Eclogite Facies Metamorphism Research Articles

Garnet and zircon geochronology of the Paleoproterozoic Kuru-Vaara eclogites, northern Belomorian Province, Fennoscandian Shield

The Belomorian Province of the Fennoscandian Shield exposes numerous Precambrian eclogites, which makes it significant for the study of early tectonic processes. The age of these eclogites has been discussed for more than 15 years and regarded as either Archean or Paleoproterozoic. In the Kuru-Vaara quarry within the northern Belomorian Province, the eclogitic assemblage is preserved in concordant mafic boudins in felsic gneisses and a partially eclogitized gabbro-norite dike cutting discordantly through the gneiss fabric. Both eclogite types preserve zircon cores with a magmatic geochemical signature that yield protolith ages of ca. 2.88 Ga for a mafic boudin and ca. 2.44 Ga for the eclogitized gabbro-norite. Ca. 1.9 Ga zircon rims and grains from the eclogites show low Th/U ratio and HREE depletion, reflecting the growth of metamorphic zircon in equilibrium with garnet. In the eclogite boudin, the Archean zircon cores yield δ18O = 5.1–5.9‰ typical of mantle melts; the oxygen isotope composition of garnet (δ18O = 4.0–5.0‰) is in equilibrium with that of the 1.9 Ga zircon (δ18O = 4.5–5.4‰). Garnet Lu-Hf geochronology coupled with U-Pb zircon geochronology constrains prograde metamorphism for the Kuru-Vaara eclogites at 1.92–1.89 Ga. Mineral inclusions of garnet, zoisite, plagioclase, kyanite, amphibole, quartz, and low-Na clinopyroxene in ca. 1.9 Ga zircon from the eclogite boudin imply epidote–amphibolite/amphibolite facies conditions for the prograde metamorphism. All data point to a Paleoproterozoic age of the eclogite facies metamorphism.

The subduction and exhumation history of the Voltri Ophiolite, Italy: Evaluating exhumation mechanisms for high-pressure metamorphic massifs

The problem of how dense high- (HP) and ultrahigh-pressure (UHP) metamorphosed oceanic mafic and ultramafic rocks are exhumed from deep within subduction zones is crucial to understanding processes occurring at the interface between the subducting slab and the overlying plate and mantle wedge. In this study, we use the P-T-t evolution of metamorphosed FeTi gabbros hosted in meta-serpentinite from the Voltri Ophiolite (Ligurian Alps, Italy) to evaluate potential exhumation mechanisms and assess whether HP mafic rocks were exhumed as blocks within a complex serpentinite-hosted mélange or as coherent sections of oceanic lithosphere. A number of observations provide evidence for subduction and exhumation of the central portion of Voltri Ophiolite as a coherent package of ultramafic and mafic material: (1) a similar evolution in mineral assemblages, textures and compositions suggesting peak metamorphism in the eclogite-facies, followed by a retrograde evolution through blueschist-, amphibolite- and greenschist-facies conditions; (2) uniform P-T conditions of both initial garnet growth (~475–480 °C and 2.0–2.2 GPa) and peak metamorphism (~500–525 °C and 2.3–2.5 GPa); and (3) bulk SmNd garnet ages that reveal a very tight grouping of ages for four well-equilibrated foliated FeTi meta-gabbros clustering at 39.98 ± 0.84 Ma. A massive undeformed gabbro also from the central Voltri area gave a slightly younger age of 38.15 ± 0.89 Ma, which is interpreted to reflect kinetic overstepping and delayed growth of garnet. By contrast, an SmNd garnet age from the NW portion of the Voltri Ophiolite indicates peak eclogite-facies metamorphism occurring ~10–12 Ma earlier at 49.68 ± 0.35 Ma. A compilation of the current geochronological data suggests that the NW portion of the Voltri Ophiolite underwent peak eclogite-facies metamorphism first at ~50 Ma, the sedimentary portion of the Voltri Unit (the Voltri-Rossiglione meta-sediments) experienced peak metamorphism around ~47–44 Ma, whilst subduction and peak metamorphism of the central part of the ophiolite occurred later at 41–38 Ma. Alongside recent studies of other Alpine ophiolites (Zermatt Saas, Monviso, Alpine Corsica), this study suggests that detachment and buoyant exhumation of large kilometric-scale coherent sections of slab material, rather than exhumation within a chaotic serpentinite mélange, is the dominant mechanism of preserving and returning eclogite-facies metamorphic rocks to the surface in the Western Alps. In the case of the Voltri Ophiolite, which lacks any associated continental massif, this exhumation was likely aided by the buoyancy of the serpentinite-dominated oceanic lithosphere, rather than by coupling to continental crust.

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.

Constraints on retrograde metamorphism of UHP eclogites in North Qinling, Central China, from 40Ar/39Ar dating of amphibole and phengite

Coesite- and microdiamond- bearing ultra-high pressure (UHP) eclogites in the North Qinling terrane have been widely retrogressed to amphibolites. Previous geochronological studies on these UHP rocks mainly focused on the timing of peak eclogite facies metamorphism. The Kanfenggou UHP metamorphic domain is one of the best-preserved coesite-bearing eclogite occurrences in the North Qinling terrane. In this study, mafic amphibolites and host schists from this domain were collected for 40Ar/39Ar dating to constrain their retrograde evolution. Two generations of amphibole are recognized based on their mineral parageneses and 40Ar/39Ar ages. A first generation of amphibole from garnet amphibolites yielded irregularly-shaped age spectra with anomalously old apparent ages. Isochron ages of 484–473 Ma and initial 40Ar/36Ar ratios of 3695–774 are obtained from this generation of amphibole, indicating incorporation of excess argon. Second generation amphibole occurs in epidote amphibolites yielded flat age spectra with plateau ages of 464–462 Ma without evidence for excess argon. These ages suggest that the amphibolite-facies metamorphism has taken place as early as 484 Ma and lasted until 462 Ma for the North Qinling UHP metamorphic rocks. Phengite from the country-rock schists yielded 40Ar/39Ar plateau ages of 426–396 Ma, with higher phengite Si contents associated with the older the plateau ages. Based on our new 40Ar/39Ar ages and previous zircon UPb geochronological data, we construct a new detailed pressure-temperature-time (P-T-t) path illustrating the retrograde metamorphism and exhumation rate of the North Qinling eclogites and host schists. The P-T-t path suggests that these UHP metamorphic rocks experienced initial medium-to-high exhumation rates (ca. 8.7 mm/yr) during the Early Ordovician (489–484 Ma), which was mainly derived from buoyancy forces. Subsequently, the exhumation rate decreased gradually from ~0.8 to 0.3 mm/yr from 484 to 426 Ma, which was probably governed by extension and/or erosion.

Muztaghata Dome Miocene Eclogite Facies Metamorphism: A Record of Lower Crustal Evolution of the NE Pamir

AbstractThe Pamir gneiss domes represent the most extensive exposure of mid to lower crustal rocks in the Himalayan‐Tibetan orogen north of the India‐Asia suture zone. Unlike other domes in the Central and Southern Pamir, the Muztaghata dome stands out due to its higher metamorphic grade, more complex structural elements, and variable timing of metamorphism. In order to unravel the P‐T‐t history of the Muztaghata dome and better constrain the timing of peak metamorphism, we applied petrologic modeling in concert with geochronology to samples from the structure. The Muztaghata gneiss dome is composed of a structurally higher metapelite‐dominated terrane in the west and a structurally lower orthogneiss terrane in the east. Our results from the western terrane indicate high‐pressure eclogite facies peak conditions of ~800°C/22 kbar at ~25–20 Ma. Zircon grains from metapelitic samples from the western terrane also yield Early Jurassic metamorphic U‐Pb ages with REE signals that indicate coeval garnet growth. Our results from the eastern terrane record high‐pressure amphibolite facies peak conditions of ~650°C/14 kbar at ~24–20 Ma, noticeably lower than the structurally higher western terrane indicating structural juxtaposition during Miocene exhumation. Peak metamorphic conditions from the eastern terrane indicate depths below the current Moho, supporting the interpretation that the Early Miocene Pamir crust was thicker than present. This was followed by rapid exhumation from depths of ~75–80 km and partial westward collapse of the Pamir after 20 Ma, possibly driven in part by regional lithospheric delamination.

Cambrian eclogite-facies metamorphism in the central Transantarctic Mountains, East Antarctica: Extending the record of early Palaeozoic high-pressure metamorphism along the eastern Gondwanan margin

The Nimrod Complex in the Miller and Geologists ranges of the central Transantarctic Mountains comprises Mesoarchean and Palaeoproterozoic crust that was incorporated into the Ross Orogen during metamorphism and deformation along the eastern Gondwanan convergent margin in the late Neoproterozoic and early Palaeozoic. In the Miller Range, boudinaged mafic domains are hosted within upper-amphibolite- to granulite-facies mylonitic rocks that were deformed during the Ross Orogeny. The mafic rocks preserve evidence of a precursor eclogite-facies mineral assemblage containing garnet, omphacite, amphibole, quartz and rutile. Mineral equilibria forward modelling and Zr-in-rutile thermometry indicate that the eclogite assemblage records peak pressure-temperature conditions of 16–18 kbar and 675–760 °C. A secondary metamorphic assemblage of clinopyroxene-plagioclase symplectite, plagioclase coronae rimming garnet, fine-grained amphibole, and interstitial ilmenite orthopyroxene overprint the high-pressure assemblage. Compositions and modal proportions of retrograde garnet, orthopyroxene and amphibole suggest near isothermal exhumation to conditions of 8–10 kbar and 660–840 °C, followed by a long-lived mid-crustal residence. In-situ LA–ICP–MS UPb dating of zircon and rutile yields concordant ages of 536 ± 14 Ma and 536 ± 20 Ma, respectively. The Cambrian-aged zircons show an absent to weakly negative Eu anomaly signature and flat heavy rare earth element (HREE) distributions that are typical of zircons crystallised in the absence of plagioclase and presence of garnet, and which is diagnostic of eclogite-facies conditions. The Cambrian-aged zircon population contrasts with older, highly discordant Palaeoproterozoic zircons that have negative Eu anomalies and are HREE enriched. These zircons are interpreted to be protolithic, and their ages are consistent with existing geochronology from host rocks in the region. The Cambrian-aged zircon and rutile are inferred to represent eclogite-facies metamorphism associated with Ross orogenesis. The early Palaeozoic eastern Gondwanan margin contains a diverse array of eclogite precursors that document contrasting geodynamic styles. Eclogites in northern Victoria Land and central Tasmania record comparatively warm thermal gradients, short-lived high-pressure conditions, and rapid exhumation within their continental crust hosts, whereas oceanic eclogites in the southern New England Fold Belt record refrigerated, long-lived convergent-margin metamorphism. The Miller Range eclogite-facies rocks in this study appear to have been derived from shallow subduction of ancient continental crust, after which they stalled at thermally elevated mid-crustal depths prior to their final exhumation. The discovery of Cambrian-aged eclogite-facies rocks in the Miller Range expands the spatial footprint of high-pressure metamorphism during the Ross Orogeny along the early Palaeozoic eastern Gondwanan margin.

Pre-Alpine high-pressure metamorphism in the Gemer unit: mineral textures and their geodynamic implications for Variscan Orogeny in the Western Carpathians

We report mineral textures of retrogressed eclogites from the gneiss–amphibolite complex, which is represented by a series of isolated bodies that were thrusted over the greenschist-facies Rakovec group in the Gemer unit (Western Carpathians). Peak pressure and temperature conditions of about 2.0 GPa and 650 °C are estimated for the eclogite-facies assemblage (omphacite, garnet and rutile). The eclogites show strong amphibolite-facies re-equilibration that is characteristic for the rest of gneiss–amphibolite complex rocks. Pressure and temperature conditions for both eclogite and subsequent amphibolite-facies stages are comparable with those reported from the retrogressed eclogites in the Vepor and Tatra units. The underlying Rakovec group rocks indicate greenschist-facies conditions, but relicts of high-pressure (blueschist facies) metamorphism are also preserved. The Pre-Alpine age of the eclogite-facies metamorphism in the gneiss–amphibolite complex and blueschist-facies metamorphism in the Rakovec group is confirmed by the presence of their pebbles in the overlaying upper Carboniferous conglomerates. The origin of eclogites and blueschists in the Gemer unit and their relationship to eclogites in the Vepor and Tatra units are discussed with respect to the subduction zone along which they originated, but also in relation to the high-pressure rocks occurring along the European Variscan Belt. As the whole Gemer unit underwent Alpine metamorphism, the retrogressed eclogites and associated gneisses were also studied to clarify possible degree of this metamorphic overprint in the gneiss–amphibolite complex rocks.

High-pressure Cu–Fe–S Phase Equilibria: some Experimental and Thermodynamic Constraints on Sulfides in Subduction Zones and the Lithospheric Mantle

Abstract High-pressure phase relations for much of the Cu–Fe–S system have not previously been determined experimentally. Experimental studies have concentrated on low-pressure phase relations and cannot explain high-pressure sulfide mineral inclusion assemblages in some natural blueschists and eclogites. In particular, the coexistence of pyrite + covellite at 1·0 GPa, and pyrite + bornite at 1·9 GPa, observed in New Caledonian rocks, is precluded by tie-lines between S and bornite, and S and the intermediate solid solution (iss), in the published low-pressure experimental topologies at corresponding temperatures. In addition, the Cu content (up to ∼10 at%) of pyrrhotite in eclogite exceeds the experimentally determined maximum for Cu in solid solution with pyrrhotite at low pressures and at corresponding temperatures. We have performed six experiments in which natural chalcopyrite starting material was equilibrated at conditions ranging from 1·0 to 1·7 GPa and 500 to 650 °C. At 1 GPa chalcopyrite is replaced by iss. The iss phase undergoes a terminal breakdown reaction between 1·0 and 1·7 GPa, being replaced by a new assemblage of bornite, pyrite, and pyrrhotite. Our experimental results confirm predictions from the SUPCRT thermodynamic database (Johnson et al., 1992; Computers & Geosciences 18, 899–947) but not that of Robie & Hemingway (1995; US Geological Survey Bulletin 2131). The former database is therefore recommended for calculation of high-pressure sulfide phase relations. Chalcopyrite and its high-temperature, low-fS2 equivalent, iss are not stable at pressures corresponding to much of blueschist–eclogite-facies metamorphism. These results are also applicable to sulfide assemblages in the lithospheric mantle along both oceanic and continental geotherms; the subsolidus Cu-rich mineral in the lithosphere at depths of 30 to >65 km must be bornite–digenite solid solution (bn-ss) rather than iss as is commonly assumed.

The Belomorian eclogite province (eastern Fennoscandian Shield, Russia): Meso-Neoarchean or Late Paleoproterozoic?

A critical discussion of competing models of the geodynamic nature (oceanic or continental subduction) and age (Meso-Neoarchean or Late Paleoproterozoic) of the eclogite facies metamorphism in the Belomorian eclogite province (BEP) is based on the systematic analysis of the sum of previously known and newly obtained data characterizing the geological structure of the Salma eclogite association and features of zircons from eclogites, including the isotopegeochronological and geochemical characteristics, composition and distribution of mineral inclusions. Regular changes in the REE trends and crystallization-recrystallization temperature of porous zircons in eclogite-metagabbro illustrate the sequence of magmatic and metamorphic events in the Meso-Neoarchean and Paleoproterozoic. The susceptibility to recrystallization of zircons is due to partial metamictness and porous structure. The earliest (~2.9 Ga) zircon zones retain mag-matic-type REE trends. The microinclusions of the prenite-pumpelliite and greenschist facies minerals and the increase in the LREE and MREE concentrations indicate hydrothermal metamorphism in the spreading ridge and on the ocean floor at 2.9–2.82 Ga. Prenite, pumpelliite, albite, actinolite, chlorite, diaspore and saponite also form inclusions in the eclogitic garnet. An increase of LREE and MREE, the disappearance of the Ce positive anomaly, a change from negative to positive Eu anomaly at 2.82–2.78 Ga indicate that plagioclase was removed during the formation of the ‘garnet + omphacite’ eclogite association and the replacement of sphene with rutile. The eclogite facies metamorphism linked with subduction of the oceanic crust is also indicated by the microinclusions of garnet and rutile in zircon. The crystallization temperature in 700–900 °C range of the round-oval zircons from eclogites-metagabbronorites records the Neoarchean granulite facies metamorphism at 2.77–2.70 Ga, the negative Eu anomalies in the cores and rims of zircons indicate the participation of plagioclase in the metamorphic crystallization. Late (2.1–1.7 Ga) rims of porous zircons that occurred at 600–680 °C are distinguished by minimal REE concentrations, a change from a positive Eu anomaly to a negative one, and the appearance of a negative Ce anomaly, which indicates the presence of plagioclase, reducing type of fluids and, accordingly, low water activity that is characteristic of high-temperature metamorphism under stretching condition and mantle-plume activity. The deep Sm-Nd system reworking in the Belomorian tectonic province, including BEP, at ~1.9 Ga was caused by the crustal heating that spread from the Lapland granulite belt border in the west-south-westward direction. The Lu-Hf system in zircon reworking with a significant increase in radiogenic Hf indicates the recrystallization of a long-existing garnet, in which a significant amount of radiogenic 176Hf accumulated by 1.9 Ga as a result of the 176Lu decay. This contradicts the earlier suggestion of the eclogite garnet primary crystallization in the late Paleoproterozoic (1.94–1.89 Ga).

P–T–tevolution of the Cycladic Blueschist Unit in Western Anatolia/Turkey: Geodynamic implications for the Aegean region

AbstractEclogite and blueschist facies rocks occurring as a tectonic unit between the underlying Menderes Massif (MM) and the overlying Afyon Zone/Lycian Nappes and the Bornova Flysch Zone in western Anatolia represent the eastward continuation of the Cycladic Blueschist Unit (CBU) in Turkey. This high‐Punit is attributed to the closure of the Pindos Ocean and consists of (a) a Triassic to Upper Cretaceous coherent series derived from passive continental margin sediments and (b) the tectonically overlying Upper Cretaceous Selçuk mélange with eclogite blocks embedded in a pelitic epidote‐blueschist matrix. The coherent series has experienced epidote‐blueschist facies metamorphism (490 ± 25°C/11.5 ± 1.5 kbar; 38 km depth).40Ar/39Ar white mica and206Pb/238U monazite dating of quartz metaconglomerate from coherent series yielded middle Eocene ages of 44 ± 0.3 and 40.1 ± 3.1 Ma for epidote‐blueschist facies metamorphism, respectively. The epidote‐blueschist facies metamorphism of the matrix of the Selçuk mélange culminates at 520 ± 15°C/13 ± 1.5 kbar, 43 km depth, and is dated at 57.5 ± 0.3–54.5 ± 0.1 Ma (40Ar/39Ar phengite). Eclogite facies metamorphism of the blocks (570 ± 30°C/18 ± 2 kbar, 60 km depth) is early Eocene and dated at 56.2 ± 1.5 Ma by206Pb/238U zircon. Eclogites experienced a nearly isothermal retrogression (490 ± 40°C/~6 to 7 kbar) during their incorporation into the Selçuk mélange. The retrograde overprints of the coherent series (410 ± 15°C/7 ± 1.5 kbar from Dilek Peninsula and 485 ± 33°C/~6 to 7 kbar from Selçuk–Tire area) and the Selçuk mélange (510 ± 15°C/6 ± 1 kbar) are dated at 35.8 ± 0.5–34.3 ± 0.1 Ma by40Ar/39Ar white mica and 31.6 ± 6.6 Ma by206Pb/238U allanite dating methods, respectively. Regional geological constrains reveal that the contact between the MM and the CBU originally formed a lithosphere‐scale transform fault zone.40Ar/39Ar white mica age from the contact indicates that the CBU and the MM were tectonically juxtaposed under greenschist facies conditions during late Eocene, 35.1 ± 0.3 Ma.

Paleoproterozoic oceanic subduction in the North China Craton: Insights from the metamorphic P–T–t paths of the Chicheng Mélange in the Hongqiyingzi Complex

How and when modern-style plate tectonics started in the Earth’s history is one of the most important scientific issues. The Chicheng mélange in the north margin of North China Craton (NCC) presents a new insight into the Paleoproterozoic cold oceanic subduction and the beginning of modern-style plate tectonic regime. The mélange comprises blocks of retrograde eclogites, amphibolites and peridotites in a matrix of metasediments. The retrograde eclogites mainly contain garnet, amphibole and symplectites consisting of fine-grained plagioclase, clinopyroxene and amphibole. Three metamorphic stages were derived, involving the prograde, peak, and post-peak decompression stages. The P–T conditions for the prograde and peak stages range from ~17 kbar/660 °C to >22/~750 °C kbar based on mineral composition isopleths in P–T pseudosections, involving core–to–mantle increasing pyrope and decreasing grossular in garnet and Ti content in amphibole. The post-peak decompression is proposed to be isothermal under melt (fluid)-absent conditions, recorded a pressure of 8–11 kbar on the basis of symplectite assemblages. For the amphibolites pre-peak heating, peak temperature, and post-peak cooling metamorphic stages were derived. They experienced anataxis during this overprinting metamorphism when heated to above the solidus. The peak temperature conditions are constrained to be 5–7 kbar/780–830 °C on the basis of the contours of the maximum Ti in amphibole and anorthite of plagioclase in P–T pseudosections. Zircon U–Pb dating for the selected samples and a summary of published age data reveal that the eclogite facies metamorphism may have occurred at 1.85–1.80 Ga, and the eclogitic rocks exhumed to a lower crust level during or after subduction to be overprinted by multi-phase thermal events at ~1.66 Ga and 460–420 Ma with undefined conditions, and at 360–270 Ma under amphibolite facies. The eclogite facies metamorphism from the Chicheng mélange shows a geothermal gradient of ~9 °C/km, pointing to a cold oceanic subduction at late Paleoproterozoic that occurred along the northern margin of the NCC. The overprinting metamorphism with anataxis recognized from the amphibolites represents a low-pressure type with a geothermal gradient of ~35 °C/km, which was interpreted to occur at 360–270 Ma governed by extension.

The Theodul Glacier Unit, a slab of pre-Alpine rocks in the Alpine meta-ophiolite of Zermatt-Saas, Western Alps

The Theodul-Glacier-Unit (TGU) is a 100 m thick and 2 km long slab of pre-Alpine schist, gneiss and mafic rocks tectonically emplaced in the eclogite-facies Zermatt-Saas meta-ophiolite nappe (ZSU). The meta-sedimentary rocks occur mostly as garnet-phengite schists with locally cm-sized garnet porphyroblasts. The metavolcanic basic rocks are present as variably retrogressed eclogites showing a continental basalt signature and contain abundant zircon, which is unusual for basalts. The zircons dated with the U–Pb system yield an upper intercept age of 295 ± 16 Ma and a lower intercept age of 145 ± 34 Ma. The early Permian age is interpreted to represent the age of high-grade granulite facies metamorphism, evidence of which is also preserved in the cores of garnet porphyroblasts of the Grt-Ph schists. The lower intercept age corresponds to the time of continental breakup and the initiation of the Tethys in the Mid-Jurassic; these events may have created the TGU as an extensional allochton. Eclogite facies metamorphism recorded by the TGU rocks occurred during Alpine subduction at 57 Ma, the Lu–Hf age of TGU eclogite garnets. The TGU reached a depth of about 53 km at P–T conditions of 1.7 GPa and 520 °C derived from both, eclogite and Grt-Ph schist. This is in contrast to the ZSU surrounding the TGU with a reported subduction depth of more than 80 km at 43 Ma. It is proposed here that TGU and ZSU were subducted separately out of sequence. After juxtaposition of the two units during late Alpine thrusting and folding forming the present day geometry of nappes in the Zermatt-Saas region both units were progressively metamorphosed to about 650 MPa and 470 °C. This late prograde metamorphism at 34 Ma produced oligoclase + magnesio-hornblende in the matrix of Grt-Ph schists and eclogites. The derived TGU data document a complete Wilson Cycle.

Granulites record the tectonic evolution from collisional thickening to extensional thinning of the Tongbai orogen in central China

AbstractA combined study of petrology and geochemistry was carried out for granulites from the Tongbai orogen in central China. The results reveal the tectonic evolution from collisional thickening to extensional thinning of the lithosphere at the convergent plate boundary. Petrographic observations, zircon U–Pb dating, and pseudosection calculations indicate that the granulites underwent four metamorphic stages, which are categorized into two cycles. The first cycle occurred at 490–450 Ma and involves high‐P (HP) metamorphism (M1) at 785–815°C and 10–14 kbar followed by decompressional heating to 840–880°C and 8–9 kbar for medium‐pressure granulite facies metamorphism (M2), defining a clockwise P–T path. The high pressure is indicated by the occurrence of inclusions of rutile+kyanite+K‐feldspar in the garnet mantle. The second cycle occurred at c. 440 Ma and shows an anticlockwise P–T path with continuous heating to ultrahigh‐temperature (UHT) metamorphism (M3) at 890–980°C and 9–11 kbar, followed by decompressional cooling to 740–880°C and 7–9 kbar (M4) till 405 Ma. The HP metamorphism is synchronous with the ultrahigh‐pressure eclogite facies metamorphism in the Qinling orogen, indicating its relevance to the continental collision in the Cambrian. The UHT metamorphism took place at reduced pressures, indicating thinning of the collision‐thickened orogenic lithosphere. Therefore, the Tongbai orogen was initially thickened by the collisional orogeny and then thinned, possibly as a result of foundering of the orogenic root. Such tectonic evolution may be common in collisional orogens where compression during continental collision switched to extension during continental rifting.

Timing of Paleozoic Exhumation and Deformation of the High-Pressure Vestgӧtabreen Complex at the Motalafjella Nunatak, Svalbard

The Vestgӧtabreen Complex exposed in the Southwestern Caledonian Basement Province of Svalbard comprises two Caledonian high-pressure units. In situ white mica 40Ar/39Ar and monazite Th-U-total Pb geochronology has resolved the timing of the tectonic evolution of the complex. Cooling of the Upper Unit during exhumation occurred at 476 ± 2 Ma, shortly after eclogite-facies metamorphism. The two units were juxtaposed at 454 ± 6 Ma. This was followed by subaerial exposure and deposition of Bullbreen Group sediments. A 430–400 Ma late Caledonian phase of thrusting associated with major sinistral shearing throughout Svalbard deformed both the complex and the overlying sediments. This phase of thrusting is prominently recorded in the Lower Unit, and is associated with a pervasive greenschist-facies metamorphic overprint of high-pressure lithologies. A c. 365–344 Ma geochronological record may represent an Ellesmerian tectonothermal overprint. Altogether, the geochronological evolution of the Vestgӧtabreen Complex, with previous petrological and structural studies, suggests that it may be a correlative to the high-pressure Tsäkkok Lens in the Scandinavian Caledonides. It is suggested that the Vestgӧtabreen Complex escaped to the periphery of the orogen along the sinistral strike-slip shear zones prior to, or during the initial stages of continental collision between Baltica and Laurentia.

Reply to “Comments to high-pressure eclogite facies metamorphism and decompression melting recorded in Paleoproterozoic accretionary wedge adjacent to probable ophiolite from Itaguara (southern São Francisco Craton - Brazil)”

Unlike the question brought by Massonne (2020), mica-quartz schist as a source of Córrego do Peixoto granite from Itaguara Sequence is clearly confirmed by geochemical new data. It is a peraluminous (S-type) syn-collisional granite generated by partial melting of the schist as a result of orogen exhumation during Paleoproterozoic continental collision, i.e during the decompression part of clockwise P-T-time path. Preserved in mica-quartz schist, garnet-chloritoid-phengite-kyanite mineral assemblage, typical of high-pressure metapsammopelites, is in equilibrium close to the metamorphic peak suggested by de Oliveira Chaves et al. (2019), according to new pseudosection of this rock. The new discovery of retroeclogite in the IS (Chaves and Porcher, in review) close to schist and granite is sufficient to oppose the comment of Massonne (2020), which sustains that mica-quartz schist from IS has never reached conditions of the eclogite-facies. The geotectonic setting proposed by de Oliveira Chaves et al. (2019) for Itaguara Sequence seems to indicate a reliable Paleoproterozoic accretionary prism in a paleosubduction zone, which can correspond to one of the earliest records of subduction process followed by continental collision in world.

Synchronous A-type and adakitic granitic magmatism at ca. 2.2 Ga in the Jiao–Liao–Ji belt, North China Craton: Implications for rifting triggered by lithospheric delamination

The Jiao–Liao–Ji belt (JLJB) underwent back-arc or intra-continental rifting during the early Palaeoproterozoic but the underlying mechanism remains unclear. To address this issue, this study focuses on the petrogenesis of widespread Palaeoproterozoic granitoids on Liaodong Peninsula, China. Zircon U–Pb dating and geochemical and Hf isotopic data suggest that these granitoids were formed at ca. 2.2 Ga and they can be divided into A-type and adakitic granites. The A-type granites have relatively high SiO2, K2O, Zr, Ga, Nb and Y contents, and low CaO, MgO, Al2O3 and Sr contents. Based on these characteristics, εHf(t) values (−0.8 to +2.8) and TDM2 ages (2797–2569 Ma), the A-type granites are inferred to be derived from Neoarchaean tonalite–trondhjemite–granodiorites in a rift setting. In contrast, the coeval adakitic granites have relatively high Na2O, Al2O3 and Sr contents and high Sr/Y and LaN/YbN ratios, but low MgO, Ni, Yb and Y contents, suggesting a magma source with garnet but no plagioclase in the residue beneath thickened lower crust. Their variable εHf(t) values (−8.5 to +6.9) and TDM2 ages (3276–2305 Ma) indicate that the adakitic magmas were mainly generated by the partial melting of Meso–Neoarchaean crust. The co-existence of contemporaneous ca. 2.2 Ga adakitic and A-type granites provides evidence that the JLJB underwent regional rifting in the Palaeoproterozoic, most likely due to delamination of a thickened lithosphere, as follows. (1) In the Neoarchaean, the lower crust of the Longgang Block underwent granulite- and/or eclogite-facies metamorphism due to continuous thickening, which resulted in increasing lower crustal density. (2) The thickened lower crust became unstable and delaminated at ca. 2.2 Ga. Asthenospheric upwelling following this lithospheric delamination triggered the thinning of the overlying lithosphere. The upper crust and remnant thickened lower crust were partially melted to form the A-type and adakitic granites, respectively. The rifting likely resulted in the separation of the Longgang and Nangrim blocks, prior to their amalgamation in the late Palaeoproterozoic to form the JLJB.

Oceanic-arc subduction, stagnation, and exhumation: zircon U–Pb geochronology and trace-element geochemistry of the Sanbagawa eclogites in central Shikoku, SW Japan

In the Iratsu and the quartz-bearing eclogite bodies of the Sanbagawa high-pressure type metamorphic belt, southwest Japan, zircon U–Pb dating and trace-element analysis of the mafic gneiss combined with its geologic structure revealed that the protolith basaltic rock constituted the topographic high on a seafloor in relation to intra-oceanic arc magmatism at ca. 195 Ma. Moreover, the metamorphic zircon U–Pb data and the rare-earth element patterns obtained from the subordinated metasedimentary rocks of the Iratsu and the quartz-bearing eclogite bodies indicate that both bodies were subducted from a trench at ca. 120 Ma and underwent the eclogite facies metamorphism between ca. 120 and 90 Ma. This study, combined with previous studies for the Sanbagawa rocks and the Jurassic-Cretaceous accretionary complexes in Japan, identifies the following constraints that led to the tectonic evolution of the Sanbagawa eclogites: 1) the metamorphic unit including the Iratsu and the quartz-bearing eclogite bodies (the Besshi unit) was subducted from a trench at ca. 120 Ma. 2) This unit was stagnated at the depth of the eclogite–facies condition between ca. 120 and 90 Ma. 3) The eclogites in the Besshi unit was exhumed with the younger metamorphic rocks which were subducted at ca. 100–90 Ma (Asemi-gawa unit). 4) The Besshi unit is a high-pressure metamorphic equivalent of the non- or weakly metamorphic Sanbosan accretionary complex and the Mikabu greenstones from a standpoint of age similarity on accretion. The probable mechanism for the stagnation of the Besshi unit at the depth of the eclogite–acies condition needs 1) the detachment of oceanic-arc material from the subducting slab, driven by the resistance against the subduction of the topographic-high part underneath the forearc, and 2) the oceanward movement of the entire arc-trench system, which might have depressed the subduction of the Besshi unit into a deeper depth than its eclogite depth.

Comments to “high-pressure eclogite facies metamorphism and decompression melting recorded in Paleoproterozoic accretionary wedge adjacent to probable ophiolite from Itaguara (southern São Francisco Craton - Brazil)”

The comments to the publication by de Oliviera Chaves et al. (2019) refer to the derivation of eclogite-facies conditions for micaschist from the Paleoproterozoic Itaguara Sequence in the southern São Francisco Craton in Brazil. These authors derived peak pressures of 18.5 kbar at 626 °C for this rock using phengite geobarometry. A scrutiny of their analytical data and the applied geothermobarometry led to the conclusion that these pressures were at most 13–14 kbar and possibly even below 10 kbar. Temperatures did not exceed about 600 °C. Thus, melting of the studied micaschist is unlikely. A subduction-related process in the Paleoproterozoic to form the Itaguara Sequence, as suggested by de Oliviera Chaves et al. (2019), must be called in question.

Chapter 15 Polyphase (1.9–1.8, 1.5–1.4 and 1.0–0.9 Ga) deformation and metamorphism of Proterozoic (1.9–1.2 Ga) continental crust, Eastern Segment, Sveconorwegian orogen

Abstract The Eastern Segment in the Sveconorwegian orogen comprises Paleoproterozoic–Mesoproterozoic magmatic suites, which formed along an active continental margin, and Mesoproterozoic suites emplaced during intracratonic extension. Zn–Pb sulphide and Fe oxide mineralizations in 1.9 Ga metavolcanic rocks form a significant mineral resource cluster in the northeastern part. Deformation and metamorphism under low-pressure (≤5 kbar) and variable-temperature conditions, including anatexis and granulite facies, prevailed during 1.9–1.8 Ga (Svecokarelian) and 1.5–1.4 Ga (Hallandian) accretionary orogenies. Sveconorwegian tectonothermal reworking initiated at c. 0.99–0.98 Ga in structurally lower levels. Crustal shortening, underthrusting with eclogite facies metamorphism (18 kbar), exhumation by eastwards thrusting (D 1 ) during continued shortening and high-pressure granulite (8–12 kbar) to upper amphibolite facies metamorphism prevailed. Anatexis and folding around east–west axial surfaces with west-northwesterly constrictional strain (D 2 ) followed at c. 0.98–0.95 Ga, being consanguineous with crustal extension. Structurally higher levels, northwards and eastwards, consist of high-pressure (10–12 kbar) orthogneisses, not affected by anatexis but also showing polyphase deformation. Sveconorwegian convergence ceased with upright folding along north–south axial surfaces and, in the uppermost frontal part, greenschist facies shearing with top-to-the-foreland normal followed by reverse displacement after 0.95 Ga. The normal shearing detached the upper compartment from the underlying gneisses.

Flow of partially molten crust controlling construction, growth and collapse of the Variscan orogenic belt: the geologic record of the French Massif Central

We present here a tectonic-geodynamic model for the generation and flow of partially molten rocks and for magmatism during the Variscan orogenic evolution from the Silurian to the late Carboniferous based on a synthesis of geological data from the French Massif Central. Eclogite facies metamorphism of mafic and ultramafic rocks records the subduction of the Gondwana hyperextended margin. Part of these eclogites are forming boudins-enclaves in felsic HP granulite facies migmatites partly retrogressed into amphibolite facies attesting for continental subduction followed by thermal relaxation and decompression. We propose that HP partial melting has triggered mechanical decoupling of the partially molten continental rocks from the subducting slab. This would have allowed buoyancy-driven exhumation and entrainment of pieces of oceanic lithosphere and subcontinental mantle. Geochronological data of the eclogite-bearing HP migmatites points to diachronous emplacement of distinct nappes from middle to late Devonian. These nappes were thrusted onto metapelites and orthogneisses affected by MP/MT greenschist to amphibolite facies metamorphism reaching partial melting attributed to the late Devonian to early Carboniferous thickening of the crust. The emplacement of laccoliths rooted into strike-slip transcurrent shear zones capped by low-angle detachments from c. 345 to c. 310 Ma is concomitant with the southward propagation of the Variscan deformation front marked by deposition of clastic sediments in foreland basins. We attribute these features to horizontal growth of the Variscan belt and formation of an orogenic plateau by gravity-driven lateral flow of the partially molten orogenic root. The diversity of the magmatic rocks points to various crustal sources with modest, but systematic mantle-derived input. In the eastern French Massif Central, the southward decrease in age of the mantle- and crustal-derived plutonic rocks from c. 345 Ma to c. 310 Ma suggests southward retreat of a northward subducting slab toward the Paleotethys free boundary. Late Carboniferous destruction of the Variscan belt is dominantly achieved by gravitational collapse accommodated by the activation of low-angle detachments and the exhumation-crystallization of the partially molten orogenic root forming crustal-scale LP migmatite domes from c. 305 Ma to c. 295 Ma, coeval with orogen-parallel flow in the external zone. Laccoliths emplaced along low-angle detachments and intrusive dykes with sharp contacts correspond to the segregation of the last melt fraction leaving behind a thick accumulation of refractory LP felsic and mafic granulites in the lower crust. This model points to the primordial role of partial melting and magmatism in the tectonic-geodynamic evolution of the Variscan orogenic belt. In particular, partial melting and magma transfer (i) triggers mechanical decoupling of subducted units from the downgoing slab and their syn-orogenic exhumation; (ii) the development of an orogenic plateau by lateral flow of the low-viscosity partially molten crust; and, (iii) the formation of metamorphic core complexes and domes that accommodate post-orogenic exhumation during gravitational collapse. All these processes contribute to differentiation and stabilisation of the orogenic crust.