High-pressure Metamorphism Research Articles

Lightning-induced high temperature and pressure microstructures in surface and subsurface fulgurites

Cloud-to-ground lightning causes both high-temperature and high-pressure metamorphism of rocks, forming rock fulgurite. We demonstrate that a range of microstructural features indicative of high temperatures and pressures can form in fulgurites at the surface and in fractures up to several meters below the surface. In comparison to a granite reference sample collected from a borehole at a depth of 138 m, microstructures in both the surface and fracture fulgurite are characterized by: (i) the presence of glass, (ii) a phase transformation in K-feldspar with the presence of exsolution lamellae of plagioclase, and (iii) high residual stresses up to 1.5 GPa. Since this is the first time that fracture-related fulgurite has been described, we also carried out a 1-D numerical model to investigate the processes by which these can form. The model shows that the electric current density in fractures up to 40 m from the landing point can be as high as that on the surface, providing an explanation for the occurrence of fracture-related fulgurites. Our work broadens the near-surface environments in which rock fulgurite has been reported, and provides a detailed description of microstructures that can be compared to those formed during other types of extreme metamorphic events.

Permian to Cretaceous tectonic evolution of the Jiamusi and Songliao blocks in NE China: Transition from the closure of the Paleo-Asian Ocean to the subduction of the Paleo-Pacific Ocean

Zhangguangcailing Orogen constitutes a significant part of the Eastern Asian Continental margin. Its evolution was constrained by the oceanic consumption processes between the Jiamusi and Songliao blocks during late Paleozoic and Mesozoic time, resulted from the closure of the Paleo-Asian Ocean and subsequent Paleo-Pacific Ocean subduction. Considerable and continuing controversy has surrounded when, where, and how the ocean was subducted and the two blocks were eventually amalgamated, which plays a crucial role in understanding the tectonic transition from the closure of the Paleo-Asian ocean to the subduction of the Paleo-Pacific Plate, especially the initial formation of the Mesozoic foreland basin of the Songliao Basin. This review synthesizes the most recent data and geologic evidence that place critical constraints on the evolution of this oceanic domain.Available data and geological evidence indicate that there exists a stable micro-continent consisting of several microblocks of NE China and adjacent area within the Paleo-Asian Ocean regime during the Late Paleozoic. In the early Permian, voluminous magmatism occurred in an extensional setting induced by the subduction of the Paleo-Asian Ocean to the south and the Mongol-Okhotsk Ocean to the northwest. As the crustal extension continued, a continental rifting was initiated and further developed into an oceanic domain (the Heilongjiang Ocean or the Mudanjiang Ocean) separating the Jiamusi and Songliao blocks during Permian and Early Triassic time. Later, an active continental margin was situated along the Zhangguangcailing Orogen due to the subduction of the Heilongjiang oceanic plate in the Late Triassic, followed by a change in the regional stress regime controlled by Paleo-Pacific plate subduction. The high-pressure metamorphism occurred synchronously with the subduction/collision between the Jiamusi and Songliao blocks, forming the Heilongjiang Complex. The oceanic closure and collision events were sequentially followed by the A-type granitic magmatism, which commenced in the earliest Cretaceous.

The Mesozoic magmatic, metamorphic, and tectonic evolution of the eastern Gangdese magmatic arc, southern Tibet

ABSTRACTMagmatic arcs are natural laboratories for studying the growth of continental crusts. The Gangdese arc, southern Tibet, is an archetypal continental magmatic arc that formed due to Mesozoic subduction of the Neo-Tethyan oceanic lithosphere; however, its formation and evolution remain controversial. In this contribution, we combine newly reported and previously published geochemical and geochronological data for Mesozoic magmatic rocks in the eastern Gangdese arc to reveal its magmatic and metamorphic histories and review its growth, thickening, and fractionation and mineralization processes. Our results show that: (1) the Gangdese arc consists of multiple Mesozoic arc-type magmatic rocks and records voluminous juvenile crustal growth. (2) The Mesozoic magmatic rocks experienced Late Cretaceous granulite-facies metamorphism and partial melting, thus producing hydrous and metallogenic element-rich migmatites that form a major component of the lower arc crust and are a potential source for the Miocene ore-hosting porphyries. (3) The Gangdese arc witnessed crustal thickening and reworking during the Middle to Late Jurassic and Late Cretaceous. (4) Crystallization-fractionation of mantle-derived magmas and partial melting of thickened juvenile lower crust induced intracrustal chemical differentiation during subduction. We suggest that the Gangdese arc underwent the following main tectonic, magmatic, and metamorphic evolution processes: normal subduction and associated mantle-derived magmatism during the Late Triassic to Jurassic; shallow subduction during the Early Cretaceous and an associated magmatic lull; and mid-oceanic ridge subduction, high-temperature metamorphism and an associated magmatic flare-up during the early Late Cretaceous, and flat subduction, high-temperature and high-pressure metamorphism, partial melting, and associated crust-derived magmatism during the late Late Cretaceous. Key issues for further research include the temporal and spatial distributions of Mesozoic magmatic rocks, the evolution of the components and compositions of arc crust over time, and the metallogenic processes that occur in such environments during subduction.

The Formation and Evolution of Uvarovite in UHP Serpentinite and Rodingite and its Constraints on Chromium Mobility in the Oceanic Subduction Zone

AbstractThe uvarovite‐andradite and uvarovite‐andradite‐grossular solid‐solution series are rare in nature. The discovery of uvarovite‐andradite in serpentinite and rodingite from the ultra‐high pressure (UHP) metamorphic belt in southwestern Tianshan provided an opportunity to investigate its behavior in the subduction zone. Uvarovite (defined as chromium‐garnet) from serpentinite is homogeneous in a single grain, covering compositions in the uvarovite‐andradite solid solution series of Adr58–66Uv33–41, with few grossular components. Uvarovite from rodingites contain various Cr2O3 contents (1.7–17.9 wt%) and mineral compositions being in the range of Adr21–31Uv41–50Grs22–37, Adr52–90Uv5–25Grs0–21 and Adr19–67Uv3–63Grs13–42. Discontinuous chemical variation of uvarovite from core to rim indicates that uvarovite formed by consuming andradite and chromite, which could provide Ca, Cr, Al and Fe. Raman signals of water were identified for uvarovite from both serpentinite and rodingite, with high water content in uvarovite from serpentinite. The high pressure mineral assemblage, as well as the association with perovskite, indicated that the studied uvarovite from serpentinite and rodingite was formed through high pressure metamorphism, during the subduction zone serpentinization and rodingitization. High alkaline and highly reduced fluids released from serpentinization or rodingitization in the oceanic subduction zone promote the mobility of chromium and enable its long‐distance migration.

Paleogeographic position of the central Dodecanese Islands, southeastern Greece: The push-pull of Pelagonia

Abstract The paleogeographic position of the central Dodecanese Islands at the transition between the Aegean and Anatolian plates plays a considerable role in understanding the link between both geologically unique domains. In this study, we investigate the tectonic history of the central Dodecanese Islands and the general correlation with the Aegean and western Anatolian and focus on the poorly studied islands of Kalymnos and Telendos. Three different major tectonic units were mapped on both islands from bottom to top: (1) The Kefala Unit consists of late Paleozoic, fossil-rich limestones, which have been deformed into a SE-vergent fold-and-thrust belt sealed by an up to 200-m-thick wildflysch-type olistostrome with marble and ultramafic blocks on a scale of tens of meters. (2) The Marina Basement Unit consists of a Variscan amphibolite facies basement with garnet mica schists, quartzites, and amphibolites. (3) Verrucano-type formation violet shales and Mesozoic unmetamorphosed limestones form the Marina Cover Unit. Correlation of these units with other units in the Aegean suggests that Kalymnos is paleogeographically located at the southern margin of the Pelagonian domain, and therefore it was in a structurally upper tectonic position during the Paleogene Alpine orogeny. New white mica 40Ar/39Ar ages confirm the Carboniferous deformation of the Marina Basement Unit followed by a weak Triassic thermal event. Single-grain white mica 40Ar/39Ar ages from pressure solution cleavage of the newly defined Telendos Thrust suggest that the Marina Basement Unit was thrusted toward the north on top of the Kefala Unit in the Paleocene. Located at a tectonically upper position, the units exposed in the central Dodecanese escaped subduction and the syn-orogenic, high-pressure metamorphism. However, these units were affected by post-orogenic extension, and the contact between the Marina Basement Unit and the non-metamorphic Marina Cover Unit has been reactivated by the cataclastic top-to-SSW, low-angle Kalymnos Detachment. Zircon (U-Th)/He ages from the Kefala and Marina Basement Units are ca. 30 Ma, which indicates that exhumation and cooling below the Kalymnos Detachment started in the Oligocene. Conjugate brittle high-angle normal fault systems, which resulted in the formation of four major WNW-ESE–trending graben systems on Kalymnos, localized mainly in the Marina Cover Unit and probably rooted in the mechanically linked Kalymnos Detachment. Since Oligo-Miocene deformation in the northern Dodecanese records top-to-NNE extension and the Kalymnos Detachment accommodated top-to-SSW extension, we suggest that back-arc extension in the whole Aegean realm and transition to the Anatolian plate is bivergent.

Behavior of barium isotopes during high-pressure metamorphism and fluid evolution

Barium (Ba) and its stable isotopes are powerful geochemical tracers for the recycling of subducted materials into the convective mantle. However, this requires a good understanding of the behavior of Ba isotopes during subduction-zone fluid processes. Here we present the first integrated study on Ba isotopes for two eclogite-vein systems (Ganghe and Hualiangting) that formed at high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic conditions from the Dabie orogen, central China. The Ganghe system displays constant δ138/134Ba (−0.01‰ to 0.03‰) in the UHP eclogites but relatively high δ138/134Ba (0.17‰) in the HP vein. In contrast, in the Hualiangting system, δ138/134Ba of the UHP eclogites vary from −0.14‰ to 0.05‰, while δ138/134Ba of the multiple HP veins and the retrograded amphibolites range from −0.17‰ to 0.34‰ and from −0.11‰ to 0.46‰, respectively.The combined results suggest that Ba isotopes can be considerably fractionated during subduction-zone fluid processes. HP-UHP metamorphic dehydration of the eclogites during the early stages of exhumation can produce an isotopically heavy initial vein-forming fluid due to the preferential leaching of epidote enriched with heavy Ba isotopes from the host eclogites. However, such processes have limited effects on the bulk eclogites because the Ba-rich phengite exists as a residual phase. During vein formation, light Ba isotopes are preferentially enriched in minerals relative to the fluids, resulting in the residual fluids with high δ138/134Ba. In addition, the fluids released from the gneisses have negative δ138/134Ba which is different from the eclogites, indicating that Ba isotopes can be used to trace the source of the metasomatic fluids. This study demonstrates that the UHP metamorphic rocks and HP veins can record Ba isotope fractionation during subduction-zone metamorphism and fluid-rock interaction, and recycling of the metamorphic rocks could modify Ba isotopic composition of the mantle wedge and arc volcanics.

Petrochronology of Monazite-Bearing Garnet Micaschists as a Tool to Decipher the Metamorphic Evolution of the Alpine Basement

Garnet-bearing metapelites in the Helvetic and Austroalpine pre-Mesozoic polymetamorphic basement are characterised by pressure-temperature path segments reconstructed by microstructurally controlled geothermobarometry, and the Th-U-Pb monazite age distribution pattern revealed by the electron probe microanalyser (EPMA). In the Helvetic Aiguilles Rouges Massif and the Austroalpine Oetztal-Stubai basement to the NW an Ordovician-to-Silurian high temperature event preceded a pressure-dominated Carboniferous metamorphism. In the Austroalpine basement units to the south of the Tauern Window, the maximal pressures of the Carboniferous amphibolite-facies metamorphism range from 12 to 6 kbar. The decompressional P-T path segments signal a transition to low pressure conditions. A subsequent high pressure overprint is restricted to the Prijakt Subgroup unit in the Schobergruppe and documented by Cretaceous monazite crystallisation at 88 ± 6 Ma. In the Austroalpine Saualpe basement to the SE, a distinct early Permian metamorphism which started at low pressures of ~4 kbar/500 °C and reached maximal 6 kbar/600–650 °C predated the intrusion of Permian pegmatites. Permian monazite crystallised in line with the intrusion of pegmatites. Corona microstructures around the Permian monazites indicate retrogression previous to a Cretaceous high pressure metamorphism. That way, pressure-temperature-time paths resolve the spatial and temporal evolution in the polymetamorphic Alpine basement prior to the Tertiary collision.

Early cretaceous accretionary complex of the valaisan ocean, western alps?

Based on detailed field investigations and structural mapping in the Petit-Saint Bernard Pass area (French-Italian border), the Versoyen Complex is newly defined as an imbrication of four structural units. The first one is composed of the remnant of the Valaisan ocean floor (“basalt” and “black schist” formation) and the other three units -grouped under the name of “schistes a blocs”- contain blocks with different origin: oceanic, continental or a mix of both, embedded in a matrix of grey micaschists. Kinematic analysis highlights the compressional style of this complex with a well-developed stack of tectonic slivers. Rocks in the tectonic melange include Paleozoic green gneiss and grey-green micaschists blocks, together with Upper Paleozoic granitoid and layered gabbro, which display a typical orogenic calc-alkaline signature, comparable to that of coeval plutonic rocks of the Brianconnais continental basement attributed to a late Variscan back-arc environment. A new isotopic dating on a calc-alkaline layered gabbro block gave a U/Pb zircon age of 310 ± 4 Ma, in the range of other dated plutonic rocks such as the Punta Rossa and Aiguille du Clapet megablocks. These Paleozoic continental basement rocks of the Versoyen Complex were initially rift allochthons, but are now outcropping as a tectonic melange, with imbricated blocks and slivers within an Early Cretaceous grey micaschists matrix. Radiolarians sampled by Beltrando et al. (2012) within the grey micaschists have been attributed to the Late Jurassic - Early Cretaceous. Additional stratigraphic and geodynamic constraints allow us to restrict the deposition of the Versoyen sediments from the Aptian to Cenomanian times. The Versoyen Complex is unconformably overlain by the Valaisan Trilogy (Aroley-Marmontains-Saint Christophe) of Cretaceous age. The Turonian Aroley strata deposited on the Versoyen “schistes a blocs” units and the absence of any Aroley limestones within the underlying tectonic melange testify a clear stratigraphic unconformity, mapped regionally, between the Versoyen Complex and Valaisan Trilogy. Both are affected by a high-pressure metamorphism related to the Alpine collision around 40 Ma and trace of former metamorphism are nowhere observed suggesting that the former melange did not result in a significant tectonic thickening. We interpret the Versoyen Complex, as a pre-Alpine Cretaceous subduction-related accretionary prism, formed during the closure of the Valaisan Ocean. Valaisan rifting initiated during the Late Jurassic, as a consequence of an eastwards propagation of the North-Atlantic rifting towards the Alpine region. This propagation followed pre-existing Variscan structures along the Zone Houillere, explaining the present marked differences between Variscan basements on each side of the Valaisan suture (External Crystalline Massifs and Brianconnais basement). Due to the southward subduction of the Valaisan Ocean, this passive margin was later on deformed through compressional shearing tectonics, developing a “schistes a blocs” fabric and imbricating the former rift allochthons. During the Tertiary Alpine collision, Versoyen and Valaisan Trilogy units were deeply underthrusted, and then exhumed under the Subbrianconnais units - including the Petit-Saint Bernard and Arguerey Liassic calcschists, which underthrusted in turn the Brianconnais Front.

Metamorphic evolution and P–T path of the Posada Valley amphibolites: new insights on the Variscan high pressure metamorphism in NE Sardinia, Italy

Metamorphic evolution and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>P</mml:mi></mml:math>–<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>T</mml:mi></mml:math> path of the Posada Valley amphibolites: new insights on the Variscan high pressure metamorphism in NE Sardinia, Italy

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.

Thallium behavior during high-pressure metamorphism in the Western Alps, Europe

Thallium (Tl) is a highly incompatible element which can be redistributed during various igneous and hydrothermal events such as mineral dehydration and low-temperature hydrothermal alteration. Despite the strong influence fluid-rock interactions can have on Tl redistribution and the importance of fluids in subduction-zone volatiles cycling, the geochemical behavior of Tl during high-P/T metamorphism has not yet been quantified. Here we provide new Tl isotope composition (ε205Tl) and concentration ([Tl]) data for a suite of 18 metapelite and metacarbonate rocks from the Schistes Lustrés Complex and the nearby Lago di Cignana exposure in the western Alps. This suite represents metamorphism over a range of peak metamorphic P-T conditions of 300–550 °C and 1.5–3.0 GPa (similar to the range of P-T experienced in most modern subduction zones).With increasing metamorphic grade, [Tl] shows no statistical variation while ε205Tl initially remains fairly constant (in order of increasing grade, Fraiteve (unit A) ε205Tlavg = −2.1 (± 0.3); Assietta (unit B) ε205Tlavg = −1.6 (± 0.6); Albergian (unit C) ε205Tlavg = −1.9 (± 0.5)) before shifting to more negative values during higher grade metamorphism (Finestre (unit D) ε205Tlavg = −2.4 (± 0.3); Cignana (unit E) ε205Tlavg = −3.5 (± 0.3)). Although it appears likely that Tl is largely retained in such rocks to depths approaching those of arc magma generation, the mineralogical hosts for Tl change during prograde devolatilization. This may contribute to a significant shift in ε205Tl related to fractionation induced by only minor amounts of loss due to fluid-mineral partitioning. Prograde metamorphism results in the breakdown of chlorite and other minor mineralogical constituents, releasing chlorite-borne Tl that could be in part incorporated into phengite and in part lost to fluids. This study emphasizes the need for prograde mineral reaction history to be considered in studies of the retention or loss of trace elements such as Tl.

On the origin of high-pressure mafic granulite in the Eastern Himalayan Syntaxis: Implications for the tectonic evolution of the Himalayan orogen

The Himalayan orogen, resulting from the Early Cenozoic collision of the Indian and Asian plates, is an ideal vehicle to study active orogenic processes and test geodynamic models of how the crust responds to collisional orogeny. This paper focused on migmatitic high-pressure (HP) mafic granulite and associated leucosome from the Greater Himalayan Crystallines (GHC) in the Eastern Himalayan Syntaxis (EHS) in order to understand the conditions and timescales over which high-grade rocks and partial melts were produced during the Himalayan orogeny. Combining with previous study results from the Western and Central Himalayas and Trans-Himalayan magmatic arc, we obtained the following conclusions: (1) The mafic granulites from the EHS underwent HP and high-temperature (HT) granulite facies metamorphism and partial melting, with peak metamorphic conditions of 15–17 kbar and 820–880 °C. The GHC, at least its western part of the EHS, underwent coherent HP granulite-facies metamorphism. (2) The HP mafic granulites experienced long-lived dehydration melting of amphibole from ~40 Ma to ~20 Ma during prograde metamorphism and generated up to ~16 vol% partial melt. The variable degrees of dehydration melting of the HP mafic, pelitic and felsic granulites in the EHS generated voluminous granitic melts with distinct compositions, and provided the source for the Himalayan granites. (3) Peak metamorphic pressure of the GHC gradually decreases, whereas the metamorphic temperature progressively increases from the Western to Eastern Himalayas. This indicates that the Indian continental crust deeply subducted into the mantle in the Western Himalaya after the Indo-Asia collision, whereas the Indian crust underthrusted or relaminated beneath the Asian continental crust, and formed the thickened lower crust in the Central and Eastern Himalayas and Gangdese arc. (4) The melts derived from the underthrusted Indian crust probably resulted in isotopic compositional enrichment of the Early Cenozoic mantle- and juvenile crust-derived magmatic rocks of the Gangdese arc.

Rodingitization records from ocean-floor to high pressure metamorphism in the Xigaze ophiolite, southern Tibet

Abundant rodingite was found within serpentinized peridotite and spatially close to gabbro and diabase dykes in the Xigaze ophiolite of the Yarlung Zangbo suture zone. Two types of rodingite are identified based on field- and textural relationships as well as on mineral- and bulk-rock compositions. Type I rodingite with porphyroblastic texture occurs along extensional fractures within massive serpentinized peridotite and contains two subtypes representing the early stage of rodingitization. Type II rodingite is associated with a foliated serpentinized host rock and contains three subtypes, characterized by prehnite-absent mineral assemblages with fine-grained porphyritic to aphanitic texture. All rodingite II subtypes experienced a higher degree of rodingitization than Type I. Type I rodingite was formed by metasomatism of CaO-rich fluids from serpentinized peridotite during ocean floor metamorphism at ~120–130 Ma. Thermodynamic modeling shows that Type I rodingite originated at low P-T conditions of <~4 kbar and ~200–300 °C during limited desilication and calcium enrichment. within a highly reducing environment (logƒO2 < −35.3). Type II rodingite originated during subduction starting at ~120 Ma with most intense desilication and calcium increase under peak P-T conditions up to ~13.5–21.5 kbar and ~450–550 °C. Modeled values of logƒO2 for the formation of rodingite II subtypes vary from highly oxidized conditions of logƒO2 = −19.7 to −18.6 to relatively reducing conditions of logƒO2 < ~−26. P-XCO2 pseudosection modelling predicted Type II rodingite to have formed under CO2-poor conditions at XCO2 < ~1 × 10-4 during subduction to the maximum depth. Hence the rodingite of the Xigaze ophiolite provides important fingerprints recording events during ocean floor spreading and different stages of subduction as well as late exhumation.

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.

HP–UHT granulites in the East Kunlun Orogen, NW China: Constraints on the transition from compression to extension in an arc setting of the Proto‐Tethys Ocean

AbstractThe East Kunlun Orogen (EKO) of the northwest part of the Central China Orogenic belt records long‐lived orogenic events from the Cambrian to the Triassic, involving the tectonic evolution from Proto‐Tethys to Palaeo‐Tethys. Although high‐ and ultra high pressure (HP‒UHP) metamorphism has been recognized in the EKO, other metamorphic events associated with the orogenic history are little known. The metamorphic history of high‐ and ultra high temperature granulites and their pressure–temperature–time (P‒T–t) path is keys to reconstructing tectonic events associated with a subduction‐to‐collision transition. In this contribution, we present P‒T–t results for the mafic and pelitic granulite facies metamorphic rocks in the Qingshuiquan region of the EKO, determined using phase equilibrium modelling and zircon geochronology. The Jingshuikou granulite terrane records a clockwise P–T path with high‐P and ultra high temperature (HP–UHT) conditions at Pmax ~900 to 950°C at 13 kbar, at Tmax of ~950°C at 9‒12 kbar, and a final stage of 800‒895°C at 6‒10 kbar. LA‐ICP‐MS U–Pb dating of zircons provides systemic constraints on a long‐lived metamorphic age span of 530‒465 Ma, with a peak of c. 503 Ma. The HP–UHT metamorphism of the Qingshuiquan granulite terrane reveals a complete process from arc compression at the early stage of subduction to extension on the active continental margin related to rollback of the subducted slab in the Early Palaeozoic, driven by heat transfer from upwelling of the sub‐arc hot asthenosphere. The timing and P‒T conditions of the high geothermal gradient metamorphism can be used to understand the subduction and collision process of the Proto‐Tethys Ocean, including the relationship between high‐P and high‐ to ultra‐temperature metamorphism in the East Kunlun and other orogenic belts.

Paleoproterozoic medium to high pressure metamorphism in the Wanzi supracrustal association, Trans-North China Orogen: New insights from the gedrite-bearing gneiss, gedrite-free gneiss, and amphibolite

The Wanzi supracrustal association locates in the Fuping Complex, middle Trans-North China Orogen, and it was generally considered before as a lithological unit that underwent medium pressure metamorphism. The newly discovered gedrite-bearing gneiss, gedrite-free gneiss, and amphibolite of the Wanzi supracrustal association show abundant mineral assemblages with complex reaction textures, which are critical clues to retrieve the metamorphic evolution of this area. Gedrite-bearing/free gneiss samples mainly consist of garnet porphyroblast plus matrix minerals including biotite, kyanite, sillimanite, staurolite, ±gedrite, plagioclase, quartz, as well as accessory minerals such as ilmenite, magnetite, apatite, monazite, and zircon. Amphibolite samples are generally composed of garnet porphyroblast plus matrix minerals amphibole, ±biotite, plagioclase, quartz, as well as accessory minerals including ilmenite, magnetite, and zircon. Two to four generations of metamorphic mineral assemblages formed at the prograde (M1), peak (M2), and retrograde (M3-M4) stages have been identified from these samples, recording clockwise P-T paths including sequential near-isothermal decompression (ITD) and near-isobaric cooling (IBC) segments, indicative of convergence tectonic settings. The peak P-T conditions of these samples are different, ranging from 8.2 to 13 kbar and 695 to 822 °C. SIMS/SHRIMP U-Pb dating of metamorphic zircon yields 207Pb/206Pb age of 1838–1821 Ma, representing the metamorphic event. Combined with the previous data, the Wanzi supracrustal association is inferred to have undergone middle to high pressure metamorphism during 1.84–1.82 Ga, which was involved in the subduction and/or collision event between the Western and Eastern Blocks and led to the final assembly of the North China Craton.

Isotope-Geochemical Evidence of the Nature of the Protoliths of Diamondiferous Rocks of the Kokchetav Subduction–Collision Zone (Northern Kazakhstan)

Abstract —The isotope-geochemical features of diamondiferous metamorphic rocks of the Kokchetav subduction–collision zone (KSCZ) show that both the basement rocks and the sediments of the Kokchetav massif were their protoliths. A whole-rock Sm–Nd isochron from the diamondiferous calc-silicate, garnet–pyroxene rocks and migmatized granite-gneisses of the western block of the KSCZ yielded an age of 1116 ± 14 Ma, while an age of 1.2–1.1 Ga was obtained by U–Pb dating of zircons from the granite-gneiss basement of the Kokchetav microcontinent. Based on these data, we assume that the protoliths of the calc-silicate, garnet–pyroxene rocks and the granite-gneisses of the KSCZ were the basement rocks sharing an initially single Nd source, which was not influenced by high- to ultrahigh-pressure metamorphism (~530 Ma). Therefore, their geochemical features are probably not directly related to ultrahigh-pressure metamorphism. The corresponding rock associations lack isotope-geochemical evidence of partial melting that would occur during ultrahigh-pressure metamorphism, which suggesting that they were metamorphosed under granulite-facies conditions. At the same time, the high-alumina diamondiferous rocks of the Barchi area (garnet–kyanite–mica schists and granofelses), which were depleted to different degrees in light rare-earth elements (REE) and K, have yielded a Sm–Nd whole-rock isochron age of 507 ± 10 Ma indicating partial melting of these rocks during their exhumation stage. The close ɛNd (1100) values of the basement rocks and garnet–kyanite–mica schist with geochemical characteristics arguing against its depletion during high-pressure metamorphism indicate that the basement rocks were a crustal source for high-alumina sediments.

Metasediments Covering Ophiolites in the HP Internal Belt of the Western Alps: Review of Tectono-Stratigraphic Successions and Constraints for the Alpine Evolution

Ophiolites of the Alpine belt derive from the closure of the Mesozoic Tethys Ocean that was interposed between the palaeo-Europe and palaeo-Adria continental plates. The Alpine orogeny has intensely reworked the oceanic rocks into metaophiolites with various metamorphic imprints. In the Western Alps, metaophiolites and continental-derived units are distributed within two paired bands: An inner band where Alpine subduction-related high-pressure (HP) metamorphism is preserved, and an outer band where blueschist to greenschist facies recrystallisation due to the decompression path prevails. The metaophiolites of the inner band are hugely important not just because they provide records of the prograde tectonic and metamorphic evolution of the Western Alps, but also because they retain the signature of the intra-oceanic tectono-sedimentary evolution. Lithostratigraphic and petrographic criteria applied to metasediments associated with HP metaophiolites reveal the occurrence of distinct tectono-stratigraphic successions including quartzites with marbles, chaotic rock units, and layered calc schists. These successions, although sliced, deformed, and superposed in complex ways during the orogenic stage, preserve remnants of their primary depositional setting constraining the pre-orogenic evolution of the Jurassic Tethys Ocean.

First report of phengites in the Longyou paragneiss in the northern Early Paleozoic Wuyi-Yunkai orogen, South China: P-T conditions, zircon U-Pb ages and tectonic implications

Our recent investigations into the Longyou paragneiss (country gneiss of the Longyou garnet amphibolite) in the northern Wuyi-Yunkai orogen of South China lead to a recognition of high-pressure metamorphic phengites in the paragneiss. The paragneiss consists of phengite, muscovite, biotite, K-feldspar, plagioclase, quartz, and minor graphite and ilmenite or magnetite. Mineral compositions show that Si contents in phengites are between 3.40 and 3.25 (p.f.u., per formula unit), belonging to high-pressure metamorphic phengites. P-T estimates suggest that peak high-pressure metamorphic stage (M2) has P-T conditions of ~ 13.0 kbar and ~ 690 °C, whereas post-peak retrograde stage (M3) has lower P-T conditions of ~ 6.0 kbar and ~ 695 °C. The two-stage P-T conditions define a clockwise P-T path of post-peak isothermal decompression, reflecting a typical collisional orogenic process. SIMS zircon U-Pb age data show a weighted mean 206Pb/238U age of 445 ± 3 Ma, consistent with the metamorphic age of the Longyou garnet amphibolite, suggesting that the peak high-pressure metamorphic event occurred in the Early Palaeozoic orogeny in the Cathaysia block in South China. Thus, we consider that both the Longyou paragneiss and enclosed garnet amphibolite together experienced a phase of high-pressure metamorphism during the Early Palaeozoic continent–continent collisional orogeny likely along the Zhenghe-Dapu fault between the South China block and an unknown block on the northern margin of eastern Gondwana continent.

Absence of evidence for Palaeoproterozoic eclogite-facies metamorphism in East Antarctica: no record of subduction orogenesis during Nuna development

The cratonic elements of proto-Australia, East Antarctica, and Laurentia constitute the nucleus of the Palaeo-Mesoproterozoic supercontinent Nuna, with the eastern margin of the Mawson Continent (South Australia and East Antarctica) positioned adjacent to the western margin of Laurentia. Such reconstructions of Nuna fundamentally rely on palaeomagnetic and geological evidence. In the geological record, eclogite-facies rocks are irrefutable indicators of subduction and collisional orogenesis, yet occurrences of eclogites in the ancient Earth (> 1.5 Ga) are rare. Models for Palaeoproterozoic amalgamation between Australia, East Antarctica, and Laurentia are based in part on an interpretation that eclogite-facies metamorphism and, therefore, collisional orogenesis, occurred in the Nimrod Complex of the central Transantarctic Mountains at c. 1.7 Ga. However, new zircon petrochronological data from relict eclogite preserved in the Nimrod Complex indicate that high-pressure metamorphism did not occur in the Palaeoproterozoic, but instead occurred during early Palaeozoic Ross orogenesis along the active convergent margin of East Gondwana. Relict c. 1.7 Ga zircons from the eclogites have trace-element characteristics reflecting the original igneous precursor, thereby casting doubt on evidence for a Palaeoproterozoic convergent plate boundary along the current eastern margin of the Mawson Continent. Therefore, rather than a Palaeoproterozoic (c. 1.7 Ga) history involving subduction-related continental collision, a pattern of crustal shortening, magmatism, and high thermal gradient metamorphism connected cratons in Australia, East Antarctica, and western Laurentia at that time, leading eventually to amalgamation of Nuna at c. 1.6 Ga.