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.     

The Habutengsu metapelites and metagreywackes in western Tianshan,China: metamorphic evolution and tectonic implications

The HP‐UHP metamorphic belt of western Tianshan in northwestern China is a rarely preserved oceanic UHP terrane which consists predominantly of meta‐siliciclastic rocks, occasionally accompanied by lens‐shaped metabasites. The metapelites and metagreywackes from the Habutengsu Valley and adjacent area within this belt contain quartz, albite, garnet, white mica, chlorite and rutile/titanite, with or without minor amounts of barroisite, glaucophane, clinozoisite, allanite, graphite, carbonate and tourmaline. Included in coarse‐grained garnet, pseudomorphs of clinozoisite + paragonite after lawsonite are common, seldom also together with inclusions of chloritoid, jadeite and glaucophane. In the northern Habutengsu area, garnet is compositionally characterized by similar cores with consistently low‐Ca content. Similar garnet armouring coesite has been reported in UHP schists from the same area. Deduced P–T conditions during formation of these Ca‐poor garnet cores are 25–31 kbar and 430–510 °C, which are consistent with the computed stability of the observed assemblage Grt + Gln + Lws ± Jd ± Cld in the coesite stability field. Thus, the occurrences of the UHP metapelites and metagreywackes define an internally coherent UHP unit in the north of the Habutengsu area, the spatial extension of which is much larger than previously known. Compared with the northern ones, the southern metapelites and metagreywackes in the Habutengsu area consist of similar minerals and have similar bulk rock compositions but significantly different garnet chemistry, indicating an abrupt variation in P–T conditions during garnet growth. The derived conditions initiating the garnet growth for the southern rocks in a similar range (18–21 kbar and 450–500 °C) and thus constrain a coherent HP unit in the south of the Habutengsu area. The juxtaposition of two exhumed slices of contrasting metamorphic grades probably indicates the change of subduction dynamics of the palaeo‐Tianshan oceanic crust, the subduction polarity (from south to north) of which accounts for the spatial relationship between these two units.     

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.     

An (in‐)coherent metamorphic evolution of high‐P eclogites and their host rocks in the Chinese southwest Tianshan?

In the (ultra‐)high‐P–low‐T metamorphic terrane of the Chinese South Tianshan, discontinuous mafic blocks and boudins (former upper oceanic crust) are now embedded in voluminous (mainly metasedimentary) host rocks. Two different models were proposed and relate the occurrence of both high‐P and ultra‐high‐P mafic and metasedimentary rocks to either (i) a tectonic mélange style exhumation, with no exhumation of coherent units, but different lithologies derived from different depths juxtaposed and intermingled during exhumation in the subduction channel, or (ii) the evolution of two coherent metamorphic belts: one with high‐P and the other with ultra‐high‐P conditions. In contrast to most previous studies in the Chinese South Tianshan which focused either on single eclogites or metasedimentary rocks (assumed as representative), this study concentrates on the systematic investigation of both mafic boudins and their immediate sedimentary host rocks, because the investigation of both lithologies and the comparison of their metamorphic evolution is crucial to reconstruct the geodynamical context of the whole (ultra‐)high‐P–low‐T metamorphic complex. Several sample pairs consisting of both lithologies were geochemically investigated and their respective metamorphic evolution was reconstructed using geothermobarometry and thermodynamic modelling. The latter approach considers changes in the mineral assemblage during the metamorphic evolution, as well as changes in mineral composition, which may help to determine the metamorphic history of a rock despite the preservation of critical mineral assemblages. All samples experienced a clockwise P–T path with overall maximum P–T conditions of 540–550 °C and 1.9–2.25 GPa for the host rocks, and 555–575 °C and 2.2–2.5 GPa for the eclogites. Peak‐metamorphic temperatures of ~525–540 °C of the metasedimentary host rocks were also confirmed by Raman spectroscopy of carbonaceous material. Results from thermobarometry and thermodynamic modelling are consistent with the observation that none of the samples contains mineral relicts indicating UHP conditions (like coesite in garnet) and neither conventional thermobarometry, nor thermodynamical modelling resulted in P–T conditions in the stability field of coesite. Thus, no evidence of ultra‐high‐P conditions was found. Given that the whole sampled river valley lies within the proposed ‘ultra‐high‐P sub‐belt’ and considering former studies, which showed that at other places within this ‘unit’ both ultra‐high‐P and high‐P rocks are now juxtaposed on a small scale, the formation of the whole (ultra‐)high‐P–low‐T metamorphic belt in the Chinese South Tianshan as a tectonic mélange style exhumation is more convincing than the formation and juxtaposition of two coherent metamorphic units with high‐P and ultra‐high‐P conditions respectively.     

东准噶尔扎河坝蛇绿混杂岩中的石榴角闪岩

继报道了超高压成因的石榴辉石岩和石英菱镁岩之后,作者在扎河坝地区又发现了具有超高压成因特征的石榴角闪岩。扎河坝石榴角闪岩主要组成矿物为角闪石、石榴石、辉石和钠长石,它不但含硬玉、多硅白云母等高压变质矿物,而且还残留着具有超高压成因特征的超硅石榴石和超硅辉石。根据野外产状和矿物组成特征,扎河坝石榴角闪岩可以细分成含硬玉石榴角闪岩和含多硅白云母石榴角闪岩两类,它们的原岩为大洋玄武岩。石榴角闪岩的厘定进一步证实在扎河坝蛇绿混杂岩带内确实存在着超深俯冲后折返的洋壳,同时超深俯冲洋壳的组成也较复杂,包括沉积岩、辉长岩和大洋玄武岩等。石榴角闪岩、石英菱镁岩及石榴辉石岩等超高压变质岩的存在表明扎河坝蛇绿岩带是一条大洋板块的超深俯冲带,它可能通过克拉玛依蛇绿岩带西延出境,形成一条横贯东西准噶尔北缘的超深俯冲带。这条超深俯冲带的发现为研究新疆北部早古生代洋陆转换过程及机制提供了新的视野和思路,对深入探讨准噶尔盆地北缘及阿尔泰南缘早古生代壳幔相互作用及中亚造山带形成的动力学机制具有重要的启示。     

The metahyaloclastitic matrix of a unique metavolcanic block reveals subduction in the Somozas Mélange (Cabo Ortegal Complex,NW Iberia): tectonic implications for the assembly of Pangea

The allochthonous Cabo Ortegal Complex (NW Iberian Massif) contains a ~500 m thick serpentinite‐matrix mélange located in the lowest structural position, the Somozas Mélange. The mélange occurs at the leading edge of a thick nappe pile constituted by a variety of terranes transported to the East (present‐day coordinates; NW Iberian allochthonous complexes), with continental and oceanic affinities, and represents a Variscan suture. Among other types of metaigneous (calcalkaline suite dated at 527–499 Ma) and metasedimentary blocks, it contains close‐packed pillow‐lavas and broken pillow‐breccias with a metahyaloclastitic matrix formed by muscovite–paragonite–margarite–garnet–chlorite–kyanite–hematite–epidote–quartz–rutile. Pseudosection modelling in the MnCNTKFMASHO system indicates metamorphic peak conditions of ~17.5–18 kbar and ~550 °C followed by near‐isothermal decompression. This P–T evolution indicates subduction/accretion of an arc‐derived section of peri‐Gondwanan transitional crust. Subduction below the Variscan orogenic wedge evolved to continental collision with important dextral component. Closure of the remaining oceanic peri‐Gondwanan domain and associated release of fluid led to hydration of the overlying mantle wedge and the formation of a low‐viscosity subduction channel, where return flow formed the mélange. The submarine metavolcanic rocks were deformed and detached from the subducting transitional crust and eventually incorporated into the subduction channel, where they experienced fast exhumation. Due to the cryptic nature of the high‐P metamorphism preserved in its tectonic blocks, the significance of the Somozas Mélange had remained elusive, but it is made clear here for the first time as an important tectonic boundary within the Variscan Orogen formed during the late stages of the continental convergence leading to the assembly of Pangea.     

Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.     

Sm-Nd Isotope Tracer Study of UHP Metamorphic Rocks: Implications for Continental Subduction and Collisional Tectonics

Sm-Nd isotope tracer techniques are powerful tools in identification of the protolith nature of UHP and HP rocks and can be used to constrain modeling of tectonic processes of continental collision. UHP rocks may have diverse origins, and not all of them carry the same significance for subduction of continental blocks. In this paper, Sm-Nd isotopic data are compiled for UHP and HP rocks, mostly represented by eclogites and garnet peridotites, from the Alpine, Hercynian (Variscan), and Caledonian belts of western Europe; the Pan-African belts of northern Africa; and the Ross belt of Antarctica. These data then are compared with the isotopic characteristics of the UHP rocks from the Dabie orogen of central China. Except for the coesite-bearing quartzitic metasediments of Dora-Maira (Western Alps), which are clearly of continental origin, all HP and UHP rocks (eclogites and ultramafic rocks) from the Alpine, Hercynian, and Pan-African belts have oceanic affinities with the characteristic positive ε_Nd(T) values (= metamorphic initial ¹⁴³Nd/¹⁴⁴Nd ratios). They represent segments of oceanic lithosphere that were subducted, underwent eclogite-facies metamorphism, and later were tectonically transported into orogenic zones during continental collisions. By contrast, the majority of UHP rocks from the European Caledonide and the Dabie orogen have negative ε_ND(T) values, indicating continental affinity. This suggests that these mafic and ultramafic rocks have had a long crustal residence time and that their UHP metamorphism is indicative of subduction of ancient and cold continental blocks, as represented by some Precambrian gneiss terranes containing mafic components including greenschists, amphibolites, or basic granulites.

In the Dabie orogen, none of the UHP eclogites analyzed thus far have shown oceanic affinity; thus they do not represent subducted Tethys Ocean crust. The preservation of ultrahigh ε_ND(0) values (+170 to +260) in eclogites of very low Nd concentrations (average 0.5 ppm) from the Weihai region and of the extraordinarily low δ¹⁸O in many eclogites and gneisses, the general absence of syntectonic granites in the Dabie Shan, and the available age data obtained by different techniques all point to a rapid rate of exhumation and the absence of a pervasive aqueous fluid phase during the entire process of subduction and exhumation of the Dabie UHP terrane.     

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.     

大陆俯冲带大规模的变质流体活动:来自大别造山带超高压硬玉石英岩的记录

硬玉石英岩是一种稀少且与流体作用相关的变质岩,同时出露于高压或超高压洋壳和陆壳俯冲带中.通过对中国东部大别造山带中出露达50 km2的含柯石英的超高压硬玉石英岩进行研究,综合全岩主微量元素、矿物Mg-O同位素和锆石学研究.结果表明,硬玉石英岩的原岩为古元古代TTG岩石,经历过弱化学风化和强物理风化作用,然后在三叠纪时期受到围岩富黑云母片麻岩分解脱水而产生的大量重Mg同位素流体交代,从而形成硬玉石英岩.考虑到这种受流体交代成因的硬玉石英岩在大别山广泛出露,表明其在三叠纪大陆深俯冲过程中存在着大规模的变质流体活动,这项研究首次报道了大陆俯冲带有大规模的流体活动存在,同时也挑战了传统观点认为的大陆俯冲带缺乏岛弧岩浆作用主要原因是缺乏足够量的流体活动.      

Graphite‐schist blocks in the Franciscan Mélange,San Simeon,California: Evidence of high‐P metamorphism

The seacliff exposure at San Simeon, California, contains graphite‐schist blocks in a shale‐matrix, an undocumented lithology within the Franciscan mélange. Thirty graphite‐schist blocks were studied to discover all the varieties in this classic locality of mélange. Based on their mineralogical assemblage and composition, and textural characteristics the graphite‐schists in San Simeon are subdivided into two main types (Type I and II) with two subdivisions each (A and B). Type IA and IIA blocks are the most abundant. Type IA graphite‐schists are siltstone/fine greywacke‐like, preserve sedimentary textures, and lack lawsonite. Type IB graphite‐schists are mineralogical and texturally similar to Type IA schists, but are finer grained siltstone and shale. Type IIA graphite‐schists are compositionally layered and contain quartz‐ and albite‐rich layers and dark graphite‐ and intergrown mica/chlorite‐rich layers. Nine out of the 15 Type IIA blocks contain lawsonite. Two Type IIA blocks also contain aragonite (+calcite) in veins. Type IIB graphite‐schists are mostly composed of quartz and minor graphite, intergrown chlorite and white mica, and white mica pseudomorphs after lawsonite. The phengite content of mica in Type IIA blocks is higher than that of mica in Type IA graphite‐schists, confirming they were metamorphosed under high‐P/low‐T conditions. Type IA blocks were recrystallized between 200 and 250 °C at <~3 kbar; whereas, Type IIA blocks were metamorphosed under higher pressure conditions, probably at 250–300 °C and 3–5 kbar. Most likely both types of graphite‐schists were derived from a similar layered siltstone/fine greywacke/shale protolith. Organic matter‐rich sediments deposited in the trench axis were subducted along with oceanic crust during Franciscan subduction. Type I graphite‐schists were subducted to depths <10 km, whereas Type II graphite‐schists were subducted to depths ~15 km where they were underplated under high‐P conditions. The graphitic metasedimentary rocks were juxtaposed with mafic lithologies from the subducted oceanic crust that were metamorphosed to blueschist facies and retrograded to greenstone as they returned to the surface in the subduction channel shear zone.     

Origin and evolution of the Escambray Massif (Central Cuba): an example of HP/LT rocks exhumed during intraoceanic subduction

High‐pressure (HP) metabasites from the Sancti Spiritus dome (Escambray massif, Central Cuba) have been studied in order to better understand the origin and evolution of the Northern Caribbean boundary plate during the Cretaceous, in a global subduction context. Geochemical and petrological studies of these eclogites reveal two groups with contrasting origins and pre‐subduction metamorphic histories. Eclogites collected from exotic blocks within serpentinite (mélange zone) originated from a N‐MORB type protolith, do not record pre‐eclogitic metamorphic history. Conversely eclogites intercalated in Jurassic metasedimentary rocks (non‐mélange zone) have a calc‐alkaline arc‐like origin and yield evidence for a pre‐subduction metamorphic event in the amphibolite facies. However, all the studied Escambray eclogites underwent the same eclogitic peak (around 600 °C at 16 kbar), and followed a cold thermal gradient during their exhumation (estimated at around 13.5 °C km^?1), which can suggest that this exhumation was coeval with subduction. Concordant geochronological data (Rb/Sr and Ar/Ar) support that the main exhumation of HP/LT rocks from the Sancti Spiritus dome occurred at 70 Ma by top to SW thrusting. The retrograde trajectory of these rocks suggests that the north‐east subduction of the Farallon plate continued after 70 Ma. The set‐off to the exhumation can be correlated with the beginning of the collision between the Bahamas platform and the Cretaceous island arc that induced a change of the subduction kinematics. The contrasting origin and ante‐subduction history of the analysed samples imply that the Escambray massif consists of different geological units that evolved in different environments before their amalgamation during exhumation to form the present unit III of the massif.     

The importance of talc and chlorite “hybrid” rocks for volatile recycling through subduction zones; evidence from the high-pressure subduction mélange of New Caledonia

The transfer of fluid and trace elements from the slab to the mantle wedge cannot be adequately explained by simple models of slab devolatilization. The eclogite-facies mélange belt of northern New Caledonia represents previously subducted oceanic crust and contains a significant proportion of talc and chlorite schists associated with serpentinite. These rocks host large quantities of H₂O and CO₂ and may transport volatiles to deep levels in subduction zones. The bulk-rock and stable isotope compositions of talc and chlorite schist and serpentinite indicate that the serpentinite was formed by seawater alteration of oceanic lithosphere prior to subduction, whereas the talc and chlorite schists were formed by fluid-induced metasomatism of a mélange of mafic, ultramafic and metasedimentary rocks during subduction. In subduction zones, dehydration of talc and chlorite schists should occur at sub-arc depths and at significantly higher temperatures (∼ 800°C) than other lithologies (400–650°C). Fluids released under these conditions could carry high trace-element contents and may trigger partial melting of adjacent pelitic and mafic rocks, and hence may be vital for transferring volatile and trace elements to the source regions of arc magmas. In contrast, these hybrid rocks are unlikely to undergo significant decarbonation during subduction and so may be important for recycling carbon into the deep mantle. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Reworking of the Gangdese magmatic arc,southeastern Tibet: post‐collisional metamorphism and anatexis

The Gangdese magmatic arc, southeastern Tibet, was built by mantle‐derived magma accretion and juvenile crustal growth during the Mesozoic to Early Cenozoic northward subduction of the Neo‐Tethyan oceanic slab beneath the Eurasian continent. The petrological and geochronological data reveal that the lower crust of the southeastern Gangdese arc experienced Oligocene reworking by metamorphism, anatexis and magmatism after the India and Asia collision. The post‐collisional metamorphic and migmatitic rocks formed at 34–26 Ma and 28–26 Ma respectively. Meta‐granitoids have protolith ages of 65–38 Ma. Inherited detrital zircon from metasedimentary rocks has highly variable ages ranging from 2708 to 37 Ma. These rocks underwent post‐collisional amphibolite facies metamorphism and coeval anatexis under P–T conditions of ~710–760 °C and ~12 kbar with geothermal gradients of 18–20 °C km ^{? 1}, indicating a distinct crustal thickening process. Crustal shortening, thickening and possible subduction erosion due to the continental collision and ongoing convergence resulted in high‐P metamorphic and anatectic reworking of the magmatic and sedimentary rocks of the deep Gangdese arc. This study provides a typical example of the reworking of juvenile and ancient continental crust during active collisional orogeny.     

Olivine‐bearing symplectites in fractured garnet from eclogite,Moldanubian Zone (Bohemian Massif) – a short‐lived,granulite facies event

Recent petrological studies on high‐pressure (HP)–ultrahigh‐pressure (UHP) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P–T loop for the HP–UHP metamorphism and amphibolite–granulite facies overprint in the Moldanubian Zone. Here further evidence is presented that the granulite facies metamorphism occurred after the HP–UHP rocks had been exhumed to different levels of the middle or upper crust. A medium‐temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well‐preserved eclogite facies assemblage with omphacite and prograde zoned garnet. Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of ~600 °C at 2.0–2.2 GPa. The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca‐rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel. Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine‐spinel equilibria and pseudosection calculations indicate temperatures of ~850–900 °C at pressure below 0.7 GPa. The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short‐lived process. The new results point to a geodynamic model where HP–UHP rocks are exhumed to amphibolite facies conditions with subsequent granulite facies heating by mantle‐derived magma in the middle and upper crust.     

西南天山造山带超高压变质作用研究新进展

低温超高压变质岩具有极低的地热梯度,其变质演化对于认识板块间相互作用的动力学过程以及弧地壳的生长机制具有重要意义。西南天山造山带发育了世界上少有的经历过深俯冲作用且具有洋壳属性的典型低温超高压变质地质体。近几年来对该造山带中的超高压变质岩开展了大量深入细致的岩石学研究工作,取得了一系列新进展。变基性岩和变沉积岩系岩石中柯石英的普遍发现,直接证明西南天山变质蛇绿混杂岩曾经俯冲到上地幔深度,且具有极低的地热梯度,与热力学模拟结果一致。柯石英的稀少以及大量不同类型柯石英假像的存在,说明在折返过程中发生了强烈的退变质叠加,只有刚性较大且没有经历碎裂-愈合作用的矿物(如石榴石)才有可能保存柯石英。综合岩相学证据和相平衡计算结果,确定西南天山造山带北部的高压地质体(即哈布腾苏-科布尔特单元)整体经历过超高压变质作用,南部的高压地质体峰期压力未达到柯石英稳定域。超高压和高压变质地体的空间分布特点指示了古天山洋由南向北的俯冲极性。这些基础岩石学研究工作的开展对于揭示冷俯冲带的深部物理化学过程以及建立中亚南天山造山带演化的精细模型具有重要意义。     

北秦岭榴辉岩及相关岩石年代学的进一步确定及其对板片俯冲属性的约束

在岩相学观察和锆石CL图像研究的基础上,利用LA ICP MS原位分析方法,对北秦岭官坡超高压榴辉岩和伴生的石榴石角闪岩(榴闪岩)进行了详细的锆石微区微量元素和U Th Pb同位素分析,在榴辉岩样品中得到变质年龄为(502±11)Ma,原岩结晶年龄>(657±18)Ma;在榴闪岩样品中得到原岩结晶年龄为(791±6)Ma,变质年龄为487~503 Ma,角闪岩相退变质年龄为(366±4)Ma。岩石地球化学研究显示,北秦岭官坡地区的榴闪岩具有低Si(SiO2质量分数为4916%~5078%),高Ti(TiO2质量分数为228%~283%)、富集LREE、LILE和大部分的HFSE元素,不显Nb、Ta负异常的板内玄武岩特征,与北秦岭超高压榴辉岩地球化学特征一致。结合两者的野外产状、岩相学特征、锆石形貌和年代学研究结果,表明本文研究的官坡地区的榴闪岩是超高压榴辉岩在抬升过程中在角闪岩相条件下退变质的产物。综合两者的年代学研究结果,得到北秦岭地区超高压榴辉岩的变质年龄为(502±11)Ma,原岩结晶年龄为(791±6)Ma,角闪岩相退变质年龄为(366±4)Ma。研究得到的(502±11)Ma的榴辉岩相变质年龄与前人得到的该榴辉岩围岩超高压泥质片麻岩的变质年龄(507±38)Ma以及北秦岭松树沟地区的超高压长英质片麻岩的变质年龄485~514 Ma一致,表明它们经历了同期超高压变质作用。而且,榴辉岩(502±11)Ma的变质年龄与其原岩的结晶年龄(791±6)Ma存在近300 Ma的时间间隔,表明原岩具有板内玄武岩性质的北秦岭官坡超高压榴辉岩不可能是秦岭古生代大洋板块深俯冲的产物,而可能是已构造就位的古洋壳或裂谷火山岩在古生代随陆壳一起发生大陆深俯冲作用的产物。     

New Data Regarding Hotly Debated Topics Concerning UHP Metamorphism of the Dabie-Sulu Belt,East-Central China

Critical but controversial problems in the study of UHP metamorphic rocks from the Dabie-Sulu region include: (1) the possible existence of ophiolitic mélange; (2) the “in situ” versus “foreign” origin of UHP eclogites and their enclosing gneisses; (3) the possible presence and role of fluids during ultrahigh-pressure (UHP) recrystallization; (4) the timing of collision between the Yangtze and Sino-Korean continental blocks; (5) the polarity of syncollisional subduction; and (6) a single-versus multistage exhumation scenario for the UHP rocks. These questions are discussed in light of new geological, geochemical, and isotopic constraints.

Our conclusions for the Dabie-Sulu belt are as follows: (1) Mafic-ultramafic blocks are of two distinct origins: one group samples lithosphere of the suprasubductionzone mantle wedge, whereas the second group represents postcollisional magmatic intrusions. Neither lithologic group represents true oceanic crust. (2) Quartzofeldspathic gneisses enveloping the eclogites are of two types— metasedimentary “in situ” and igneous “foreign.” The paragneisses contain UHP garnets + white micas, and are uniformly older (235 ± 5 Ma) than the orthogneisses (210 ± 5 Ma), which are devoid of UHP mineralogic indicators. (3) Fluids were active under UHP conditions and allowed the formation of UHP hydrous phases such as phengite and zoisite. However, the aqueous fluids may have been restricted to certain channels/pathways during exhumation. External fluids were absent until ascent of the UHP rocks to middle-crustal levels. (4) The Yangtze and Sino-Korean continental blocks collided during 230 to 240 Ma, when supracontinental rocks experienced UHP metamorphism. The HP metamorphic event dated as >400 Ma might record a subduction of oceanic crust during the Paleozoic. (5) An ancient mantle wedge is revealed by geochemical characteristics of Mesozoic magmatic rocks developed on the southern margin of the Sino-Korean craton, the hanging wall of the UHP-rock-bearing unit. Seismic tomography images reveal that the Yangtze block extends beneath the Dabie-Sulu orogenic belt. This indicates that both oceanic and continental crust had a northward subduction polarity. (6) Taking petrologic and geochronological data into account, we prefer a multistage exhumation model. The UHP rocks were exhumed rapidly during the first stage (230 to 200 Ma), perhaps reflecting a corner-flow mechanism. Then, buoyancy and mantle upwelling brought the UHP rocks up to middle-crustal levels during the second stage (200 to 170 Ma). Extension and thermal uplift, as well as erosion, eventually exposed the UHP rocks to the surface in the third stage (170 to 120 Ma).     

Petrology and metamorphism of glaucophane eclogites in Changning-Menglian suture zone,Bangbing area,southeast Tibetan Plateau: An evidence for Paleo-Tethyan subduction

High/ultrahigh-pressure (HP/UHP) metamorphic complexes, such as eclogite and blueschist, are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones. Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary mélange in the Changning-Menglian suture zone, at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau. The authors report the result of petrological, mineralogical and metamorphism investigations of these rocks, and discuss their tectonic implications. The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists. The major mineral assemblage of the eclogites includes garnet, omphacite, glaucophane, phengite, clinozoisite and rutile. Eclogitic garnet contains numerous inclusions, such as omphacite, glaucophane, rutile, and quartz with radial cracks around. Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation. Four stages of metamorphic evolution can be determined: The prograde blueschist facies (M₁), the peak eclogite facies (M₂), the decompression blueschist facies (M₃) and retrograde greenschist facies (M₄). Using the Grt-Omp-Phn geothermobarometer, a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658°C was determined, which is typical of low-temperature ultrahigh-pressure metamorphism. The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic mélange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean. The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone.©2021 China Geology Editorial Office.     

Ultrahigh-pressure metamorphism and exhumation of garnet-bearing ultramafic rocks from the Lanterman Range (northern Victoria Land, Antarctica)

Northern Victoria Land is a key area for the Ross Orogen – a Palaeozoic foldbelt formed at the palaeo‐Pacific margin of Gondwana. A narrow and discontinuous high‐ to ultrahigh‐pressure (UHP) belt, consisting of mafic and ultramafic rocks (including garnet‐bearing types) within a metasedimentary sequence of gneisses and quartzites, is exposed at the Lanterman Range (northern Victoria Land). Garnet‐bearing ultramafic rocks evolved through at least six metamorphic stages. Stage 1 is defined by medium‐grained garnet + olivine + low‐Al orthopyroxene + clinopyroxene, whereas finer‐grained garnet + olivine + orthopyroxene + clinopyroxene + amphibole constitutes the stage 2 assemblage. Stage 3 is defined by kelyphites of orthopyroxene + clinopyroxene + spinel ± amphibole around garnet. Porphyroblasts of amphibole replacing garnet and clinopyroxene characterize stage 4. Retrograde stages 5 and 6 consist of tremolite + Mg‐chlorite ± serpentine ± talc. A high‐temperature (～950 °C), spinel‐bearing protolith (stage 0), is identified on the basis of orthopyroxene + clinopyroxene + olivine + spinel + amphibole inclusions within stage 1 garnet. The P–T estimates for stage 1 are indicative of UHP conditions (3.2–3.3 GPa and 764–820 °C), whereas stage 2 is constrained between 726–788 °C and 2.6–2.9 GPa. Stage 3 records a decompression up to 1.1–1.3 GPa at 705–776 °C. Stages 4, 5 and 6 reflect uplift and cooling, the final estimates yielding values below 0.5 GPa at 300–400 °C. The retrograde P–T path is nearly isothermal from UHP conditions up to deep crustal levels, and becomes a cooling–unloading path from intermediate to shallow levels. The garnet‐bearing ultramafic rocks originated in the mantle wedge and were probably incorporated into the subduction zone with felsic and mafic rocks with which they shared the subsequent metamorphic and geodynamic evolution. The density and rheology of the subducted rocks are compatible with detachment of slices along the subduction channel and gravity‐driven exhumation.