Metamorphic P–T–t–d evolution of (U)HP metabasites from the South Tianshan accretionary complex (NW China) — Implications for rock deformation during exhumation in a subduction channel

The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.     

Exhumation of oceanic blueschists and eclogites in subduction zones: Timing and mechanisms

High-pressure low-temperature (HP–LT) metamorphic rocks provide invaluable constraints on the evolution of convergent zones. Based on a worldwide compilation of key information pertaining to fossil subduction zones (shape of exhumation P–T–t paths, exhumation velocities, timing of exhumation with respect to the convergence process, convergence velocities, volume of exhumed rocks,…), this contribution reappraises the burial and exhumation of oceanic blueschists and eclogites, which have received much less attention than continental ones during the last two decades.Whereas the buoyancy-driven exhumation of continental rocks proceeds at relatively fast rates at mantle depths (≥ cm/yr), oceanic exhumation velocities for HP–LT oceanic rocks, whether sedimentary or crustal, are usually on the order of the mm/yr. For the sediments, characterized by the continuity of the P–T conditions and the importance of accretionary processes, the driving exhumation mechanisms are underthrusting, detachment faulting and erosion. In contrast, blueschist and eclogite mafic bodies are systematically associated with serpentinites and/or a mechanically weak matrix and crop out in an internal position in the orogen.Oceanic crust rarely records P conditions > 2.0–2.3 GPa, which suggests the existence of maximum depths for the sampling of slab-derived oceanic crust. On the basis of natural observations and calculations of the net buoyancy of the oceanic crust, we conclude that beyond depths around 70 km there are either not enough serpentinites and/or they are not light enough to compensate the negative buoyancy of the crust.Most importantly, this survey demonstrates that short-lived (< ～ 15 My), discontinuous exhumation is the rule for the oceanic crust and associated mantle rocks: exhumation takes place either early (group 1: Franciscan, Chile), late (group 2: New Caledonia, W. Alps) or incidentally (group 3: SE Zagros, Himalayas, Andes, N. Cuba) during the subduction history. This discontinuous exhumation is likely permitted by the specific thermal regime following the onset of a young, warm subduction (group 1), by continental subduction (group 2) or by a major, geodynamic modification of convergence across the subduction zone (group 3; change of kinematics, subduction of asperities, etc).Understanding what controls this short-lived exhumation and the detachment and migration of oceanic crustal slices along the subduction channel will provide useful insights into the interplate mechanical coupling in subduction zones.     

An early Paleozoic Subduction Channel in the North Qilian Mountains,Northwestern China: Structural,Petrological and Chronological Constraints

Detailed geological mapping, structural, petrological and chronological investigation allow us to place new constraints on the tectono‐thermal evolution of the North Qilian high pressure/low temperature (HP/LT) metamorphic belt. The North Qilian HP/LT metamorphic belt manly consists of eclogite, blueschist, metasedimentary rocks and serpentinite. Most of eclogites and mafic blueschists occur as lenses within metasedimentary rocks, and minor eclogites within serpentinite. Petrological and geochemistical data indicate that the protoliths of eclogite and mafic blueschist includes E‐, N‐MORB, OIB and arc basalt. Geochronology and Lu‐Hf isotope of detrital zircons from metasedimentary rocks indicate the detritus materials are derived from Qilian block and likely deposit in continental margin or fore‐arc basin. Zircon U‐Pb datings show that the protolith ages of eclogites vary between 500 Ma and 530 Ma, and the metamorphic age of eclogite between 460 and 489 Ma. The detrital zircon ages of metasedimentary rocks distribute between 532 and 2700 Ma. The structural data show that the deformation related to the subduction during prograde is recorded in eclogite blocks. In contrast, the dominant deformation structures are characterized by tight fold, sheath fold and penetrative foliation and lineation, which are recorded in various rocks, reflecting a top‐to‐the‐south shear sense and representing the deformation related to the exhumation. The petrological data suggest that the different rocks in the North Qilian HP/LT metamorphic belt equilibrated at different peak metamorphic conditions and recorded different P‐T path. Synthesizing the structural, petrological, geochemical and geochronological data suggest a subduction channel model related to oceanic subduction during Paleozoic in the North Qilian Mountains. The different HP/LT metamorphic rocks formed in different settings with various protolith ages were carried by the subducted oceanic crust into different depth in subduction channel, and experienced independent tectono‐thermal evolution inside subduction channel. The North Qilian HP/LT mélange reflects a fossil oceanic subduction channel.     

Iranshahr blueschist: subduction of the inner Makran oceanic crust

The Makran accretionary prism in SE Iran and SW Pakistan is one of the most extensive subduction accretions on Earth. It is characterized by intense folding, thrust faulting and dislocation of the Cenozoic units that consist of sedimentary, igneous and metamorphic rocks. Rock units forming the northern Makran ophiolites are amalgamated as a mélange. Metamorphic rocks, including greenschist, amphibolite and blueschist, resulted from metamorphism of mafic rocks and serpentinites. In spite of the geodynamic significance of blueschist in this area, it has been rarely studied. Peak metamorphic phases of the northern Makran mafic blueschist in the Iranshahr area are glaucophane, phengite, quartz±omphacite+epidote. Post peak minerals are chlorite, albite and calcic amphibole. Blueschist facies metasedimentary rocks contain garnet, phengite, albite and epidote in the matrix and as inclusions in glaucophane. The calculated P–T pseudosection for a representative metabasic glaucophane schist yields peak pressure and temperature of 11.5–15 kbar at 400–510 °C. These rocks experienced retrograde metamorphism from blueschist to greenschist facies (350–450 °C and 7–8 kbar) during exhumation. A back arc basin was formed due to northward subduction of Neotethys under Eurasia (Lut block). Exhumation of the high‐pressure metamorphic rocks in northern Makran occurred contemporarily with subduction. Several reverse faults played an important role in exhumation of the ophiolitic and HP‐LT rocks. The presence of serpentinite shows the possible role of a serpentinite diapir for exhumation of the blueschist. A tectonic model is proposed here for metamorphism and exhumation of oceanic crust and accretionary sedimentary rocks of the Makran area. Vast accretion of subducted materials caused southward migration of the shore.     

High-temperature,high-pressure granulites (retrogressed eclogites) in the central region of the Lewisian,NW Scotland: Crustal-scale subduction in the Neoarchaean

Eclogites and associated high-pressure (HP) rocks in collisional and accretionary orogenic belts preserve a record of subduction and exhumation, and provide a key constraint on the tectonic evolution of the continents. Most eclogites that formed at high pressures but low temperatures at > 10–11 kbar and 450–650 °C can be interpreted as a result of subduction of cold oceanic lithosphere. A new class of high-temperature (HT) eclogites that formed above 900 °C and at 14 to 30 kbar occurs in the deep continental crust, but their geodynamic significance and processes of formation are poorly understood. Here we show that Neoarchaean mafic–ultramafic complexes in the central granulite facies region of the Lewisian in NW Scotland contain HP/HT garnet-bearing granulites (retrogressed eclogites), gabbros, lherzolites, and websterites, and that the HP granulites have garnets that contain inclusions of omphacite. From thermodynamic modeling and compositional isopleths we calculate that peak eclogite-facies metamorphism took place at 24–22 kbar and 1060–1040 °C. The geochemical signature of one (G-21) of the samples shows a strong depletion of Eu indicating magma fractionation at a crustal level. The Sm–Nd isochron ages of HP phases record different cooling ages of ca. 2480 and 2330 Ma. We suggest that the layered mafic–ultramafic complexes, which may have formed in an oceanic environment, were subducted to eclogite depths, and exhumed as HP garnet-bearing orogenic peridotites. The layered complexes were engulfed by widespread orthogneisses of tonalite–trondhjemite–granodiorite (TTG) composition with granulite facies assemblages. We propose two possible tectonic models: (1) the fact that the relicts of eclogitic complexes are so widespread in the Scourian can be taken as evidence that a > 90 km × 40 km-size slab of continental crust containing mafic–ultramafic complexes was subducted to at least 70 km depth in the late Archaean. During exhumation the gneiss protoliths were retrogressed to granulite facies assemblages, but the mafic–ultramafic rocks resisted retrogression. (2) The layered complexes of mafic and ultramafic rocks were subducted to eclogite-facies depths and during exhumation under crustal conditions they were intruded by the orthogneiss protoliths (TTG) that were metamorphosed in the granulite facies. Apart from poorly defined UHP metamorphic rocks in Norway, the retrogressed eclogites in the central granulite/retrogressed eclogite facies Lewisian region, NW Scotland have the highest crustal pressures so far reported for Archaean rocks, and demonstrate that lithospheric subduction was transporting crustal rocks to HP depths in the Neoarchaean.     

Coupled Lu–Hf and Sm–Nd geochronology constrains blueschist-facies metamorphism and closure timing of the Qilian Ocean in the North Qilian orogen

The Qilian–Qaidam orogenic belt at the northern edge of the Tibetan Plateau has received increasing attention as it recorded a complete history from continental breakup to opening and closure of ocean basin, and to the ultimate continental collision in the time period from the Neoproterozoic to the Paleozoic. Determining a geochronological framework of the initiation and termination of the fossil Qilian Ocean subduction in the North Qilian orogenic belt plays an essential role in understanding the whole tectonic process. Dating the high-pressure metamorphic rocks in the North Qilian orogenic belt, such as blueschist and eclogite, is the key in this respect. A blueschist from the southern North Qilian orogenic belt was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, and Sm–Nd multichronometric approaches. Pseudosection modeling indicates that the blueschist was metamorphosed under peak P–T conditions of 1.4–1.6 GPa and 530–550 °C. Zircon U–Pb ages show no constraints on the metamorphism due to the lack of metamorphic growth of zircon. Lu–Hf and Sm–Nd ages of 466.3 ± 2.0 Ma and 462.2 ± 5.6 Ma were obtained for the blueschist, which is generally consistent with the U–Pb zircon ages of 467–489 Ma for adjacent eclogites. Lutetium and Sm zoning profiles in garnet indicate that the Lu–Hf and Sm–Nd ages are biased toward the formation of the garnet inner rim. The ages are thus interpreted to reflect the time of blueschist-facies metamorphism. Previous ⁴⁰Ar/³⁹Ar ages of phengitic muscovite from blueschist/eclogite in this area likely represent a cooling age due to the higher peak metamorphic temperature than the argon retention temperature. The differences of peak metamorphic conditions and metamorphic ages between the eclogites and adjacent blueschists indicate that this region likely comprises different tectonic slices, which had distinct P–T histories and underwent high-pressure metamorphism at different times. The initial opening of the Qilian Ocean could trace back to the early Paleozoic, and the ultimate closure of the Qilian Ocean was no earlier than c. 466 Ma.     

Paleo-Tethyan tectonic evolution of Lancangjiang metamorphic complex: Evidence from SHRIMP U-Pb zircon dating and 40Ar/39Ar isotope geochronology of blueschists in Xiaoheijiang-Xiayun area,Southeastern Tibetan Plateau

Blueschists are sporadically exposed as lenses within the Lancangjiang metamorphic complex, and represent unique components of the Paleo-Tethys. In this paper, we present geochemical and geochronological results of blueschists to decipher their origin and tectonic significance. The whole-rock geochemical analyses revealed strong similarities with ocean island basalt (OIB), and further discrimination diagrams confirm an affinity to a within-plate setting. Combined studies on blueschists using cathodoluminescence (CL) imaging, SHRIMP U-Pb dating of zircon domains and ⁴⁰Ar/³⁹Ar dating of phengite and glaucophane provide evidence of their magmatic origin and metamorphic evolution. Slightly oscillatory zoned or compositionally homogeneous zircon grains/domains, as well as structureless zircon rims, yield ages from 231.6 ± 3.7 to 225.3 ± 4.8 Ma, recording the blueschist facies metamorphic event. In contrast, the captured zircon grains and cores with a major age peak at ~241 Ma as well as several minor older age peaks indicate the multiple provenance of the zircons. ⁴⁰Ar/³⁹Ar step heating analyses on single grains of phengite and glaucophane separated from blueschists yield plateau ages ranging from 242.5 ± 1.4 to 228.7 ± 1.5 Ma which are interpreted to reflect high-pressure metamorphism.This study provides geochemical and geochronological constraints on the tectonic evolution of the Paleo-Tethyan ocean, which was closed and subsequently subducted as a result of the collision of the Simao and Baoshan Blocks. During subduction in Trassic (243 to 225 Ma), the protoliths of blueschists underwent blueschist facies conditions.     

Initial water budget: The key to detaching large volumes of eclogitized oceanic crust along the subduction channel?

We herein investigate the extent to which extensive hydration of the oceanic lithosphere influences the preservation and exhumation of large-scale ophiolite bodies from subduction zones. The Zermatt–Saas ophiolite (ZS, W. Alps), which was subducted during the late stages of oceanic subduction, preserves a complete section of Mesozoic Tethys oceanic lithosphere and particularly fresh eclogites, and represents, so far, the largest and deepest known portion of exhumed oceanic lithosphere. Pervasive hydrothermal processes and seafloor alteration led to the incorporation of large amounts of fluid bound in the hydrated upper layers of the oceanic crust (now as lawsonite eclogites, glaucophanites, and chloritoschists) and in associated ultramafic rocks.Internally, the ZS ophiolite is made up of a series of tectonic slices of oceanic crust (150–300 m thick) which are systematically separated by a 5 to 100 m thick layer of serpentinite. This stack of slices is separated from the underlying eclogitized continental crust (e.g., Monte Rosa) by a thick (~ 500 m) serpentinite sole. Field observations, textural relationships and pseudosection modelling reveal that lawsonite was abundant and widespread in mafic eclogites when the ophiolite detached from the slab at around 550 °C and 24 kbar.Comparison between fresh eclogitic samples and pseudosection modelling shows that (i) water remained in excess from burial to eclogitic peak conditions, (ii) the lightest eclogitized metabasalts correspond to the portions of oceanic crust where metasomatism was the strongest, (iii) crystallization of widespread hydrated parageneses (such as lawsonite, glaucophane and phengite) instead of garnet and omphacite decreased by 5 to 10% the rock density and subsequently enhanced its buoyancy.We propose that this density decrease acted as a ‘float’ which prevented the slices from an irreversible sinking in the mantle. These slices were subsequently detached from the downgoing slab and stacked in the serpentinized subduction channel at pressures between 15 and 20 kbar, in the epidote blueschist facies. Exhumation of the underlying, positively buoyant continental crust dragged this “frozen” nappe-stack from the subduction channel towards the surface.     

A high-pressure folded klippe at Tehuitzingo on the western margin of an extrusion zone,Acatlán Complex,southern México

High-pressure (HP) rocks at Tehuitzingo, on the western margin of the HP belt within the Paleozoic Acatlán Complex (southern México), occur in a klippe that was thrust over low-grade clastic rocks. The youngest detrital zircon cluster in the low-grade rocks yielded U-Pb ages of 481 ± 16 Ma, which provide an older limit for deposition. The HP rocks are composed of metabasites, serpentinite, granite (482 ± 3 Ma) and mica schist (youngest concordant detrital zircon: 433 ± 3 Ma). The schist and granite are inferred to be high-grade equivalents of lower Paleozoic, low-grade rocks exposed elsewhere in the Acatlán Complex, from which they are inferred to have been removed by subduction erosion. Mineral analyses indicate that the subducted rocks underwent HP metamorphism and polyphase deformation at depths of ~ 50 km (~ 16 kbar and 750 °C: eclogite facies). Subsequent retrogression passed through epidote-amphibolite to greenschist facies, which was synchronous with W-vergent thrusting over the low-grade clastic rocks. Deposition of the low-grade rocks and thrusting are bracketed between either 481–329 Ma (Ordovician-Mississippian), and was followed by F₃ synformal folding. Cooling through ca. 385 °C is indicated by 329 ± 1 and 316–317 ± 2 Ma, ⁴⁰Ar/³⁹Ar muscovite plateau ages in HP rocks, which are 5–17 my younger than those of the adjacent Piaxtla eclogites suggesting younger exhumation. The petrology, P-T conditions and ages of the Piaxtla Suite is consistent with an extrusion channel within the Acatlán Complex along the active western margin of Pangea during the Carboniferous. Detrital zircon populations in the low-grade psammite (ca. 481, 520–650, 720, 750, 815, 890, 1050 and 2750 Ma) and the HP schist (ca. 457–480, 534, 908, 954–1150, 1265, 1845 and 2035 Ma) indicate derivation from the Ordovician Acatlán granitoids, Neoproterozoic Brasiliano orogens, 900–750 Ma Goiás arc (Amazonia), 1–1.3 Ma Oaxaquia, and more ancient sources in Oaxaquia/Amazonia.     

The Meso-Neoarchaean Belomorian eclogite province: Tectonic position and geodynamic evolution

The aim of this paper is to review the main features of the Meso-Neoarchaean Belomorian eclogite province (BEP) in the northeastern Fennoscandian Shield, including regional and local geology, geochemistry, petrology and geochronology and to compare the Belomorian eclogites with Precambrian eclogites elsewhere. Two eclogite associations have been recognized within Belomorian TTG gneisses: (1) the subduction-type Salma association and (2) Gridino eclogitized mafic dykes. Protoliths of the Salma eclogites represent a sequence comprising gabbro, Fe–Ti gabbro and troctolites, formed at ~ 2.9 Ga in a slow-spreading ridge setting (like the Southwest Indian Ridge). The main subduction and eclogite-facies events occurred between ~ 2.87 and ~ 2.82 Ga. Injection of mafic magma into an active continental margin setting, recorded by the Gridino dyke swarm, is attributed to subduction of a mid-ocean ridge, commencing at 2.87 Ga. Crustal delamination of the active margin and subsequent involvement of the lower crust in subduction between 2.87 and 2.82 Ga ago caused high-pressure metamorphism of the Gridino dykes, culminating in eclogite-facies conditions between 2.82 and 2.78 Ga and accompanying amalgamation of the Karelia, Kola and Khetolamba blocks and formation of the Mesoarchaean Belomorian accretionary–collisional orogen. The clockwise P–T paths of the Salma and Gridino associations cross the granulite-facies P–T field. Detailed metamorphic studies indicate a complicated post-eclogite history with thermal events and fluid infiltration, related to plume activity at 2.72–2.70, ~ 2.4 and ~ 1.9 Ga. The eclogite assemblages were exhumed to mid-to-lower crustal depths at ~ 1.7 Ga, while erosion or younger tectonic events were responsible for final exhumation to the surface. Comparison of P–T–t paths and data for peak metamorphic parameters demonstrates the general similarity of the Archaean and Palaeoproterozoic eclogites worldwide and their association with anomalously “hot” environments. The occurrence of high-T conditions during eclogite-facies metamorphism can be attributed to either subduction of a mid-ocean ridge (Archaean, BEP) or to interaction with mantle plumes (Proterozoic).     

Middle Carboniferous-Early Triassic eclogite–blueschist blocks within a serpentinite mélange at Port Macquarie,eastern Australia: Implications for the evolution of Gondwana's eastern margin

The New England Orogen of easternmost Australia is dominated by suites of Palaeozoic to earliest Mesozoic rocks that formed in supra-subduction zone settings at Gondwana's eastern margin. On the northern New South Wales coast at Rocky Beach, Port Macquarie, a serpentinite mélange carries rare tectonic blocks of low-grade, high-pressure, metamorphic rocks derived from sedimentary and igneous protoliths. Dominant assemblages are glaucophane + phengite ± garnet ± lawsonite ± calcite ± albite blueschists and lawsonite-bearing retrogressed garnet + omphacite eclogites. In some blocks with sedimentary protoliths, eclogite forms folded layers within the blueschists, which is interpreted as Mn/(Mn + Fe) compositional control on the development of blueschist versus eclogite assemblages. Review of previous studies indicates pressure–temperature conditions of 0.7–0.5 GPa and ≤ 450 °C. Three samples of high-pressure metasedimentary rocks contain Archaean to 251 ± 6 Ma (Permo-Triassic) zircons, with the majority of the grains being Middle Devonian to Middle Carboniferous in age (380–340 Ma). Regardless of age, all grains show pitting and variable rounding of their exteriors. This morphology is attributed to abrasion in sedimentary systems, suggesting that they are all detrital grains. New in situ metamorphic zircon growth did not develop because of the low temperature (≤ 450 °C) of metamorphism. The Permo-Triassic, Devonian and Carboniferous zircons show strong heavy rare earth element enrichment and negative europium anomalies, indicating that they grew in low pressure igneous systems, not in a garnet-rich plagioclase-absent high pressure metamorphic environment. Therefore the youngest of these detrital zircons provides the maximum age of the metamorphism. A titanite + rutile porphyroblast within an eclogite has a U–Pb age of 332 ± 140 Ma (poor precision due to very low U abundances of mostly < 1 p.p.m.) and provides an imprecise direct age for metamorphism. In the south of the Port Macquarie area, the Lorne Basin ≥ 220 Ma Triassic sedimentary and volcanic rocks unconformably overlie serpentinite mélange, and provide the minimum age of the high-pressure metamorphism. Our preferred interpretation is that the 251 Ma zircons are detrital and thus the Port Macquarie high-pressure metamorphism is constrained to the end of the Permian–Early Triassic. Emplacement of the serpentinite mélange carrying the Rocky Beach high-pressure rocks might have been due to docking of a Permian oceanic island arc (represented by the Gympie terrane in southern Queensland?) and an Andean-style arc at the eastern Australian margin (expressed in the New England Orogen by 260–230 Ma north-south orientated magmatic belts). Alternatively, if the 251 Ma grains are regarded as having grown in thin pegmatites, then the dominant Devonian–Carboniferous detrital population still indicates a maximum age for the high pressure metamorphism of ca. 340 Ma. A ≤ 340 Ma age of metamorphism would still be much younger than the previously suggested ca. 470 Ma (Ordovician) age, which was based on Ar–Ar dating of phengites.     

The origin and geochemical evolution of the Woodlark Rift of Papua New Guinea

Geochemical, isotopic, and geochronologic data for exhumed rocks in the Woodlark Rift of Papua New Guinea (PNG) allow a tectonic link to be established with the Late Cretaceous Whitsunday Volcanic Province (WVP) of northeastern Australia. Most of the metamorphic rocks in the Woodlark Rift have Nd isotopic compositions (ε_Nd = + 1.7 to + 6.2) similar to the Nd isotopic compositions of rocks in the WVP (ε_Nd = + 1.3 to + 6.6; Ewart et al., 1992), and contain inherited zircons with 90 to 100 Ma U–Pb ages that overlap the timing of magmatism in the WVP. None of the metamorphic rocks in the Woodlark Rift have the highly evolved Hf and Nd isotopic compositions expected of ancient continental crust. Magmas were erupted in the WVP during the middle Cretaceous as eastern Gondwana was rifted apart. The protoliths of felsic and intermediate metamorphic rocks in the Woodlark Rift are interpreted to be related to the magmatic products produced during this Cretaceous rifting event. Some mafic metamorphic rocks exposed in the western Woodlark Rift (eclogites and amphibolites) are not related to the WVP and instead could have originated as basaltic lavas crystallized from mantle melts at (U)HP depths in the Late Cenozoic, or as fragments of Mesozoic aged oceanic lithosphere.Isotopic and elemental comparisons between basement gneisses and Quaternary felsic volcanic rocks demonstrate that felsic lavas in the D'Entrecasteaux Islands did not form solely from partial melting of metamorphic rocks during exhumation. Instead, the isotopic compositions and geochemistry of Quaternary felsic volcanic rocks indicate a significant contribution from the partial melting of the mantle in this region. When combined with geophysical data for the western Woodlark Rift, this suggests that future seafloor spreading will commence south of Fergusson Island, and west of the present-day active seafloor spreading rift tip.     

Variable P–T paths and HP-UHP metamorphism in a Precambrian terrane,Gridino, Russia: Petrological evidence and geodynamic implications

The Gridino Complex represents one of the oldest eclogite-facies terranes on Earth. It consists of blocks, boudins and lenses of eclogites, pyroxenites, and epidosites as well as deformed eclogitized dikes within biotite-amphibole gneisses. Detailed petrological studies of the pyroxenites and different types of eclogites reveal considerable diversity in metamorphic pressure (P) – temperature (T) conditions (from 1.3 GPa at 660 °C to 3.0 GPa at 660 °C) and fluid regimes (wet vs. dry) experienced by these rocks. Dike-related rocks escaped prograde metamorphism and reached higher pressures than the lenses and blocks that experienced considerable prograde metamorphic reworking. The variability in P–T conditions and the shapes of P–T paths are in agreement with the results of thermomechanical modeling and data from (U)HP metamorphic rocks exhumed during continent–continent collision in the Phanerozoic. The T/P ratio estimated for an eclogitized dike from Eclogitovii Island of the Gridino Complex corresponds to the gradients of < 350 °C/GPa attributed to high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belts, which are often considered as representative of modern style plate tectonics operating in the Phanerozoic Eon. The data presented in this paper suggest that occurrences of HP-UHP metamorphic terrains might be extended back towards the time of either the assembly of Columbia in the Paleoproterozoic or Kenorland in the Neoarchean.     

Combined U-Pb,Lu-Hf,Sm-Nd and Ar-Ar multichronometric dating on the Bailang eclogite constrains the closure timing of the Paleo-Tethys Ocean in the Lhasa terrane,Tibet

The Lhasa terrane, the main tectonic component of the Himalayan–Tibetan orogen, has received much attention as it records the entire history of the orogeny. The occurrence of Permian to Triassic high-pressure eclogites has a significant bearing on the understanding of the Paleo-Tethys subduction and plate suturing processes in this area. An eclogite from the Bailang, eastern Lhasa terrane, was investigated with a combined metamorphic P–T and U–Pb, Lu–Hf, Sm–Nd and Ar–Ar multichronometric approach. Pseudosection modeling combined with thermobarometric calculations indicate that the Bailang eclogite equilibrated at peak P–T conditions of ~ 2.6 GPa and 465–503 °C, which is much lower than those of Sumdo and Jilang eclogites in this area. Garnet–whole rock–omphacite Lu–Hf and Sm–Nd ages of 238.1 ± 3.6 Ma and 230.0 ± 4.7 Ma were obtained on the same sample, which are largely consistent with the corresponding U–Pb age of 227.4 ± 6.4 Ma for the metamorphic zircons within uncertainty. The peak metamorphic temperature of the sample is lower than the Lu–Hf and Sm–Nd closure temperatures in garnet. This, combined with the core-to-rim decrease in Mn and HREE concentrations, the slightly U-shaped Sm zonation across garnet and the exclusive occurrence of omphacite inclusion in garnet rim, are consistent with the Lu–Hf system skewing to the age of the garnet core and the Sm–Nd system favoring the rim age. The Sm–Nd age was thus interpreted as the age of eclogite-facies metamorphism and the Lu–Hf age likely pre-dated the eclogite-facies metamorphism. ⁴⁰Ar/³⁹Ar dating of hornblende from the eclogite yielded ages about 200 Ma, which is interpreted as a cooling age and is probably indicative of the time of exhumation to the middle crust. The difference of peak eclogite-facies metamorphic conditions and the distinct metamorphic ages for the Bailang eclogite (~ 2.6 GPa and ~ 480 °C; ca. 230 Ma), the Sumdo eclogite (~ 3.4 GPa and ~ 650 °C; ca. 262 Ma) and Jiang eclogite (~ 3.6 GPa and ~ 750 °C; ca. 261 Ma) in the same (ultra)-high-pressure belt indicate that this region likely comprises different slices that had distinct P–T histories and underwent (U)HP metamorphism at different times. The initiation of the opening the Paleo-Tethys Ocean in the Lhasa terrane could trace back to the early Permian. The ultimate closure of the Paleo-Tethys Ocean in the Lhasa terrane was no earlier than ca. 230 Ma.     

Subducted seamounts in an eclogite-facies ophiolite sequence: the Andean Raspas Complex, SW Ecuador

The metamorphic Raspas Complex of southwest Ecuador consists of high-pressure mafic, ultramafic, and sedimentary rocks. The Lu–Hf ages of a blueschist, a metapelite, and an eclogite overlap at around 130 Ma and date high-pressure garnet growth. Peak metamorphic conditions in the eclogites reached 1.8 GPa at 600°C, corresponding to a maximum burial depth of ~60 km. The geochemical signatures of the eclogites suggest that their protoliths were typical mid-ocean ridge basalts (MORB), whereas the blueschists exhibit seamount-like characteristics, and the eclogite-facies peridotites seem to represent depleted, MORB-source mantle. That these rocks were subjected to similar peak PT conditions contemporaneously suggests that they were subducted together as an essentially complete section within the slab. We suggest that this section became dismembered from the slab during burial at great depth—perhaps as a consequence of scraping off the seamounts. The spatially close association of MORB-type eclogite, seamount-type blueschist, serpentinized peridotite, and metasediments points to an exhumed high-pressure ophiolite sequence.     

Palaeozoic to Early Jurassic history of the northwestern corner of Gondwana,and implications for the evolution of the Iapetus,Rheic and Pacific Oceans

The Palaeozoic to Mesozoic igneous and metamorphic basement rocks exposed in the Mérida Andes of Venezuela and the Santander Massif of Colombia are generally considered to define allochthonous terranes that accreted to the margin of Gondwana during the Ordovician and the Carboniferous. However, terrane sutures have not been identified and there are no published isotopic data that support the existence of separate crustal domains. A general paucity of geochronological data led to published tectonic reconstructions for the evolution of the northwestern corner of Gondwana that do not account for the magmatic and metamorphic histories of the basement rocks of the Mérida Andes and the Santander Massif. We present new zircon U–Pb (ICP-MS) data from 52 igneous and metamorphic rocks, which we combine with whole rock geochemical and Pb isotopic data to constrain the tectonic history of the Precambrian to Mesozoic basement of the Mérida Andes and the Santander Massif. These data show that the basement rocks of these massifs are autochthonous to Gondwana and share a similar tectono-magmatic history with the Gondwanan margin of Peru, Chile and Argentina, which evolved during the subduction of oceanic lithosphere of the Iapetus Ocean. The oldest Palaeozoic arc magmatism is recorded at ~ 500 Ma, and was followed shortly by Barrovian metamorphism. Peak metamorphic conditions at upper amphibolite facies are recorded by anatexis at ~ 477 Ma and the intrusion of synkinematic granitoids until ~ 472 Ma. Subsequent retrogression resulted from localised back-arc or intra-arc extension at ~ 453 Ma, when volcanic tuffs and interfingered sedimentary rocks were deposited over the amphibolite facies basement. Continental arc magmatism dwindled after ~ 430 Ma and terminated at ~ 415 Ma, coevally with most of the western margin of Gondwana. After Pangaea amalgamation in the Late Carboniferous to Early Permian, a magmatic arc developed on its western margin at ~ 294 Ma as a result of subduction of oceanic crust of the palaeo-Pacific ocean. Intermittent arc magmatism recorded between ~ 294 and ~ 225 Ma was followed by the onset of the Andean subduction cycle at ~ 213 Ma, in an extensional regime. Extension was accompanied by slab roll-back which led to the migration of the arc axis into the Central Cordillera of Colombia in the Early Jurassic.     

LP/HT metamorphism as a temporal marker of change of deformation style within the Late Palaeozoic accretionary wedge of central Chile

A Late Palaeozoic accretionary prism, formed at the southwestern margin of Gondwana from Early Carboniferous to Late Triassic, comprises the Coastal Accretionary Complex of central Chile (34–41°S). This fossil accretionary system is made up of two parallel contemporaneous metamorphic belts: a high‐pressure/low temperature belt (HP/LT – Western Series) and a low pressure/high temperature belt (LP/HT – Eastern Series). However, the timing of deformation events associated with the growth of the accretionary prism (successive frontal accretion and basal underplating) and the development of the LP/HT metamorphism in the shallower levels of the wedge are not continuously observed along this paired metamorphic belt, suggesting the former existence of local perturbations in the subduction regime. In the Pichilemu region, a well‐preserved segment of the paired metamorphic belt allows a first order correlation between the metamorphic and deformational evolution of the deep accreted slices of oceanic crust (blueschists and HP greenschists from the Western Series) and deformation at the shallower levels of the wedge (the Eastern Series). LP/HT mineral assemblages grew in response to arc‐related granitic intrusions, and porphyroblasts constitute time markers recording the evolution of deformation within shallow wedge material. Integrated P–T–t–d analysis reveals that the LP/HT belt is formed between the stages of frontal accretion (D₁) and basal underplating of basic rocks (D₂) forming blueschists at c. 300 Ma. A timeline evolution relating the formation of blueschists and the formation and deformation of LP/HT mineral assemblages at shallower levels, combined with published geochronological/thermobarometric/geochemistry data suggests a cause–effect relation between the basal accretion of basic rocks and the deformation of the shallower LP/HT belt. The S₂ foliation that formed during basal accretion initiated near the base of the accretionary wedge at ~30 km depth at c. 308 Ma. Later, the S₂ foliation developed at c. 300 Ma and ~15 km depth shortly after the emplacement of the granitoids and formation of the (LP/HT) peak metamorphic mineral assemblages. This shallow deformation may reflect a perturbation in the long‐term subduction dynamics (e.g. entrance of a seamount), which would in turn have contributed to the coeval exhumation of the nearby blueschists at c. 300 Ma. Finally, ⁴⁰Ar–³⁹Ar cooling ages reveal that foliated LP/HT rocks were already at ~350 °C at c. 292 Ma, indicating a rapid cooling for this metamorphic system.     

Middle Paleozoic mafic magmatism and ocean plate stratigraphy of the South Tianshan,Kyrgyzstan

The tectonic history of the Kyrgyz South Tianshan in the western Central Asian Orogenic Belt (CAOB) remains controversial, first of all, due to the limited amount of geochemical and isotope data. Our paper presents the first results of a detailed geochemical study (major and trace elements, Sr, Nd and Pb isotopes) of Middle Paleozoic mafic volcanic and subvolcanic rocks of the Ferghana and Atbashi–Kokshaal accretionary belts of the South Tianshan orogen in Kyrgyzstan, which formed during the evolution of the Turkestan Ocean. A special focus is given to the relation between magmatic rocks and sedimentary units of marine origin, chert, siliceous shale/mudstone, volcanogenic–carbonate clastics, seamount carbonates, and turbidites, which we consider as elements of Ocean Plate Stratigraphy (OPS). The age range of marine sediments is Late Silurian to Early Carboniferous, but the age of the most volcanic rocks associated with fossil-bearing OPS sediments is Devonian. The magmatic rocks have geochemical affinity to oceanic island basalts (OIB-type) and, to a lesser extent, mid-oceanic ridge (MORB-type) basalts associated with hemipelagic siliceous mudstone and pelagic chert. The rocks with OIB-type affinity are associated with chert, siliceous shale and carbonaceous clastics and carbonates. They are enriched in TiO₂, LREE (La/Sm_n = 1.9), and Nb (Zr/Nb_av. = 10), have differentiated HREE (Gd/Yb_n = 2.0), medium to low εNd (~ 5.7) and are characterized by clear Nb positive anomalies in normalized multi-element plots (Nb/Th_pm = 1.3, Nb/La_pm = 1.1). The OIBs formed by relatively low degrees of melting (< 5%) of mantle sources in the garnet stability field and erupted in an oceanic island setting. The MORB-type samples associated with siliceous mudstone and chert are less enriched in incompatible elements, possess flat REE and multi-element pattern, and show higher εNd values (~ 9.1); they were probably produced by high-degree melting of spinel lherzolite and/or harzburgite and erupted in a mid-oceanic ridge setting. The geological, lithological and geochronological data suggest that the OPS units with dominantly OIB-type basalts formed at one or several seamount chains of the Turkestan Ocean, which were accreted to the Kazakhstan continent, and thus contribute to our understanding of the Paleozoic tectonic evolution of the western CAOB during the Serpukhovian–Bashkirian.     

The 40Ar/39Ar and Rb–Sr chronology of the Precambrian Aksu blueschists in western China

Three metapelite samples from the Aksu blueschist terrane, Xinjiang, China, were dated by the ⁴⁰Ar/³⁹Ar method on separated phengite grains, obtaining plateau ages in the range of 741−757 Ma. In contrast, the measured Rb and Sr isotope data for the three samples yielded isochron ages ranging from 630 Ma to 900 Ma, suggesting large heterogeneity in the blueschist protolith and suppression of diffusional exchange owing to the low-temperature metamorphic conditions. Because the protolith of Aksu blueschist is composed of oceanic materials that formed ⁴⁰Ar-free phengite during HP and UHP metamorphism and the apparent ⁴⁰Ar/³⁹Ar plateaus ages in this study are similar to previous K–Ar and Rb–Sr ages, the existence of excess argon in these rocks is considered to be insignificant. As a result, the ⁴⁰Ar/³⁹Ar plateau ages in this study (ca. 750 Ma) likely represent the approximate time for peak metamorphism, given the low peak metamorphic temperatures for the Aksu blueschist terrane (300−400 °C). This strongly implies that modern style, cold subduction tectonics operated along the margin of the Aksu terrane no later than 750 Ma, in Neoproterozoic time.     

Metamorphic history of glaucophane‐paragonite‐zoisite eclogites from the Shanderman area,northern Iran

The Shanderman eclogites and related metamorphosed oceanic rocks mark the site of closure of the Palaeotethys ocean in northern Iran. The protolith of the eclogites was an oceanic tholeiitic basalt with MORB composition. Eclogite occurs within a serpentinite matrix, accompanied by mafic rocks resembling a dismembered ophiolite. The eclogitic mafic rocks record different stages of metamorphism during subduction and exhumation. Minerals formed during the prograde stages are preserved as inclusions in peak metamorphic garnet and omphacite. The rocks experienced blueschist facies metamorphism on their prograde path and were metamorphosed in eclogite facies at the peak of metamorphism. The peak metamorphic mineral paragenesis of the rocks is omphacite, garnet (pyrope‐rich), glaucophane, paragonite, zoisite and rutile. Based on textural relations, post‐peak stages can be divided into amphibolite and greenschist facies. Pressure and temperature estimates for eclogite facies minerals (peak of metamorphism) indicate 15–20 kbar at ~600 °C. The pre‐peak blueschist facies assemblage yields <11 kbar and 400–460 °C. The average pressure and temperature of the post‐peak amphibolite stage was 5–6 kbar, ~470 °C. The Shanderman eclogites were formed by subduction of Palaeotethys oceanic crust to a depth of no more than 75 km. Subduction was followed by collision between the Central Iran and Turan blocks, and then exhumation of the high pressure rocks in northern Iran.