Age Of Eclogite-facies Metamorphism Research Articles

Paleo-Tethys subduction and arc-continent collision: Evidence from zircon U-Pb chronology, geochemistry and Sr-Nd-Hf isotopes of eclogites in western Yunnan, bangbing area, southeastern Tibetan Plateau

The Changning-Menglian suture zone (CMSZ) in the southeastern Tibetan Plateau is a newly discovered HP-UHP metamorphic zone. The eclogites therein are the key evidence constraining the main suture of the Proto- and Paleo-Tethys Ocean in western Yunnan. Targeting the weakly studied Bangbing eclogites, we developed a comprehensive study on the whole-rock compositions, Sr-Nd isotope and zircon U-Pb ages, zircon trace elements and Lu-Hf isotope to reveal the subduction and arc-land collision. The eclogites occur as massive blocks or lenses and embedded in garnet phengite quartz schists of Lancang Group, Early Paleozoic accretionary complex. Their geochemistry is similar to E-MORB, and exhibit isotopic εNd(t) values of 3.14–4.49 and εHf(t) of 14.64–16.41, respectively. The Nb-enriched mafic protoliths suggested they were probably generated by partial melting of the enriched oceanic mantle within the spinel stability field and emplaced or erupted as mid-ocean ridge in the Paleo-Tethys Ocean. By LA-ICP-MS zircon U-Pb age testing, the magmatic zircon grains separated from the eclogites yield a wide range of ages, which may be capture zircon ages rather than protolith crystallization. We infer the age of eclogite-facies metamorphism to be 238 ± 2 Ma based on CL images, zircon trace element analysis, and that this metamorphism marks the collision between the Eastern Lincang magmatic arc, the Simao block and the Western Baoshan block. Thus, exhumation of the eclogites occurred only 7 to 23 Ma later, according to age 231–215 Ma for post-collisional volcanic and granitic rocks east of the CMSZ. Conclusively, the continued subduction of the Paleo-Tethys oceanic crust occurred during the Early-Middle Triassic, and rapid exhumation in the Late Triassic. The Changning-Menglian suture zone is a typical oceanic subduction-accretionary orogeny belt.

Multi-stage metamorphism and deformation of the North Qinling Orogenic Belt: Constraints from petrology, geochronology, and structural analysis of the Qinling Complex

The Qinling Complex is the only continental nucleus that outcrops in the North Qinling Orogenic Belt (NQOB) and preserves valuable evolutionary information of NQOB during the early Paleozoic. There are many high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic rocks distributed in the northern and southern parts of the Qinling Complex. Whether the central part (namely the Qihe tectonic slab) underwent a similar HP–UHP metamorphism and what the relationship among different tectonic slabs of Qinling Complex is, are the key to revealing the formation and exhumation process of these HP–UHP rocks. Four episodes of deformation (D1–4) have been recognized in the Qinling Complex. The metamorphic pressure‐temperature conditions of syn-D3 amphibolite in the Qihe tectonic slab are limited to 3.3–6.4 Kbar and 479–538 ℃ based on equilibrium modeling, corresponding to an extensional tectonic setting. The U-Pb dating of the amphibolite yielded a protolith age of 714 ± 46 Ma, an eclogite-facies metamorphic age of 514 ± 4 Ma, and two stages of retrograde metamorphism at 462 ± 3 Ma and 418 ± 5 Ma. The parametamorphic rocks of different tectonic slabs have similar detrital zircon age spectra, which is a very different pattern from adjacent areas of the Qinling Complex. The consistency in the protolith ages, peak metamorphic ages, two stages of retrograde metamorphism, and detrital zircon age from different tectonic slabs in the Qinling Complex indicate that HP–UHP metamorphic rocks from Qinling Complex formed during the same tectonic event. Therefore, the Qinling Complex evolved in deep subduction and underwent eclogite facies metamorphism, followed by superimposed deformation structures and retrograde metamorphism during the two stages of exhumation.

Zircon U–Pb Dating and Lu-Hf Isotope of the Retrograded Eclogite from Chicheng, Northern Hebei Province, China

We report zircon U–Pb ages and Lu-Hf isotopic data from two sample of the retrograded eclogite in the Chicheng area. Two groups of the metamorphic zircons from the Chicheng retrograded eclogite were identified: group one shows characteristics of depletion in LREE and flat in HREE curves and exhibit no significant Eu anomaly, and this may imply that they may form under eclogite facies metamorphic condition; group two is rich in HREE and shows slight negative Eu anomaly indicated that they may form under amphibolite facies metamorphic condition. Zircon Lu-Hf isotopic of εHf from the Chicheng eclogite has larger span range from 6.0 to 18.0, which suggests that the magma of the eclogite protolith may be mixed with partial crustal components. The peak eclogite facies metamorphism of Chicheng eclogite may occur at 348.5–344.2 Ma and its retrograde metamorphism of amphibolite fancies may occur at ca. 325.0 Ma. The Hongqiyingzi Complex may experience multistage metamorphic events mainly including Late Archean (2494–2448 Ma), Late Paleoproterozoic (1900–1734 Ma, peak age = 1824.6 Ma), and Phanerozoic (495–234 Ma, peak age = 323.7 Ma). Thus, the metamorphic event (348.5–325 Ma) of the Chicheng eclogite is in accordance with the Phanerozoic metamorphic event of the Hongqiyingzi Complex. The eclogite facies metamorphic age of the eclogite is in accordance with the metamorphism (granulite facies or amphibolite facies) of its surrounding rocks, which implied that the tectonic subduction and exhumation of the retrograded eclogite may cause the regional metamorphism of garnet biotite plagioclase gneiss.

Age of eclogites formed by the subduction of the Mesoarchaean oceanic crust (Salma, Belomorian Eclogite Province, eastern Fennoscandian Shield, Russia): A synthesis

Competing evolutionary models and age of eclogite facies metamorphism, Mesoarchaean, Neoarchaean or Palaeoproterozoic, of the subducted Mesoarchaean oceanic crust (Salma association, Belomorian Eclogite Province) are discussed on a basis of systematic analysis of previously known and newly obtained data. Four main types of zircons were distinguished in eclogites: porous crystals with numerous inclusions from eclogite-metagabbro; wide-rimmed zircons with relict porous cores similar to previous type separated from garnetites; round-oval zircons from eclogite-metagabbronorite that are characteristic for granulite facies rocks and zircons with euhedral oscillatory zoning cores and oval grains that are characteristic for the eclogite facies pillow basalts. Regular changes in REE patterns and in crystallization-recrystallization temperatures of certain domains of the porous zircons display sequence of magmatic and metamorphic events. The ∼ 2.9 Ga domains retain magmatic-type REE patterns. Low- and medium-temperature inclusions of prenite, pumpelliite, albite, actinolite, chlorite, diaspore and saponite in garnet and abundant microinclusions of the prenite-pumpelliite and greenschist facies in zircons with LREE-MREE enrichment indicate hydrothermal metamorphism in the spreading ridge and ocean floor at 2.9–2.82 Ga. Disappearance of Ce positive anomaly from REE pattern in zircon, change negative to positive Eu anomaly and LREE-MREE enrichment caused by plagioclase removal and replacement of rutile with sphene evidence eclogite facies metamorphism linked with subduction at 2.82–2.78 Ga. Temperatures in the 700–900 °C range of the round-oval zircons from eclogite-metagabbronorite records the Neoarchaean granulite facies overprint at 2.77–2.70 Ga. Series of the high temperature Palaeoprpoterozoic events was terminated by 2.1–1.7 Ga event marked by the rims with lowest REE that frame all types of zircons. Change from positive to negative Eu anomaly, retrieval of negative Ce anomaly indicate the presence of plagioclase, reduction type of fluids and low water activity characteristic of high-temperature metamorphism under stretching condition and mantle-plume activity. The deep reworking of the Sm-Nd isotope system in the Belomorian tectonic province at ∼ 1.9 Ga, including the Salma eclogite association, was caused by the enormous crustal heating that spread from the Lapland granulite belt southward. Radiogenic 176Hf enrichment of 1.9 Ga zircon indicates recrystallization of a long-existed garnet with release of significant amount of 176Hf.

Multi-system geochronology of North Dabie eclogite: Ineffective garnet ‘shielding’ on rutile inclusions under multi-thermal conditions

It has been shown that certain host minerals can robustly shield mineral inclusions and prevent loss of petrochronological information during subsequent thermal events. Rutile inclusions in garnet may crystallize under a wide range of metamorphic conditions and thus rutile is expected to be a useful geochronometer for constraining different stages of a metamorphic P-T path. Hence, it is important to ascertain whether rutile inclusions shielded by garnet retain the original crystallization age information despite later retrograde cooling or subsequent new thermal events. In this study, we test the “shielding effect” of garnet on rutile inclusions from the Baizhangya eclogite in the North Dabie Zone (China), which underwent a multi-stage thermal evolution. Secondary ion mass spectrometry (SIMS) UPb dating of inherited zircon core domains constrains a Neoproterozoic formation age (~770 Ma) of the eclogite protolith. SIMS UPb dating of metamorphic zircon and complementary thermal-ionization mass spectrometry (TIMS) SmNd isochron dating of garnet revealed Triassic eclogite-facies metamorphic ages (245–210 Ma). An Early Cretaceous SIMS UPb age of ca. 130 Ma, yielded by matrix rutile grains and rutile inclusions, suggests significant isotopic alteration since crystallization. This indicates that shielding was ineffective, since the UPb ages derived from the rutile inclusions, although hermetically armored by garnet, were influenced by a subsequent Early Cretaceous tectono-thermal event. Further transmission electron microscope (TEM) analysis revealed the presence of hematite microlites along the garnet-rutile boundaries, which may have acted as a potential reservoir to store the expelled radiogenic Pb from rutile. This study suggests that newly formed minerals located along mineral boundaries seem to be crucial for understanding the shielding effect.

The geodynamic setting of Dulan eclogite-type rutile deposits in the North Qaidam orogen, western China

Abundant eclogite-type rutile deposits have recently been discovered along the North Qaidam ultrahigh-pressure (UHP) metamorphic belt in China. The identified titanium reserves exceed 3 million tons and range in grade from 1.0% to 4.5%. The rutile deposits of economic interest are found within two types of eclogite. However, the origin and original geodynamic setting of the two types of rutile-bearing eclogites remain controversial. This study therefore investigates ore and mineralization bodies belonging to the two types by way of detailed field work and geochronological, geochemical and Sr–Nd–Hf isotopic compositional analysis, taking the Dulan eclogite-type rutile deposit as its case study. Type-1 rutile-bearing eclogites enclosed in granitic gneisses are found to have Neoproterozoic protolith ages of 845 ± 4 Ma and an eclogite-facies metamorphic age of ca. 433 Ma. Their high TiO2, TFe2O3, and P2O5 contents cannot have been produced by rutile-formation during UHP metamorphism but could have arisen through the accumulation of ilmenite and apatite in a high-Ti protolith. They have ocean-island basalt (OIB)-like trace element patterns, with enrichment in light rare-earth elements (LREEs) and high field strength elements (HFSEs) (Nb, Ta, Zr, and Hf). The Hf isotopic compositions of the residual magmatic zircon cores in Type-1 rutile-bearing eclogites (εHf (845) = +1.4 to +6.0, TDM = 1.2–1.5 Ga) indicate that their parental magmas were derived from the Proterozoic mantle. However, they show initial 87Sr/86Sr values of 0.71059–0.71326 and εNd (845) values of −1.5 to −2.5, suggesting significant crustal contamination. Thus, the Type-1 rutile-bearing eclogites may have originated through partial melting of an OIB-type mantle plume source during the Neoproterozoic rifting of the Rodinia supercontinent. In contrast, the Type-2 rutile-bearing eclogites, which are hosted in a mafic–ultramafic ophiolitic complex, have protolith ages of 449 ± 3 Ma and eclogite-facies metamorphic ages of ca. 435–433 Ma. Correlations between the whole-rock MgO and the major and trace element contents of Type-2 rutile-bearing eclogites suggest that they are derived from cumulates with alternating olivine and pyroxene-rich and ilmenite-rich layers. They have mid-ocean-ridge basalt (MORB) and OIB-like trace element patterns, indicating extensive fractional crystallization and the possibility that there were multiple injections of magma during the formation of the protolith. Meanwhile, the +0.6 to +4.3 εNd (449) values and −1.8 to +11.8 εHf (449) values of the Type-2 rutile-bearing eclogites indicate that the protoliths may have been generated by partial melting of a mantle wedge source metasomatized by slab-derived components. The short interval (ca. 14 my) between magma generation and eclogite-facies metamorphism, in combination with the type of rock assemblage within which they are found, indicates that the Type-2 rutile-bearing eclogites originated in a supra-subduction zone (SSZ) ophiolite. Type-1 and Type-2 rutile-bearing eclogites have similar eclogite-facies metamorphic ages of ca. 435–433 Ma, indicating that the Type-1 and SSZ-related Type-2 protoliths were incorporated into a subduction mélange that was dragged down the subduction channel and underwent eclogite-facies metamorphism via subduction of the Qaidam Block, eventually forming eclogite-type rutile deposits.

Late Cretaceous age of eclogite facies metamorphism of the Sanbagawa belt in the Asemi River area, Shikoku (SW Japan): Evidence from detrital zircon

Within the Sanbagawa metamorphic belt, the timing of high-pressure and retrograde metamorphism is controversial. Recently, an area in the Asemi River region (central Shikoku, Japan) was delineated that had been subjected to ecologite facies metamorphism prior to retrograde metamorphism in the epidote-amphibolite facies which provides a new opportunity to constrain the age of high-pressure metamorphism. Detrital zircon from this area yields ages as young as 84 ± 4 Ma. Hence, the protoliths were deposited not earlier than ca. 88 Ma and thus, eclogite facies and retrograde metamorphism occurred after 88 Ma, probably at 86–82 Ma as indicated by the age of metamorphic zircon and K-Ar and Ar-Ar ages for white mica.

Dynamics of ketamine-related resting state functional connectivity changes in healthy controls

It has been shown that certain host minerals can robustly shield mineral inclusions and prevent loss of petrochronological information during subsequent thermal events. Rutile inclusions in garnet may crystallize under a wide range of metamorphic conditions and thus rutile is expected to be a useful geochronometer for constraining different stages of a metamorphic P-T path. Hence, it is important to ascertain whether rutile inclusions shielded by garnet retain the original crystallization age information despite later retrograde cooling or subsequent new thermal events. In this study, we test the “shielding effect” of garnet on rutile inclusions from the Baizhangya eclogite in the North Dabie Zone (China), which underwent a multi-stage thermal evolution. Secondary ion mass spectrometry (SIMS) UPb dating of inherited zircon core domains constrains a Neoproterozoic formation age (~770 Ma) of the eclogite protolith. SIMS UPb dating of metamorphic zircon and complementary thermal-ionization mass spectrometry (TIMS) SmNd isochron dating of garnet revealed Triassic eclogite-facies metamorphic ages (245–210 Ma). An Early Cretaceous SIMS UPb age of ca. 130 Ma, yielded by matrix rutile grains and rutile inclusions, suggests significant isotopic alteration since crystallization. This indicates that shielding was ineffective, since the UPb ages derived from the rutile inclusions, although hermetically armored by garnet, were influenced by a subsequent Early Cretaceous tectono-thermal event. Further transmission electron microscope (TEM) analysis revealed the presence of hematite microlites along the garnet-rutile boundaries, which may have acted as a potential reservoir to store the expelled radiogenic Pb from rutile. This study suggests that newly formed minerals located along mineral boundaries seem to be crucial for understanding the shielding effect.

Subduction of the Mesoarchaean spreading ridge and related metamorphism, magmatism and deformation by the example of the Gridino eclogitized mafic dyke swarm, the Belomorian Eclogite Province, eastern Fennoscandian Shield

A model of Mesoarchaean spreading ridge subduction and related metamorphism, magmatism and deformation in an active continental margin is presented here. The eclogites exposed along northeastern boundary of the Belomorian orogen in the eastern Fennoscandian Shield were formed as a result of Mesoarchaean–Neoarchaean subduction and collision. The Belomorian eclogite province contains two eclogite associations have been recognized within TTG gneisses: (1) the subduction-type Salma association and (2) the Gridino mafic dykes. Protoliths of the Salma eclogites represent a sequence comprising oceanic layered gabbro, formed at ∼2.9 Ga in a slow-spreading ridge. The subduction began in a time span 2.9–2.87 Ga, and eclogite-facies events occurred between ca.2.87 and ca.2.78 Ga. Injection of a mafic magma into an active continental margin, recorded by the Gridino dyke swarm, is attributed to subduction of a mid-ocean ridge, commencing at 2.87 Ga. The dykes represent typical fissure intrusions and underwent metamorphism to eclogite, granulite and amphibolite facies. The dykes divide into two compositions (Fe-Ti gabbro and Mg-Cr gabbronorite) which represent alternating episodes of arc and slab window magmatism. The Fe-Ti gabbros have high Ti and low abundances of fluid-mobile and other LIL elements compared to HFSE and plot in the MORB and within-plate oceanic basalt fields. The Fe-Ti gabbros formed from low-degree of partial melting from a depleted mantle source. This petrogenesis is consistent with slab window model in which magmas were derived by decompression melting of suboceanic mantle that upwelled into the opening beneath continental margin following spreading ridge subduction. The Mg-Cr gabbronorites have low Ti along with high abundances of fluid-mobile elements and other LILE compared to HFSE and plot in arc basalt fields that imply melting of subduction-modified hydrous subcontinental mantle.As the mid-ocean spreading ridge entered a trench, continental rocks above subduction zone underwent deformations with pseudotachylyte-bearing strike-slip faults development that is an evidence of a fossile earthquake. Paleoseismic event was limited to crustal conditions above the brittle-ductile transition, corresponding to temperatures below 450 °C and depths above 15 km. The slab window opening induced upwelling of anhydrous mantle and formation of the Fe-Ti gabbro dykes. The Mg-Cr gabbronorite was formed at slab window edges from metasomatized mantle.Summarizing evidences, such as ophiolites, eclogite, arc magmatism, we suggest that the modern style of subduction and plate tectonics began already in Mezoarchaean time 2.9-2.8 Ga ago. Special aspect of the paper is an argumentation of Meso-Neoarchaean age of eclogite facies metamorphism in the Belomorian eclogite province.

Petrology and geochemistry of high niobium eclogite in the North Qaidam orogen, Western China: Implications for an eclogite facies metamorphosed island arc slice

To better understand the evolution of the North Qaidam orogen, eclogite that forms lens-like bodies in granitic gneiss was studied in the Lüliangshan terrane, North Qaidam ultrahigh-pressure (UHP) metamorphic belt. Petrographic and mineralogical studies as well as pressure-temperature calculations showed that the eclogite underwent at least three metamorphic stages. Peak eclogite-facies metamorphism at 647–768 °C and ≥3.0 GPa was followed by high-pressure (HP) granulite-facies metamorphism at 815–929 °C/1.61–1.75 GPa and cooling to amphibolite-facies metamorphism at 606–720 °C/0.41–0.56 GPa. Three age groups can be identified via LA-ICP-MS zircon U–Pb eclogite dating: (1) an eclogite-facies metamorphic age of ca. 432–428 Ma, (2) an HP granulite-facies retrograde age of ca. 424–420 Ma, and (3) ages of 415–382 Ma, which constrained the timing of amphibolite-facies metamorphism. The studied eclogite is characterized by high field strength element enrichment, particularly in Nb content (57.8–124 ppm) and flat rare earth element patterns similar to high Nb island arc basalts described in many modern arcs that feature subduction of oceanic slabs. The samples have positive εNd (t) values (+2.1 to +5.1) and positive εHf (t) values (+4.9 to +10.0), indicating that the protolith of the high Nb eclogite was directly derived from the depleted mantle with little crustal contamination. These island arc basalts have been interpreted to have arisen from the partial melting of mantle wedge garnet-amphibole or garnet peridotite metasomatized by slab-dehydrated melts of a subducted oceanic slab. They were emplaced onto the continental block by arc magmatism prior to the main collision event. Then, the island arc materials were dragged into the subduction channel with eclogite-facies metamorphism via subduction of the Qaidam Block.

Newly discovered Late Triassic Baqing eclogite in central Tibet indicates an anticlockwise West\u2013East Qiangtang collision

The Triassic eclogite-bearing central Qiangtang metamorphic belt (CQMB) in the northern Tibetan Plateau has been debated whether it is a metamorphic core complex underthrust from the Jinsha Paleo-Tethys or an in-situ Shuanghu suture. The CQMB is thus a key issue to elucidate the crustal architecture of the northern Tibetan Plateau, the tectonics of the eastern Tethys, and the petrogenesis of Cenozoic high-K magmatism. We here report the newly discovered Baqing eclogite along the eastern extension of the CQMB near the Baqing town, central Tibet. These eclogites are characterized by the garnet + omphacite + rutile + phengite + quartz assemblages. Primary eclogite-facies metamorphic pressure–temperature estimates yield consistent minimum pressure of 25 ± 1 kbar at 730 ± 60 °C. U–Pb dating on zircons that contain inclusions (garnet + omphacite + rutile + phengite) gave eclogite-facies metamorphic ages of 223 Ma. The geochemical continental crustal signature and the presence of Paleozoic cores in the zircons indicate that the Baqing eclogite formed by continental subduction and marks an eastward-younging anticlockwise West–East Qiangtang collision along the Shuanghu suture from the Middle to Late Triassic.

Rapid formation of eclogites during a nearly closed ocean: Revisiting the Pianshishan eclogite in Qiangtang, central Tibetan Plateau

The Pianshishan eclogites discovered in central Qiangtang are a key piece of evidence constraining the main suture of the Paleo-Tethys Ocean in Tibet. Based on the eclogite-facies metamorphic inclusions found in them, a previous study interpreted the zircon grains as metamorphic in origin, although their catholuminescence images were similar to zircons of igneous origin. Distinguishing metamorphic from igneous zircons can be difficult, however, especially in high-pressure metamorphic rocks. In this study, we report new SIMS zircon U-Pb dates along with zircon O isotope and trace element analyses, to constrain the origin of these zircons and the interpretation of zircon dates. Twenty-five of 26 zircons analyzed from three samples yielded zircon U-Pb dates ranging from 239 to 236Ma with an average of 238±1Ma. The zircon rare earth element (REE) patterns are characterized by positive Ce and negative Eu anomalies, strongly indicating an igneous origin. This interpretation is also supported by the high Ti-in-zircon temperatures (>700°C) and depleted mantle-like zircon O isotope ratios (5.15±0.57‰, 2SD). Thus, we conclude that the protoliths of the Pianshishan eclogites were generated at ~238Ma. Their geochemistry is similar to the oceanic island basalts (OIB), and they were probably generated by small extents partial melting of enriched oceanic asthenospheric mantle within the garnet stability field and emplaced or erupted as a seamount in the Paleo-Tethys Ocean. We infer the age of eclogite-facies metamorphism to be ~233Ma based on previously reported Lu-Hf isochron ages, and that this metamorphism marks the collision between the Southern and Northern Qiangtang terranes. Thus, metamorphism of the eclogites occurred within 5My of protolith generation and exhumation occurred only 13 to 19 My later, based on the 40Ar/39Ar dates of 220–214Ma. The time between protolith generation and exhumation was thus <24My. This rare rapid evolution may be due to the protoliths having formed as a “petit-spot” seamount in response to outer rise flexure as the Paleo-Tethys Ocean entered the Longmu Co-Shuanghu subduction zone.

Whole-rock and zircon geochemical distinction between oceanic- and continental-type eclogites in the North Qaidam orogen, northern Tibet

Continental ultrahigh-pressure (UHP) metamorphic belts were tectonically developed from oceanic subduction to continental subduction. Petrological records of the tectonic transition can be preserved if the subducted oceanic crust is exhumed together with dominant crustal rocks of continental origin in collisional orogens. This is illustrated by the coexistence of continental-type and oceanic-type UHP eclogites in the North Qaidam orogen, northern Tibet. An integrated study of whole-rock major-trace elements, Sr-Nd isotopes and mineral O isotopes for eclogites, as well as zircon U-Pb ages and trace elements was carried out for UHP eclogites and gneisses from the North Qaidam orogen. The results allow discriminating between the two types of eclogites in their protolith origin. The continental-type eclogites are dominant there and characterized by Neoproterozoic protolith ages of ca. 850Ma, versatile trace element compositions, mostly positive εNd(t) values, and normal mantle-like δ18O values. Their protoliths are continental mafic igneous rocks that are originated from multiple mantle sources with different compositions. In comparison, the oceanic-type eclogites are minor there and have protolith ages of ca. 500Ma, positive εNd(t) values, and lower δ18O values than normal mantle δ18O values. Their protoliths are a kind of oceanic mafic igneous rocks that experienced high-T seafloor hydrothermal alteration. Both IAB-like and MORB- to OIB-like trace element compositions occur in the oceanic-type eclogites, suggesting their origination from a backarc basin close to the continental margin. Zircon U-Pb dating of the oceanic-type eclogites, continental-type eclogites and gneisses yields similar eclogite-facies metamorphic ages of 427–439Ma, with coesite inclusions in metamorphic zircons from one gneiss. The identical metamorphic ages for the two types of eclogites indicate that the exposed eclogites of oceanic origin were primarily residing in the continental margin and they were entrained by the UHP metamorphic rocks of continental origin for exhumation along the same subduction channel toward the surface. Therefore, identifying the occurrence of oceanic-type eclogites in the continental-type eclogite-gneiss associations is a valid means to decipher the tectonic development from oceanic subduction to continental collision.

Petrology and Geochemistry of the lawsonite (pseudomorph)-bearing eclogite in Yuka terrane, North Qaidam UHPM belt: An eclogite facies metamorphosed oceanic slice

Although lawsonite eclogite is predicted to be common at depth of ~45–300km in subduction zones, it is rarely exposed at Earth's surface. In this study, we identified a lawsonite (pseudomorph)-bearing eclogite in Yuka terrane, North Qaidam UHPM belt. It is mainly composed of garnet, amphibole, clinozoisite, quartz and rutile with minor omphacite relics and talc. Petrographic investigation identified abundant rectangular- or rhombic- shaped mineral aggregates composed of polycrystalline clinozoisite or polyphase clinozoisite+paragonite±phengite±quartz and clinozoisite+amphibole±quartz±Na-rich plagioclase occurring in garnet porphyroblasts. The peculiar shape, mineral assemblages and reconstructed compositions of these aggregates show that they are pseudomorphs after lawsonite. Phase equilibrium modeling predicts that the lawsonite (pseudomorph)-bearing eclogite experienced a clockwise P–T evolution with a peak stage at 570°C, 27kbar, near the quartz–coesite transition boundary. Geochemical data show that the lawsonite (pseudomorph)-bearing eclogite has a MORB-like affinity with flat Chondrite–normalized rare earth element (REE) distribution patterns. Zircons from this sample have similar δ18O values (5.38±0.10‰) with the normal mantle (5.3±0.3‰), indicating that the protolith of the lawsonite (pseudomorph)-bearing eclogite is derived directly from mantle rock without continental crust contamination. The MORB-like character and zircon O isotopic composition of the lawsonite (pseudomorph)-bearing eclogite indicate that its protolith is likely to be an oceanic crustal slice. SIMS and LA–ICP–MS zircon U–Pb dating yielded a 436–438Ma eclogite-facies metamorphic age for the lawsonite (pseudomorph)-bearing eclogite, similar to the continental subduction age of the North Qaidam UHPM belt, but did not obtain its protolith age. These data indicate that the protolith of the lawsonite (pseudomorph)-bearing eclogite may be either a piece of Proterozoic ocean that was emplaced in the active continental margin of the Qaidam block during the amalgamation of the Rodinia supercontinent, or the early Paleozoic oceanic crust in the transition zone between the Qilian Ocean and the Qaidam block.

The timing of metamorphism in the Odenwald–Spessart basement, Mid-German Crystalline Zone

New in situ electron microprobe monazite and white mica 40Ar/39Ar step heating ages support the proposition that the Odenwald–Spessart basement, Mid-German Crystalline Zone, consists of at least two distinct crustal terranes that experienced different geological histories prior to their juxtaposition. The monazite ages constrain tectonothermal events at 430 ± 43 Ma, 349 ± 14 Ma, 331 ± 16 Ma and 317 ± 12 Ma/316 ± 4 Ma, and the 40Ar/39Ar analyses provide white mica ages of 322 ± 3 Ma and 324 ± 3 Ma. Granulite-facies metamorphism occurred in the western Odenwald at c. 430 and 349 Ma, and amphibolite-facies metamorphism affected the eastern Odenwald and the central Spessart basements between c. 324 and 316 Ma. We interpret these data to indicate that the Otzberg–Michelbach Fault Zone, which separates the eastern Odenwald–Spessart basement from the Western Odenwald basement, is part of the Rheic Suture, which marks the position of a major Variscan plate boundary separating Gondwana- and Avalonia-derived crustal terranes. The age of the Carboniferous granulite-facies event in the western Odenwald overlaps with the minimum age of eclogite-facies metamorphism in the adjacent eastern Odenwald. The granulite- and eclogite-facies rocks experienced contrasting pressure–temperature paths but occur in close spatial proximity, being separated by the Rheic Suture. As high-pressure and high-temperature metamorphisms are of similar age, we interpret the Odenwald–Spessart basement as a paired metamorphic belt and propose that the adjacent high-pressure and high-temperature rocks were metamorphosed in the same subduction zone system. Juxtaposition of these rocks occurred during the final stages of the Variscan orogeny along the Rheic Suture.

The tectonic transition from oceanic subduction to continental subduction: Zirconological constraints from two types of eclogites in the North Qaidam orogen, northern Tibet

In the plate tectonics theory, continental subduction is pulled by subduction of dense oceanic crust. In practice, however, it is not easy to demonstrate that preceding oceanic crust exposes as oceanic-type eclogite together with continental-type eclogite in collisional orogens. The North Qaidam orogen in northern Tibet is an ultrahigh-pressure (UHP) metamorphic belt that contains the two types of eclogites, providing us with an excellent opportunity to study the tectonic transition from oceanic subduction to continental subduction. In order to constrain the protolith nature and metamorphic evolution of eclogites, we performed a combined study of zircon U–Pb ages, trace elements, mineral inclusions and O–Hf isotopes for various eclogites from the orogen. We discriminate the two types of eclogites by their differences in zircon U–Pb ages and O–Hf isotopes. CL-dark zircon domains exhibit high Th/U ratios, steep HREE patterns and significantly negative Eu anomalies, indicating that they are protolith zircons of magmatic origin with different extents of metamorphic recrystallization. Relict magmatic zircon domains in Type I eclogites yield Neoproterozoic protolith ages of >830Ma and Hf model ages of 850–1100Ma, whereas those in Type II eclogites yield Cambrian protolith U–Pb ages of >489Ma and Hf model ages of 500–650Ma. Most of the CL-bright zircon domains show low Th/U ratios, flat HREE patterns and no negative Eu anomalies, and contain mineral inclusions of garnet, omphacite and rutile, indicating their growth under eclogite-facies metamorphic conditions. These metamorphic domains have consistent eclogite-facies metamorphic ages of 433–440Ma throughout the North Qaidam orogen, regardless of the eclogite types and locations. The metamorphic zircon domains in Type I eclogites mostly exhibit δ18O values higher than normal mantle values, whereas Type II eclogites mostly have δ18O values lower than the normal mantle values. The difference in the δ18O values indicates that their protoliths underwent different temperatures of hydrothermal alteration at different tectonic settings. Combining zircon U–Pb ages and O–Hf isotope compositions with local tectonics, it is inferred that Type I eclogites were metamorphosed from Neoproterozoic continental mafic rocks, whereas Type II eclogites were metamorphosed from oceanic mafic rocks that were subducted prior to the continental subduction. The consistent eclogite-facies metamorphic ages for the two types of eclogites indicate that the exhumed oceanic-type eclogite was detached from the subducted oceanic crust and then entrained by the exhuming continental crust. Therefore, the coexistence of oceanic- and continental-type eclogites in the North Qaidam orogen demonstrates the tectonic transition from oceanic subduction to continental collision in the early Paleozoic.

Multi-stage metamorphic evolution of retrograde eclogite with a granulite-facies overprint in the Zhaigen area of the North Qinling Belt, China

Retrograde eclogite from the central part of the Qinling Complex, Zhaigen area of the North Qinling Belt, was studied using integrated petrology, mineral chemistry, pseudosection modeling, and geochronology. Microstructures and mineral relationships reveal five metamorphic stages and associated mineral assemblages as follows: (1) pre-peak stage M1, which is recorded by the inner cores of garnets together with mineral inclusions of clinopyroxene (Cpx1)+amphibole (Am1)+plagioclase (Pl1)±quartz±rutile, occurred under conditions of 760–770°C and 11.4–14.0kbar; (2) eclogite-facies stage M2, recorded by garnet cores+relic omphacite (with a high jadeite content up to 31%)+rutile+quartz under conditions of >16.7kbar and 679–765°C; (3) high-pressure granulite-facies stage M3, characterized by clinopyroxene (Cpx2)+plagioclase (Pl2) symplectites under conditions of 14.5–15.6kbar and 800–850°C; (4) medium-pressure granulite-facies stage M4, characterized by the growth of plagioclase+orthopyroxene coronas around garnet under conditions of 8.3–10kbar and 795–855°C; and (5) retrogressive amphibolite-facies stage M5, which is represented by amphibole (Am3)+plagioclase (Pl3) kelyphitic rims around garnet at conditions of <4kbar and <620°C. Based on Laser Raman analysis of mineral inclusions, cathodoluminescence images, in situ trace element concentrations from different domains within zircon grains, and LA-ICP-MS and SHRIMP U–Pb dating, the protolith age of the Zhaigen retrograde eclogite is suggested at 786±10Ma and the eclogite-facies metamorphic age recorded by metamorphic zircon cores is limited within 501–497Ma. The retrograde zircon rims display ages of 476–447Ma and 425Ma that probably reflect the timing of two stages of retrograde metamorphism, respectively. The mineral assemblages, P–T conditions, and zircon U–Pb data define a clockwise P–T–t path for the retrograde eclogite, suggesting that the Neoproterozoic protolith of the retrograde eclogite might evolved into continental subduction and eclogite-facies metamorphism during 501–497Ma before undergoing retrograde metamorphism during an initial stage of exhumation to middle–upper crust level at 474–447Ma and subsequent exhumation to shallow upper crust at ~420Ma.

Syn-collapse eclogite metamorphism and exhumation of deep crust in a migmatite dome: The P–T–t record of the youngest Variscan eclogite (Montagne Noire, French Massif Central)

In many orogens, high-pressure (HP) metamorphic rocks such as eclogite occur as lenses in quartzofeldspathic gneiss that equilibrated at much lower pressures. The pressure–temperature–time (P–T–t) history of eclogite relative to host gneiss provides information about mechanisms and timescales of exhumation of orogenic crust. The Montagne Noire of the southern Massif Central, France, is an eclogite–bearing gneiss (migmatite) dome located at the orogen-foreland transition of the Variscan belt. Results of our study show that it contains the youngest eclogite in the orogen, similar in age to migmatite and granite that crystallized under low-pressure conditions. P–T conditions for an exceptionally unaltered eclogite from the central Montagne Noire were estimated using a pseudosection supplemented by garnet–clinopyroxene and Zr-in-rutile thermometry. Results indicate peak P∼1.4 GPa and T∼725°C for Mg-rich garnet rim (50 mol% pyrope) + omphacite (36 mol% jadeite) + rutile + quartz. U–Pb geochronology (LA-ICP-MS) of 16 zoned zircon grains yielded ∼360 Ma (4 cores) and ∼315 Ma (12 rims and cores). Rare earth element abundances determined by LA-ICP-MS for dated zircon are consistent with crystallization of ∼315 Ma zircon under garnet-stable, plagioclase-unstable conditions that we interpret to indicate high pressure; in contrast, the ∼360 Ma zircon core corresponds to crystallization under lower pressure plagioclase-stable conditions. Based on garnet zoning and inclusion suites, rutile textures and Zr zoning, P–T results, and zircon petrochronology, we interpret the ∼315 Ma date as the age of eclogite-facies metamorphism that only slightly preceded dome formation and crystallization at 315–300 Ma. This age relation indicates that eclogite formation at high pressure and migmatite dome emplacement at low pressure were closely spaced in time. We propose that collapse-driven material transfer from the hot orogen to the cool foreland resulted in thickening of the orogen edge, leading to eclogite facies metamorphism of the deep crust. Soon after, the low-viscosity partially molten crust flowed from the plateau toward the foreland, incorporating and exhuming eclogite. The P–T–t history of the Montagne Noire eclogite shows that some dome material ascended from >40 km depth to shallow crustal levels, likely in a single decompression event, and that migmatite domes are therefore very efficient at exhuming the deep crust.

Petrology and geochronology of ultrahigh-pressure granitic gneiss from South Dulan, North Qaidam belt, NW China

abstractAn integrated study including petrography, mineral chemistry, metamorphic P–T path modelling, and zircon U–Pb dating was conducted on a granitic gneiss and enclosed eclogite from South Dulan, North Qaidam UHP (ultrahigh-pressure) belt. The result shows that the granitic gneiss underwent a clockwise P–T path with a peak-P stage at 655–745°C, 30–34 kbar, and a subsequent peak-T stage at 815–870°C, 14–18 kbar, which is similar to the P–T estimates reported for coesite-bearing continental-type eclogites in this region. The enclosed eclogite resembles an olivine–pyroxene-rich cumulate in Qaidam block. It has a similar prograde P–T path with the country gneiss and experienced a peak-P stage of 682–748°C at 27–34 kbar. Zircon U–Pb dating yields an eclogite-facies metamorphic age of 447 ± 2 Ma for the granitic gneiss and 445 ± 6 Ma for the enclosed eclogite. These ages agree with metamorphic ages obtained from paragneisses (427–439 Ma), coesite-bearing continental-type eclogites (430–451 Ma), and UHPM (ultrahigh-pressure metamorphic) oceanic crust–mantle sequence (440–445 Ma) from South Dulan, as well as UHP eclogites, garnet peridotite, and gneisses from other units (460–420 Ma) within this belt reported by others. Similar metamorphic ages as well as P–T evolution documented in gneisses and intercalated eclogites imply that both rocks experienced a coeval UHP event. Summarizing all the published geochronology data, we argue that the North Qaidam UHP belt was mainly formed by continental deep subduction at ~460 to ~420 Ma. The UHPM oceanic crust-mantle sequence in South Dulan may represent oceanic lithosphere in the transition zone between oceanic and continental crust, which was dragged upward by the exhumed continental rocks after break-off of the dense oceanic crust.

Lu-Hf geochronology of Mississippian high-pressure metamorphism in the Acatlán Complex, southern México

Since the first discovery of eclogite in the Acatlán Complex in southern México, the age of the high-pressure metamorphism has been a matter of debate. Several attempts to date high-pressure metamorphic rocks of the Acatlán Complex have been made using the U-Pb and 40Ar-39Ar methods. The resulting dates, however do not correspond unambiguously to the time of eclogite facies metamorphism. In this study, the age of high-pressure metamorphism in the Acatlán Complex has been determined by Lu-Hf garnet-whole rock geochronology. This paper presents four high precision, 4- to 7-point garnet-whole rock isochrons of amphibolitized eclogite from the Piaxtla Suite and the Asís Lithodeme, in the Acatlán Complex. The four dates agree within uncertainties, yielding a weighted mean of 352.5±1.6Ma, which we interpret to be the age of eclogite facies metamorphism in the Piaxtla Suite and the Asís Lithodeme, marking active subduction in the Acatlán Complex in the Carboniferous. Mississippian high-pressure metamorphism in turn, might be related to the closure of the Rheic Ocean and the assembly of Pangea.