Eclogite Facies Metamorphism Research Articles

Repeated Caledonian burial and ultrafast cooling and exhumation of high‐pressure granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft, Western Gneiss Region, Norway

AbstractThe allochthonous Blåhø Nappe in the Nordøyane ultra high pressure (UHP) domain, Western Gneiss Region in Norway, acts as a window to examine geological processes occurring in continent–continent collisional zones, but many aspects regarding its tectonometamorphic evolution remain debated and elusive. In this contribution, an integrated study including major‐ and trace‐element zoning in garnet, phase equilibrium modelling and the simulation of cation diffusion in garnet was conducted on two high‐pressure (HP) granulite facies rocks from the Blåhø Nappe on the island of Fjørtoft. The results shed new light on the complex geodynamic processes that act in continent–continent collisional zones and finally shape collisional orogens. Phengite, biotite, amphibole, zoisite‐allanite and low‐Zr rutile enclosed in garnet likely attest to a prograde eclogite facies metamorphism for the studied rocks. Pressure–temperature (P–T) conditions of ~1.5–1.6 GPa and 615–670°C were retrieved for this stage. An extensive re‐equilibration under peak HP granulite facies conditions of ~1.5 GPa and 925 ± 50°C followed. Subsequently, the rocks were cooled and reburied to eclogite facies conditions of ~1.8–1.9 GPa and 805–825°C. This was followed by a final stage of decompression and cooling to amphibolite facies conditions of ~650–780°C and 0.5–1.0 GPa. Cooling and exhumation rates of >400°C/Ma and >75 km/Ma, respectively, indicating an ultrafast temperature and pressure decrease are estimated for this stage from simulations of cation diffusion in garnet. The anticlockwise P–T path obtained here is relatively complete and compatible with a repeated burial history during the Caledonian orogeny but not with UHP conditions proposed for the Blåhø Nappe. Our model proposes that the rocks later forming the Blåhø Nappe were buried to lower crustal depths of approximately 55 km equating to a geothermal gradient of ~13°C/km during the early Caledonian orogeny. Subsequent heating of these rocks to HP granulite facies conditions was likely driven by slab break‐off and hot mantle upwelling. Baltica underthrusting during the Scandian continent–continent collision cooled and transported the Blåhø Nappe to greater depths. The obtained cooling and exhumation rates indicate ultrafast exhumation, presumably in an exhumation channel.

Metamorphic Evolution of the Garnet Amphibolite Schist from Mawat Ophiolite, Kurdistan Region, Northeast Iraq: Geochemistry, Mineral Chemistry and Thermodynamic Approach

The garnet amphibolite schist is associated with the gabbroic unit of the Mawat ophiolite, Kurdistan region, Northeast Iraq, in the imbricated zone of the Zagros orogenic belt. Field observation, petrographic study, mineral chemistry, geochemical data, and thermodynamic modeling were used to demonstrate mineral identification, predict the observed stable mineral assemblages, and validate the protolith. The mineral assemblages of prograde are garnet (pyrope-almandine), chlorite (clinochlore), amphibole (grunerite), plagioclase (anorthite), iron oxides while the garnet (almandine-pyrope), chlorite (chamosite), amphibole (cummingtonite), and iron oxide represent retrograde assemblage. The relict pod of metagabbro in the center of the garnet amphibolite schist body consists mainly of plagioclase, amphibole, pyroxene, and iron oxide. Geochemical data reveals that the rock is depleted in SiO2 and CaO and enriched in refractory elements compared to the metagabbro. This may be attributed to the generation of silicic melts by partial melting and producing nearby plagiogranite. The rare earth elements (REE) patterns show that the garnet amphibolite schist is depleted in light rare earth elements (LREE) and gradually enriched in heavy rare earth elements (HREE). Thermodynamic modeling, field observation, combined with petrographic study constrains two metamorphic stages and one possible anticlockwise P-T-t path for garnet amphibolite schist, with increasing pressure and temperature of gabbroic rocks units of Mawat ophiolite. The rocks reached peak metamorphic conditions at more than 800 °C and 8–10 kbar, which partially melted the rock produced prograde resitite and melt. Later, the resitite rock retrograded to the lower stage at temperatures ranging from 620 to 650 °C and pressures ranging from 6.5 to 6.8 kbar. Anticlockwise P-T-t paths suggested based on geochemical and thermodynamic data for garnet amphibolite schist of the Mawat ophiolite. Moreover, the gabbro is considered to be the protolith of garnet amphibolite schist, which have been undergone eclogite facies metamorphism later overprinted by amphibolite facies.

A re-evaluation of the peak P-T conditions of eclogite-facies metamorphism of the Paleozoic Acatlán Complex (Mexico) reveals deeper subduction.

Eclogites in the Acatlán Complex, southern Mexico, record the subduction history of the complex. Previous studies indicate that the proto-Acatlán Complex reached < 50km depth during subduction. Yet, a recent study reported higher pressures for a single eclogite, questioning the maximum depth reached by the complex during subduction. In this work, I re-calculate eclogite pressure and temperature (P-T) conditions using thermobarometric methods applicable to eclogite-facies mafic rocks to a set of eclogites cropping out throughout the high-pressure belt of the Acatlán Complex-the Piaxtla Suite. I find that Acatlán eclogites record substantially-and systematically-greater pressures than previously reported. Calculations show that eclogites from the central part of the Piaxtla Suite (in the Piaxtla area) record consistent pressures of ~ 2.0GPa and temperatures ranging between 460 and 675°C. Eclogites from the northern part of the Piaxtla Suite (Mimilulco and Santa Cruz Organal areas) lack phengite, thus pressures were not calculated; temperatures calculated for these rocks at a fixed pressure (2.0GPa) yield contrasting temperatures (511°C and 870°C, respectively). Mimilulco eclogite likely records similar pressures (~ 2.0GPa) to other Piaxtla eclogites, whereas the pressures of Santa Cruz Organal eclogites might have been different, and likely experiencing a different thermal history compared to the rest of the eclogites from the Piaxtla Suite. Overall, these results indicate that the Acatlán Complex subducted to greater depths than previously thought implying a faster burial-exhumation cycle of the proto-Acatlán Complex.

Tectonic setting of metamorphism and exhumation of eclogite-facies rocks in the South Beishan orogen, northwestern China

AbstractHigh-pressure metamorphic rocks occur as distinct belts along subduction zones and collisional orogens or as isolated blocks within orogens or mélanges and represent continental materials that were subducted to deep depths and subsequently exhumed to the shallow crust. Understanding the burial and exhumation processes and the sizes and shapes of the high-pressure blocks is important for providing insight into global geodynamics and plate tectonic processes. The South Beishan orogen of northwestern China is notable for the exposure of early Paleozoic high-pressure (HP), eclogite-facies metamorphic rocks, yet the tectonism associated with the HP metamorphism and mechanism of exhumation are poorly understood despite being key to understanding the tectonic evolution of the larger Central Asian Orogenic System. To address this issue, we examined the geometries, kinematics, and overprinting relationships of structures and determined the temperatures and timings of deformation and metamorphism of the HP rocks of the South Beishan orogen. Geochronological results show that the South Beishan orogen contains ca. 1.55–1.35 Ga basement metamorphic rocks and ca. 970–866 Ma granitoids generated during a regional tectono-magmatic event. Ca. 500–450 Ma crustal thickening and HP metamorphism may have been related to regional contraction in the South Beishan orogen. Ca. 900–800 Ma protoliths experienced eclogite-facies metamorphism (~1.2–2.1 GPa and ~700–800 °C) in thickened lower crust. These HP rocks were subsequently exhumed after ca. 450 Ma to mid-crustal depths in the footwall of a regional detachment fault during southeast-northwest–oriented crustal extension, possibly as the result of rollback of a subducted oceanic slab. Prior to ca. 438 Ma, north-south–oriented contraction resulted in isoclinal folding of the detachment fault and HP rocks. Following this contractional phase in the middle Mesozoic, the South Beishan orogen experienced thrusting interpreted to be the response to the closure of the Tethyan and Paleo-Asian Ocean domains. This contractional phase was followed by late Mesozoic extension and subsequent surface erosion that controlled exhumation of the HP rocks.

The emplacement, alteration, subduction and metamorphism of metagranites from the Tso Morari Complex, Ladakh Himalaya

AbstractEclogite-facies mineral assemblages are commonly preserved in mafic protoliths within continental terranes. It is widely accepted that the entirety of these continental terrains must also have been subducted to eclogite-facies conditions. However, evidence that the felsic material transformed at eclogite-facies conditions is lacking. Low-strain metagranites of the ultrahigh-pressure metamorphic Tso Morari Complex in Ladakh, Himalaya, are host to eclogite-facies mafic sills and preserve evidence of subduction to eclogite-facies conditions. Following the eclogite-facies metamorphism, the granites and their gneissic equivalents were overprinted by amphibolite-facies Barrovian metamorphism, obscuring their earlier metamorphic history. We present evidence that the Tso Morari metagranites preserve a complex magmatic, hydrothermal and polymetamorphic history that involved four stages. Stage 1 was magmatic crystallisation, a record of which is preserved in the primary igneous mineralogy and relict igneous microstructures. Monazite grains record a U–Pb age of 474.0 ± 11.6 Ma, concurrent with a published zircon crystallisation age. Stage 2 represents pervasive late-magmatic hydrothermal alteration of the granite during emplacement and is evident in the mineral composition, particularly in the white micas preserved in the igneous domains. Stage 3 involved the (ultra)high-pressure metamorphism of these granite bodies during the Himalayan subduction of continental material. The high-pressure stage of the metamorphic history (&gt;25 kbar at 550–650°C) is preserved as thin coronas of garnet and phengite around igneous biotite, garnet with kyanite inclusions in pseudomorphs after cordierite, and rare palisade quartz textures after coesite. Stage 4 was a result of Barrovian metamorphism of the Tso Morari Complex and is evident in the replacement of garnet by biotite. Many of these features are preserved in localised textural domains in the rock, where local equilibrium was important and the anhydrous conditions limited reaction progress, though aided preservation potential. Collectively, these four stages record a 480 Myr history of metamorphism and reworking of the northernmost Indian plate.

Isothermal compression of an eclogite from the Western Gneiss Region (Norway)

AbstractIn the Western Gneiss Region in Norway, mafic eclogites form lenses within granitoid orthogneiss and contain the best record of the pressure and temperature evolution of this ultrahigh‐pressure (UHP) terrane. Their exhumation from the UHP conditions has been extensively studied, but their prograde evolution has been rarely quantified although it represents a key constraint for the tectonic history of this area. This study focused on a well‐preserved phengite‐bearing eclogite sample from the Nordfjord region. The sample was investigated using phase‐equilibrium modelling, trace‐element analyses of garnet, trace‐ and major‐element thermobarometry and quartz‐in‐garnet barometry by Raman spectroscopy. Inclusions in garnet core point to crystallization conditions in the amphibolite facies at 510–600°C and 11–16 kbar, whereas chemical zoning in garnet suggests growth during isothermal compression up to the peak pressure of 28 kbar at 600°C, followed by near‐isobaric heating to 660–680°C. Near‐isothermal decompression to 10–14 kbar is recorded in fine‐grained clinopyroxene–amphibole–plagioclase symplectites. The absence of a temperature increase during compression seems incompatible with the classic view of crystallization along a geothermal gradient in a subduction zone and may question the tectonic significance of eclogite facies metamorphism. Two end‐member tectonic scenarios are proposed to explain such an isothermal compression: Either (1) the mafic rocks were originally at depth within the lower crust and were consecutively buried along the isothermal portion of the subducting slab or (2) the mafic rocks recorded up to 14 kbar of tectonic overpressure at constant depth and temperature during the collisional stage of the orogeny.

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.

Trench retreat recorded by a subduction zone metamorphic history

Abstract Upper amphibolite-facies metamorphism in subduction zone rocks may occur under exceptional tectonic settings. Differentiating competing mechanisms for its occurrence requires carefully integrated, high-resolution thermobarometric and geochronologic studies of mélange rocks with well-defined field relationships. We present new pressure, temperature, and age data from the classic Cretaceous Catalina Schist in southern California (USA) that allow us to establish a plausible model for its high-temperature metamorphic history. Our results indicate that garnet-amphibolite blocks in the structurally highest amphibolite-facies mélange preserve evidence of three stages of tectonic evolution: (1) prograde lawsonite eclogite-facies metamorphism that peaked at 2.4–2.7 GPa with temperatures &amp;gt;580 °C during fixed-trench subduction (120–115 Ma); (2) post-peak epidote eclogite-facies metamorphism followed by amphibolite-facies metamorphism at 1.4–1.3 GPa with temperatures of 740–790 °C during trench retreat (115–105 Ma); and (3) isothermal decompression (1.3 GPa to &amp;lt;1.0 GPa at temperatures of ~780 °C) and cooling during trench advance and slab-flattening subduction (ca. 105–100 Ma). Our model implies the presence of a continuous Cordilleran subduction system in the Cretaceous, which had varying tectonic regimes through episodes of trench retreat/advance and slab shallowing/steepening that, in turn, dictated the development of the Cordilleran arc system.

Multi‐episodic formation of baddeleyite and zircon in polymetamorphic anorthosite and rutile‐bearing ilmenitite from the Chiapas Massif Complex, Mexico

AbstractMassif‐type anorthosite and comagmatic associations of rutile‐bearing ilmenitite (RBI) and oxide‐apatite‐rich amphibolite (OARA) from the Chiapas Massif Complex (CMC) in southeastern Mexico display a protracted billion‐year accessory mineral record encompassing magmatic crystallization at c. 1.0 Ga to recent ductile shear deformation at c. 3.0 Ma. Multiple discrete zircon populations between these age end‐members resulted from neoformation/recrystallization during local to regional metamorphism that affected the southeastern portion of the CMC. The ubiquitous presence of relict baddeleyite (ZrO2), along with various zircon generations spatially associated with pristine to partly retrogressed Zr‐bearing igneous and metamorphic minerals (e.g., ilmenite, rutile, högbomite and garnet), suggests significant Zr diffusive re‐equilibration (exsolution) during slow cooling and mineral breakdown followed by crystallization of baddeleyite. The subsequent transformation of baddeleyite into zircon was likely driven by reaction with Si‐bearing fluids in several geochronologically identified metamorphic stages. Strikingly contrasting compositional signatures in coeval zircon from anorthosite (silicate‐dominated) and comagmatic RBI (Ti‐Fe‐oxide‐dominated) indicate a major role of fluids locally equilibrating with the rock matrix, as indicated by distinct zircon trace element and oxygen isotopic compositions. A high‐grade metamorphic event at c. 950 Ma is likely responsible for the formation of coarse‐grained rutile (~0.1–10 mm in diameter), srilankite, zircon and garnet with rutile inclusions as well as metamorphic högbomite surrounding Fe‐Mg spinel. Zr‐in‐rutile minimum temperatures suggest &gt;730°C for this event, which may correlate to rutile‐forming granulite facies metamorphism in other Grenvillian‐aged basement rocks in Mexico and northern South America. A younger generation of baddeleyite exsolution occurred during post‐peak cooling of coarse‐grained rutile, reflected in rimward Zr depletion and formation of discontinuous baddeleyite coronas. Baddeleyite around rutile was then transformed into zircon possibly during subsequent metamorphism at c. 920 or 620 Ma, resulting from syn‐kinematic and contact metamorphism, respectively. Regional metamorphism at c. 450 and 250 Ma extensively overprinted the existing zircon population, especially during the Triassic event, as suggested by a significant presence of zircon with this age. Nearly pristine baddeleyite occurring interstitial to ilmenite yielded an isochron age of c. 232 Ma according to in situ U–Pb secondary ion mass spectrometry (SIMS), suggesting either formation during metamorphic peak conditions or post‐peak cooling. Zircon with ages of c. 80–100 Ma in anorthosite is identified for the first time within the CMC and coincides with cooling ages of c. 100 Ma for coarse‐grained rutile. This age is similar to those of rocks occurring ~200 km further to the east in Guatemala, which are also bounded to the Polochic fault system but overprinted by eclogite facies metamorphism. A high‐pressure event in the southern CMC after 200 Ma, however, is presently unsupported. Although the abundance of rutile and ilmenite is unusually high in the CMC anorthosite assemblage compared with common igneous rocks, the reactions documented here nonetheless stress the importance of these phases for generating Zr‐bearing accessory minerals over a wide range of metamorphic conditions.

Metamorphic evolution of the Sittampundi Layered Complex, India, during the Archaean–Proterozoic boundary: insight from pseudosection modelling and zircon U–Pb SHRIMP geochronology

AbstractIn the Palghat–Cauvery Shear/Suture Zone of the Southern Granulite Terrane, the Sittampundi Layered Complex occurs as a mappable unit. The Sittampundi Layered Complex consists of mafic, ultramafic and anorthositic rocks with chromitite layers and has been interpreted as an arc/ophiolite complex that formed in an Archaean suprasubduction zone arc setting. In the Sittampundi Layered Complex, reddish-black metabasites occur as layers or boudins with a rim of amphibolite within anorthosite. The peak metamorphic assemblage of the metabasites is garnet + clinopyroxene + quartz + rutile ± plagioclase ± orthopyroxene, and symplectite consisting of amphibole and plagioclase formed around the garnet during retrograde metamorphism. The protolith of metabasite may have intruded in a suprasubduction zone arc setting during the late Neoarchaean (c. 2540–2520 Ma), and then underwent high-pressure granulite-facies peak metamorphism (900–800 °C and 11–14 kbar) in the early Palaeoproterozoic (c. 2460–2440 Ma), followed by amphibolite-facies metamorphism (550–480 °C and 5.5–4.5 kbar) in the middle Palaeoproterozoic (c. 1900–1850 Ma). The results obtained in this study, together with previous studies, indicate that: (1) the high-pressure granulite-facies metamorphism in the study area indicates that subduction occurred during the Archaean–Proterozoic boundary with a higher apparent average geothermal gradient (∼20–16 °C km−1) than the modern-day Earth, and (2) the apparent average geothermal gradient of the subduction zone was ∼29–14 °C km−1 during the Archaean–Proterozoic boundary, which was still too high to enter the realm of eclogite-facies metamorphism.

Identification of UHT Granulites in the Pan-African Dahomeyide Suture Zone in SE Ghana: Implications for Evolution of Collisional Orogens

AbstractThis study presents the petrology, geochemistry, U–Pb ages, Lu–Hf and oxygen isotope compositions of Adaklu mafic granulites (ADMGs), from the Pan-African Dahomeyide suture zone in southeastern Ghana. The ADMGs show mafic precursor with low-K tholeiitic affinity. They display convex rare earth and trace elements characteristics without any obvious anomalies of Eu, Ti, Nb and Ta. The geochemical characteristics of ADMGs mimic those of N-MORB. Zircon U–Pb dating on the ADMGs reveals granulite facies metamorphic ages of ca. 595–602 Ma. However, a few zircons yield relatively older apparent 206Pb/238U ages of ca. 620 Ma, representing prograde metamorphic age. ADMGs preserve mean zircon εHf (t) values of +7.0 to +9.7 and δ18O values of 6.1–8.0‰. Based on petrographic observations, geothermobarometric calculations using conventional thermobarometry, mineral equilibria modeling, Ti-in-zircon and Zr-in-rutile thermometers reveal peak granulite facies P–T conditions of 0.95–1.2 GPa/940–1000°C, and retrograde amphibolite facies conditions of 0.83–0.93 GPa/575–710°C. The prograde metamorphic stage is inferred to be amphibolite or eclogite facies metamorphism. Hence, a clockwise a P–T–t path is proposed for the ADMGs. The overall results indicate that ADMGs are ultra-high temperature (UHT) granulites, and the protolith is altered oceanic crust consumed during the Pan-African collisional events. Asthenospheric upwelling induced by lithospheric delamination in the earliest extension of the thickened orogen, or by slab break-off in the background of collision may have provided the additional heat for UHT granulite facies metamorphism.

Formation of an Intracontinental Orogen Above the Permo-Triassic Mantle Convection Cell in the Paleo-Tethys Tectonic Realm due to Far-Field Stress Derived From Continental Margins

The identification of intraplate orogens seemingly poses challenges to the plate tectonic theory. Delineating the formation processes of intraplate orogens can provide clues for the better understandings of the above issue. Although still controversial, the Indosinian (Permo-Triassic) orogeny in the South China Block (SCB) is potentially a good example of intracontinental orogen. In this paper, we carry out studies on the Indosinian high-grade rocks in the northeastern Cathaysia Block of the SCB, hoping to cast light on the features and formation processes of intraplate orogenic belts. These rocks exhibit HP/HT granulite facies mineral assemblages and reaction textures imply that they witnessed eclogite-facies metamorphism. Their clockwise P-T trajectories with isothermal decompression stages suggest significant crustal thickening followed by quick orogenic collapse. Immobile whole-rock trace elements indicate basaltic protoliths features, resembling E-MORB and OIB, respectively. SIMS zircon U-Pb age dating confirms Indosinian metamorphic ages of ∼248 Ma and a protolith age of ∼953 Ma. The mantle-like O isotopic compositions of the Neoproterozoic magmatic zircon cores further attest that they were primarily mantle derived rocks. The whole-rock Sm-Nd isotopic compositions show more enriched features because of metamorphic alteration, while zircon Lu-Hf isotopic results show primitive characteristics with Neoproterozoic model ages. These features suggest that the high-grade mafic rocks, as well as the metamorphosed early Precambrian metasedimentary rocks hosting them, are all continental crust components and juvenile oceanic crust components featuring plate margins are absent during the SCB Indosinian orogeny. Characteristics of these high-grade rocks and their spatial occurrences are both consistent with the proposal of an intracontinental orogen. After summarizations and comparisons of the Indosinian plate margin activities around the SCB, we suggest that this northeast-southwest trending orogenic belt is geometrically consistent with two mantle convection cells, with one conveying the SCB northward to collide with the North China Craton, and the other conveying the Paleo-Pacific plate northwestwards to form an active continental margin along the southeast SCB. The driving mechanism of the formation of the SCB Indosinian intracontinental orogenic belt could have broad implications for other intraplate orogens around the world.

Middle Triassic remnant of the Palaeo‐Tethys Ocean, central Tibet: Constraints from the Pianshishan retrograded eclogite‐type rocks

The high-pressure and low-temperature metamorphic rocks constitute the important parts of the Longmu Co-Shuanghu suture zone (LSS) in Tibet, providing a natural window for unveiling the tectonic evolution of the Palaeo-Tethys Ocean. In this contribution, geochemical studies, including the whole-rock geochemistry, Sr-Nd isotopes and zircon U–Pb geochronology, were performed on the Pianshishan garnet amphibolites and retrograded eclogites, to reconstruct the genesis of the protoliths and to reveal tectonic evolution of the Longmu Co-Shuanghu Palaeo-Tethys Ocean (LSPTO). The garnet amphibolites show characteristics similar to the retrograded eclogites, representing different degrees of retrograde metamorphism of the Pianshishan eclogites. The protoliths of garnet amphibolites and retrograded eclogites are alkaline basalts, with geochemical characteristics similar to ocean island basalts (OIB) without Nb, Ta or Ti negative anomalies. Furthermore, geochemical data imply that the protoliths were formed by fractional crystallization of the magma derived from partial melting of garnet lherzolites without crustal contamination. The parental magma was probably generated in a relatively depleted but heterogeneous asthenosphere mantle source. Their protoliths erupted as a seamount within the LSPTO. Garnet amphibolites and retrograded eclogites yielded zircon U–Pb weighted mean ages of 240.2 ± 1.7 Ma and 239.1 ± 2.7 Ma, respectively. Combining with previous studies, we suggest that the LSPTO remained open in the Pianshishan area during the Middle Triassic, and the whole ocean might be gradually closed during the Early to Late Triassic. The small remnant ocean basin during the Middle Triassic can well explain a rapid evolution of the Pianshishan eclogite-type rocks, that is, from generation to eclogite-facies metamorphism and then to exhumation-related retrograde metamorphism within ~19 Myr.

Contrasting mechanisms and timescales of subduction and exhumation as recorded by Paleoproterozoic and late Paleozoic high-pressure granulites in the North China Craton

Abstract The North China Craton records multiple metamorphic events related to supercontinent assembly during the Paleoproterozoic, forming Columbia, and again during the late Paleozoic, forming Pangea. Here we show that the Paleoproterozoic high-pressure granulites (HPGs) formed from enriched mid-ocean ridge basalt protoliths and record a clockwise pressure-temperature-time (P–T–t) path with prograde metamorphism at 7.8–10.0 kbar and 780–820 °C, peak granulite-facies metamorphism at 12–12.3 kbar and ∼860–880 °C, and retrograde metamorphism at 8.7–9.1 kbar and 850–855 °C. Subduction initiated prior to 1.90 Ga, with final collision and orogeny at 1.88 Ga, followed by post-collision/exhumation at 1.80 Ga, defining a prolonged exhumation period (∼90 m.y.) that occurred at a slow velocity of ∼0.16 ± 0.08 mm/y. Late Paleozoic HPGs are normal mid-ocean ridge basalt type and record a near clockwise P–T–t path, with peak/post-peak amphibolitefacies metamorphism at 11.0–12.5 kbar and 860–890 °C, isothermal decompression to 7.2–7.5 kbar and 810–820 °C, and retrogression to 5.5–7.2 kbar and 805–850 °C. Subduction initiated earlier than ca. 340 Ma, exhumation and uplift initiated at 335–309 Ma and continued to 297–287 Ma. The exhumation was short-lived (∼50 m.y.) and relatively fast (0.38 ± 0.14 mm/y). When compared to granulite-facies metamorphism documented in many Paleoproterozoic HPGs, late Paleozoic HPGs appear to commonly form with an initial period of steep subduction leading to eclogite-facies metamorphism, with subsequent exhumation to middle/lower levels of the crust. Our results further reveal that the exhumation velocity for supercontinent collision was facilitated and duration shortened through time, and that the exhumation mechanism might have been controlled by subduction angle, compression pressure, and temperature.

Archean eclogite-facies oceanic crust indicates modern-style plate tectonics

SignificanceThe onset time of plate tectonics is highly debated in the Earth sciences. A key indicator of modern-style plate tectonics, with deep subduction of oceanic plates, is the presence of eclogite (oceanic crust metamorphosed at high-pressure and low-temperature) in orogenic belts. Since no orogenic eclogites older than 2.1 billion y are currently documented, many scientists argue that modern plate tectonics started only 2.1 billion y ago (Ga). We document an Archean orogenic eclogite, providing robust evidence that subduction of oceanic crust reached to at least 65 to 70 km in depth at circa 2.5 Ga. This extends the known age of subduction-related eclogite-facies metamorphism back 400 My, showing that modern-style plate tectonics operated by the close of the Archean.

Reconstruction of the mid-Devonian HP-HT metamorphic event in the Bohemian Massif (European Variscan belt)

Mid-Devonian high-pressure (HP) and high-temperature (HT) metamorphism represents an enigmatic early phase in the evolution of the Variscan Orogeny. Within the Bohemian Massif this metamorphism is recorded mostly in allochthonous complexes with uncertain relationship to the major tectonic units. In this regard, the Mariánské Lázně Complex (MLC) is unique in its position at the base of its original upper plate (Teplá-Barrandian Zone). The MLC is composed of diverse, but predominantly mafic, magmatic-metamorphic rocks with late Ediacaran to mid-Devonian protolith ages. Mid-Devonian HP eclogite-facies metamorphism was swiftly followed by a HT granulite-facies overprint contemporaneous with the emplacement of magmatic rocks with apparent supra-subduction affinity. New Hf in zircon isotopic measurements combined with a review of whole-rock isotopic and geochemical data reveals that the magmatic protoliths of the MLC, as well as in the upper plate Teplá-Barrandian Zone, developed above a relatively unaltered Neoproterozoic lithospheric mantle. They remained coupled with this lithospheric mantle throughout a geological timeframe that encompasses separate Ediacaran and Cambrian age arc magmatism, protracted early Paleozoic rifting, and the earliest phases of the Variscan Orogeny. These results are presented in the context of reconstructing the original architecture of the Variscan terranes up to and including the mid-Devonian HP-HT event.

Tectonothermal transition from continental collision to post‐collision: Insights from eclogites overprinted in the ultrahigh‐temperature granulite facies (Yadong region, central Himalaya)

AbstractEclogites and granulites are fossil records of orogenies and provide crucial evidence of geodynamic processes. Incomplete knowledge on both hampers the establishment of detailed tectonothermal models for the central Himalaya. Utilizing detailed petrography, pseudosection modelling, mineral thermometers, and zircon petrochronology, the granulitized eclogites discovered firstly at Yadong region at the southern tip of the Yadong–Gulu Rift record five metamorphic stages. The epidote–amphibole eclogite facies metamorphism (M1) is preserved in the garnet core including the relics of omphacite. The peak eclogite facies metamorphism (M2) is represented by the garnet rim and melt‐related polymineralic inclusions. From M1 to M2, the eclogites experienced a prograde path from 1.7 GPa/620°C to 2.1 GPa/750–770°C. The clockwise pressure–temperature path, the occurrence as coherent layers in gneiss, and zircon U–Pb dating imply that the Indian crust subducted to a maximum depth of ~60 km in the central Himalaya, close to the Moho of southern Lhasa block at c. 17 Ma. Afterwards, the exhumation started with decompressional heating. In the high‐pressure granulite facies metamorphism (M3), almost all the omphacite broke down to the symplectites of diopside and plagioclase. The subsequent two‐pyroxene granulite facies metamorphism (M4) is characterized by the intergrowth of clinopyroxenes and orthopyroxenes, high‐Ti amphibole, and antiperthite. The exhumed eclogites eventually reached ultrahigh‐temperature conditions of ~0.8 GPa/950–1,000°C in M4 and were near completely transformed into mafic granulites. At the final stage, these granulitized eclogites cooled in the middle crust to amphibolite facies (M5) of 0.6 GPa/700–750°C. Given the spatio‐temporal consistencies between the granulitized eclogites, post‐collisional magmatism, and north‐trending rifts, the heat source of the ultrahigh‐temperature metamorphism was attributed to the upwelling of asthenospheric mantle due to slab tear of subducted Indian lithosphere. The granulitized eclogites from Yadong evolved from the high‐pressure eclogite to the ultrahigh‐temperature granulite facies, which recorded the transitions from collisional convergence to post‐collisional extension during their exhumation.

Comment on Volodichev et al. Archean Zircons with Omphacite Inclusions from Eclogites of the Belomorian Province, Fennoscandian Shield: The First Finding. Minerals 2021, 11, 1029

Volodichev et al. (Volodichev et al., 2021) reported on the first finding of omphacite (23%–25% Jd) inclusions in 2.68 Ga metamorphic zircons from Gridino eclogites and presented it as evidence for Archean eclogite-facies metamorphism in the Belomorian Mobile Belt. We believe that the Archean age of the garnets referred to by the above authors was estimated incorrectly. Our interpretation is that omphacite origin is related to Archean high-pressure granulite-facies metamorphism.

Reply to Skublov et al. Comment on “Volodichev et al. Archean Zircons with Omphacite Inclusions from Eclogites of the Belomorian Province, Fennoscandian Shield: The First Finding. Minerals 2021, 11, 1029”

Early Precambrian retrogressed eclogites are abundant in the Archean Belomorian Province of the Fennoscandian Shield. Archean zircons with inclusions of omphacite have been found in these eclogites. Similar Archean zircons from the retrogressed eclogites also contain garnet inclusions. The Archean zircons display no negative Eu anomaly, which indicates their crystallization in plagioclase-free rock. Garnet, omphacite and clinopyroxene-plagioclase symplectite as a proxy of omphacite compose ≥75% of the studied rocks, with garnet and omphacite being major constituents and associating with rutile and quartz. These data strongly suggest that the studied rock is eclogite. In the majority of petrogenetic grids, P-T parameters calculated for these rocks fall in the eclogite-facies field. Thus, these findings and data provide evidence that eclogite-facies metamorphism occurred in the Neoarchean.

Decoupling of U–Pb ages and compositional zoning of garnet in a high–pressure marble from the eastern Iratsu body, Sanbagawa metamorphic terrane, Japan

Here we first report the in situ U–Pb dating of metamorphic grossular garnet (Grs) with distinction between internal zonation textures. The studied Grs occurs in an eclogite–facies marble collected from the eastern Iratsu body of the Sanbagawa metamorphic terrane, Japan. The Grs has a patchy texture, predominantly with pure Grs cores and andradite (Adr)–rich rims formed during eclogite–facies and exhumation stages, respectively. The U–Pb ages for the Grs core and Adr–rich rim were 97 ± 10 and 106 ± 16 Ma (95% confidence level), respectively. Despite the compositional zoning formed under different P–T conditions, the U–Pb ages of the core and rim were in similar values within analytical uncertainties. This decoupling of chemical zonation and U–Pb ages implies that the U–Pb chronological signatures of rims were inherited from cores owing to the redistribution of radiogenic Pb in cores during the rim formation through fluid–mediated dissolution and reprecipitation. The Grs U–Pb age (97 ± 10 Ma) thus directly corresponds to previously reported P–T conditions of the core formation during the eclogite–facies metamorphism. This advantage of Grt petrochronology as the combination of radiometric ages obtained by in situ analysis and P–T conditions deduced from paragenesis can contribute to reconstruct reliable metamorphic histories.