Unusual exsolution phenomena in omphacite and partial replacement of phengite by phlogopite + kyanite in an eclogite from the Erzgebirge

In the Erzgebirge Crystalline Complex, eclogites occur in three different high pressure (HP) units (1, 2 and 3) recording contrasting pressure (P)–temperature (T) conditions. Eclogites from HP-unit 1 experienced peak metamorphic conditions in the coesite stability field at about 33 kbar/850 °C. Commonly, these eclogites from HP-unit 1 are all very similar, with an eclogitic peak assemblage of omphacite–garnet–coesite–K-feldspar, rarely accompanied by kyanite, and omphacites systematically deviating from a stoichiometric composition. In contrast, an eclogite recently found near Blumenau, is mineralogically and geochemically different from the typical eclogites of HP-unit 1. This unusual eclogite reveals the eclogitic equilibrium assemblage omphacite–garnet–coesite–phengite–phlogopite–kyanite, and yields metamorphic peak conditions of 870 °C and >29 kbar. There is clear textural evidence of the formation of phlogopite and kyanite under partial consumption of phengite and garnet. Moreover, the omphacite is stoichiometric and contains abundant exsolution lamellae, the thickest of which were identified as quartz by the electron microprobe. The finer lamellae were studied by transmission electron microscopy (TEM). Oligoclase was identified as an exsolution phase. Other lamellae proved to consist of K-white mica, also interpreted as exsolution. Prior to exsolution, the omphacite composition must have been cation-deficient, as that of the other, common HP-unit 1 eclogites. These non-stoichiometric compositions are ascribed to partial substitution by the Ca-Eskola pyroxene component, which calculates to an average of 8 mol% for omphacite in HP-unit 1 eclogites. According to experiments, this substitution becomes significant at P > 30 kbar. Exsolution of K-white mica may indicate hydroxyl defects in the original omphacite, also favoured by high pressure. Oligoclase and K-white mica exsolution from Ca-Eskola-rich clinopyroxene has not previously been reported. The omphacite has a disordered C2/c structure; and in just one case very small (a few tens of nanometres) antiphase domains, resulting from the C2/c to P2/n transformation, are present. These features may indicate a brief thermal history and rapid tectonic processes. Received: 4 January 1999 / Accepted: 20 April 2000     

Talc-, Magnesite- and Ti-Clinohumite-Bearing Ultrahigh-Pressure Meta-Mafic and Ultramafic Complex in the Dabie Mountains, China

The Bixiling mafic-ultramafic metamorphic complex is a 1•5km² tectonic block within biotite gneiss in the southern Dabieultrahigh-pressure terrane, central China. The complex consistsof banded eclogites that contain thin layers of garnet-bearingcumulate ultramafic rock. Except for common eclogitic phases(garnet, omphacite, kyanite, phengite, zoisite and rutilc),banded eclogites contain additional talc and abundant coesiteinclusions in omphacite, zoisite, kyanite and garnet. Some metaultramaficrocks contain magnesite and Ti-clinohumite. Both eclogites andmeta-ultramafic rocks have undergone multi-stage metamorphism.Eclogite facies metamorphisrn occurred at 610–700C andP>27 kbar, whereas amphibolite facies retrograde metamorphismis characterized by symplectites of plagioclase and hornblendeafter omphacite and replacement of tremolite after talc at P<6–15kbar and T <600C. The meta-ultramafic assemblages such asolivine + enstatite + diopside + garnet and Ti-clinohumite +diopside + enstatite + garnet + magnesite olivine formed at700–800C and 47–67 kbar. Investigation of the phaserelations for the system CaO-MgO-SiO₂-H₂O-CO₂ and the experimentallydetermined stabilities of talc, magnesite and Ti-clinohumiteindicate that (1) UHP talc assemblages are restricted to Mg-Algabbro composition and cannot be an important water-bearingphase in the ultramafic mantle, and (2) Ti-clinohumite and magnesiteare stable H₂O-bearing and CO₂-bearing phases at depths >100km. The mafic-ultramafic cumulates were initially emplaced atcrustal levels, then subducted to great depths during the Triassiccollision of the Sine-Korean and Yangtze cratons. KEY WORDS: eclogite; magnesite; meta-ultramafics; talc; ultrahigh-P metamorphism ^*Corresponding author     

Petrology and phase relations of the kyanite-eclogites from eastern Turkey

Eclogites are found as lenses or layers in the Precambrian gneiss terrain of the Bitlis Massif in eastern Turkey. Kyanite-eclogites from the region of Gablor Hill in the Bitlis Massif exhibit relatively minor alteration, and consist of garnet, omphacite, kyanite, zoisite, calcic amphibole, phengite, rutile and quartz. In terms of geological setting and mineral compositions, Gablor eclogites are very similar to eclogites from high-grade gneiss terrains. Metamorphic conditions during the eclogite crystallisation are determined as 625±35° C and 16±3 kbars. The coexistence of white mixa, omphacite and kyanite constrains between 0.4 and 1. Primary calcic amphiboles from the Gablor eclogites exhibit conflicting textures, indicating stable coexistence with, as well as growth from omphacite and garnet. This is explained by a buffering reaction between amphibole, garnet, omphacite, zoisite and kyanite during the eclogite crystallisation, whereby is controlled and buffered by the mineral assemblage.     

大别山北部榴辉岩的退变质特征及其地质意义

研究了大别山北部榴辉岩的变质岩岩石学。结果表明，该区榴辉岩相变质作用可分为早期(超高压)和晚期(高压)两个阶段，并在折返过程中形成了一系列特征性的退变质显微构造。其中，退变质结构主要包括：(1)由于压力降低而出溶形成的一些定向针状或叶片状矿物包裹体，如钠质单斜辉石中石英及石榴子石中的金红石、单斜辉石和磷灰石等；(2)冠状体或后成合晶，特别是石榴子石外围发育两期(“双层”)后成合晶；(3)反应边或退变边，如绿辉石的透辉石退变边、透辉石的角闪石退变边和金红石的钛铁矿退变边等。这些退变质结构为本区榴辉岩高级变质岩的快速折返过程和抬升历史提供了强有力的岩石学依据；石榴子石中针状矿物出溶体进一步证明研究区榴辉岩早期经历了超高压变质作用，峰期变质压力应大干4．0GPa，甚至可能达到5～7GPa或更高。     

Retrogressive Microstructures of the Eclogites from the Northern Dabie Mountains, Central China： Evidence for Rapid Exhumation

The ultrahigh-pressure eclogites from the northern Dabie Mountains in central China occurred as tectonic lens or blocks within granitic gneisses or meta-peridotites. Petrologic studies suggest that the studied eclogites experienced strongly retrogressive metamorphism and produced a series of characteristic retrogressive microstructures. The retrograde structures mainly include: (1) oriented needle mineral exsolution, e. g. , quartz needles in Na-clinopyroxene and rotile, clinopyroxene and apatite exsolution in garnet formed under decreasing pressure conditions during exhumation; (2) symplectite, especially, two generations of symplectites developed outside the garnet grains, which are called ““double symplectite““ here; (3) compositional zoning of minerals such as garnet and clinopyroxene; (4) minerals with a reaction rim or retrograde rim, e.g. , omphacite with a diopside rim, diopside with an amphibole rim and rutile with a rim of ilmenite. These retrograde textures, especially mineral zoning and symplectite, provide important petrologic evidence for the exhumation process and uplift of high-grade metamorphic rocks such as eclogite in the northern Dabie Mountains, indicating a rapid exhumation process.     

Petrology of ultrahigh-pressure eclogites from the ZK703 drillhole in the Donghai, eastern China

The 558-m-deep ZK703 drillhole located near Donghai in the southern part of the Sulu ultrahigh-pressure metamorphic belt, eastern China, penetrates alternating layers of eclogites, gneisses, jadeite quartzites, garnet peridotites, phengite–quartz schists, and kyanite quartzites. The preservation of ultrahigh-pressure metamorphic minerals and their relics, together with the contact relationship and protolith types of the various rocks indicates that these are metamorphic supracrustal rocks and mafic-ultramafic rock assemblages that have experienced in-situ ultrahigh-pressure metamorphism. The eclogites can be divided into five types based on accessory minerals: rutile eclogite, phengite eclogite, kyanite–phengite eclogite, quartz eclogite, and common eclogite with rare minor minerals. Rutile eclogite forms a thick layer in the drillhole that contains sufficient rutile for potential mining. Two retrograde assemblages are observed in the eclogites: the first stage is characterized by the formation of sodic plagioclase+amphibole symplectite or symplectitic coronas after omphacite and garnet, plagioclase+biotite after garnet or phengite, and plagioclase coronas after kyanite; the second stage involved total replacement of omphacite and garnet by amphibole+albite+epidote+quartz. Peak metamorphic P–T conditions of the eclogites were around 32 to 40 kbar and 720°C to 880°C. The retrograde P–T path of the eclogites is characterized by rapidly decreasing pressure with slightly decreasing temperature. Micro-textures and compositional variations in symplectitic minerals suggest that the decompression breakdown of ultrahigh-pressure minerals is a domainal equilibrium reaction or disequilibrium reaction. The composition of the original minerals and the diffusion rate of elements involved in these reactions controlled the symplectitic mineral compositions.     

Relics of eclogite facies metamorphism in the Austroalpine basement,Hochgrössen (Speik complex), Austria

Summary Retrograde eclogites and serpentinites from the Hochgr?ssen massif, Styria, are parts of the Speik complex in the Austroalpine basement nappes of the Eastern Alps. They are in tectonic contact with pre-Alpine gneisses, amphibolites, and Permo-Triassic quartz phyllites (Rannach Series). The eclogites are derived from ocean-floor basalts with affinities to mid-ocean ridge and back-arc basin basalts. Fresh eclogites are rare and contain omphacite with a maximum of 39 mol% jadeite content, garnet (Py_15–19) and amphibole. Retrograde eclogites consist of amphibole and symplectites of Na-poor clinopyroxene (5–8 mol% Jd) + albite ± amphibole. Amphiboles are classified as edenite, pargasite, tschermakite, magnesiohornblende and actinolite. In relatively fresh eclogite, edenite is a common amphibole and texturally coexists with omphacite and garnet. An average temperature of 700 °C was obtained for eclogite facies metamorphism using garnet-pyroxene thermometry. A minimum pressure of 1.5 GPa is indicated by the maximum jadeite content in omphacite. Thermobarometric calculations using the TWEEQ program for amphibole in textural equilibrium with omphacite and garnet give pressures of 1.8–2.2 GPa at 700 °C. The equilibrium assemblage of Na-poor clinopyroxene, albite, amphibole and zoisite in the symplectites gives a pressure of about 0.6–0.8 GPa at 590–640 °C. ⁴⁰Ar/³⁹Ar radiometric dating of edenitic amphibole in textural equilibrium with omphacite gave a plateau age of 397.3 ± 7.8 Ma, and probably indicates retrograde cooling through the closure temperature for amphibole (∼500 °C). The age of the high-pressure metamorphism thus must be pre-Variscan and points to one of the earliest metamorphic events in the Austroalpine nappes known to date. Received June 11, 2000; revised version accepted January 2, 2001     

Origin of retrograde fluid in ultrahigh-pressure metamorphic rocks: Constraints from mineral hydrogen isotope and water content changes in eclogite-gneiss transitions in the Sulu orogen

By taking advantage of having depth profiles between contrasting lithologies from core samples of the Chinese Continental Scientific Drilling (CCSD) project, a combined study was carried out to examine changes in mineral H isotope, total water and hydroxyl contents in garnet and omphacite across the contacts between ultrahigh-pressure (UHP) eclogite and gneiss in the Sulu orogen, east-central China. The samples of interest were from two continuous core segments from the CCSD main hole at depths of 734.21-737.16 and 929.67-932.86 m, respectively. The results show δD values of −116‰ to  − 64‰ for garnet and −104‰ to −82‰ for omphacite, consistent with incorporation of meteoric water into protoliths of UHP metamorphic rocks by high-T alteration. Both equilibrium and disequilibrium H isotope fractionations were observed between garnet and omphacite, suggesting fluid-assisted H isotope exchange at local scales during amphibolite-facies retrogression. While bulk water analysis gave total H₂O concentrations of 522-1584 ppm for garnet and 1170-20745 ppm for omphacite, structural hydroxyl analysis yielded H₂O contents of 80-413 ppm for garnet and 228-412 ppm for omphacite. It appears that significant amounts of molecular H₂O are present in the minerals, pointing to enhanced capacity of water storage in the UHP eclogite minerals. Hydrogen isotope variations in the transition between eclogite and gneiss show correlations with variations in their water contents. Petrographically, the degree of retrograde metamorphism generally increases with decreasing distance from the eclogite-gneiss boundary. Thus, retrograde metamorphism results in mineral reactions and H isotope variation. Because hydroxyl solubility in nominally anhydrous minerals decreases with dropping pressure, significant amounts of water are expected to be released from the minerals during decompression exhumation. Decompression exsolution of structural hydroxyl from 1 m³ volume of eclogite composed of only garnet and omphacite results in release of a quantitative estimate of 3.07-3.44 kg water that can form 140-156 kg amphibole during exhumation. Therefore, it is concluded that fluid for retrogression of the eclogites away from the eclogite-gneiss boundary was derived from the decompression exsolution of structural hydroxyl and molecular H₂O in nominally anhydrous minerals. For the eclogites adjacent to gneiss, in contrast, the retrograde metamorphism was principally caused by aqueous fluid from the gneiss which is relatively rich in water. Consequently, both the origin and availability of metamorphic fluid during exhumation of deeply subducted continental crust are deciphered by this combined study focusing on the transitions and the retrograde processes between the felsic and mafic UHP rocks.     

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.     

Trace element distribution in Central Dabie eclogites

Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content of the host phases, with D _REE ^Cpx/Grt decreasing with decreasing D _Ca . This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids will be strongly depleted in LREE–MREE. Received: 11 February 1999 / Accepted: 31 January 2000     

Relationships between eclogites and certain megacrysts from the Jagersfontein kimberlite, South Africa

Some garnet and omphacite megacrysts which are compositionally similar to phases in the eclogite xenoliths from the Jagersfontein kimberlite are ascribed to fragmentation of coarse-grained eclogites. They differ compositionally from other megacrysts from Jagersfontein and those found at other localities that are believed to be high-pressure phenocrysts precipitating from kimberlite. The eclogite suite differs from those in other southern African kimberlites, mainly in lacking eclogites containing Ca-rich garnets.     

Chemistry,mineralogy and petrology of an eclogite from the type locality (Saualpe,Austria)

An eclogite and five of its coexisting minerals (omphacite, garnet, carinthine, kyanite and zoisite) from the probable type locality of eclogites (Kupplerbrunn, Saualpe, Austria) described by Haüy (1822) have been analysed. Optical and X-ray data for these minerals are also given. Comparison of the Kupplerbrunn rock with those of other eclogites from the Saualpe region indicates they all have roughly similar compositions. When plotted on an A-C-F diagram the majority of these analyses fall in the region of kyanite-bearing eclogites suggested by Tilley (1936) although the Kupplerbrunn rock is the only sample containing kyanite; the others containing zoisite. The garnet and omphacite compositions of the Kupplerbrunn rock differ markedly from those of other Saualpe eclogites, possibly due to different metamorphic conditions of their formation. Carinthine analyses are all very similar for eclogites from Saualpe. On the basis of geological, analytical and limited experimental evidence, it is postulated that the Kupplerbrunn eclogite was derived from an original gabbroic rock low in water content such that amphibole and zoisite formed from plagioclase, pyroxene and water; omphacite, garnet and kyanite formed from plagioclase and pyroxene, once all the water was used up in the form of amphibole and zoisite. These reactions are believed to have taken place at 5–8 kb pressure at around 600° C; a value close to that suggested by Lodemann (1966) from field data.     

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.     

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.     

Partial eclogitisation of gabbroic rocks in a late Precambrian subduction zone (Zambia): prograde metamorphism triggered by fluid infiltration

Gabbros and eclogites occur closely associated in a 200-km-long and up to 40-km-wide area of the Zambezi Belt in central Zambia. This area is interpreted to represent part of a late Precambrian suture zone, with the mafic rocks being relics of subducted oceanic crust. Gradual stages of prograde transformation from gabbro to eclogite are preserved by disequilibrium textures of incomplete reactions. This resulted in kyanite–omphacite-bearing assemblages for eclogites that have Al-poor bulk compositions. Undeformed eclogites typically preserve features of a former gabbroic texture, reflected by replacements of plagioclase and magmatic pyroxene by eclogite facies minerals. Textures of deformed eclogites range from sheared porphyroclastic to porphyroblastic. Relics of magmatic pyroxene are common and complete eclogitisation occurred only in millimetre to centimetre-scale domains in most of the rocks. No evidence for prograde blueschist or amphibolite facies mineral assemblages was found in eclogites. In contrast, the fine grained intergrowth of omphacite, garnet, kyanite and quartz, which replace former plagioclase or was formed in the pressure shadow of magmatic pyroxene relics, indicates that eclogitisation might have affected the gabbroic protoliths directly without any significant intervening metamorphic reactions. Eclogitisation took place under P–T conditions of 630–690 °C and 26–28 kbar, suggesting a large overstepping (>10 kbar) of reaction boundaries. Eclogitisation was initialised and accompanied by a channelised fluid flow resulting in veins with large, subhedral grains of omphacite, kyanite and garnet. The gabbro-to-eclogite transformation was enhanced by a fluid which allowed the necessary material transport for the dissolution–precipitation mechanism that characterises the metamorphic mineral replacements. The process of eclogitisation was limited by reaction kinetics and dissolution–precipitation rates rather than by the metamorphic P–T conditions. Even though ductile deformation occurred and equilibrium phase boundaries were overstepped, the infiltration of fluids was necessary for triggering the gabbro-to-eclogite transformation.Editorial responsibility: J. Hoefs     

A ternary solid solution model for omphacite and its application to geothermobarometry of eclogites from the Middle Adula nappe (Central Alps, Switzerland)

A ternary solid solution model for omphacite with the end-members jadeite (NaAlSi₂O₆), diopside (CaMgSi₂O₆) and hedenbergite (CaFeSi₂O₆) was derived from experimental data from the literature. The subregular solution model, fitted by linear programming, is best suited to omphacites with very little aegirine component in common eclogites. Applying this solution model to the calculation of equilibrium phase diagrams of eclogites from the Adula nappe (Central Alps, Switzerland) results in large stability fields for common eclogite assemblages (garnet+omphacite+quartz+H₂O±kyanite). Within this field the compositions of garnet and omphacite show very little variation. A precise determination of the peak-pressure and temperature is not possible. The occurrence of amphibole, overgrowing the peak-pressure assemblage in fresh eclogite, suggests retrograde re-equilibration, still under eclogite facies conditions. The computation of isopleths for garnet and pyroxene end-members allows the estimation of the pressure and temperature conditions of this re-equilibration event (19–21  kbar, c .  700 °C).     

High-pressure eclogites in northern Jiangsu – southern Shandong province, eastern China

Abstract Eclogites are distributed for more than 500 km along a major tectonic boundary between the Sino-Korean and Yangtze cratons in central and eastern China. These eclogites usually have high-P assemblages including omphacite + kyanite and/or coesite (or its pseudomorph), and form a high-P eclogite terrane. They occur as isolated lenses or blocks 10 cm to 300 m long in gneisses (Type I), serpentinized garnet peridotites (Type II) and marbles (Type III). Type I eclogites were formed by prograde metamorphism, and their primary metamorphic mineral assemblage consists mainly of garnet [pyrope (Prp) = 15–40 mol%], omphacite [jadeite (Jd) = 34–64 mol%], pargasitic amphibole, kyanite, phengitic muscovite, zoisite, an SiO₂ phase, apatite, rutile and zircon. Type II eclogites characteristically contain no SiO₂ phase, and are divided into prograde eclogites and mantle-derived eclogites. The prograde eclogites of Type II are petrographically similar to Type I eclogites. The mantle-derived eclogites have high MgO/(FeO + Fe₂O₃) and Cr₂O₃ compositions in bulk rock and minerals, and consist mainly of pyrope-rich garnet (Prp = 48–60 mol%), sodic augite (Jd = 10–27 mol%) and rutile. Type III eclogites have an unusual mineral assemblage of grossular-rich (Grs = 57 mol%) garnet + omphacite (Jd = 30–34 mol%) + pargasite + rutile. Pargasitic and taramitic amphiboles, calcic plagioclase (An₆₈), epidote, zoisite, K-feldspar and paragonite occur as inclusions in garnet and omphacite in the prograde eclogites. This suggests that the prograde eclogites were formed by recrystallization of epidote amphibolite and/or amphibolite facies rocks with near-isothermal compression reflecting crustal thickening during continent–continent collision of late Proterozoic age. Equilibrium conditions of the prograde eclogites range from P > 26 kbar and T= 500–750°C in the western part to P > 28 kbar and T= 810–880°C in the eastern part of the high-P eclogite terrane. The prograde eclogites in the eastern part are considered to have been derived from a deeper position than those in the western part. Subsequent reactions, manifested by (1) narrow rims of sodic plagioclase or paragonite on kyanite and (2) symplectites between omphacite and quartz are interpreted as an effect of near-isothermal decompression during the retrograde stage. The conditions at which symplectites re-equilibrated tend to increase from west (P < 10 kbar and T < 580°C) to east (P > 9 kbar and T > 680°C). Equilibrium temperatures of Type II mantle-derived eclogites and Type III eclogite are 730–750°C and 680°C, respectively.     

Metamorphic evolution of a newly identified Mesoproterozoic oceanic slice in the Yuka terrane and its implications for a multi‐cyclic orogenic history of the North Qaidam UHPM belt

The North Qaidam Orogenic Belt (NQOB), lying at the northern margin of the Tibet Plateau, records two orogenic cycles: A Proterozoic cycle related to the amalgamation and breakup of the supercontinent Rodinia, and an Early Palaeozoic cycle including oceanic subduction and continental deep subduction. At present, the only information about the Proterozoic cycle is the concurrent c. 1,000–900 Ma magmatic and metamorphic events, which limited the understanding of the Proterozoic evolution of NQOB and the relationship between the Qaidam Block and other Rodinia fragments. In this study, a kyanite‐bearing eclogite was identified in Yuka terrane. It has positive‐slope chondrite‐normalized rare earth element distribution patterns, similar to present‐day N‐MORB. LA–ICP–MS zircon U–Pb dating obtained a protolith age of 1,273 Ma and an eclogite facies metamorphic age of 437 Ma, which is similar to the continental deep subduction age of the Yuka terrane. Zircon Lu–Hf analysis show that the magmatic zircon cores have high εHf(t) of 8.36–15.98 and T_DM1 of 1,450–1,131 Ma (M = 1,303 ± 55 Ma, consistent with its protolith age within error), indicating a juvenile crust protolith of the eclogite. The MORB‐like whole‐rock composition and zircon U–Pb and Lu–Hf analysis indicate that the protolith of the kyanite‐bearing eclogite was a Mesoproterozoic oceanic slice. P–T pseudosection analysis shows that the kyanite‐bearing eclogite experienced four metamorphic stages: (1) a prograde stage with the assemblage garnet+omphacite+talc+lawsonite+phengite+quartz at 22.4–23.2 kbar and 585°C; (2) a peak stage with the assemblage garnet+omphacite+lawsonite+phengite+coesite at 32.5 kbar and 670°C; (3) an early retrograde stage with the assemblage garnet+omphacite+kyanite+phengite+coesite/quartz±lawsonite at 27.1–30.0 kbar and 670–690°C; and (4) a late retrograde stage with the assemblage garnet+omphacite+epidote+hornblende+phengite+quartz at <18.0 kbar. The established clockwise P–T path is similar with other continental‐type eclogites in this area. On the basis of the geochemical and geochronological data, as well as the P–T path, we suggest that the protolith of the kyanite‐bearing eclogite was emplaced in the active margin of the Qaidam Block during the assembly of Rodinia and underwent continental deep subduction in the Early Palaeozoic. We conclude that (1) the Qaidam Block participated in the assembly of the Rodinia supercontinent. It was situated at or proximal to the margin of the supercontinent and probably close to India, east Antarctica and Tarim; and (2) both Mesoproterozoic and Early Palaeozoic oceanic crust slices occur in the NQOB. Thus, special caution is needed when using the metamorphic ages of oceanic affinity eclogites without protolith ages to constrain the evolution history of the North Qaidam UHPM belt.     

Metabasites with eclogite facies relics from Variscides in Sardinia,Italy: a review

Metabasites with eclogite facies relics occur in northern Sardinia as massive to strongly foliated lenses or boudins embedded within low- to medium-grade rocks (Anglona) and migmatites (NE Sardinia). U–Pb zircon dating yielded 453 ± 14, 457 ± 2 and 460 ± 5 Ma as the protolith ages; 400 ± 10 and 403 ± 4 Ma have been interpreted as the ages of the HP event and 352 ± 3 and 327 ± 7 Ma as the ages of the main Variscan retrograde events. A pre-eclogite stage is documented by the occurrence of tschermakite, zoisite relics within garnet porphyroblasts (Punta de li Tulchi) and an edenite–andesine inclusion within a relict kyanite porphyroblast (Golfo Aranci). Four main metamorphic stages have been distinguished in the eclogite evolution: (1) eclogite stage, revealed by the occurrence of armoured omphacite relics within garnet porphyroblasts. The Golfo Aranci eclogites also include kyanite, Mg-rich garnet and pargasite; (2) granulite stage, producing orthopyroxene and clinopyroxene–plagioclase symplectites replacing omphacite. At Golfo Aranci, the symplectitic rims around relict kyanite consist of sapphirine, anorthite, corundum and spinel; (3) amphibolite stage, leading to the formation of amphibole–plagioclase kelyphites between garnet porphyroblasts and pyroxene–plagioclase symplectites and to the growth of cummingtonite on orthopyroxene. Tschermakite to Mg-hornblende, plagioclase, cummingtonite, ilmenite, titanite and biotite are coexisting phases; (4) greenschist to sub-greenschist stage, defined by the appearance of actinolite, chlorite, epidote ss, titanite, sericite and prehnite. The following P–T ranges have been estimated for the different stages. Eclogite stage 550–700°C; 1.3–1.7 GPa; granulite stage 650–900°C; 0.8–1.2 GPa, clustering in the range 1.0–1.2 GPa; amphibolite stage 550–740°C; 0.3–0.7 GPa; greenschist stage 300–400°C; 0.2–0.3 GPa. Comparable ranges characterise the other Variscan massifs in Europe; eclogite stage: T = 530–800°C; P from 0.7–1.1 to 1.7 ± 0.3 GPa; granulite stage T = 760–870°C and P from 1.1–1.4 to 7.2–9.9 GPa, clustering around 1.0–1.2 GPa. Whole-rock chemistry: Sardinian eclogites are N- to T-MORB; European ones N- to E-MORB or calc-alkaline.     

Xenoliths of Eclogites with Diamonds from the Yubileinaya Kimberlite Pipe,Yakutia

The first results of study of minerals and diamonds of diamond-bearing eclogites from kimberlites of the Yubileinaya pipe with a variable percent amount of clinopyroxene and garnet are presented. Samples with a garnet content from 30 to 90% of the xenolith volume are dominant among the round to oval xenoliths with diamonds. Five eclogite samples contain grains of accessory rutile, as well as corundum and kyanite. Some samples host two or more diamond crystals.