Metamorphic Core Complexes and Its Significance in the Continental Crustal Evolution

Metamorphic core complexes are a basic structural pattern related toextensional tectonics.Several characteristics of different scales of metamorphiccore complexes in the Fangshan and Yunmengshan(Beijing),Zhongtiaoshan(Shanxi),and Dengfong(Henan)are examined.A three-layer model formetamorphic core complexes is suggested.The conclusion is that metamorphiccore complexes are the result of multiphase intracontinental crustal extensionsand are an important tectonic pattern.which exposes the basementmetamorphic rocks to the ground surface in the intracontinental cover.     

Why are Q-Ratios High in the Sulu UHP Metamorphic Rocks?

I found high Q values (Q-ratio=Jn/Ji, Jn, Ji are remanent magnetization and induced magnetization) in the Sulu ultrahigh pressure (UHP) metamorphic rocks, eastern China which is the world's largest UHP     

Fluid Activity and Tectonic Evolution in the Northern Qilian High-pressure Metamorphic Belt

The Northern Qilian high-pressure metamorphic belt has experienced multiple deformation-metamorphism, which consists of at least four stages.In 550.8-526 Ma, eclogites were formed. High temperature and pressure caused the escape of a large quantity of gas-liquid fluids from rocks while silicate melt was generated. In the late stage, small amounts of CO2 and H2O infiltrating along fractures were introduced.In the formation of glaucophane schist (447-362 Ma), devolatilization reactions were dominated during the subduction-uplift stage of the paleoplate.In the uplift-exhumation stage (400-380 Ma) the increase of internal space of fractures in the rocks favoured fluid infiltration and concentration. These fluids participated in hydration reactions in the retro-metamorphism. The fluids participating in the mineral reactions have the compositions of CaCl2-NaCl-H2O.In subsequent thrusting (<380 Ma), the metamorphic terrain was uplifted to the shallower crust and ductile-shearing deformation took place, which c     

U–Pb SHRIMP ages and tectonic setting of the Munster Suite of the Margate Terrane of the Natal Metamorphic Belt

New U–Pb Zircon SHRIMP ages of 1091 ± 7.1 Ma and 1093.1 ± 5.8 Ma have been determined for two discrete phases of the Munster Suite. The Munster Suite is a calc-alkaline mafic to intermediate suite of intrusive igneous rocks that form part of the southern-most tectonic Terrane of the Mesoproterozoic-aged Natal Metamorphic Belt. Previously published geochemical data indicate that the discrete phases of the Suite are consanguineous and that these rocks originated within an oceanic island arc environment. The new age determinations now show that the different phases are also coeval. Moreover, the ages also indicate that the intrusions were, within statistical error, coeval with S-type granites within the Terrane. This is interpreted to indicate that magmatic underplating provided both the magma by way of a number of progressively more evolved pulses to produce the Munster Suite, as well as the heat necessary for crustal melting to produce the S-type granites within an island arc environment. Therefore, these new age determinations indicate a period of crustal growth at circa 1090 Ma. This moreover, is a maximum constraint on the age of the northward-verging structures within the Margate Terrane.     

Metamorphic Characteristics of Granulite Facies Rocks in the Laixi-Pingdu Area in Eastern Shandong Provincc

Granulite in eastern Shandong is mainly exposed in Laixi. Pingdu, Changyi and Anqiu, and the diagnosticmineral assemblage is Opx+Cpx+Hb+PI±Q±Sca. The appearance of orthopyroxene and Its coexistence withhornblende indicate that the reaction Hb+Q=Opx+Cpx+PI+H_2O did not proceed completely and thereforethese rocks belong to the amphibolite-granulite transition facies. i.e., belonging to hornblende-granulitesubfacics. According to the data obtained from such geothermometers and geobarometers as Opx-Cpx,Opx-Hb. Cpx-Hb, Hb-PI. Sca-PI and Fe-Ti oxides, it has been determined that the temperature of the mainmetamorphic stage was 720-810℃, the pressure 0.5 GPa and fo_210~(-15.5), showing a geothermal gradient of41-46℃/km, and thus the rocks belong to "low-temperature" and low-pressure granulite facies.     

Post-Collisional Ductile Extensional Tectonic Framework in the UHP and HP Metamorphic Belts in the Dabie-Sulu Region, China

The present-day observable tectonic framework of the ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic belts in the Dabie-Sulu region was dominantly formed by an extensional process, mostly between 200 and 170 Ma, following the Triassic collision between the Sino-Korean and Yangtze cratons. The framework that controls the present spatial distribution of UHP and HP metamorphic rocks in particular displays the typical features of a Cordilleran-type metamorphic core complex, in which at least four regional-scale, shallow-dipping detachment zones are recognized. Each of these detachment zones corresponds to a pressure gap of 0.5 to 2.0 GPa. The detachment zones separate the rocks exposed in the region into several petrotectonic units with different P-T conditions. The geometry and kinematics of both the detachment zones and the petrotectonic units show that the exhumation of UHP and HP metamorphic rocks in the Dabie-Sulu region was achieved, at least in part, by non-coaxial ductile flow in the mul     

Formation Environment of the High-Pressure Metamorphic Rocks, Western Tianshan, Xinjiang, China

1.IntroductionFromexperimefltalphaseequilibrium,stableisotOPe,andthermo-barometricstudies,ProgradebineschistdineralparageneseshavebeenproducedexclusivelyatrelativelyhighPadratios(DeRoever,1956;Miyashiro,1961;DobretsovandSobolev,1984;Emst,1973,1988;Maruyamaetal.,1996).InthelastthreedeCades,withtheadvanceofplatetectonics,manygeologistssuggestedthatblueschists,representinghigh-Pressurelow-tCmperamre~rphism,areformedbysubductionofoceanicplate(Emst,1973).Blueschistshavealsobeenregardedasoneof…     

Abiotic Methane Reservoirs in the Western Tianshan HP–UHP Metamorphic Belt,China

Natural gas, consisting primarily of methane(CH₄), has become a major source of clean energy in modern society in many parts of the globe. Recent experimental observations and discoveries of deep-sourced abiotic CH₄ in cold subduction zones indicate the important ability of cold subducted slabs to generate natural gas reservoirs. However, most CH₄ flux and reservoirs remain unknown and their potential is overlooked in global carbon flux estimations. Massive abiot...     

Metamorphic response within different subduction–obduction settings preserved on the NE Arabian margin

Metamorphic rocks form a minor component of the NE Arabian margin in Oman and the United Arab Emirates (UAE). Conditions span almost the entire range of crustal metamorphism from very high-P/low-T eclogite and blueschist to high-P/moderate-T epidote- to upper-amphibolite and low-P/high-T granulite facies. The NE Arabian margin experienced at least six metamorphic events, each characterized by distinct peak metamorphic temperature, depth of burial, average thermal gradient and timing. Synthesis of the available metamorphic data defines five different tectonic settings that evolved during the middle Cretaceous: [1] The Saih Hatat window exposes former continental margin crust that was buried and metamorphosed in a SW-dipping subduction system. Lower-plate units in the window include relict oceanic crust with eclogite (M1–M2) parageneses that recrystallized at pressures of ~14–23 kbar under very low thermal gradients of 7–10 °C/km. Peak metamorphism occurred at ~110 Ma. Peak assemblages were overprinted by garnet–glaucophane-blueschist foliations (M3) at about ~104–94 Ma that formed at ~10–15 kbar and 10–15 °C/km during the first-stage of isothermal exhumation. [2] Metamorphic soles in the footwall of the Semail ophiolite experienced a two-stage history of deep burial and peak metamorphism at ~96–94 Ma, followed by retrogression during obduction onto the continental margin between ~93 and 84 Ma. Peak metamorphic garnet–clinopyroxene–hornblende–plagioclase assemblages (M4s), exposed at highest structural levels, formed at 743 ± 13 °C and 10.7 ± 0.4 kbar, indicating Barrovian thermal regimes of 20.0 ± 2.2 °C/km. Burial of seafloor sediments and oceanic crust to ~38 km depth, was attained within a short-lived, NE-dipping intra-oceanic subduction system. The relatively high average thermal gradient during the peak of metamorphism was the result of heating after subcretion onto the base of hanging-wall oceanic lithosphere. [3] The Bani Hamid terrane consists of seafloor cherts and calcareous turbidites, metamorphosed to low-P/high-T granulite condition at ~96–94 Ma. Diagnostic assemblages (M4b) such as orthopyroxene–cordierite–quartz–plagioclase and orthopyroxene–sapphirine–hercynite–quartz–plagioclase, formed at conditions averaging ~915 ± 35 °C, ~6.1 ± 0.9 kbar and ~42.9 ± 6.5 °C/km. The elevated average thermal gradient, combined with significant depths of burial, is anomalous for typical oceanic settings. This suggests that these sea-floor sediments were buried to ~22 km depths within the intra-oceanic subduction system, accreted onto the hanging-wall, and metamorphosed at high-T during subduction of a recently active spreading ridge. [4] A plausible plate tectonic arrangement that can account for the different metamorphic elements on the Arabian margin is one composed of divergent subduction systems: a relatively long-lived SW-dipping subduction zone at the continental margin, and a short-lived, NE-dipping intra-oceanic subduction system. Consumption of the intervening oceanic crust led to obduction of the Semail ophiolite and accreted metamorphic soles from the upper-plate of the floundered outboard subduction system. SW-directed obduction was initiated between 93.7 and 93.2 Ma and continued until ~84 Ma, producing lower-amphibolite to sub-greenschist facies retrograde fabrics in the metamorphic soles (M5) and sub-metamorphic melange in the footwall. [5] The lower-plate of the Saih Hatat window was reworked by top-to-NE extensional shear at epidote-greenschist facies grades (M6) between ~84 and 76 Ma. Crustal-scale structures were reactivated as extensional detachments that telescoped the continental margin, leading to isothermal decompression and development of an asymmetric core complex that segmented the Semail ophiolite and formed the Saih Hatat domal window.     

Two Discrete UHP and HP Metamorphic Belts in the Central Orogenic Belt, China

INTRODUCTION Anew ultrahigh pressure ( UHP) metamorphicbelt ,the South Altun-North Qaidam-North QinlingUHP metamorphic belt ,has been recently discoveredand widely discussed by different workers (Yang J Set al .,2003 ,2002 ,2001 ,2000 ,1998 ;Zhang J Xetal .,2002 ,1999 ; Zhang G et al .,2001 ; Hu et al .,1996 ,1995 ,1994) . Detailed studies have also beencarried out onthe Dabie-Sulu UHP/ HP metamorphicbelt inthe central orogenic belt (COB) of China (Gaoet al .,2002 ;Sun et al …     

eo-Variscan (Devonian?) melting in the High Grade Metamorphic Complex of the NE Sardinia Belt (Italy)

New structural data pointed out the presence of an older scattered migmatization event (Devonian?, M1) overcome by the well known Variscan migmatization event (Lower-Middle Carboniferous, M2) related to the Late extensional tectonic that affected the High Grade Metamorphic Complex (HGMC) in the Variscan Belt of Sardinia (Italy). The M1 event is only recognizable in the kyanite – amphibole bearing migmatitic gneiss. Both migmatization events (M1 and M2) are characterized by a syn-tectonic non coaxial deformations (D1 and D2 deformational events). D1 shows a top to NW sense of shear while the D2 event a top to NE/SE sense of shear (the shear senses are considered at the present Sardinia – Corsica block position in the Mediterranean sea). The M2+D2 is characterized by a complicate, composite normal shear network; the M1+D1 by inverse shear zones. The M2+D2 is transposed by the late D3 strike slip shear event: the D3 is characterized by strike slip shear zones syn-kinematic to the emplacement of Late Carboniferous granitoids (320 Ma – 300 Ma). Despite the absence of geochronological data about the M1+D1 event, the field relationships suggest, for first time, an older migmatization process (Devonian?) syn-tectonic with the late stage of thickness of the Sardinia Variscan Belt. Similar evolutions are recognized in different segments of the Variscan Belt such as the Massif Central (France) or in the eastern mid-European Variscides.     

Mineral Composition and Metamorphic P-T Conditions of the Eclogitic Rocks in the Eastern Dabie Mountains and Their Genetic Implication

There are obvious differences in the mineral assemblage and metamorphic P-T conditions between the eclogites from the northern and southern parts of the eastern Dabie Mountains. Those from the northern part of the mountains are developed in Alpine peridotite and gneiss. They have a mineral assemblage of garnet+diopside with no quartz, and were formed at temperatures of 600℃-740℃. Those from the southern part are developed in gneiss and marble. They consist of garnet+omphacite+less quartz and were metamorphosed at temperatures in the range of 650°-800℃. These differences suggest that the former may be formed during the metamorphism of the deep subducted oceanic crust, whereas the latter may be genetically related to the subduction of the continental crust in this area.     

Metamorphic degassing of carbonates in the contact aureole of the Aguablanca Cu–Ni–PGE deposit,Spain

Analysis of magmatic and sedimentary rocks of several large igneous provinces has demonstrated that the release of gas during plutonic-metamorphic processes may be linked to global climate change and mass extinctions. Aguablanca, one of the largest Cu–Ni–PGE deposits in Europe, formed during the Variscan orogeny when a mafic magma intruded limestones and shales, creating a contact aureole composed of marble, skarn and hornfels. Our petrological and geochemical investigation of the aureole provides evidence that a combination of the two processes led to the formation of the ore deposit: The assimilation of terrigenous sediments supplied S to the magma while the assimilation of carbonates changed the oxygen fugacity and decreased the solubility of sulfur in the magma. The metamorphic assemblages in the contact aureole are directly related to heterogeneity of the protolith and particularly to the original proportions of calcite and clay. We modeled carbon dioxide degassing during contact metamorphism and showed that pure limestone is relatively unproductive because of its high reaction temperature. The presence of clay, however, leads to the formation of calc-silicates and significantly enhances CO₂ degassing. Our estimations suggest that degassing of the Aguablanca contact aureole released about 74.8 Mt of CO₂, a relatively low volume that we attribute to the composition of the host rock, mainly a pure limestone. A far larger volume of carbon dioxide was emitted by the contact metamorphism of dolostones in the contact aureole of Panzhihua (part of Emeishan large igneous province, SW China). We propose that the level of emission of carbon dioxide depends strongly on the nature of the protolith and has to be considered when predicting environmental impact during the emplacement of large igneous provinces.     

Petrogenesis of Eclogites in the Light of Punctuated Metamorphic Evolution in Dabie Terrane，China

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Petrology of the Non-mafic UHP Metamorphic Rocks from a Drillhole in the Southern Sulu Orogenic Belt, Eastern-Central China

The Drillhole ZK703 with a depth of 558 m is located in the Donghai area of the southern Sulu ultrahigh-pressure (UHP) metamorphic belt, eastern China, and penetrates typical UHP eclogites and various non-mafic rocks, including peridotite, gneiss, schist and quartzite. Their protoliths include ultramafic, mafic, intermediate, intermediate-acidic, acidic igneous rocks and sediments. These rocks are intimately interlayered, which are meters to millimeters thick with sharp and nontectonic contacts, suggesting in-situ metamorphism under UHP eclogite facies conditions. The following petrologic features indicate that the non-mafic rocks have experienced early-stage UHP metamorphism together with the eclogites: (1) phengite relics in gneisses and schists contain a high content of Si, up to 3.52 p.f.u. (per formula unit), while amphibolite-facies phengites have considerably low Si content (<3.26 p.f.u.); (2) jadeite relics are found in quartzite and jadeitite; (3) various types of symplectitic coronas and pseud     

Discovery of Coesite in the North Qaidam Caledonian Ultrahigh-High Pressure (UHP-HP) Metamorphic Belt in the NE Qinghai-Tibet Plateau

Coesite was discovered as inclusions in zircon separates from paragneiss associated with a large eclogite body in the North Belt of the Qaidam UHP terrane on the Chinese New Year of 2001 by Laser Raman Spectrometer at Tokyo Institute of Technology. Some graphite inclusions also found. However, extensive search did not reveal the occurrence of microdiamond. This finding confirms our previous suggestion that the North Qaidam UHP terrane was recrystallized at P below the diamond stability…     

Structure of the Soresat Metamorphic Complex,North Sanandaj–Sirjan Zone,northwest Iran

The metasedimentary and granitoid rocks of the Soresat Metamorphic Complex occur along the northern margin of the Sanandaj–Sirjan Zone in northwest Iran. Four different deformational events (D1–D4) are recorded in the Soresat Metamorphic Complex. The D1 and D2 progressive deformation events resulted from north-northeast–south-southwest regional horizontal shortening due to the subduction of Neo-Tethys oceanic lithosphere beneath the Sanandaj–Sirjan Zone. Post-suturing convergence between Arabia and Iran, which resulted in a right lateral-reverse displacement along the suture caused the north-northwest–south-southeast horizontal shortening of D3. D4 is recorded by normal faulting. Andalusite, cordierite and sillimanite (fibrolite) record the thermal peak (with a geothermal gradient >30°C/km). Field and microscopic studies of intruded granitoid rocks in the Soresat Metamorphic Complex divide them into three major groups: (i) syn-deformation (syn-D2) granitic gneiss; (ii) late- to post-deformation (late- to post-D2) granites and granodiorites; and (iii) post-deformation (post-D2) alkali granites.     

Li and B Isotope Systematics of Ultrahigh-pressure Metamorphic Rocks from the Chinese Continental Scientific Drilling Program

1 Introduction Recent improvements in the precision of Li and B isotope measurements have demonstrated the potential of these elements in tracing a wide range of geological processes. The Li and B isotope systematics of ultrahigh-pressure (UHP) metamorphic rocks provides a unique opportunity to investigate the behaviour of Li and B during fluid-rock interaction at high temperatures and very high pressures and to constrain the fluid budget and the recycling of subducted crustal materials into the mantle during UHP metamorphism.     

Formation Time of the Kalaqin Metamorphic Core Complex: Constraints from Zircon DatingEI北大核心CSCD

The Kalaqin metamorphic core complex, located on the northern margin of the Yanshan Tectonic Belt, is an important structure to understand the Late Mesozoic destruction processes of the North China Craton. In this study, structural analysis and geochronological investigation were conducted for the Anjiayingzi pluton and associated granodiorite dikes that intruded the core complex. Field observations demonstrated that emplacement of the pluton and dikes took place after the early stage extensional deformation, and intrusions are products of syn-tectonic magmatism during the late stage extension deformation. LA-ICP-MS zircon U-Pb dating yielded ages of 133-131 Ma for the Anjiayingzi pluton and 135 Ma for the dike, which demonstrated that the early extensional deformation took place at 156-135 Ma, rather than the Early Cretaceous as proposed previously. By integrating with other dating results, the early extensional deformation occurred at 156-150 Ma (Late Jurassic) and led to the exhumation of the core complex. The core complex was overprinted by a late extension in the Early Cretaceous. The revision of formation time for the Kalaqin metamorphic core complex further confirms that the extensional deformation took place in the Late Jurassic in the Yanshan tectonic belt, and therefore, it is likely that the northern margin of the North China Craton might have destructed since the Late Jurassic. © 2018, Science Press. All right reserved.