Retrogressive development of glaucophane in some eclogites from “Massif Armoricain” (East of Nantes,France)

A petrological and mineralogical study, using an electron microprobe, of a blue-amphibole eclogite occurring near Nantes (Massif Armoricain, France) has enabled us to characterize this amphibole as glaucophane resulting from a secondary reaction in the rock. This sodic amphibole was formed at the expense of primary eclogite paragenesis including omphacite, garnet and quartz, according to a sliding reaction which it was possible to study quantitatively: 3.24 omphacite+0.90 SiO₂+0.76 garnet+1.08 H₂O =1 glaucophane+0.55 grossular (S.S. in the garnet) +0.04 paragonite.This reaction is accompanied by a variation in the distribution of iron and magnesium between the amphibole, the garnet and the omphacite.The appearance of the glaucophane can be explained as the beginning of a retromorphic evolution from the stable physical conditions of the primary eclogite paragenesis (650±100° C; minimum pressure 15 Kb).     

Calculated phase equilibria for MORB compositions: a reappraisal of the metamorphic evolution of lawsonite eclogite

Pseudosections calculated with thermocalc predict that lawsonite‐bearing assemblages, including lawsonite eclogite, will be common for subducted oceanic crust that experiences cool, fluid‐saturated conditions. For glaucophane–lawsonite eclogite facies conditions (500–600 °C and 18–28 kbar), MORB compositions are predicted in the NCKMnFMASHO system to contain glaucophane, garnet, omphacite, lawsonite, phengite and quartz, with chlorite at lower temperature and talc at higher temperature. In these assemblages, the pyrope content in garnet is mostly controlled by variations in temperature, and grossular content is strongly controlled by pressure. The silica content in phengite increases linearly with pressure. As the P–T conditions for these given isopleths are only subtly affected by common variations in bulk‐rock compositions, the P–T pseudosections potentially present a robust geothermobarometric method for natural glaucophane‐bearing eclogites. Thermobarometric results recovered both by isopleth and conventional approaches indicate that most natural glaucophane–lawsonite eclogites (Type‐L) and glaucophane–epidote eclogites (Type‐E) record similar peak P–T conditions within the lawsonite stability field. Decompression from conditions appropriate for lawsonite stability should result in epidote‐bearing assemblages through dehydration reactions controlled by lawsonite + omphacite = glaucophane + epidote + H₂O. Lawsonite and omphacite breakdown will be accompanied by the release of a large amount of bound fluid, such that eclogite assemblages are variably recrystallized to glaucophane‐rich blueschist. Calculated pseudosections indicate that eclogite assemblages form most readily in Ca‐rich rocks and blueschist assemblages most readily in Ca‐poor rocks. This distinction in bulk‐rock composition can account for the co‐existence of low‐T eclogite and blueschist in high‐pressure terranes.     

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

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

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.     

Glaucophanites and eclogites from Val Chiusella,Sesia-Lanzo zone (Italian Alps)

In the metabasites of Val Chiusella, metamorphic assemblages are present, corresponding to the glaucophane schist facies, i.e. garnet glaucophanites to omphacite-garnet glaucophanites, as well as to the eclogite facies, i.e., glaucophane eclogites, eclogites, and omphacite felses. Both groups of assemblages are divided by the critical reaction 1 zoisite +1 glaucophane 1.2 omphacite+0.8 garnet+0.7 paragonite +1.4 quartz+0.8 H₂O. From textural evidence it is clear that in the investigated area this reaction proceeded to the right according to a prograde metamorphism. Correspondingly, K ^garn-cpx _D(Fe/Mg) values of coexisting garnet-omphacite pairs in the glaucophane schist facies assemblages are higher than in the eclogite facies assemblages and reflect a temperature increase from about 450 ° C to about 550 ° C at minimum water vapour pressures of 12 to 16 kb.     

Phase Equilibria of Hornblende‐Bearing Eclogite in the Western Dabie Mountain,Central China

The high-pressure (HP) eclogite in the western Dabie Mountain encloses numerous hornblendes, mostly barroisite. Opinions on the peak metamorphic P-T condition, PT path and mineral paragenesis of it are still in dispute. Generally, HP eclogite involves garnet, omphacite, hornblendes and quartz, with or without glaucophane, zoisite and phengite. The garnet has compositional zoning with XMg increase, XCa and XMn decrease from core to rim, which indicates a progressive metamorphism. The phase equilibria of the HP eclogite modeled by the P-T pseudosection method developed recently showed the following: (1) the growth zonation of garnet records a progressive metamorphic PT path from pre-peak condition of 1.9–2.1 GPa at 508°C–514°C to a peak one of 2.3–2.5 GPa at 528°C–531°C for the HP eclogite; (2) the peak mineral assemblage is garnet+omphacite+glaucophane+quartz±phengite, likely paragenetic with lawsonite; (3) the extensive hornblendes derive mainly from glaucophane, partial omphacite and even a little garnet due to the decompression with some heating during the post-peak stage, mostly representing the conditions of about 1.4–1.6 GPa and 580°C–640°C, and their growth is favored by the dehydration of lawsonite into zoisite or epidote, but most of the garnet, omphacite or phengite in the HP eclogite still preserve their compositions at peak condition, and they are not obviously equilibrious with the hornblendes.     

Transformation of garnet epidote amphibolite to eclogite, western Dabie Mountains, China

Omphacite and garnet coronas around amphibole occur in amphibolites in the Hong'an area, western Dabie Mountains, China. These amphibolites consist of an epidote–amphibolite facies assemblage of amphibole, garnet, albite, clinozoisite, paragonite, ilmenite and quartz, which is incompletely overprinted by an eclogite facies assemblage of garnet, omphacite and rutile. Coronas around amphibole can be divided into three types: an omphacite corona; a garnet–omphacite–rutile corona; and, a garnet–omphacite corona with less rutile. Chemographic analysis for local reaction domains in combination with petrographical observations show that reactions Amp + Ab + Pg = Omp +Czo + Qtz + H₂O, and Amp + Ab = Omp ± Czo + Qtz + H₂O may lead to the development of omphacite coronas. The garnet–omphacite–rutile corona was formed from the reaction Amp + Ab + Czo + Ilm ± Qtz = Omp + Grt + Rt + H₂O. In garnet–omphacite coronas, the garnet corona grew during an early stage of epidote amphibolite facies metamorphism, whereas omphacite probably formed by the reactions forming the omphacite corona during the eclogite facies stage. It is estimated that these reactions occurred at 0.8–1.4 GPa and 480–610 °C using the garnet–clinopyroxene thermometer and omphacite barometer in the presence of albite.     

Eclogites from the south Tianshan, NW China: petrological characteristic and calculated mineral equilibria in the Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O system

Eclogites from the south Tianshan, NW China are grouped into two types: glaucophane and hornblende eclogites, composed, respectively, of garnet + omphacite + glaucophane + paragonite + epidote + quartz and garnet + omphacite + hornblende (sensu lato) + paragonite + epidote + quartz, plus accessory rutile and ilmenite. These eclogites are diverse both in mineral composition and texture not only between the two types but also among the different selected samples within the glaucophane eclogite. Using thermocalc 3.1 and recent models of activity–composition relation for minerals, a P–T projection and a series of P–T pseudosections for specific samples of eclogite have been calculated in the system Na₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O (NCFMASH) with quartz and water taken to be in excess. On the basis of these phase diagrams, the phase relations and P–T conditions are well delineated. The three selected samples of glaucophane eclogite AK05, AK11 and AK17 are estimated to have peak P–T conditions, respectively, of 540–550 °C at c. 16 kbar, c. 560 °C at 15–17 kbar and c. 580 °C at 15–19 kbar, and two samples of hornblende eclogite AK10 and AK30 of 610–630 °C and 17–18 kbar. Together with H₂O‐content contours in the related P–T pseudosections and textural relations, both types of eclogite are inferred to show clockwise P–T paths, with the hornblende eclogite being transformed from the glaucophane eclogite assemblage dominantly through increasing temperature.     

Paragonite–hornblende assemblages and their petrological significance: an example from the Austroalpine Schneeberg Complex, Southern Tyrol, Italy

ABSTRACT Paragonite-bearing amphibolites occur interbedded with a garbenschist-micaschist sequence in the Austroalpine Schneeberg Complex, southern Tyrol. The mineral assemblage mainly comprises paragonite + Mg-hornblende/tschermakite + quartz + plagioclase + biotite + ankerite + Ti-phase + garnet ± muscovite. Equilibrium P–T conditions for this assemblage are 550–600°C and 8–10 kbar estimated from garnet–amphibole–plagioclase–ilmenite–rutile and Si contents of phengitic muscovites. In the vicinity of amphibole, paragonite is replaced by symplectitic chlorite + plagioclase + margarite +± biotite assemblages. Muscovite in the vicinity of amphibole reacts to form plagioclase + biotite + margarite symplectites. The reaction of white mica + hornblende is the result of decompression during uplift of the Schneeberg Complex. The breakdown of paragonite + hornblende is a water-consuming reaction and therefore it is controlled by the availability of fluid on the retrogressive P–T path. Paragonite + hornblende is a high-temperature equivalent of the common blueschist-assemblage paragonite + glaucophane in Ca-bearing systems and represents restricted P–T conditions just below omphacite stability in a mafic bulk system. While paragonite + glaucophane breakdown to chlorite + albite marks the blueschist/greenschist transition, the paragonite + hornblende breakdown observed in Schneeberg Complex rocks is indicative of a transition from epidote-amphibolite facies to greenschist facies conditions at a flatter P–T gradient of the metamorphic path compared to subduction-zone environments. Ar/Ar dating of paragonite yields an age of 84.5 ± 1 Ma, corroborating an Eoalpine high-pressure metamorphic event within the Austroalpine unit west of the Tauern Window. Eclogites that occur in the Ötztal Crystalline Basement south of the Schneeberg Complex are thought to be associated with this Eoalpine metamorphic event.     

P–T path of high-pressure/low-temperature rocks and tectonic implications in the western Tianshan Mountains, NW China

The Chinese western Tianshan high-pressure/low-temperature (HP–LT) metamorphic belt, which extends for about 200 km along the South Central Tianshan suture zone, is composed of mainly metabasic blueschists, eclogites and greenschist facies rocks. The metabasic blueschists occur as small discrete blocks, lenses, bands, laminae or thick beds in meta-sedimentary greenschist facies country rocks. Eclogites are intercalated within blueschist layers as lenses, laminae, thick beds or large massive blocks (up to 2 km² in plan view). Metabasic blueschists consist of mainly garnet, sodic amphibole, phengite, paragonite, clinozoisite, epidote, chlorite, albite, accessory titanite and ilmenite. Eclogites are predominantly composed of garnet, omphacite, sodic–calcic amphibole, clinozoisite, phengite, paragonite, quartz with accessory minerals such as rutile, titanite, ilmenite, calcite and apatite. Garnet in eclogite has a composition of 53–79 mol% almandine, 8.5–30 mol% grossular, 5–24 mol% pyrope and 0.6–13 mol% spessartine. Garnet in blueschists shows similar composition. Sodic amphiboles include glaucophane, ferro-glaucophane and crossite, whereas the sodic–calcic amphiboles mainly comprise barroisite and winchite. The jadeite content of omphacite varies from 35–54 mol%. Peak eclogite facies temperatures are estimated as 480–580 °C for a pressure range of 14–21 kbar. The conditions of pre-peak, epidote–blueschist facies metamorphism are estimated to be 350–450 °C and 8–12 kbar. All rock types have experienced a clockwise P–T path through pre-peak lawsonite/epidote-blueschist to eclogite facies conditions. The retrograde part of the P–T path is represented by the transition of epidote-blueschist to greenschist facies conditions. The P–T path indicates that the high-pressure rocks formed in a B-type subduction zone along the northern margin of the Palaeozoic South Tianshan ocean between the Tarim and Yili-central Tianshan plates.     

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.     

Metamorphic history of glaucophane‐paragonite‐zoisite eclogites from the Shanderman area,northern Iran

The Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields <11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.     

Petrogenesis of lawsonite and epidote eclogite and blueschist, Sivrihisar Massif, Turkey

The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre‐ to centimetre‐scale eclogite pods occur in blueschist facies metabasalt, marble and quartz‐rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P–T conditions: ～26 kbar, 500 °C (lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (lawsonite blueschist); and 15–16 kbar, 480–500 °C (lawsonite‐epidote blueschist). Pressures for the Sivrihisar lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of lawsonite ± epidote define P–T conditions and paths. For example, in some lawsonite‐bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by lawsonite suggest an anticlockwise P–T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the lawsonite stability field. Results of the P–T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P–T conditions were tectonically juxtaposed as kilometre‐scale slices and associated high‐P pods, although all shared the same exhumation path from ～9–11 kbar, 300–400 °C. Within the tectonic slices, alternating millimetre–centimetre‐scale layers of eclogite and blueschist formed together at the same P–T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in fO₂ or other chemical factors.     

Phase relations in the greenschist-blueschist-amphibolite-eclogite facies in the system Na2O-CaO-FeO-MgO-Al2O3-SiO2-H2O (NCFMASH), with application to metamorphic rocks from Samos, Greece

Calculated phase equilibria among the minerals sodic amphibole, calcic amphibole, garnet, chloritoid, talc, chlorite, paragonite, margarite, omphacite, plagioclase, carpholite, zoisite/clinozoisite, lawsonite, pyrophyllite, kyanite, sillimanite, quartz and H₂O are presented for the model system Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O (NCFMASH), which is relevant for many greenschist, blueschist, amphibolite and eclogite facies rocks. Using the activity-composition relationships for multicomponent amphiboles constrained by Will and Powell (1992), equilibria containing coexisting calcic and sodic amphiboles could be determined. The blueschist–greenschist transition reaction in the NCFMASH system, for example, is defined by the univariant reaction sodic amphibole + zoisite = calcic amphibole + chlorite + paragonite + plagioclase (+ quartz + H₂O) occurring between approximately 420 and 450 °C at 9.5 to 10 kbar. The calculated petrogenetic grid is a valuable tool for reconstructing the PT-evolution of metabasic rocks. This is shown for rocks from the island of Samos, Greece. On the basis of mineral and whole rock analyses, PT-pseudosections were calculated and, together with the observed mineral assemblages and reaction textures, are used to reconstruct PT-paths. For rocks from northern Samos, pseudomorphs after lawsonite preserved in garnet, the assemblage sodic amphibole-garnet-paragonite-chlorite-zoisite-quartz and the retrograde appearance of albitic plagioclase and the formation of calcic amphibole around sodic amphibole constrain a clockwise PT-path that reaches its thermal maximum at some 520 °C and 19 kbar. The derived PT-trajectory indicates cooling during exhumation of the rocks and is similar to paths for rocks from the western part of the Attic-Cycladic crystalline complex. Rocks from eastern Samos indicate lower pressures and are probably related to high-pressure rocks from the Menderes Massif in western Turkey. Received: 8 July 1997 / Accepted: 11 February 1998     

Calculated stabilities of sodic phases in the Sambagawa metapelites and their implications

Pressure (P)–temperature (T) pseudosection analyses were carried out on metapelites from Sambagawa belt by using Perple_X 07 so as to determine mineral equilibria and the stability of sodic phases, in the model system MnO–Na₂O–K₂O–CaO–FeO–MgO–Al₂O₃–SiO₂–H₂O–(CO₂) under high‐pressure (HP) conditions (0.5–2.5 GPa/400–600 °C). A pressure–X_NA [=Na/(Na+Al–2K–1.5Ca)] pseudosection at 500 °C is also calculated to examine the effect of Na/Al value of the bulk‐rock composition on the stabilities of sodic minerals. The bulk‐rock compositions of Sambagawa metapelite are variable in X_NA values. The calculation results of stable assemblages in metapelites under the blueschist and eclogite facies conditions indicate that: (i) paragonite and glaucophane are stable throughout the wide X_NA range of bulk‐rock compositions of host rocks; (ii) stable P–T conditions of sodic pyroxene enlarge with increasing X_NA value; and (iii) the stability field of paragonite enlarges with the presence of CO₂ in the metamorphic fluid. The suggested wide stability of paragonite in metapelites and the relationships between the stability of sodic pyroxene and the bulk‐rock compositions explain the reasons why (i) the occurrence of omphacite in metapelites from several subduction‐related terranes is rare; and (ii) paragonite commonly occurs as inclusions in garnet of metapelites from the Besshi region of the Sambagawa belt. Paragonite is an important sodic phase of HP metapelites, and a combination of paragonite and quartz with high residual pressure included in garnet may be a useful indicator to verify the evidence for the eclogite facies metamorphism recorded in metapelites.     

Eclogite and carpholite-bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for Early Palaeozoic cold oceanic subduction and water transport into mantle

Low‐temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high‐pressure low‐temperature (HPLT) subduction complex (mélange) in the North Qilian oceanic‐type suture zone, NW China. Phengite–eclogite (type I) and epidote–eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet–omphacite–phengite geothermobarometry yields metamorphic conditions of 460–510 °C and 2.20–2.60 GPa for weakly deformed eclogite, and 475–500 °C and 1.75–1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet–omphacite–phengite–glaucophane schist and various chloritoid‐bearing schists. Mg‐carpholite was identified in some high‐Mg chloritoid schists. P–T estimates yield 2.60–2.15 GPa and 495–540 °C for Grt–Omp–Phn–Gln schist, and 2.45–2.50 GPa and 525–530 °C for the Mg‐carpholite schist. Mineral assemblages and P–T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi‐pelagic sediments have been subducted to the mantle depths (≥75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454–446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P–Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction‐zone gradients (6–7 °C km^?1), and this cold subduction brought a large amount of H₂O to the deep mantle in the Early Palaeozoic times.     

Petrology and metamorphism of glaucophane eclogites in Changning-Menglian suture zone,Bangbing area,southeast Tibetan Plateau: An evidence for Paleo-Tethyan subduction

High/ultrahigh-pressure (HP/UHP) metamorphic complexes, such as eclogite and blueschist, are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones. Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary mélange in the Changning-Menglian suture zone, at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau. The authors report the result of petrological, mineralogical and metamorphism investigations of these rocks, and discuss their tectonic implications. The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists. The major mineral assemblage of the eclogites includes garnet, omphacite, glaucophane, phengite, clinozoisite and rutile. Eclogitic garnet contains numerous inclusions, such as omphacite, glaucophane, rutile, and quartz with radial cracks around. Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation. Four stages of metamorphic evolution can be determined: The prograde blueschist facies (M₁), the peak eclogite facies (M₂), the decompression blueschist facies (M₃) and retrograde greenschist facies (M₄). Using the Grt-Omp-Phn geothermobarometer, a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658°C was determined, which is typical of low-temperature ultrahigh-pressure metamorphism. The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic mélange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean. The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone.©2021 China Geology Editorial Office.     

Phase equilibria modelling of blueschist and eclogite from the Sanbagawa metamorphic belt of southwest Japan reveals along‐strike consistency in tectonothermal architecture

The Sanbagawa metamorphic belt of southwest Japan is one of the type localities of subduction‐related high‐P metamorphism. However, variable pressure–temperature (P–T) paths and metabasic assemblages have been reported for eclogite units in the region, leading to uncertainty about the subduction zone paleo‐thermal structure and associated tectonometamorphic conditions. To analyse this variation, phase equilibria modelling was applied to the three main high‐P metabasic rock types documented in the region – glaucophane eclogite, barroisite eclogite and garnet blueschist – with modelling performed over a range of P, T, bulk rock H₂O and bulk rock ferric iron conditions using thermocalc . All samples are calculated to share a common steep prograde P–T path to similar peak conditions of ～16–20 kbar and 560–610 °C. The results establish that regional assemblage variation is systematic, with the alternation in peak amphibole phase due to peak conditions overlapping the glaucophane–barroisite solvus, and bulk composition effects stabilizing blueschist v. eclogite facies assemblages at similar P–T conditions. Furthermore, the results reveal that a steep prograde P–T path is common to all eclogite units in the Sanbagawa belt, indicating that metamorphic conditions were consistent along strike. All localities are compatible with predictions made by a ridge approach model, which attributes eclogite facies metamorphism and exhumation of the Sanbagawa belt to the approach of a spreading ridge.     

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.     

Metamorphic evolution of relict lawsonite‐bearing eclogites from the (U) HP metamorphic belt in the Chinese southwestern Tianshan

In the Chinese southwestern Tianshan (U)HP belt, former lawsonite presence has been predicted for many (U)HP metamorphic eclogites, but only a very few lawsonite grains have been found so far. We discovered armoured lawsonite relicts included in quartz, which, on its part, is enclosed in porphyroblastic garnet in an epidote eclogite H711‐14 and a paragonite eclogite H711‐29. H711‐14 is mainly composed of garnet, omphacite, epidote and titanite, with minor quartz, paragonite and secondary barroisite and glaucophane. Coarse‐grained titanite occasionally occurs in millimetre‐wide veins in equilibrium with epidote and omphacite, and relict rutile is only preserved as inclusions in matrix titanite and garnet. H711‐29 shows the mineral assemblage of garnet, omphacite, glaucophane, paragonite, quartz, dolomite, rutile and minor epidote. Dolomite and rutile are commonly rimed by secondary calcite and titanite respectively. Porphyroblastic garnet in both eclogites is compositionally zoned and exhibits an inclusion‐rich core overgrown by an inclusion‐poor rim. Phase equilibria modelling predicts that garnet cores formed at the P‐peak (490–505 °C and 23–25.5 kbar) and coexisted with the lawsonite eclogite facies assemblage of omphacite + glaucophane + lawsonite + quartz. Garnet rims (550–570 °C and ~20 kbar) grew subsequently during a post‐peak epidote eclogite facies metamorphism and coexisted with omphacite + quartz ± glaucophane ± epidote ± paragonite. The results confirm the former presence of a cold subduction zone environment in the Chinese southwestern Tianshan. The P–T evolution of the eclogites is characterized by a clockwise P–T path with a heating stage during early exhumation (thermal relaxation). The preservation of lawsonite in these eclogites is attributed to isolation from the matrix by quartz and rigid garnet, which should be considered as a new type of lawsonite preservation in eclogites. The complete rutile–titanite transition in H711‐14 took place in the epidote eclogite facies stage in the presence of an extremely CO₂‐poor fluid with X(CO₂) [CO₂/(CO₂ + H₂O) in the fluid] <<0.008. In contrast, the incomplete rutile–titanite transition in H711‐29 may have occurred after the epidote eclogite facies stage and the presence of dolomite reflects a higher X(CO₂) (>0.01) in the coexisting fluid at the epidote eclogite facies stage.