Constraining peak P–T conditions in UHP eclogites: calculated phase equilibria in kyanite‐ and phengite‐bearing eclogite of the Tromsø Nappe,Norway

Kyanite‐ and phengite‐bearing eclogites have better potential to constrain the peak metamorphic P–T conditions from phase equilibria between garnet + omphacite + kyanite + phengite + quartz/coesite than common, mostly bimineralic (garnet + omphacite) eclogites, as exemplified by this study. Textural relationships, conventional geothermobarometry and thermodynamic modelling have been used to constrain the metamorphic evolution of the Tromsdalstind eclogite from the Tromsø Nappe, one of the biggest exposures of eclogite in the Scandinavian Caledonides. The phase relationships demonstrate that the rock progressively dehydrated, resulting in breakdown of amphibole and zoisite at increasing pressure. The peak‐pressure mineral assemblage was garnet + omphacite + kyanite + phengite + coesite, inferred from polycrystalline quartz included in radially fractured omphacite. This omphacite, with up to 37 mol.% of jadeite and 3% of the Ca‐Eskola component, contains oriented rods of silica composition. Garnet shows higher grossular (X_Grs = 0.25–0.29), but lower pyrope‐content (X_Prp = 0. 37–0.39) in the core than the rim, while phengite contains up to 3.5 Si pfu. The compositional isopleths for garnet core, phengite and omphacite constrain the P–T conditions to 3.2–3.5 GPa and 720–800 °C, in good agreement with the results obtained from conventional geothermobarometry (3.2–3.5 GPa & 730–780 °C). Peak‐pressure assemblage is variably overprinted by symplectites of diopside + plagioclase after omphacite, biotite and plagioclase after phengite, and sapphirine + spinel + corundum + plagioclase after kyanite. Exhumation from ultrahigh‐pressure (UHP) conditions to 1.3–1.5 GPa at 740–770 °C is constrained by the garnet rim (X_Ca^Grt = 0.18–0.21) and symplectite clinopyroxene (X_Na^Cpx = 0.13–0.21), and to 0.5–0.7 GPa at 700–800 °C by sapphirine (X_Mg = 0.86–0.87) and spinel (X_Mg = 0.60–0.62) compositional isopleths. UHP metamorphism in the Tromsø Nappe is more widespread than previously known. Available data suggest that UHP eclogites were uplifted to lower crustal levels rapidly, within a short time interval (452–449 Ma) prior to the Scandian collision between Laurentia and Baltica. The Tromsø Nappe as the highest tectonic unit of the North Norwegian Caledonides is considered to be of Laurentian origin and UHP metamorphism could have resulted from subduction along the Laurentian continental margin. An alternative is that the Tromsø Nappe belonged to a continental margin of Baltica, which had already been subducted before the terminal Scandian collision, and was emplaced as an out‐of‐sequence thrust during the Scandian lateral transport of nappes.     

Mineral evolution of a garnet-pyroxenite nodule within eclogite, eastern Sulu ultrahigh-pressure metamorphic terrane, East China

Detailed microtextural observations and bulk chemical analysis were undertaken on a garnet‐pyroxenite nodule within retrograde eclogites from the NE Sulu ultrahigh‐pressure metamorphic (UHPM) terrane. The results suggest that the protolith was a cumulate from a gabbroic body. The nodule consists primarily of coarse clinopyroxene grains with a very high content of the Ca‐Tschermakite molecule. Microscopic observations and back‐scattered electron images (BSE) demonstrate a complicated intergrowth of clinopyroxene, garnet and ilmenite, which represents the peak metamorphic assemblage. The primary clinopyroxene grains are armoured with a thin garnet corona up to 0.5 mm wide that forms an interconnected network. Within the clinopyroxene grains, four sets of garnet lamellae are distributed along crystallographic planes; locally, a vermicular intergrowth of garnet and diopside is developed. Besides the garnet, parallel arrays of ilmenite blebs are common within the clinopyroxene. Hydrous minerals such as amphibole, zoisite and titanite formed at later stages, and replaced diopside, garnet and ilmenite respectively. The P–T conditions determined for the formation of the garnet lamellae indicate that the garnet pyroxenite experienced UHP metamorphism at the same peak P–T condition as its host eclogite. The very high Ca‐Tschermakite content (31–34 mol.%) of the primary clinopyroxene indicates crystallization at about 9–17 kbar and 1250–1450 °C, and together with the microtextural observations, suggests that the protolith of the garnet pyroxenite was a cumulate from a former gabbroic body, in which case, the host eclogite might represent the gabbroic body.     

Middle Ordovician subduction of continental crust in the Scandinavian Caledonides: an example from Tjeliken,Seve Nappe Complex,Sweden

The Seve Nappe Complex of the Scandinavian Caledonides is thought to be derived from the distal passive margin of Baltica which collided with Laurentia in the Scandian Phase of the Caledonian Orogeny at 430–400 Ma. Parts of the Seve Nappe Complex were affected by pre-Scandian high- and ultrahigh-pressure metamorphism, in a tectonic framework that is still unclear, partly due to uncertainties about the exact timing. Previous age determinations yielded between ~ 505 and ~ 446 Ma, with a general trend of older ages in the North (Norrbotten) than in the South (Jämtland). New age determinations were performed on eclogite and garnet–phengite gneiss at Tjeliken in northern Jämtland. Thermodynamic modelling yielded peak metamorphic conditions of 25–27 kbar/680–760 °C for the garnet–phengite gneiss, similar to published peak metamorphic conditions of the eclogite (25–26 kbar/650–700 °C). Metamorphic rims of zircons from the garnet–phengite gneiss were dated using secondary ion mass spectrometry and yielded a concordia age of 458.9 ± 2.5 Ma. Lu–Hf garnet-whole rock dating yielded 458 ± 1.0 Ma for the eclogite. Garnet in the eclogite shows prograde major-element zoning and concentration of Lu in the cores, indicating that this age is related to garnet growth during pressure increase, i.e. subduction. The identical ages from both rock types, coinciding with published Sm–Nd ages from the eclogite, confirm subduction of the Seve Nappe Complex in Northern Jämtland during the Middle Ordovician in a fast subduction–exhumation cycle.     

An anticlockwise P–T–t path at high‐pressure,high‐temperature conditions for a migmatitic gneiss from the island of Fjørtoft,Western Gneiss Region,Norway, indicates two burial events during the Caledonian orogeny

The Blåhø Nappe on the island of Fjørtoft, which represents an isolated portion of the Seve Nappe Complex in the Western Gneiss Region, Norway, has been suggested to have experienced two deep burial cycles during the Caledonian orogeny. However, evidence on this multiple burial process by the derivation of a pressure–temperature–time (P–T–t) path has never been given in the literature. In this study, the ‘diamondiferous’ kyanite–garnet gneiss from the Blåhø Nappe on Fjørtoft was revisited to determine if such a process was correct. Two types of garnet, porphyroblastic garnet‐1 and fine‐grained garnet‐2, were recognized in the gneiss. The core of garnet‐1 is poor in Ca and documents P–T conditions of 1.2–1.3 GPa at c. 880°C based on pseudosection modelling. The inner rims of garnet‐1 and the core of garnet‐2 are both richer in Ca, recording P–T conditions of 1.35–1.45 GPa and 770–820°C. Application of conventional geothermobarometry on the outer rim of garnet‐1 and the rim of garnet‐2 yielded retrograde P–T conditions of 0.75–0.90 GPa and 610–685°C. These estimates define an anticlockwise P–T path at pressures below 1.5 GPa. Accessory monazite was dated with the electron microscope. Relicts of detrital monazite in the gneiss point to Sveconorwegian and possibly also Cryogenian provenance for the detritus of the sedimentary protolith. Metamorphic monazite in the gneiss records a wide age range from 460 to 380 Ma, with a peak c. 435 Ma and a shoulder at 395 Ma. These data suggest that the original (Ediacaran?) Baltica margin sediment (gneiss protolith) was transported to the base of an overlying plate during the early Caledonian (pre‐Scandian) orogeny. A long residence time of the metasedimentary rock at this base caused its heating to 880°C and homogenization of the early garnet chemistry. The late Caledonian (Scandian) collision between Baltica and Laurentia led to further burial, during which the studied gneiss was close to the former surface of the downgoing continental plate and, thus, cooled. The reconstructed P–T–t path confirms the multiple burial history of the Blåhø Nappe but contradicts previous ideas of deep burial of the Fjørtoft gneiss to more than 100 km.     

Petrology and zircon U–Pb dating of well‐preserved eclogites from the Thongmön area in central Himalaya and their tectonic implications

The discovery of eclogites is reported within the Great Himalayan Crystalline Complex in the Thongmön area, central Himalaya, and their metamorphic evolution is deciphered by petrographic studies, pseudosection modelling, and zircon dating. For the first time, omphacite has been found in the matrix of eclogites taken from a metamorphic mafic lens. Two groups of garnet have been identified in the Thongmön eclogites on the basis of major and rare earth elements and mineral inclusions. Core and intermediate sections of garnet represent Grt I, in which the major elements (Ca, Mg, and Fe) show a nearly homogenous distribution with little or weak zonation. This Grt I displays an almost flat chondrite‐normalized HREE pattern, and the main inclusions are amphibole, apatite, quartz, and abundant omphacite. Grt II, forms thin rims on large garnet grains, and is characterized by rim‐ward Ca decrease and Mg increase and MREE enrichment relative to HREE and LREE. No amphibole inclusions are found in Grt II, indicating the decomposition of amphibole contributed to its MREE enrichment. Two metamorphic stages, recorded by matrix minerals and inclusions in garnet and zircon, outline the burial of the Thongmön eclogites and progressive metamorphic processes to the pressure peak: (a) the assemblage of amphibole–garnet–omphacite–phengite–rutile–quartz, with the phengite interpreted as having been replaced by Bt+Pl symplectites, represents the prograde amphibole eclogite facies stage M1₍₁₎, (b) in the peak eclogite facies [stage M1₍₂₎], amphibole was lost and melting started. Based on the compositions of garnet and omphacite inclusions, M1₍₁₎ is constrained to 19–20 kbar and 640–660°C and M1₍₂₎ occurred at >21 kbar, >750°C, with appearance of melt and its entrapment in metamorphic zircon. SHRIMP U–Pb dating of zircon from two eclogite samples yielded consistent metamorphic ages of 16.7 ± 0.6 Ma and 17.1 ± 0.4 Ma respectively. The metamorphic zircon grew concurrently with Grt II in the peak eclogite facies. Thongmön eclogites characterized by the prograde metamorphism from amphibolite facies to eclogite facies were formed by the continuing continental subduction of Indian plate beneath the Euro‐Asian continent in the Miocene.     

北苏鲁荣成地区超高压变质带大理岩中石榴辉石岩的退变质过程

北苏鲁荣成地区超高压变质带大理岩中的退变质石榴辉石岩包裹体具有“核-幔-边”结构,核部主要由石榴子石与单斜辉石组成,幔部含有由细小角闪石与绿帘石组成的后成合晶以及颗粒较大的角闪石与单斜辉石,通过详细的岩相学分析以及矿物成分分析,认为这些后成合晶是由石榴子石、单斜辉石以及来自围岩的流体共同反应而产生,大颗粒的角闪石主要是由辉石转变而来的,在幔部这个转变并不彻底,仍有一些残余辉石颗粒。边部主要由角闪石和绿帘石组成。该石榴辉石岩曾经历榴辉岩相超高压变质阶段。没有柯石英超高压代表性矿物（采样处的其他类型岩石都含有柯石英）的原因是石榴辉石岩的原岩为超基性岩。     

Kyanite eclogite xenolith from the orthogneiss terrane of the Tisza Megaunit, Jánoshalma area, crystalline basement of southern Hungary

New evidence for high-pressure, eclogite facies metamorphism in the crystalline basement of the Tisza Megaunit (southern Hungary) is reported. The retrogressed mafic eclogite forms a small lens in the orthogneiss and it was found in the borehole near Jánoshalma. The carbonated eclogite contains the peak metamorphic assemblage omphacite + garnet + phengite + kyanite + clinozoizite + rutile + K-feldspar + quartz. Omphacite (Xjd_0.40–0.41Xdio_0.52–0.53Xhd_0.05Xaug_1.55–2.85) occurs in the matrix and as inclusions in garnet (Xpy_0.37–0.38Xgrs_0.21–0.22Xalm_0.39–0.40Xsps_0–0.01Xadr_0–0.01) and kyanite. Thermobarometry based on net-transfer reactions between garnet, omphacite, kyanite and phengite yields P–T conditions of 710 ± 10 °C and 2.6 ± 0.75 GPa. Retrogression during decompression is manifested by formation of symplectites; the most typical are diopside + plagioclase after omphacite, corundum + spinel + plagioclase after kyanite and biotite + plagioclase after phengite. Carbonatization along the veins of the retrogressed eclogite was probably coeval with formation of these symplectites. At places where carbonate is absent the rock was completely hydrated and retrogressed down to the greenschist facies with the development of actinolite. Similar eclogites together with abundant orthogneisses occur mainly in the eastern parts of the Tisza Megaunit, suggesting the existence of an ancient (possibly Variscan) subduction/accretionary complex.     

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan,China

A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well‐preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N₂‐bearing NaCl‐rich solutions, whereas it changed into CO₂‐dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low‐salinity fluids were involved. In situ UV‐laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ¹⁸O_VSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB‐like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra‐high‐pressure (UHP) eclogites in the Dabie‐Sulu area. However, the age‐corrected initial ε_Nd(t) is ? 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N₂ content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.     

P–T evolution of glaucophane–omphacite bearing HP–LT rocks in the western Tianshan Orogen, NW China:new evidence for 'Alpine-type' tectonics

The late Palaeozoic western Tianshan high‐pressure /low‐temperature belt extends for about 200 km along the south‐central Tianshan suture zone and is composed mainly of blueschist, eclogite and epidote amphibolite/greenschist facies rocks. P–T conditions of mafic garnet omphacite and garnet–omphacite blueschist, which are interlayered with eclogite, were investigated in order to establish an exhumation path for these high‐pressure rocks. Maximum pressure conditions are represented by the assemblage garnet–omphacite–paragonite–phengite–glaucophane–quartz–rutile. Estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 °C. Decompression caused the destabilization of omphacite, garnet and glaucophane to albite, Ca‐amphibole and chlorite. The post‐eclogite facies metamorphic conditions between 9 and 14 kbar at 480–570 °C suggest an almost isothermal decompression from eclogite to epidote–amphibolite facies conditions. Prograde growth zoning and mineral inclusions in garnet as well as post‐eclogite facies conditions are evidence for a clockwise P–T path. Analysis of phase diagrams constrains the P–T path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures. This implies that the high‐pressure rocks from the western Tianshan Orogen formed in a tectonic regime similar to ‘Alpine‐type’ tectonics. This contradicts previous models which favour ‘Franciscan‐type’ tectonics for the southern Tianshan high‐pressure rocks.     

A counter-clockwise P–T path for the Voltri Massif eclogites (Ligurian Alps, Italy)

Integrated petrological and structural investigations of eclogites from the eclogite zone of the Voltri Massif (Ligurian Alps) have been used to reconstruct a complete Alpine P–T deformation path from burial by subduction to subsequent exhumation. The early metamorphic evolution of the eclogites has been unravelled by correlating garnet zonation trends with the chemical variations in inclusions found in the different garnet domains. Garnet in massive eclogites displays typical growth zoning, whereas garnet in foliated eclogites shows rim‐ward resorption, likely related to re‐equilibration during retrogressive evolution. Garnet inclusions are distinctly different from core to rim, consisting primarily of Ca‐, Na/Ca‐amphibole, epidote, paragonite and talc in garnet cores and of clinopyroxene ± talc in the outer garnet domains. Quantitative thermobarometry on the inclusion assemblages in the garnet cores defines an initial greenschist‐to‐amphibolite facies metamorphic stage (M1 stage) at c. 450–500 °C and 5–8 kbar. Coexistence of omphacite + talc + katophorite inclusion assemblage in the outer garnet domains indicate c. 550 °C and 20 kbar, conditions which were considered as minimum P–T estimates for the M2 eclogitic stage. The early phase of retrograde reactions is polyphase and equilibrated under epidote–blueschist facies (M3 stage), characterized by the development of composite reaction textures (garnet necklaces and fluid‐assisted Na‐amphibole‐bearing symplectites) produced at the expense of the primary M2 garnet‐clinopyroxene assemblage. The blueschist retrogression is contemporaneous with the development of a penetrative deformation (D3) that resulted in a non‐coaxial fabric, with dominant top‐to‐the‐N sense of shear during rock exhumation. All of that is overprinted by a texturally late amphibolite/greenschist facies assemblages (M4 & M5 stages), which are not associated with a penetrative structural fabric. The combined P–T deformation data are consistent with an overall counter‐clockwise path, from the greenschist/amphibolite, through the eclogite, the blueschist to the greenschist facies. These new results provide insights into the dynamic evolution of the Tertiary oceanic subduction processes leading to the building up of the Alpine orogen and the mechanisms involved in the exhumation of its high‐pressure roots.     

UHP metamorphism recorded by kyanite-bearing eclogite in the Seve Nappe Complex of northern Jämtland,Swedish Caledonides

The first evidence for ultrahigh-pressure (UHP) metamorphism in the Seve Nappe Complex of the Scandinavian Caledonides is recorded by kyanite-bearing eclogite, found in a basic dyke within a garnet peridotite body exposed close to the lake Friningen in northern Jämtland (central Sweden). UHP metamorphic conditions of ~ 3 GPa and 800 °C, within the stability field of coesite, are constrained from geothermobarometry and calculated phase equilibria for the peak-pressure assemblage garnet + omphacite + kyanite + phengite. A prograde metamorphic evolution from a lower P–T (1.5–1.7 GPa and 700–750 °C) stage during subduction is inferred from inclusions of pargasitic amphibole, zoisite and kyanite in garnet cores. The post-UHP evolution is constrained from breakdown textures, such as exsolutions of kyanite and silica from the Ca-Eskola clinopyroxene. Near isothermal decompression of eclogite to lower crustal levels (~ 0.8–1.0 GPa ) led to formation of sapphirine, spinel, orthopyroxene and diopside at granulite facies conditions. Published age data suggest a Late Ordovician (460–445 Ma) age of the UHP metamorphism, interpreted to be related to subduction of Baltoscandian continental margin underneath an outboard terrane, possibly outermost Laurentia, during the final stages of closure of the Iapetus Ocean. The UHP rocks were emplaced from the hinterland collision zone during Scandian thrusting of the nappes onto the Baltoscandian foreland basin and platform. The record of P–T conditions and geochonological data from UHP rocks occurring within the allochthonous units of the Scandinavian Caledonides indicate that Ordovician UHP events may have affected much wider parts of the orogen than previously thought, involving deep subduction of the continental crust prior to final Scandian collision between Baltica and Laurentia.     

Prograde P–T path of medium-pressure granulite facies calc-silicate rocks, Higo metamorphic terrane, central Kyushu, Japan

This paper reports an occurrence of medium-pressure granulite facies calc-silicate rocks intercalated with pelitic gneisses in the Higo metamorphic terrane, central Kyushu, Japan, which is classified as a low- P /high- T (andalusite-sillimanite type) metamorphic belt. Three equilibrium stages are recognized in the calc-silicate rock based on reaction textures: M1 stage characterized by an assemblage of porphyroblastic garnet + coarse-grained clinopyroxene + plagioclase included in the clinopyroxene; M2 stage by two kinds of breakdown products of garnet, one is plagioclase + coronitic clinopyroxene within garnet and the other is plagioclase + vermicular clinopyroxene surrounding garnet; and M3 stage by amphibole replacing clinopyroxene. The key assemblage in the calc-silicate rock common to M1 and M2 stages is Grt + Cpx + Pl ± Qtz, which constrains the pressure and temperature ( P – T ) conditions for these stages by Fe–Mg exchange reaction and the two univariant net-transfer reactions: 2Grs + Alm + 3Qtz = 3Hd + 3An or 2Grs + Prp + 3Qtz = 3Di + 3An. The P – T conditions for M1 and M2 stages were estimated to be about 8.4 ± 1.9 kbar and 680 ± 122 °C, and 6.7 ± 1.9 to 8.9 ± 2.2 kbar and 700 ± 130 to 820 ± 160 °C, respectively. Estimates are consistent with an isobaric heating P – T path. The high peak temperature conditions at normal crustal depths and the prograde isobaric heating path probably require heat advection due to melt migration during the high- T metamorphism.     

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.     

Early Ordovician U–Pb metamorphic ages of the eclogite-bearing Seve Nappes, Northern Scandinavian Caledonides

Eclogite-grade metamorphism of the Seve Nappe Complex (SNC) in Norrbotten, Sweden, records the attempted subduction of the Baltic continental margin during the early Palaeozoic evolution of the Iapetus Ocean. Metamorphic titanite sampled from several calcsilicate gneisses of the SNC in Norrbotten occurs as part of a prograde, eclogite facies metamorphic mineral assemblage and yields concordant to nearly concordant U/Pb ages of 500–475  Ma. Later structural disruption of these rocks occurred during the Siluro-Devonian Scandian phase of the Caledonide orogeny, but the U/Pb systematics show no evidence of a second generation (metamorphic or recrystallized) of titanite, or of post-Early Ordovician disturbance through Pb loss. Hence the U/Pb ages are believed to record the time of prograde mineral growth during eclogite facies metamorphism of the SNC.
These results support earlier Sm/Nd and ⁴⁰Ar/³⁹Ar studies indicating an Early Ordovician metamorphic age for the eclogitic Norrbotten SNC, and confirm the Early Ordovician destruction of at least this segment of the Palaeozoic passive margin of Baltica. These results indicate that the SNC in the northern Scandinavian Caledonides was subducted and metamorphosed to high grade some 50–70  Myr prior to the high-grade metamorphism of the SNC in the central Scandinavian Caledonides. This result requires significantly different early Palaeozoic tectonic histories for rocks mapped as SNC in the northern Caledonides and those in the central Caledonides, despite a seemingly similar tectonostratigraphic position and broadly similar high-grade metamorphism.     

New P–T constraints on the Tamayen glaucophane‐bearing rocks,eastern Taiwan: Perple_X modelling results and geodynamic implications

New pseudosection modelling was applied to better constrain the P–T conditions and evolution of glaucophane‐bearing rocks in the Tamayen block of the Yuli belt, recognized as the world's youngest known blueschist complex. Based on the predominant clinoamphibole, textural relationships, and mineral compositions, these glaucophane‐bearing high‐P rocks can be divided into four types. We focused on the three containing garnet. The chief phase assemblages are (in decreasing mode): amphibole + quartz + epidote + garnet + chlorite + rutile/titanite (Type‐I), phengite + amphibole + quartz + garnet + chlorite + epidote + titanite + biotite + magnetite (Type‐II), and amphibole + quartz + albite + epidote + garnet + rutile + hematite + titanite (Type‐III). Amphibole exhibits compositional zoning from core to rim as follows: glaucophane → pargasitic amphibole → actinolite (Type‐I), barroisite → Mg‐katophorite/taramite → Fe‐glaucophane (Type‐II), glaucophane → winchite (Type‐III). Using petrographic data, mineral compositions and Perple_X modelling (pseudosections and superimposed isopleths), peak P–T conditions were determined as 13 ± 1 kbar and 550 ± 40 °C for Type‐I, 10.5 ± 0.5 kbar and 560 ± 30 °C for Type‐II (thermal peak) and 11 ± 1 kbar and 530 ± 30 °C for Type‐III. The calculations yield higher pressures and temperatures than previously thought; the difference is ~1–6 kbar and 50–200 °C. The three rock types record similar P–T retrograde paths with clockwise trajectories; all rocks followed trajectories with substantial pressure decrease under near‐isothermal conditions (Type‐I and Type‐III), with the probable exception of Type‐II where decompression followed colder geotherms. The P–T paths suggest a tectonic environment in which the rocks were exhumed from maximum depths of ~45 km within a subduction channel along a relative cold geothermal gradient of ~11–14 °C km^?1.     

P–T evolution of Precambrian eclogite in the Sveconorwegian orogen,SW Sweden

Conditions of the prograde, peak‐pressure and part of the decompressional P–T path of two Precambrian eclogites in the eastern Sveconorwegian orogen have been determined using the pseudosection approach. Cores of garnet from a Fe–Ti‐rich eclogite record a first prograde and syn‐deformational stage along a Barrovian gradient from ~670 °C and 7 kbar to 710 °C and 8.5 kbar. Garnet rims grew during further burial to 16.5–19 kbar at ~850–900 °C, along a steep dP/dT gradient. The pseudosection model of a kyanite‐bearing eclogite sample of more magnesian bulk composition confirms the peak conditions. Matrix reequilibration associated with subsequent near‐isothermal decompression and partial exhumation produced plagioclase‐bearing symplectites replacing kyanite and clinopyroxene at an estimated 850–870 °C and 10–11 kbar. The validity of the pseudosections is discussed in detail. It is shown that in pseudosection modelling the fractionation of FeO in accessory sulphides may cause a significant shift of field boundaries (here displaced by up to 1.5 kbar and 70 °C) and must not be neglected. Fast burial, exhumation and subsequent cooling are supported by the steepness of both the prograde and the decompressional P–T paths as well as the preservation of garnet growth zoning and the symplectitic reaction textures. These features are compatible with deep tectonic burial of the eclogite‐bearing continental crust as part of the underthrusting plate (Eastern Segment, continent Baltica) in a collisional setting that led to an effectively doubled crustal thickness and subsequent exhumation of the eclogites through tectonic extrusion. Our results are in accordance with regional structural and petrological relationships, which demonstrate foreland‐vergent partial exhumation of the eclogite‐bearing nappe along a basal thrust zone and support a major collisional stage at c. 1 Ga. We argue that the similarities between Sveconorwegian and Himalayan eclogite occurrences emphasize the modern style of Grenvillian‐aged tectonics.     

The P–T–X(fluid) evolution of meta‐anorthosites in the Eastern Granulites,Tanzania

Meta‐anorthosite bodies are typical constituents of the Neoproterozoic Eastern Granulites in Tanzania. The mineral assemblage (and accessory components) is made up of clinopyroxene, garnet, amphibole; scapolite, epidote, biotite, rutile, titanite, ilmenite and quartz. Within the feldspar‐rich matrix (70–90% plagioclase), mafic domains with metamorphic corona textures were used for P–T calculations. Central parts of these textures constitute high‐Al clinopyroxene – which is a common magmatic mineral in anorthosites – and is therefore assumed to be a magmatic relict. The clinopyroxene rims have a diopsidic composition and are surrounded by a garnet corona. Locally the pyroxene is surrounded by amphibole and scapolite suggesting that a mixed CO₂–H₂O fluid was present during their formation. Thermobarometric calculations give the following conditions for the metamorphic peak of the individual meta‐anorthosite bodies: Mwega: 11–13 kbar, 850–900 °C; Pare Mountains: 12–14 kbar, 850–900 °C; Uluguru Mountains: 12–14 kbar, 850–900 °C. The P–T evolution of these bodies was modelled using pseudosections. The amount and composition of the metamorphic fluid and <0.5 mol.% fluid in the bulk composition is sufficient to produce fluid‐saturated assemblages at 10 kbar and 800 °C. Pseudosection analysis shows that the corona textures most likely formed under fluid undersaturated conditions or close to the boundary of fluid saturation. The stabilities of garnet and amphibole are dependent on the amount of fluid present during their formation. Mode isopleths of these minerals change their geometry drastically between fluid‐saturated and fluid‐undersaturated assemblages. The garnet coronae developed during isobaric cooling following the metamorphic peak. The cooling segment is followed by decompression as indicated by the growth of amphibole and plagioclase. The estimated of the metamorphic fluid is ～0.3–0.5. Although the meta‐anorthosites have different formation ages (Archean and Proterozoic) they experienced the same Pan‐African metamorphic overprint with a retrograde isobaric cooling path. Similar P–T evolutionary paths are known from the hosting granulites. The presented data are best explained by a tectonic model of hot fold nappes that brought the different aged anorthosites and surrounding rocks together in the deep crust followed by an isobaric cooling history.     

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.     

Separate or shared metamorphic histories of eclogites and surrounding rocks? An example from the Bohemian Massif

Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite‐dominated volcano‐sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite‐bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet–staurolite–kyanite–biotite–plagioclase–muscovite–quartz–ilmenite ± rutile ± silli‐manite and prograde‐zoned garnet includes chloritoid–chlorite–paragonite–margarite, staurolite–chlorite–paragonite–margarite and kyanite–chlorite–rutile. In pseudosection modelling in the system Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite + paragonite (with muscovite + quartz + H₂O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet–staurolite–biotite–muscovite–quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet–amphibole–plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610–660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase–clinopyroxene symplectites, epidote and late amphibole–plagioclase domains. Garnet commonly includes rutile–quartz–epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H₂O‐undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote–amphibole–plagioclase indicating that decompression occurred under decreasing temperature into garnet‐free epidote–amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km^?1, respectively, to metamorphic pressure peaks that are 6–7 kbar apart. The eclogite–orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared.     

Jadeite–garnet glaucophane schists in the Bizan area,Sambagawa metamorphic belt,eastern Shikoku,Japan: significance and extent of eclogite facies metamorphism

Eclogite facies metamorphic rocks have been discovered from the Bizan area of eastern Shikoku, Sambagawa metamorphic belt. The eclogitic jadeite–garnet glaucophane schists occur as lenticular or sheet‐like bodies in the pelitic schist matrix, with the peak mineral assemblage of garnet + glaucophane + jadeite + phengite + quartz. The jadeitic clinopyroxene (X_Jd 0.46–0.75) is found exclusively as inclusions in porphyroblastic garnet. The eclogite metamorphism is characterized by prograde development from epidote–blueschist to eclogite facies. Metamorphic P–T conditions estimated using pseudosection modelling are 580–600 °C and 18–20 kbar for eclogite facies. Compared with common mafic eclogites, the jadeite–garnet glaucophane schists have low CaO (4.4–4.5 wt%) and MgO (2.1–2.3 wt%) bulk‐rock compositions. The P–T– pseudosections show that low X_Ca bulk‐rock compositions favour the appearance of jadeite instead of omphacite under eclogite facies conditions. This is a unique example of low X_Ca bulk‐rock composition triggered to form jadeite at eclogite facies conditions. Two significant types of eclogitic metamorphism have been distinguished in the Sambagawa metamorphic belt, that is, a low‐T type and subsequent high‐T type eclogitic metamorphic events. The jadeite–garnet glaucophane schists experienced low‐T type eclogite facies metamorphism, and the P–T path is similar to lawsonite‐bearing eclogites recently reported from the Kotsu area in eastern Shikoku. During subduction of the oceanic plate (Izanagi plate), the hangingwall cooled gradually, and the geothermal gradient along the subduction zone progressively decreased and formed low‐T type eclogitic metamorphic rocks. A subsequent warm subduction event associated with an approaching spreading ridge caused the high‐T type eclogitic metamorphism within a single subduction zone.