体的不同     

Petrology of coesite-bearing eclogite from Habutengsu Valley, western Tianshan, NW China and its tectonometamorphic implication

Coesite inclusions in garnet have been found in eclogite boudins enclosed in coesite‐bearing garnet micaschist in the Habutengsu Valley, Chinese western Tianshan, which are distinguished from their retrograde quartz by means of optical characteristics, CL imaging and Raman spectrum. The coesite‐bearing eclogite is mainly composed of porphyroblastic garnet, omphacite, paragonite, glaucophane and barroisite, minor amounts of rutile and dotted (or banded) graphite. In addition to coesite and quartz, the zoned porphyroblastic garnet contains inclusions of omphacite, Na‐Ca amphibole, calcite, albite, chlorite, rutile, ilmenite and graphite. Multi‐phase inclusions (e.g. Czo + Pg ± Qtz, Grt II + Qtz and Chl + Pg) can be interpreted as breakdown products of former lawsonite and possibly chloritoid. Coesite occurs scattered within a compositionally homogenous but narrow domain of garnet (outer core), indicative of equilibrium at the UHP stage. The estimate by garnet‐clinopyroxene thermometry yields peak temperatures of 420–520 °C at 2.7 GPa. Phase equilibrium calculations further constrain the P–T conditions for the UHP mineral assemblage Grt + Omp + Lws + Gln + Coe to 2.4–2.7 GPa and 470–510 °C. Modelled modal abundances of major minerals along a 5 °C km^?1 geothermal gradient suggests two critical dehydration processes at ～430 and ～510 °C respectively. Computed garnet composition patterns are in good agreement with measured core‐rim profiles. The petrological study of coesite‐bearing eclogite in this paper provides insight into the metamorphic evolution in a cold subduction zone. Together with other reported localities of UHP rocks from the entire orogen of Chinese western Tianshan, it is concluded that the regional extent of UHP‐LT metamorphism in Chinese western Tianshan is extensive and considerably larger than previously thought, although intensive retrogression has erased UHP‐LT assemblages at most localities.     

Partial transformation of gabbro to coesite-bearing eclogite from Yangkou, the Sulu terrane, eastern China

An ultra-high-pressure (UHP) metamorphic slab at Yangkou Beach near Qingdao in the Sulu region of China consists of blocks of eclogite facies metagabbro, metagranitoid, ultramafic rock and mylonitic orthogneisses enclosed in granitic gneiss. A gradational sequence from incipiently metamorphosed gabbro to completely recrystallized coesite eclogite formed at ultra-high-pressures was identified in a single 30 m block; metagabbro is preserved in the core whereas coesite eclogite occurs along the block margins. The metagabbro contains an igneous assemblage of Pl+Aug+Opx+Qtz+Bt+Ilm/Ti-Mag; it shows relict magmatic textures and reaction coronas. Fine-grained garnet developed along boundaries between plagioclase and other phases; primary plagioclase broke down to Ab+Ky+Ms+Zo±Grt±Amp. Augite is rimmed by sodic augite or omphacite, whereas orthopyroxene is rimmed by a corona of Cum±Act and Omp+Qtz layers or only Omp+Qtz. In transitional rocks, augite and orthopyroxene are totally replaced by omphacite, and the lower-pressure assemblage Ab+Ky+Phn+Zo+Grt coexists with domains of Omp (Jd_70–73)+Ky±Phn in pseudomorphs after plagioclase. Both massive and weakly deformed coesite-bearing eclogites contain Omp+Ky+Grt+Phn+Coe/Qtz+Rt, and preserve a faint gabbroic texture. Coesite inclusions in garnet and omphacite exhibit limited conversion to palisade quartz; some intergranular coesite and quartz pseudomorphs after coesite also occur. Assemblages of the coronal stage, transitional and UHP peak occurred at about 540±50 °C at c. 13 kbar, 600–800 °C at ≥15–25 kbar and 800–850 °C at >30 kbar, respectively. Garnet from the coronal- through the transitional- to the eclogite-stage rocks show a decrease in almandine and an increase in grossular±pyrope components; garnet in low-grade rocks contains higher MnO and lower pyrope components. The growth textures of garnet within pseudomorphs after plagioclase or along grain boundaries between plagioclase and other phases are complex; the application of garnet zoning to estimate P–T should be carried out with caution. Some garnet enclosing quartz aggregates as inclusions shows radial growth boundaries; these quartz aggregates, as well as other primary and low-P phases, persisted metastably at UHP conditions due to sluggish reactions resulting from the lack of fluid during prograde and retrograde P–T evolution.     

Phase equilibria and metamorphic evolution of glaucophane‐bearing UHP eclogites from the Western Dabieshan Terrane,Central China

Glaucophane‐bearing ultrahigh pressure (UHP) eclogites from the western Dabieshan terrane consist of garnet, omphacite, glaucophane, kyanite, epidote, phengite, quartz/coesite and rutile with or without talc and paragonite. Some garnet porphyroblasts exhibit a core–mantle zoning profile with slight increase in pyrope content and minor or slight decrease in grossular and a mantle–rim zoning profile characterized by a pronounced increase in pyrope and rapid decrease in grossular. Omphacite is usually zoned with a core–rim decrease in j(o) [=Na/(Ca + Na)]. Glaucophane occurs as porphyroblasts in some samples and contains inclusions of garnet, omphacite and epidote. Pseudosections calculated in the NCKMnFMASHO system for five representative samples, combined with petrographic observations suggest that the UHP eclogites record four stages of metamorphism. (i) The prograde stage, on the basis of modelling of garnet zoning and inclusions in garnet, involves P–T vectors dominated by heating with a slight increase in pressure, suggesting an early slow subduction process, and P–T vectors dominated by a pronounced increase in pressure and slight heating, pointing to a late fast subduction process. The prograde metamorphism is predominated by dehydration of glaucophane and, to a lesser extent, chlorite, epidote and paragonite, releasing ～27 wt% water that was bound in the hydrous minerals. (ii) The peak stage is represented by garnet rim compositions with maximum pyrope and minimum grossular contents, and P–T conditions of 28.2–31.8 kbar and 605–613 °C, with the modelled peak‐stage mineral assemblage mostly involving garnet + omphacite + lawsonite + talc + phengite + coesite ± glaucophane ± kyanite. (iii) The early decompression stage is characterized by dehydration of lawsonite, releasing ～70–90 wt% water bound in the peak mineral assemblages, which results in the growth of glaucophane, j(o) decrease in omphacite and formation of epidote. And, (iv) The late retrograde stage is characterized by the mineral assemblage of hornblendic amphibole + epidote + albite/oligoclase + quartz developed in the margins or strongly foliated domains of eclogite blocks due to fluid infiltration at P–T conditions of 5–10 kbar and 500–580 °C. The proposed metamorphic stages for the UHP eclogites are consistent with the petrological observations, but considerably different from those presented in the previous studies.     

Geothermobarometry of UHP and HP eclogites and schists – an evaluation of equilibria among garnet–clinopyroxene–kyanite–phengite–coesite/quartz

Geothermometry of eclogites and other high pressure (HP)/ultrahigh‐pressure (UHP) rocks has been a challenge, due to severe problems related to the reliability of the garnet–clinopyroxene Fe–Mg exchange thermometer to omphacite‐bearing assemblages. Likewise, reliable geobarometers for eclogites and related HP/UHP rocks are scarce. In this paper, a set of internally consistent geothermobarometric expressions have been formulated for reactions between the UHP assemblage garnet–clinopyroxene–kyanite–phengite–coesite, and the corresponding HP assemblage garnet–clinopyroxene–kyanite–phengite–quartz. In the system KCMASH, the end members grossular (Grs) and pyrope (Prp) in garnet, diopside (Di) in clinopyroxene, muscovite (Ms) and celadonite (Cel) in phengite together with kyanite and coesite or quartz define invariant points in the coesite and quartz stability field, respectively, depending on which SiO₂ polymorph is stable. Thus, a set of net transfer reactions including these end members will uniquely define equilibrium temperatures and pressures for phengite–kyanite–SiO₂‐bearing eclogites. Application to relevant eclogites from various localities worldwide show good consistency with petrographic evidence. Eclogites containing either coesite or polycrystalline quartz after coesite all plot within the coesite stability field, while typical quartz‐bearing eclogites with no evidence of former coesite fall within the quartz stability field. Diamondiferous coesite–kyanite eclogite and grospydite xenoliths in kimberlites all fall into the diamond stability field. The present method also yields consistent values as compared with the garnet–clinopyroxene Fe–Mg geothermometer for these kinds of rocks, but also indicates some unsystematic scatter of the latter thermometer. The net transfer geothermobarometric method presented in this paper is suggested to be less affected by later thermal re‐equilibration than common cation exchange thermometers.     

Paragonite eclogites from Dabie Shan,China: re-equilibration during exhumation?

Abstract Paragonite in textural equilibrium with garnet, omphacite and kyanite is found in two eclogites in the ultrahigh-pressure metamorphic terrane in Dabie Shan, China. Equilibrium reactions between paragonite, omphacite and kyanite indicate a pressure of about 19 kbar at c . 700° C. However, one of the paragonite eclogites also contains clear quartz pseudomorphs after coesite as inclusions in garnet, suggesting minimum pressures of 27 kbar at the same temperature. The disparate pressure estimates from the same rock suggest that the matrix minerals in the ultrahigh-pressure eclogites have recrystallized at lower pressures and do not represent the peak ultrahigh-pressure assemblages. This hypothesis is tested by calibrating a garnet + zoisite/clinozoisite + kyanite + quartz/coesite geobarometer and applying it to the appropriate eclogite facies rocks from ultrahigh- and high-pressure terranes. These four minerals coexist from 10 to 60 kbar and in this wide pressure range the grossular content of garnet reflects the equilibrium pressure on the basis of the reaction zoisite/clinozoisite = grossular + kyanite + quartz/coesite + H₂O. The results of the geobarometer agree well with independent pressure estimates from eclogites from other orogenic belts. For the paragonite eclogites in Dabie Shan the geobarometer indicates pressures in the quartz stability field, confirming that the former coesite-bearing paragonite-eclogite has re-equilibrated at lower pressures. On the other hand, garnets from other coesite-bearing but paragonite-free kyanite-zoisite eclogites show a very wide variation in grossular content, corresponding to a pressure variation from coesite into the quartz field. This wide variation, partly due to a rimward decrease in grossular component in garnet, is caused by partial equilibration of the mineral assemblage during the exhumation.     

Integrating X‐ray mapping and microtomography of garnet with thermobarometry to define the P–T evolution of the (near) UHP Międzygórze eclogite,Sudetes, SW Poland

Detailed X‐ray compositional mapping and microtomography have revealed the complex zoning and growth history of garnet in a kyanite‐bearing eclogite. The garnet occurs as clusters of coalesced grains with cores revealing slightly higher Ca and lower Mg than the rims forming the coalescence zones between the grains. Core regions of the garnet host inclusions of omphacite with the highest jadeite, and phengite with the highest Si, similar to values in the cores of omphacite and phengite located in the matrix. Therefore, the core compositions of garnet, omphacite, and phengite have been chosen for the peak pressure estimate. Coupled conventional thermobarometry, average P–T, and phase equilibrium modelling in the NCKFMMnASHT system yields P–T conditions of 26–30 kbar at 800–930°C. Although coesite is not preserved, these P–T conditions partially overlap the coesite stability field, suggesting near ultra‐high–pressure (UHP) conditions during the formation of this eclogite. Therefore, the peak pressure assemblage is suggested to have been garnet–omphacite–kyanite–phengite–coesite/quartz–rutile. Additional lines of evidence for the possible UHP origin of the Mi?dzygórze eclogite are the presence of rod‐shaped inclusions of quartz parallel to the c‐axis in omphacite as well as relatively high values of Ca‐Tschermak and Ca‐Eskola components. Late zoisite, rare diopside–plagioclase symplectites rimming omphacite, and minor phlogopite–plagioclase symplectites replacing phengite formed during retrogression together with later amphibole. These retrograde assemblages lack minerals typical of granulite facies, which suggests simultaneous decompression and cooling during exhumation before the crustal‐scale folding that was responsible for final exhumation of the eclogite.     

Ultrahigh pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: I. Petrology and geochemistry of the main hole (0–2,050 m)

The main hole (MH) of the Chinese Continental Scientific Drilling Project (CCSD) in southern Sulu has penetrated into an ultrahigh-pressure (UHP) metamorphic rock slice which consists of orthogneiss, paragneiss, eclogite, ultramafic rock and minor schist. Recovered eclogites have a UHP metamorphic mineral assemblage of garnet + omphacite + rutile ± phengite ± kyanite ± coesite ± epidote. Ultramafic rocks contain garnet + olivine + clinopyroxene + orthopyroxene ± Ti-clinohumite ± phlogopite. Gneisses and schists contain an amphibolite-facies paragenesis, but their zircons have coesite, garnet, omphacite (or jadeite) and phengite inclusions, indicating that eclogites and gneisses have been subjected to in situ UHP metamorphism. Using available geothermobarometers, P–T estimates of 3.1–4.4 GPa and 678–816°C for eclogites were obtained. If surface outcrops and neighboring shallow drill holes are considered together, we suggest that a huge supracrustal rock slab (> 50 km long × 100 km wide × 5 km deep) was subducted to a depth > 100 km and then exhumed to the surface. The depth interval (0–2,050 m) of the CCSD-MH can be divided into six lithological units. Unit 1 consists of alternating layers of quartz-rich and rutile-rich eclogites, with thin interlayers of gneiss and schist. Eclogites of unit 1 are characterized by Nb, Ta, Sr and Ti depletions, low Mg number and general LREE enrichment. Unit 2 comprises rutile- and ilmenite-rich eclogite and minor “normal” eclogite and is characterized by high TiO₂, total Fe, V, Co and Sr, and very low SiO₂, alkali, Zr, Ba, Nb, Ta and total REE contents, and LREE-depleted REE patterns with slightly positive Eu anomalies. Unit 3 contains ultramafic rock and minor MgO-rich eclogite. Protoliths of UHP rocks from units 1, 2 and 3 represent a layered mafic to ultramafic intrusion at crustal depth. Units 4 and 6 consist of interlayered eclogite and paragneiss; the eclogites are characterized by Th, U, Nb, Ta and Ti depletion and K enrichment and LREE-enriched REE patterns. Paragneisses show Nb, Ta, Sr and Ti depletions and LREE-enriched REE patterns occasionally with slightly negative Eu anomalies, indicating that their protoliths represent metamorphic supracrustal series. Unit 5 consists mainly of orthogneisses, showing distinct Nb, Ta, Sr and Ti depletions, and LREE-enriched REE patterns with pronounced negative Eu anomalies, suggesting granitic protoliths. In conclusion it is proposed that the southern Sulu UHP belt consists of a series of meta-supracrustal rocks, a layered mafic–ultramafic complex and granites.     

Petrology and U–Pb zircon dating of coesite-bearing metapelite from the Kebuerte Valley,western Tianshan,China

This paper deals with the petrology and U–Pb dating of coesite-bearing garnet–phengite schist from the Kebuerte Valley, Chinese western Tianshan. It mainly consists of porphyroblastic garnet, phengite, quartz and chlorite with minor amounts of paragonite, albite, zoisite and chloritoid. The well preserved coesite inclusions (∼100 μm) in garnet are encircled by a narrow rim of quartz. They were identified by optical microscopy and confirmed by Raman spectroscopy. Using the computer program THERMOCALC, the peak metamorphic conditions of 29 kbar and 565 °C were obtained via garnet isopleth geothermobarometry. The predicted UHP peak mineral assemblage comprises garnet + jadeite + lawsonite + carpholite + coesite + phengite. The metapelite records prograde quartz–eclogite-facies metamorphism, UHP coesite–eclogite-facies peak metamorphism, and a late greenschist-facies overprint. Phase equilibrium modeling predicts that garnet mainly grew in the mineral assemblages garnet + jadeite + lawsonite + chloritoid + glaucophane + quartz + phengite and garnet + jadeite + lawsonite + carpholite + glaucophane + quartz + phengite. SHRIMP U–Pb zircon dating of the coesite-bearing metapelite yielded the peak metamorphic age 320.4 ± 3.7 Ma. For the first time, age data of coesite-bearing UHP metapelite from the Chinese western Tianshan are presented in this paper. They are in accord with published ages obtained from eclogite from other localities in the Chinese western Tianshan and the Kyrgyz South Tianshan and therefore prove a widespread occurrence of UHP metamorphism.     

Ultra-high-pressure metamorphism of carbonate rocks in the Dabie Mountains, central China

Abstract Widespread ultra-high-P assemblages including coesite, quartz pseudomorphs after coesite, aragonite, and calcite pseudomorphs after aragonite in marble, gneiss and phengite schist are present in the Dabie Mountains eclogite terrane. These assemblages indicate that the ultra-high-P metamorphic event occurred on a regional scale during Triassic collision between the Sino-Korean and Yangtze cratons. Marble in the Dabie Mountains is interlayered with coesite-bearing eclogite and gneiss and as blocks of various size within gneiss. Discontinuous boudins of eclogite occur within marble layers. Marble contains an ultra-high-P assemblage of calcite/aragonite, dolomite, clinopyroxene, garnet, phengite, epidote, rutile and quartz/coesite. Coesite, quartz pseudomorphs after coesite, aragonite and calcite pseudomorphs after aragonite occur as fine-grained inclusions in garnet and omphacite. Phengites contain about 3.6 Si atoms per formula unit (based on 11 oxygens). Similar to the coesite-bearing eclogite, marble exhibits retrograde recrystallization under amphibolite–greenschist facies conditions generated during uplift of the ultra-high-P metamorphic terrane. Retrograde minerals are fine grained and replace coarse-grained peak metamorphic phases. The most typical replacements are: symplectic pargasitic hornblende + epidote after garnet, diopside + plagioclase (An₁₈) after omphacite, and fibrous phlogopite after phengite. Ferroan pargasite + plagioclase, and actinolite formed along grain boundaries between garnet and calcite, and calcite and quartz, respectively. The estimated peak P–T conditions for marble are comparable to those for eclogite: garnet–clinopyroxene geothermometry yields temperatures of 630–760°C; the garnet–phengite thermometer gives somewhat lower temperatures. The minimum pressure of peak metamorphism is 27 kbar based on the occurrence of coesite. Such estimates of ultra-high-P conditions are consistent with the coexistence of grossular-rich garnet + rutile, and the high jadeite content of omphacite in marble. The fluid for the peak metamorphism was calculated to have a very low X_CO2 (<0.03). The P–T conditions for retrograde metamorphism were estimated to be 475–550°C at <7 kbar.     

Factors affecting preservation of coesite in ultrahigh‐pressure metamorphic rocks: Insights from TEM observations of dislocations within kyanite

To understand the preservation of coesite inclusions in ultrahigh‐pressure (UHP) metamorphic rocks, an integrated petrological, Raman spectroscopic and focussed ion beam (FIB) system–transmission electron microscope (TEM) study was performed on a UHP kyanite eclogite from the Sulu belt in eastern China. Coesite grains have been observed only as rare inclusions in kyanite from the outer segment of garnet and in the matrix. Raman mapping analysis shows that a coesite inclusion in kyanite from the garnet rim records an anisotropic residual stress and retains a maximum residual pressure of ~0.35 GPa. TEM observations show quartz is absent from the coesite inclusion–host kyanite grain boundaries. Numerous dislocations and sub‐grain boundaries are present in the kyanite, but dislocations are not confirmed in the coesite. In particular, dislocations concentrate in the kyanite adjacent to the boundary with the coesite inclusion, and they form a dislocation concentration zone with a dislocation density of ~10⁹ cm^?2. A high‐resolution TEM image and a fast Fourier transform‐filtered image reveal that a tiny dislocation in the dislocation concentration zone is composed of multiple edge dislocations. The estimated dislocation density in most of the kyanite away from the coesite inclusion–host kyanite grain boundaries is ~10⁸ cm^?2, being lower than that in kyanite adjacent to the coesite. In the case of a coesite inclusion in a matrix kyanite, using Raman and TEM analyses, we could not identify any quartz at the grain boundaries. Dislocations are not observed in the coesite, but numerous dislocations and stacking faults are developed in the kyanite. The estimated overall dislocation density in the coesite‐bearing matrix kyanite is ~10⁸ cm^?2, but a high dislocation density region of ~10⁹ cm^?2 is also present near the coesite inclusion–host kyanite grain boundaries. Inclusion and matrix kyanite grains with no coesite have dislocation densities of ≤10⁸ cm^?2. Dislocation density is generally reduced during an annealing process, but our results show that not all dislocations in the kyanite have recovered uniformly during exhumation of the UHP rocks. Hence, one of the key factors acting as a buffer to inhibit the coesite to quartz transformation is the mechanical interaction between the host and the inclusion that lead to the formation of dislocations in the kyanite. The kyanite acts as an excellent pressure container that can preserve coesite during the decompression of rocks from UHP conditions. The search for and study of inclusions in kyanite may be a more suitable approach for tracing the spatial distribution of UHP metamorphic rocks.     

The Lanshantou Kyanite-bearing Eclogite with Coesite Inclusions in the Sulu Ultrahigh-Pressure Metamorphic Belt and Its PTt Path

Coesite inclusions are found in kyanite from the Lanshantou eclogite in the Sulu ultrahigh-pressure (UHP) metamorphic belt. This discovery extends the stable region of kyanite to over 2.4 GPa. As an important UHP metamorphic belt in China, the Sulu eclogite belt is the product of A-subduction induced by strong compression of the Yellow Sea terrane to the Jiaodong-northereastern Jiangsu terrane during the interaction of the Eurasian plate and Palaeo-Pacific plate in the Indosinian. It stretches about 350 km and contains over 1000 eclogite bodies. Most eclogites in this belt belong to Groups B and C in the classification of Coleman et al., and commonly contain kyanite, while the Lanshantou eclogite belongs to Group A and contains coesite. The MgO, CaO and FeO contents in garnet and pyroxene show regular variation from the core to the rim, which reveals the PTt paths of progressive metamorphism during the Early Mesozoic (240-200 Ma) and retrogressive metamorphism during the Late Mesozoic and Cenozoic exhum     

Petrological and Geochemical Characteristics of Ultrahigh-Pressure Metamorphic Rocks from the Dabie-Sulu Terrane,East-Central China

The Dabie-Sulu ultrahigh-pressure (UHP) terrane is the largest in the world. Mafic-ultramafic rocks occur as ubiquitous minor components in it, and preserve UHP index minerals such as coesite and microdiamond. Eclogites and garnet peridotites together with their country rocks including quartzofeldspathic, pelitic, psammatic, and carbonate rocks were subjected to coeval UHP meta-morphism during the Triassic collision between the Sino-Korean and Yangtze cratons. This review summarizes petrological and geochemical characteristics of eclogites, garnet peridotites, and UHP metasediments from published data and ongoing research in the Dabie-Sulu belt of east-central China.

Parageneses of minerals from coesite-bearing eclogites encased in gneiss, garnet peridotite, and marble delineate clockwise P-T paths characterized by nearly isothermal decompression. Many eclogites have been overprinted by Barrovian-zone amphibolite- to granulite-facies assemblages, whereas others preserve primary igneous minerals and textures in the cores of coesite-bearing eclogitic blocks. In addition, eclogites contain UHP hydrous and carbonate phases including talc, epidote, zoisite, magnesite, and dolomite; these together with Ti-clinohumite, phlogopite, amphibole, chlorite, and possible talc in garnet peridotites and OH-topaz in kyanite quartzites document the role of hydrous mineral transport to the deep mantle in fluid-deficient UHP metamorphic regions. Both crustal- and mantle-derived garnet peridotites from the Dabie-Sulu region were recrystallized within the diamond stability field, with the Earth's lowest recorded geothermal gradient. <5°C/km, suggesting that UHP metamorphic recrystallization took place in a previously unrecognized, forbidden P-T region.

Geochemical and isotopic data indicate that Dabie-Sulu mafic-ultramafic rocks have diverse origins; their compositions in some cases have been complicated by metamorphic recrystallization, crustal contamination, and fluid metasomatism. Nevertheless, REE geochemical and Nd isotope data clearly indicate that they have “continental” affinities and cannot represent a subducted Tethyan oceanic slab. Garnet peridotites and their enclosing eclogites display variable isotopic compositions; mantle-derived fragments preserve a mantle signature, whereas crust-hosted mafic-ultramafics display distinct crustal contamination and metasomatism. Among the many outstanding projects remaining to be investigated, geochemical and isotopic constraints of mantle-derived garnet peridotites and eclogites should provide an additional window to our understanding of mantle heterogeneity, metasomatism, slab/mantle interactions, and lithospheric evolution of the Sino-Korean craton.     

Petrology of ultrahigh-pressure rocks from the southern Su-Lu region, eastern China

Abstract In the Su-Lu ultrahigh- P terrane, eastern China, many coesite-bearing eclogite pods and layers within biotite gneiss occur together with interlayered metasediments now represented by garnet-quartz-jadeite rock and kyanite quartzite. In addition to garnet + omphacite + rutile + coesite, other peak-stage minerals in some eclogites include kyanite, phengite, epidote, zoisite, talc, nyböite and high-Al titanite. The garnet-quartz-jadeite rock and kyanite quartzite contain jadeite + quartz + garnet + rutile ± zoisite ± apatite and quartz + kyanite + garnet + epidote + phengite + rutile ± omphacite assemblages, respectively. Coesite and quartz pseudomorphs after coesite occur as inclusions in garnet, omphacite, jadeite, kyanite and epidote from both eclogites and metasediments. Study of major elements indicates that the protolith of the garnet-quartz jadeite rock and the kyanite quartzite was supracrustal sediments. Most eclogites have basaltic composition; some have experienced variable 'crustal'contamination or metasomatism, and others may have had a basaltic tuff or pyroclastic rock protolith.
The Su-Lu ultrahigh- P rocks have been subjected to multi-stage recrystallization and exhibit a clockwise P-T path. Inclusion assemblages within garnet record a pre-eclogite epidote amphibolite facies metamorphic event. Ultrahigh- P peak metamorphism took place at 700–890° C and P >28 kbar at c . 210–230 Ma. The symplectitic assemblage plagioclase + hornblende ± epidote ± biotite + titanite implies amphibolite facies retrogressive metamorphism during exhumation at c . 180–200 Ma. Metasedimentary and metamafic lithologies have similar P-T paths. Several lines of evidence indicate that the supracrustal rocks were subducted to mantle depths and experienced in-situ ultrahigh- P metamorphism during the Triassic collision between the Sino-Korean and Yangtze cratons.     

含柯石英榴辉岩形成深度的构造校正测算

应用透射电子显微镜(TEM)对大别地区英山县含柯石英榴辉岩、石英榴辉岩及石榴角闪岩中石榴子石进行研究,结果表明在强塑性变形的三类岩石中,石榴子石的超微构造特征存在明显差异。在含柯石英榴辉岩中的石榴子石位错密度比角闪岩中的低。修正石榴子石材料系数α为0.25,用石榴子石位错密度估算了相对差异应力值,并讨论了在超高压榴辉岩形成和退变质过程中各阶段的变质条件。根据石榴子石位错密度测定的差异应力,结合石榴子石的变形测量,恢复构造三维主应力及构造附加静水压力值P_S,从柯石英相的转变压力P中减去构造附加静水压力值P_S后,利用单纯由重力引起的静水压力值P_G换算上覆岩石的重力和厚度,获得大别超高压变质带含柯石英榴辉岩形成深度仅为 ≥ 32.09~32.11km.因此提出,该含柯石英榴辉岩是在这一地壳深处(或稍深一些)受强烈构造作用形成的新认识。同位素资料分析表明,大别地区榴辉岩系地壳成因,这对于榴辉岩是壳内产物的认识提供了进一步证据。      

Partial melting of metapelites at ultrahigh-pressure conditions, Greenland Caledonides

Eclogite, orthogneiss and, by association, metapelite from an island at 78°N in North‐East Greenland experienced ultrahigh‐pressure (UHP) metamorphism at approximately 970 °C and 3.6 GPa, at the end of the Caledonian collision, 360–350 Ma. Hydrous metapelites contain abundant leucocratic layers and lenses composed of medium‐grained, anhedral, equigranular quartz, antiperthitic plagioclase and K‐feldspar with minor small garnet and kyanite crystals. Leucosomes are generally parallel to the matrix foliation, are interlayered with residual quartz bands, anastomose around residual garnet and commonly cross‐cut micaceous segregations. Textures suggest that the leucosomes crystallized from a syntectonic melt, but crystallized at the end of local high‐grade deformation. The metapelite outcrop is < 1.5 km from kyanite eclogites with confirmed coesite, but the metapelites lack coesite and palisade textures diagnostic of coesite pseudomorphs. They do contain highly fractured garnet megacrysts with polycrystalline quartz inclusions (some surrounded by radial fractures) and Ti‐rich phengite inclusions that suggest the former presence of coesite. Polyphase inclusions in garnet contain reactants and products of the inferred dehydration melting reaction: Phe + Qtz = Ky + Kfs + Rt + melt. The reactants are thought to have been early inclusions of hydrous phases within garnet that melted and then crystallized new phases. Garnet surrounding these inclusions has patchy zoning with elevated Ca, consistent with experiments that produced similar patchy microstructures in garnet around inclusions with an unequivocal melt origin. The peak UHP metamorphic assemblage in these rocks is inferred to have been phengite, coesite, garnet, kyanite, rutile, fluid ± omphacite ± epidote. Phase diagrams indicate that dehydration melting of phengite in this assemblage would have occurred after decompression from peak pressure, but still above the coesite to quartz transition. Unusual crown‐ and moat‐like textures in garnet around some polycrystalline quartz inclusions are also consistent with the inference that melting took place at UHP conditions.