Eclogite Facies Stage Research Articles

Multiphase-solid fluid inclusions in HP-LT eclogite facies rock (Zavkhan Terrane, Western Mongolia): evidence for the evolution from saline to hypersaline fluids during metamorphism in subduction zone

Fluid inclusions in high- and ultrahigh-pressure metamorphic rocks provide direct information on the composition of the fluids that evolved during metamorphism and fluid-rock interactions in deep subduction zones. We investigate the fluid inclusions in the Khungui eclogite of the Zavkhan Terrane, Central Asian Orogenic Belt. Fluid inclusions are observed in garnet and quartz in the eclogite samples that underwent metamorphism during subduction. The primary fluid inclusions in quartz are composed of liquid and vapor with high salinities (15.7–16.4 wt.% NaCl eq.), whereas the secondary fluid inclusions in quartz are classified as: relatively high salinity (Type I:12.5–16.3 wt.% NaCl eq.) and low salinity (Type II:6.7–10.6 wt.% NaCl eq.). The garnet shows compositional zoning from Ca-poor cores to Ca-rich rims, and the rims that grew during the eclogite-stage metamorphism (2.1–2.2 GPa at 580–610 °C) preferentially contain numerous primary fluid inclusions. The primary fluid inclusions in garnet are commonly bi-phases (liquid and vapor); however, some are multiphase-solid fluid inclusions composed of fluids (liquid and vapor) and combinations of several minerals (halite, quartz, apatite, calcite, biotite, chlorite, and actinolite). Bi-phase fluid inclusions preferentially occur in the inner parts of the Ca-rich garnet rim, whereas multiphase-solid fluid inclusions occur along the margins of the Ca-rich rim. We hypothesize that the multiphase-solid fluid inclusions are formed via interactions between trapped fluids, trapped minerals, and the host garnet during exhumation. By combination of FIB–SEM and synchrotron X-ray CT analyses, the detailed occurrences, volumes, and compositions of the solid phases in the fluid inclusion was analyzed. We then conduct mass balance analysis to reconstruct accurate fluid compositions using data from the FIB–SEM and synchrotron X-ray CT images of the multiphase-solid fluid inclusion. The results of these analyses reveal that (1) fluid changed from an H2O-dominated saline fluid (13–16 wt. % NaCl eq.) at the prograde to the earlier eclogite stage to H2O–CO2-dominated hypersaline fluid at later eclogite stage (~ 32 wt. % NaCl eq., 7.3 wt. % CO2 and ~ 19 molal dissolved cations); (2) a variety of mineral assemblages in multiphase-solid fluid inclusions are produced by post-entrapment reactions between the trapped hypersaline fluid, trapped minerals and the fluid host mineral. The evolution of fluids from saline to hypersaline during the eclogite facies stage is probably caused by the formation of hydrous minerals (i.e., barroisite) under a near-closed system.

Oceanic subduction to continental collision in the NE Proto-Tethys revealed by early Paleozoic eclogites with high-temperature granulite-facies overprinting in the East Kunlun orogenic belt, northern Tibet

The East Kunlun orogenic belt in the northern Tibetan Plateau records a long-term accretionary and collisional history in the northeastern Proto-Tethys Ocean, which is important for reconstructing the paleogeography of early Paleozoic East Asia. Here, we present an integrated study combining petrology, geochemistry, geochronology, and metamorphic pressure−temperature (P−T) data of newly found eclogites in the middle Nuomuhong segment of the East Kunlun orogenic belt. The eclogites are composed mainly of garnet, omphacite and low-sodium clinopyroxene, amphibole and plagioclase with minor orthopyroxene, biotite, quartz, accessory rutile, ilmenite, titanite, and zircon. Detailed petrographic observations, conventional geothermobarometry, and phase equilibrium modeling point to the presence of five metamorphic mineral assemblages with corresponding P−T conditions related to: (1) prograde M1 stage P−T estimates at >14.0 kbar/∼470−506 °C; (2) Pmax M2 eclogite-facies stage P−T conditions of ∼26 kbar/∼570 °C; (3) early retrograde M3 high-pressure granulite-facies stage; (4) subsequent M4 retrograde medium-pressure granulite facies at Tmax of ∼860−900 °C; and (5) later M5 retrograde amphibolite-facies stage P−T conditions of <6.2 kbar/∼710−730 °C. These P−T estimates define a clockwise P−T path characterized by heating during the Pmax formation of the eclogite facies, to the Tmax exhumation stage of granulite-facies lithologies, the latter of which is identified for the first time in retrograde eclogites from the East Kunlun orogenic belt. Whole-rock geochemical compositions indicate a mid-oceanic-ridge basalt (MORB) affinity for the eclogite protoliths and a fragmented oceanic crust origin. Sensitive high-resolution ion microprobe (SHRIMP) zircon U−Pb isotopic analyses of the eclogite yielded two groups of weighted mean 206U/238Pb ages of 464 ± 8 Ma and 419 ± 4 Ma, which are interpreted as the age of the eclogite protoliths and the lower threshold for peak eclogite-facies metamorphism, respectively. Our new data, together with regional eclogite-facies metamorphism, suggest a ca. 520−460 Ma age for the subduction of the eastern Kunlun oceanic crust, within the northern Proto-Tethys Ocean, to a depth of ∼83 km, with early subduction−accretionary orogenesis at ca. 419 Ma. Overprinting by high-temperature granulite-facies points to exhumation of oceanic crust to the middle to shallow crustal level at this time. Collectively, the preserved eclogite and high-temperature granulite mineral assemblage provide new constraints on the tectonic evolution and the detailed accretionary-to-collisional orogenesis of the Proto-Tethys Ocean. They suggest that the ca. 428−411 Ma subduction-collisional event marked the termination of the Proto-Tethys Ocean and the eventual formation of an ∼500-km-long, high- to ultra-high−pressure metamorphic belt in the East Kunlun orogenic belt.

Tectonic erosion and deep subduction in Central Tibet: Evidence from the discovery of retrograde eclogites in the Amdo microcontinent

AbstractThe Amdo microcontinent which separates the Qiangtang terrane to the north and the Lhasa terrane to the south is a key terrane for reconstructing the tectonic evolution of Central Tibet. We report the new finding of retrograde eclogites within the Amdo microcontinent in this study. The eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, rutile and quartz and underwent a four‐stage metamorphic evolution, including a peak eclogite facies stage (M1) at ~20–24 kbar and 580–620°C, followed by an HP granulite facies decompression stage (M2) at ~13–15 kbar and 750–780°C, a subsequent MP‐UHT granulite facies heating stage (M3) at 8–10 kbar and >840°C and a final amphibolite facies retrogression (M4) at 5.3–6.0 kbar and 560–580°C. The eclogites exhibit rare earth element distribution patterns and trace element abundances similar to those of N‐MORB and arc‐related volcanics, with depleted whole‐rock εNd(t) values of 3.4 to 4.2, and are inferred to have formed in a back‐arc basin tectonic setting. Zircon and rutile U–Pb dating yields a protolith age of 226 ± 5 Ma, a peak eclogite facies metamorphic age of 190 ± 1 Ma, an HP granulite facies metamorphic age of 179 ± 1 Ma and an amphibolite facies retrograde age of 172 ± 1 Ma. The clockwise P–T–t paths and the oceanic protolith signature of retrograde eclogites suggest that part of the back arc basin was subducted to depths of ~80 km. Tectonic erosion associated with the subduction of the Amdo microcontinent beneath the Tethys Ocean accounts for the deep subduction of the back‐arc basin and the absence of arc magmatic rocks in the northern Amdo microcontinent.

Newly recognized retrograde eclogites overprinted by high‐temperature metamorphism in the Amdo microcontinent, Central Tibet: Implications for subduction erosion during continental subduction

The Amdo microcontinent located within the Bangong–Nujiang Suture Zone (BNSZ) holds one of the important keys to understand the tectonic evolution of Central Tibet before the Qiangtang‐Lhasa terrane collision during the Mesozoic. Newly discovered eclogites were reported within the Amdo microcontinent in this study, and their metamorphic evolution is deciphered by petrographic observations, pseudosection modelling, geochemistry, and zircon U–Pb dating. The eclogites are characterized by peak metamorphic mineral assemblages of garnet, omphacite, rutile, and quartz, and underwent a four‐stage metamorphic evolution, including eclogite facies stage (M1) at ~20–24 kbar and 580–620°C, followed by HP granulite facies decompression stage (M2) at ~13–15 kbar and 750–780°C, subsequent MP–HT granulite facies heating stage (M3) at 8–10 kbar and >840°C, and final amphibolite facies retrogression (M4) at 5.3–6.0 kbar and 560–580°C. The eclogites exhibit rare earth element (REE) distribution patterns and trace element abundance similar to those of N‐MORB and arc‐related volcanics, which are inferred to have formed in a back‐arc Basin tectonic setting. Zircon U–Pb dating yields eclogite facies metamorphic age of 190 ± 1 Ma. The clockwise P–T–t paths and the oceanic protolith signature of retrograde eclogites suggest that part of the back‐arc Basin was subducted to the depths of ~80 km. The subsequent ultra‐high temperature metamorphic overprinting may be related to the upwelling of the asthenosphere. Tectonic erosion associated with the subduction of the Amdo microcontinent beneath the Tethys Ocean accounts for the deep subduction of the back‐arc Basin and the absence of arc magmatic rocks in the northern Amdo microcontinent.

Deep subduction and exhumation of micro-continents in the Proto-Tethys realm: Evidence from metamorphism of HP-UHT rocks in the North Qinling Orogen, central China

The North Qinling Orogen in central China contains a typical continental high- and ultrahigh-pressure (HP-UHP) metamorphic belt, but its metamorphic evolution remains controversial. We report a combined investigation on petrology, geochronology and phase modelling for felsic granulite (samples SN1504 & SN1517) and garnet clinopyroxenite (sample SN1505) from the Songshugou area. Four stages of metamorphic evolution are constrained: (i) A possible prograde from HP amphibolite facies to peak UHP eclogite facies stage (~3–8 GPa/700–1080 °C) recorded by the chemical zoning of garnet core and mantle in sample SN1504 and also the presence of supersilicic titanite in sample SN1505. (ii) A post-peak decompression evolution is recorded by the formation of coronary garnet and plagioclase around kyanite in sample SN1504 and high-Ti amphibole and plagioclase in sample SN1505, including the evolution from eclogite facies to HP–ultrahigh temperature (UHT) conditions of 1.3–2.2 GPa/900–980 °C for sample SN1504 and 1.0–2.1 GPa/960–1010 °C for SN1505, respectively. (iii) Further decompression is revealed from the growth of corundum + spinel in local domains in sample SN1504, suggesting a low-pressure (LP) condition of ~0.8 GPa/950 °C. (iv) A subsequent cooling evolution is recorded by thin films of K-feldspar + quartz + biotite in sample SN1504 and low-Ti amphibole in sample SN1505, indicative of melt crystallization towards the solidus with conditions of 830–770 °C/0.55–0.8 GPa and 880–820 °C/0.8–0.9 GPa, respectively. Zircon and monazite U-Pb dating yields three metamorphic ages of c. 521 Ma, c. 503 Ma and c. 500–480 Ma, which are interpreted to represent the prograde HP amphibolite facies stage, the peak eclogite facies stage and the late cooling stage, respectively. The metamorphic P–T–t path suggests a complete history of deep continental subduction to depths of >90–250 km, followed by rapid exhumation to crustal depths for the Qinling micro-continent during the early Proto-Tethys evolution.

Metamorphism and geochronology of high-pressure mafic granulites (retrograded eclogites?) in East Cathaysia terrane of South China: Implications for Mesozoic tectonic evolution

AbstractHigh-pressure mafic granulites (retrograded eclogites?) were discovered as minor lenses enclosed in garnet-kyanite-cordierite gneiss from the Badu Complex of the East Cathaysia terrane in South China. These rocks consist mainly of garnet, clinopyroxene, hornblende, quartz, and rutile/ilmenite with or without omphacite pseudomorphs that are indicated by clinopyroxene + sodic plagioclase symplectic intergrowths. Mineral textures and reaction relationships suggest three metamorphic stages: (1) an eclogite-facies stage (M1) characterized by the mineral assemblage of garnet + clinopyroxene (omphacite) + hornblende + rutile + quartz; (2) a high-pressure granulite-facies (M2) stage mainly represented by garnet + clinopyroxene + plagioclase + hornblende + rutile + quartz in the matrix; and (3) an amphibolite retrograde stage (M3) defined by hornblende + plagioclase + ilmenite + quartz symplectites surrounding garnet porphyroblasts. Conventional geothermometers and geobarometers in combination with phase equilibria modeling constrain metamorphic P–T conditions of 15.8–18.2 kbar/625–690 °C (M1), 11.8–14.5 kbar/788–806 °C (M2), and 5.4–6.4 kbar/613–668 °C (M3), respectively. Two-staged decompression processes are defined after the peak pressure, which suggests a two-staged exhumation of these deeply buried rocks. Secondary ion mass spectrometry (SIMS) zircon U-Pb dating and trace element analysis show that the high-pressure metamorphism occurred at 240–244 Ma. Complete early Mesozoic orogenic processes characterized by initial subduction and/or crustal thickening and subsequent exhumation followed by rapid uplift are reconstructed for this part of the East Cathaysia terrane, South China.

Stability conditions and compositional variations of deerite in high-pressure meta-ironstone during subduction–exhumation processes (SW Tianshan, China)

Index minerals in metamorphic rocks, such as coesite, glaucophane, or lawsonite, are of great importance to understand high-pressure (HP) to ultrahigh-pressure (UHP) metamorphism, the thermal gradient, and the geodynamics of subduction zones. Deerite is thought to be diagnostic for blueschist-facies metamorphism in cold subduction environments and a sensitive indicator of the oxygen fugacity (fO2) in Fe-rich rocks. However, little is known concerning its stability and compositional variation during subduction and exhumation cycles. In this study, we provide the first detailed investigation of deerite in eclogite-facies meta‑ironstones (deerite–garnet riebeckitite) from the Southwestern Tianshan (U)HP metamorphic belt. Petrological observation and textural evidence suggest that the meta‑ironstones –enclosing eclogite blocks– form part of an initially coherent volcano-sedimentary sequence that underwent a common tectono-metamorphic evolution, including HP metamorphism and subsequent exhumation. Two types of deerite, which display a different texture (inclusion vs. matrix) and composition (TiO2 content), occur in the deerite–garnet riebeckitite. Dee_1 and aegirine–augite appear exclusively as fine-grained inclusions in garnet, and together with garnet, riebeckite, quartz and magnetite form the peak metamorphic mineral assemblages at P–T conditions of ~510 °C and 2.6 GPa. Matrix deerite (Dee_2), on the other hand, mainly occurs as coarse-grained rods and acicular needle-like grains in quartz, and rarely as inclusions in riebeckite and garnet intimately in the riebeckitite, representing the blueschist-facies retrogression of the meta‑ironstones during exhumation. The garnet–clinopyroxene oxybarometer and thermodynamic calculations show that the fO2 of the deerite–garnet riebeckitite increases from ΔFMQ–0.64 at the peak eclogite-facies stage to ΔFMQ+1.5 at the blueschist-facies retrogression. This fO2 variation corresponds to the chemical variation of deerite, whereby the TiO2 content increases gradually from <0.1 wt% in the Dee_1 in garnet to about 5 wt% in the matrix Dee_2. Our study reveals the detailed metamorphic P–T–fO2 evolution of the prograde eclogite- and retrograde blueschist-facies deerite-bearing meta‑ironstones during subduction–exhumation processes. This study further emphasizes the potential of deerite to reveal the P–T–fO2 evolution of subduction-related high-pressure rocks.

The metamorphic PT history of Precambrian Belomorian eclogites (Shirokaya Salma), Russia

AbstractThis petrological study details representative well‐preserved and partly retrogressed eclogite samples from the Shirokaya Salma locality in the Belomorian Province (Russia) to resolve ambiguity in the PT evolution of one of the oldest eclogite occurrences on Earth. Based on microstructures, eclogite and granulite facies mineral assemblages comprise omphacite, garnet (interior), quartz, rutile±phengite and diopside, garnet (modified rim), plagioclase, orthopyroxene, amphibole, quartz, rutile/ilmenite±melt respectively. Omphacite and Na‐bearing diopside feature breakdown textures with multiple oriented quartz–amphibole and orthopyroxene lamellae, while diopside forms symplectite with plagioclase. Garnet often exhibits a particular compositional annulus in both major and trace elements (Y+REE). We interpret the high‐Ca and inclusion‐rich garnet (core) with outwards Grs decrease and Prp increase as relict of prograde growth. The mantle‐rim domain of garnet, containing omphacite, diopside, plagioclase and rutile formed during eclogitization. Compositionally homogeneous garnet porphyroblasts are most likely outcomes of high‐T modifications. A progressive zonation of Y+REE from core to rim in several garnets is related to resorption at high temperatures. Both geothermometric and pseudosection calculations reveal a typical orogenic clockwise trajectory for the studied eclogites, and their metamorphic PT path includes a possible pre‐eclogitic stage (M0) at &lt;630°C, ~16–19 kbar, a peak eclogite facies stage (M1) at 19–21 kbar, 670–750°C, a granulite facies overprint (stage M2) from 14–16 kbar, 830–870°C (M2a) to 10–11 kbar, 880–910°C (M2b), and retrogression to &lt;9–11 kbar, &lt;650–680°C (stage M3). It agrees well with our previous estimates for other Salma eclogites with Palaeoproterozoic ages.

Multiple Metamorphic Events Recorded within Eclogites of the Chandman District, SW Mongolia

The eclogite-bearing Alag Khadny metamorphic complex in the Lake Zone, SW Mongolia occupies the central region of the Central Asian Orogenic Belt, the largest Phanerozoic orogenic belt in the world. The complex consists mainly of orthogneisses intercalated with eclogites and micaschists in a mélange zone. Most of eclogites are strongly amphibolitized. In this study, we examined petrography and mineral chemistry of eclogites and amphibolitized eclogites, respectively. The result of our research shows that Chandman eclogites experienced multiple events of metamorphism in throughout their subduction and subsequent collision history. We revealed that eclogites were subjected to blueschist facies metamorphism before the peak eclogite facies stage. In addition, we have studied amphibolitized eclogite, and revealed that another distinct progressive medium pressure (MP) epidote-amphibolite facies metamorphic event took place in the eclogite, consistent with collision process. The multiple events of metamorphism in eclogites have been revealed by zonation textures of HP amphiboles zoned with glaucophane→barroisite→Mg-hornblende and MP amphiboles zoned with actinolite/winchite→barroisite→Mg-hornblende/tschermakite/Fe-pargasite. These amphiboles with different zonation textures reflect their metamorphic history of subduction to collision events.

Coexisting different types of zoned garnet in kyanite‐quartz eclogites from the Sanbagawa metamorphic belt: Evidence of deformation‐induced lithological mixing during prograde metamorphism

AbstractGarnet grains in Sanbagawa quartz eclogites from the Besshi region, central Shikoku commonly show a zoning pattern consisting of core and mantle/rim that formed during two prograde stages of eclogite and subsequent epidote–amphibolite facies metamorphism, respectively. Garnet grains in the quartz eclogites are grouped into four types (I, II, III, and IV) according to the compositional trends of their cores. Type I garnet is most common and sometimes coexists with other types of garnet in a thin section. Type I core formed with epidote and kyanite during the prograde eclogite facies stage. The inner cores of types II and III crystallized within different whole‐rock compositions of epidote‐free and kyanite‐bearing eclogite and epidote‐ and kyanite‐free eclogite at the earlier prograde stage, respectively. The inner core of type IV probably formed during the pre‐eclogite facies stage. The inner cores of types II, III, and IV, which formed under different P–T conditions of prograde metamorphism and/or whole‐rock compositions, were juxtaposed with the core of type I, probably due to tectonic mixing of rocks at various points during the prograde eclogite facies stage. After these processes, they have shared the following same growth history: (i) successive crystal growth during the later stage of prograde eclogite facies metamorphism that formed the margin of the type I core and the outer cores of types II, III, and IV; (ii) partial resorption of the core during exhumation and hydration stage; and (iii) subsequent formation of mantle zones during prograde metamorphism of the epidote–amphibolite facies. The prograde metamorphic reactions may not have progressed under an isochemical condition in some Sanbagawa metamorphic rocks, at least at the hand specimen scale. This interpretation suggests that, in some cases, material interaction promoted by mechanical mixing and fluid‐assisted diffusive mass transfer probably influences mineral reactions and paragenesis of high‐pressure metamorphic rocks.

An integrated EPMA-EBSD study of metamorphic histories recorded in garnet

Growth histories recorded in garnet grains in metasedimentary rocks from the Sanbagawa belt in Japan and the Mogok belt in Myanmar were analyzed using an effective combination of electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) data. Garnet in the Sanbagawa metapelite has inner and outer zones that formed in the eclogite and epidote-amphibolite facies stages, respectively. Based on EPMA element mapping, this garnet appears to have grown as a single crystal with a temporal break in growth between the inner and outer zones that occurred during exhumation. The EBSD data, however, document that the garnet grain is composed of four domains. The misorientation angles of crystallographic orientations between the domains are as large as 59°, and domain boundaries crosscut the growth zoning and the compositional boundary between the inner and outer zones. Sets of quartz grains included in the garnets on either side of the domain boundaries sometimes share the same crystallographic orientation with misorientation angles less than 4°. The garnet grains formed via a three-step process of prograde crystallization of polycrystalline garnet during the eclogite facies stage (inner zone) → resorption around garnet rims and along domain boundaries during exhumation → crystallization of the outer zone and in the domain boundaries during the prograde epidote-amphibolite facies stage. The garnet porphyroblasts in the Mogok pelitic gneisses, which formed during prograde metamorphism to the upper amphibolite-granulite facies (0.6–1.0 GPa/780–850 °C), are now separated into segments of various sizes by mosaic or symplectite aggregates of biotite, plagioclase, and quartz or monomineralic biotite veins. The segment texture formed at about 0.3–0.4 GPa/610–650 °C or lower-grade conditions. The EBSD analysis shows that most of the segments share the same crystallographic orientation with misorientation angles less than 4° and show no evidence of deformation and/or rotation processes after segmentation. These data suggest that the Mogok sample did not experience dynamic deformation of the garnet grains after the resorption and segmentation stage and may have been exhumed under static conditions from depths of 9–12 km.

Early Mesozoic retrograded eclogite and mafic granulite from the Badu Complex of the Cathaysia Block, South China: Petrology and tectonic implications

The high-grade metamorphic terrane in the Badu region along the northeastern Cathaysia Block in South China preserves retrograded eclogites and mafic granulites. Here we present the petrology, mineral phase equilibria and P-T conditions based on pseudosection computations, as well as zircon U-Pb ages of these rocks. Mineral textures and reaction relationships suggest four metamorphic stages for the retrograded eclogite as follows: (1) eclogite facies stage (M1), (2) clinopyroxene retrograde stage (M2), (3) amphibole retrograde stage (M3), and (4) chlorite retrograde stage (M4). For the mafic granulite, three stages are identified as: (1) plagioclase-absent stage (M1), (2) granulite facies stage (M2) and (3) amphibolite facies stage (M3). Metamorphic evolution of both of the rock types follows clockwise P-T path. Conventional geothermometers and geobarometers in combination with phase equilibria modelling yield metamorphic P-T conditions for each metamorphic stage for the eclogite as 500–560°C, 23–24kbar (M1), 640–660°C, 14–16kbar (M2), 730–750°C, and 11–13kbar (M3). The chlorite retrograde stage (M4) is inferred to have occurred at lower amphibolite to greenschist facies conditions. Phase equilibria modelling of the mafic granulite shows P-T conditions for each metamorphic stage as 600–720°C, >13kbar (M1) and 860–890°C, 5–6kbar (M2) and M3 at amphibolite facies conditions. LA-ICPMS zircon U-Pb dating and trace element analysis show that the high pressure metamorphism occurred at 245–251Ma. Protolith age of the mafic granulite is 997Ma, similar to that of the mafic to ultramafic rocks widely distributed in the Cathaysia Block and also along the Jiangnan belt. Subduction of ancient oceanic lithospheric materials (or crustal thickening) during Mesozoic and formation of eclogites suggest that the Cathaysia Block was perhaps in the Tethyan oceanic domain at this time. The granulite formation might have been aided by Mesozoic mafic magma underplating associated with lithospheric delamination, heating and retrogression of the eclogite accompanied by rapid uplift.

Protracted (~ 30 Ma) eclogite-facies metamorphism in northern Victoria Land (Antarctica): Implications for the geodynamics of the Ross/Delamerian Orogen

Northern Victoria Land, Antarctica, plays a key role in any geodynamic reconstructions of the paleo-Pacific margin of Gondwana because it represents the along-strike continuation of Australia in Antarctica and hosts the only Paleozoic eclogites known along the Transantarctic Mountains. The architecture and evolution of the early Cambrian to Early Ordovician Ross–Delamerian Orogen in northern Victoria Land, however, remain controversial. Accurate geodynamic reconstructions have been hindered by a dearth of temporal metamorphic constraints. This study reports laser-ablation split-stream ICP-MS data on zircon in newly recognized eclogite, that preserves a memory of the prograde evolution, and on zircon from its country rock from the Lanterman Range of northern Victoria Land. Geochronological data are supplemented for the eclogite by whole-rock geochemical and petrological constraints, that indicate an E-MORB protolith derived from a depleted mantle source, and P–T conditions for the eclogite-facies stage of ~700°C and 1.7–2.4GPa. U–Pb ages, trace-element contents, cathodoluminescence imaging and textural/inclusion relationships testify to protracted zircon growth/recrystallization history under eclogite-facies conditions, that also included (re)crystallization during part of the prograde path. Results extend the onset of eclogite-facies metamorphism in northern Victoria Land back to ~530Ma and, together with geochronological evidence for the emplacement of calc-alkaline granitoids at ~530–520Ma, suggest that UHP–HP rocks formed by accretion of material beneath an active continental margin. Regionally, this interpretation implies that burial of mafic rocks and associated siliciclastic sequences to mantle depths commenced significantly earlier than previously believed (~500Ma), thereby contrasting with the common belief that high-pressure metamorphism was linked to arc–continent collision. New and old results reveal the coexistence along the same ridge of two eclogite types: (1) medium-grained, “colder” and undeformed eclogites, that recorded both prograde and peak conditions, and (2) finer-grained, “hotter” and slightly deformed eclogites, that recorded only metamorphic peak conditions with hints of the subsequent retrogression. We propose that “prograde/peak” eclogites formed during an active continental margin stage and “peak” eclogites during the subsequent island arc–continental arc collision.

EBSD-measured crystal preferred orientation of eclogites from the Sanbagawa metamorphic belt, central Shikoku, SW Japan

Electron back-scattered diffraction (EBSD) maps and crystal-preferred orientation (CPO) of eclogite-facies (omphacite and garnet) and amphibolite-facies (hornblende and actinolite) phases are reported for understanding the rheological behaviour of crust during subduction. Two types of eclogites from the subduction-related high-pressure/low-temperature type Sanbagawa metamorphic belt, Japan, have been investigated. Type-I eclogite (sample Sb-1) is composed of garnet, omphacite, secondary actinolite and hornblende. Type-II eclogite (samples Sb-2 and Sb-2a) are mainly composed of omphacite, garnet, and retrograde hornblende with no actinolite. Omphacite, the peak eclogite-facies phase, exhibits L-type CPO (maximum density of [001] axes parallel to and high density of {110} poles normal to the lineation) in Type-I eclogite, suggesting intra-crystalline plasticity with [001] {110} and 〈110〉{110} active slip systems, indicating a constrictive strain regime at mantle depths. Omphacite in Type-II eclogite exhibits a similar fabric but with much weaker CPO. Using the LS-index symmetry analysis (one for the end-member L-type, zero for the end-member S-type, and intermediate values for LS-types), a progressive change in LS index of 0.80 for Type-I and 0.61 to 0.44 for Type-II eclogites is observed. These values suggest a transition from axial extension parallel to the lineation for Sb-1 and weaker CPO associated with pure or simple shear for Sb-2 and Sb-2a. Garnet, the second dominant phase in the eclogite-facies stage, exhibits weak and complex fabric patterns in all eclogite types, behaved like rigid bodies and does not show plastic deformation. Amphibolite-facies phases ( e.g ., hornblende and actinolite) exhibit more than two types of CPO. Hornblende and actinolite in Type-I eclogite have a strong CPO along [001] axes aligned parallel to the lineation, indicating homotactic crystal growth probably by the replacement of omphacite during the early stages of retrogression. Type-II eclogites have weak CPO in hornblende but with characteristic alignment of [001] parallel to the lineation and other poles to planes (100), (010), and {110} normal to the lineation. This fabric might have resulted from a cataclastic deformation and could be related to the late-“D1” deformation stage in the Sanbagawa metamorphic belt.

A review of the first eclogites discovered in the Eastern Himalaya

As a tribute to Bruno Lombardo, our senior co-worker who sadly passed away in 2014, the geologic setting and metamorphic evolution of the first eclogites discovered in the Eastern Himalaya are summarised in the context of U-Pb analytical results from the original granulitized eclogite samples. These eclogites occur at the top of the Main Central Thrust (MCT) zone in the Ama Drime Range (Arun - Phung Chu Valley), southeastern Tibet. The eclogites record five superposed metamorphic stages: a first eclogite-facies stage ( P > 1.5 GPa, T > 580°C), two granulite-facies stages at decreasing pressure ( P = 0.8–1.0 GPa, T > 750°C; P ~ 0.4 GPa, T ~ 750°C), an amphibolite-facies stage ( T = 700°C, P = 0.4–0.6 GPa) and a late retrogression stage ( T < 690°C, P <0.4 GPa). The timing of eclogite-facies metamorphism in the Ama Drime Range is difficult to constrain because of the widespread high-temperature granulite-facies overprint and the poor preservation of high-pressure assemblages. Zircons separated and analysed by the U-Pb SIMS (secondary ionization mass spectrometry) method display complex and varied systematics. Low-U rim domains with low Th/U ratios (0.02–0.03) give young 206Pb/238U ages of 13–14 Ma, which are consistent with age estimates of other workers for the timing of granulite-facies overprint, but do not constrain the age of eclogitization. Proterozoic ages (1.8 Ga) are attributed to inherited components. Discordant analyses define an array toward an upper intercept value of ca . 1000 Ma. Five grains that give ages of ca . 90 Ma may record contaminants incorporated during processing, or may represent the age of the protolith and thus may be the first indication in the Himalayan basement nappes for the widespread Cretaceous Rajmahal Trap basalt volcanism of NE India.

High-pressure granulites of the Podolsko complex, Bohemian Massif: An example of crustal rocks that were subducted to mantle depths and survived a pervasive mid-crustal high-temperature overprint

The Podolsko complex, Bohemian Massif, is a mid-crustal migmatite–granite dome exposed along a tectonic boundary separating the upper crust from the deeply eroded interior of the Variscan orogen, referred to as the Moldanubian Zone. This study examines metamorphic history of mafic and felsic granulites that occur in this complex as minor lenses or layers hosted in pervasively anatectic rocks. The mafic granulite contains garnet with preserved high-Ca cores, which based on pseudosection modelling indicates pressure conditions near the coesite stability field at temperatures of ca. 550–600°C. The relicts of an earlier eclogite-facies stage have been overprinted by a later granulite-facies assemblage consisting of ternary feldspar, orthopyroxene, and spinel in the mafic granulite and sillimanite and spinel in the felsic granulite. Composition of younger garnet (in rims and as smaller grains) in both granulites suggests that a near isothermal decompression of these rocks was followed by heating that reached temperature of ca. 900°C at pressure of ca. 0.5GPa. It is thus concluded that the granulites underwent at least two temporally separate tectonometamorphic events: they were first subducted to mantle depths and exhumed rapidly at relatively low temperatures and then near isobarically heated at mid-crustal levels. The preservation of earlier eclogite-facies garnet in the mafic granulite indicates that the latter event was short-lived and was followed by near isobaric cooling. The geologically brief granulite-facies metamorphism was previously explained as a result of slab break-off and mantle upwelling after the main phase of microplate convergence in the Bohemian Massif. To put the Podolsko complex into a broader tectonic context, we synthesize the available petrologic and structural data from the correlative (U)HP assemblages of the Moldanubian Zone to suggest that they typically do not preserve structural record of the subduction stage, only rarely preserve an early exhumation phase to the middle crust, and most, if not all, have been extensively overprinted by late-stage processes at shallower crustal levels. This synthesis also reveals that kinematics and thus tectonic processes driving the final exhumation of the (U)HP rocks were diverse and may vary from one location to another. These local settings include exhumation in the footwall of major extensional detachments, due to folding and subsequent lateral extrusion, and along crustal-scale strike-slip shear zones. The extensive late-stage overprint masking the earlier (pre-340Ma) subduction history is greatly exemplified in the Podolsko complex where the granulites are only small relics in pervasively anatectic rocks melted at mid-crustal conditions and then rapidly exhumed along extensional detachments to upper-crustal levels.

The youngest eclogite in central Himalaya: P–T path, U–Pb zircon age and its tectonic implication

Relict omphacite inclusions have been discovered in mafic granulite at Dinggye of China, confirming the existence of eclogite in central Himalayan orogenic belt. Detailed petrological studies show that relict omphacite occur as inclusions in both garnets and zircons, and the peak mineral assemblage of eclogite-facies should be garnet, omphacite, rutile, muscovite and quartz which was strongly overprinted by granulite-facies minerals during the exhumation. Phase equilibria modeling and associated geothermometer predict that the minimum P–T conditions for peak eclogite-facies stage are 720–760°C and 20–21kbar, and those of overprinted granulite-facies are 750°C and 7–9kbar in water-undersaturated condition. Thus, a near isothermal decompression P–T path for central Himalayan eclogite has been obtained. Zircon SHRIMP U–Pb dating of two studied eclogite samples at Dinggye yields the peak metamorphic ages of 13.9±1.2Ma and 14.9±0.7Ma, respectively, which indicates that the Dinggye eclogite should be the youngest eclogite in Himalayan orogenic belt. Geochemical characteristics and zircon analyses show that the protoliths of eclogite in Dinggye are predicted to be continental rift-related basaltic rocks. The eclogite at Dinggye in central Himalaya should be formed by the crustal thickening during the long-lasting continental overthrusting by Indian plate beneath Euro-Asian continent, and its exhumation process may be related with channel flow and orogen-parallel extension. In the middle Miocene (~14Ma), Indian continental crust had reached at least ~65km depth in southern Tibet.

Prograde and retrograde evolution of eclogite from Adrar Izzilatène (Egéré-Aleksod terrane, Hoggar, Algeria) determined from chemical zoning and pseudosections, with geodynamic implications

Adrar Izzilatène in the Egéré-Aleksod terrane of the LATEA metacraton (Hoggar, Algeria) exposes one of the best preserved examples of eclogite facies metamorphism in Hoggar. Three distinct stages of metamorphic development are recognised, namely, the pre-peak stage (M1), characterised by garnet, amphibole, epidote, quartz and rutile, the peak eclogite facies stage (M2), consisting of omphacite, garnet, edenite, epidote, quartz and rutile, and the retrograde stage (M3), where initial decompression resulted in the appearance of plagioclase, the development of pargasite+plagioclase kelyphites and finally the formation of anhydrous plagioclase+diopside coronas. Porphyroblastic omphacite has a jadeite content of up to XJd=0.36, which is the highest yet observed for eclogite facies rocks from the Tuareg Shield. Garnet growth zoning patterns are characterised by flat profiles in the cores (XAlm=0.55–0.60; XPrp=0.12–0.16; XGrs=0.26–0.30) before showing a decrease in almandine to XAlm=0.45, coupled to an increase in pyrope to XPrp=0.29 and decrease in grossular to XGrs=0.26 at the rims. Calculated P–T–MH2O pseudosections show that the prograde M1 assemblage equilibrated at 13–14kbar and 580°C, before pressure and temperature increased to 19kbar and 650–700°C at fluid-saturated conditions during peak metamorphism. Retrogression involved near-isothermal decompression to 8–9kbar and 700–750°C at fluid-undersaturated conditions. Prograde-to-peak metamorphism of the Izzilatène eclogite could have involved either oceanic or continental subduction, followed by exhumation as the area was obducted towards the LATEA metacraton during the Pan-African orogeny and the assembly of Western Gondwana.

Petrogenesis and chemogenesis of oceanic and continental orogens in Asia: Current topics, Part I

Petrogenesis and chemogenesis of oceanic and continental orogens in <scp>A</scp>sia: Current topics, <scp>Part I</scp>

Composite metamorphic history recorded in garnet porphyroblasts of Sambagawa metasediments in the Besshi region, central Shikoku, Southwest Japan

AbstractThis report describes an investigation of the composite metamorphic history recorded in garnet porphyroblasts of Sambagawa metasediments, and presents discussions of the possible distribution of eclogite facies lithologies in the Besshi region, central Shikoku, southwest Japan. Garnet grains usually show chemically composite zoning with resolved inner and overgrown outer segments, respectively. The inner segment usually contains paragonite as a sodic phase inclusion and rarely contains omphacite or glaucophane, whereas the outer segment rarely includes albite but no paragonite. The inner segment often includes quartz grains that preserve high residual pressures corresponding to the eclogite facies conditions, whereas the outer segment includes quartz grains that preserve lower residual pressures corresponding to the epidote–amphibolite facies conditions. For this composite‐zoned garnet, the assemblage of sodic phase inclusions and the values of the residual pressure of quartz inclusions imply the following successive metamorphic path: prograde eclogite facies stage → decompression and hydration reaction stage → prograde epidote–amphibolite facies stage. Metasediments containing this composite‐zoned garnet with such evidence for eclogite facies conditions are more widely distributed than the eclogitic lithologies that contain the assemblage of omphacite + garnet. These results imply that the combination of (i) the chemical composite zoning of garnet, (ii) the nature of sodic phase inclusions, and (iii) the residual pressure of quartz inclusions is useful as criteria to ascertain the true distribution of the lithologies that have experienced eclogite facies metamorphism for the Sambagawa metasediments, which was difficult to recognize in earlier studies.