The Metamorphism of High-Pressure Basic Granulite in Laixi District, East Shandong, China

A high pressure basic granulite ultramafics belt is found in the late Archaean gneisses of Laixi Laiyang Qixia district, east Shandong. This belt has previously been considered as another eclogite belt in Sulu ultra high pressure(UHP) terrain. The typical high P granulite is characterized by mineral assemblages of three generations formed in different metamorphic episode respectively. The mineral assemblage of first episode is Grt(core)+Cpx+Pl, that of the second episode is Grt(rim)+Cpx+Opx+Pl+Amp+Q+Mt and that of the third episode is Cpx+Pl+Amp+Q+Mt. In first assemblage, the garnet porphyroblast is rich in grossular and pyrope component, and the coexisting Cpx is Al rich. Using mineral thermobarometer and Berman’s (1991) TWQ program, the P T conditions of three episodes are established, i.e. T=840℃～860℃, P>1.4GPa for first episode; T=720℃～780℃,P=0.8～1.2GPa for second episode and T=600℃～800℃,P=0.5～0.7GPa for the third episode.Considering the reaction texture, mineral Chemistry and PT estmations, a clockwise PTt path can be constructed, which shows post peak isothermal decompression at first and followed by a process of pressure and temperature decreasing. The mineral wholerock Sm Nd isochron age of the granulite is 1752Ma, which indicates its last thermal event during early middle Proterozoic stage. Therefore, it is concluded that the high pressure granulite and ultramafics of this belt represents the lower continental curst of the North China Craton during early Precambrian without any relationship to tectonic process of the Sulu UHP terrain.

The discovery of TiO_(2)-Ⅱ,the α-PbO_(2)-structured high-pressure polymorph of rutile,in the Suizhou L6 chondrite

We report the discovery of TiO_(2)-Ⅱ in the unmelted rock of the shocked Suizhou L6 chondrite.Natural TiO_(2)-Ⅱ was previously found in ultrahigh-pressure metamorphic and mantle-derived rocks,terrestrial impact structures,and tektite.Our microscopic,Raman spectroscopic,electron microprobe and transmission electron microscopic investigations have revealed:(1) All observed TiO_(2)-Ⅱ grains are related with ilmenite and pyrophanite;(2) TiO_(2)-Ⅱ occurs as needle-and leaf-shaped inclusions in llmenite and patch-,tape-shaped body in pyrophanite;(3)The composition of TiO_(2)-Ⅱ is identical with that of its precursor rutile;(4) The Raman spectrum of TiO_(2)-Ⅱ is in good agreement with that of natural and synthesized α-PbO_(2)-type TiO_(2);(5) TiO_(2)-Ⅱ occurs mainly in the form of well-ordered nano-domains and small mis-orientation among the domains can be observed.(6) All electron diffraction reflections from TiO_(2)-Ⅱ can be indexed to α-PbO_(2)structure in space group Pbcn with lattice parameters of a=4.481 ?,b=5.578 A and c=4.921 A;(7) The exsolution inclusions of rutile from host ilmenite are mostly connected with an alternation process along the lamellar twinning plane of ilmenite induced by shockinduced high pressure and high temperature;(8) The P-T regime of 20-25 GPa and 1000 ℃ estimated for the Suizhou unmelted rock is suitable for phase transition of rutile into TiO_(2)-Ⅱ phase.

Geochemical Characteristics of Pb Isotope of High-Pressure Metamorphic Rocks and Foliated Granites from HP Unit of Tongbai-Dabie Orogenic Belt

Whole-rock Pb isotopic compositions of the high-pressure (HP) metamorphic rocks, consisting of two-mica albite gneisses and eclogites, and foliated granites from the HP metamorphic unit of the Tongbai-Dabie orogenic belt are firstly reported in this paper. The results show that the HP metamorphic rocks in different parts of this orogenic belt have similar Pb isotopic compositions. The two- mica albite gneisses have 206 Pb/ 204 Pb=17.657-18.168, 207 Pb/ 204 Pb=15.318-15.573, 208 Pb/ 204 Pb=38.315-38.990, and the eclogites have 206 Pb/ 204 Pb=17.599-18.310, 207 Pb/ 204 Pb=15.465- 15.615 , 208 Pb/ 204 Pb=37.968-39.143. The HP metamorphic rocks are characterized by upper crustal Pb isotopic composition. Although the Pb isotopic composition of the HP metamorphic rocks partly overlaps that of the ultrahigh-pressure (UHP) metamorphic rocks, as a whole, the former is higher than the latter. The high radiogenic Pb isotopic composition for the HP metamorphic rocks confirms that the subducted Yangtze continental crust in the Tongbai-Dabie orogenic belt has the chemical structure of increasing radiogenic Pb isotopic composition from lower crust to upper crust. The foliated granites, intruded in the HP metamorphic rocks post the HP/UHP metamorphism, have 206 Pb/ 204 Pb=17.128-17.434, 207 Pb/ 204 Pb=15.313-15.422 and 208 Pb/ 204 Pb=37.631-38.122, which are obviously different from the Pb isotopic compositions of the HP metamorphic rocks but similar to those of the UHP metamorphic rocks and the foliated garnet-bearing granites in the UHP unit. This shows that the foliated granites from the HP and UHP units have common magma source. Combined with the foliated granites having the geochemical characteristics of A-type granites, it is suggested that the magma for the foliated granites in the UHP and HP unit would be derived from the partial melting of the retrometamorphosed UHP metamorphic rocks exhumed into middle to lower crust, and partial magmas were intruded into the HP unit.

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

The Northern Qilian high-pressure metamorphic belt has experienced multipledeformation-metamorphism, which consists of at least four stages. In 550.8-526 Ma, eclogites wereformed. High temperature and pressure caused the escape of a large quantity of gas-liquid fluidsfrom rocks while silicate melt was generated. In the late stage, small amounts of CO_2 and H_2Oinfiltrating 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. Inthe uplift-exhumation stage (400-380 Ma) the increase of internal space of fractures in the rocksfavoured fluid infiltration and concentration. These fluids participated in hydration reactions inthe retro-metamorphism. The fluids participating in the mineral reactions have the compositions ofCaCl_2-NaCl-H_2O. In subsequent thrusting (<380 Ma), the metamorphic terrain was uplifted to theshallower crust and ductile-shearing deformation took place, which caused mainly dehydrationreactions of minerals. In a near-surface environment the metamorphic terrain experienced brittledeformation, forming many accompanying fractures. Immiscible CO_2 and low-salinity aqueous fluidsoccurred in these secondary microfractures and were trapped and sealed. The thermodynamic conditionsof different deformation-metamorphic stages of the metamorphic terrain were calculated and thecorresponding P-T-t path was deduced, showing that the metamorphic terrain has experienced aclockwise path indicated by T- and P-rising, and T- and P-falling processes. This reveals that thesubduction zone has undergone multiple tectono-dynamic processes, i.e. initial deep burial,subsequent quick uplift and near-surface tectonism.

Metamorphic gradient modification in the Early Cretaceous Northern Andes subduction zone:A record from thermally overprinted high-pressure rocks

New field observations and petrological data from Early Cretaceous metamorphic rocks in the Central Cordillera of the Colombian Andes allowed the recognition of thermally overprinted high-pressure rocks derived from oceanic crust protoliths.The obtained metamorphic path suggests that the rocks evolved from blueschist to eclogite facies towards upper amphibolite to high-pressure granulite facies transitional conditions.Eclogite facies conditions,better recorded in mafic protoliths,are revealed by relic lawsonite and phengite,bleb-to worm-like diopside-albite symplectites,as well as garnet core composition.Upper amphibolite to high pressure granulite facies overprinting is supported by coarse-grained brown-colored Ti-rich amphibole,augite,and oligoclase recrystallization,as well as the record of partial melting leucosomes.Phase equilibria and pressure-temperature(P-T)path modeling suggest initial high-pressure metamorphic conditions M1 yielding 18.2-24.5 kbar and 465-580℃,followed by upper amphibolite to high pressure granulite facies overprinting stage M2 yielding 6.5-14.2 kbar and 580-720℃.Retrograde conditions M3 obtained through chlorite thermometry yield temperatures ranging around 286-400℃at pressures below 6.5-11 kbar.The obtained clockwise P-T path,the garnet zonation pattern revealing a decrease in X_(grs)/X_(prp)related to Mg#increment from core to rim,the presence of partial melting veins,as well as regional constraints,document the modification of the thermal structure of the active subduction zone in Northern Andes during the Early Cretaceous.Such increment of the metamorphic gradient within the subduction interface is associated with slab roll-back geodynamics where hot mantle inflow was triggered.This scenario is also argued by the reported trench-ward magmatic arc migration and multiple extensional basin formation during this period.The presented example constitutes the first report of Cretaceous roll-back-related metamorphism in the Caribbean and Andean realms,representing an additional piece of evidence for a margin-scale extensional event that modified the northwestern border of South America during the Early Cretaceous.

The Key Influence of Fluid in Metamorphic Rock Preserves, Mineral Assemblages, Compositions and Structures: Study from High-Pressure Eclogite and Its Amphibolization in the Western Dabie Mountains,Central China

The most of high/ultrahigh-pressure(HP/UHP)terranes of the world are characterized by the occurrence of numerous pods,lenses or layered blocks of eclogite and amphibolites(e.g.O’Brien,1997;Elvevold and Gilotti,2000;Zhang et al.,2003;and references there in).Field and petrological features suggest that amphibolites should

Tectonic Evolution of an Early Precambrian High-Pressure Granulite Belt in the North China Craton

A large-scale high-pressure granulite belt (HPGB), more than 700 km long, is recognized within the metamorphic basement of the North China craton. In the regional tectonic framework, the Hengshan-Chengde HPGB is located in the central collision belt between the western block and eastern block, and represents the deep crustal structural level. The typical high-pressure granulite (HPG) outcrops are distributed in the Hengshan and Chengde areas. HPGs commonly occur as mafic xenoliths within ductile shear zones, and underwent multipile deformations. To the south, the Hengshan-Chengde HPGB is juxtaposed with the Wutai greenstone belt by several strike-slip shear zones. Preliminary isotopic age dating indicates that HPGs from North China were mainly generated at the end of the Neoarchaean, assocaited with tectonic assembly of the western and eastern blocks.

Blueschist:A window into high-pressure/low-temperature metamorphism and subduction zone dynamics

Blueschist is a regional metamorphic rock formed under high-pressure(HP)low-temperature(LT)conditions.It is formed in the subduction zone environment with low geothermal gradients(4-14℃km^(−1)),and is characterized by the presence of HP/LT index minerals like glaucophane,lawsonite,aragonite,jadeite,and deerite.In general,blueschist-facies rocks are stable in subduction zones at depths of 30-60 km,and transform to eclogite-facies rocks at greater depths.The preservation of blueschists requires a fast exhumation rate.Based on protolith and tectonic setting,blueschists can be grouped into Type-A and Type-B,but some metasomatic blueschists also occur.Blueschist belts distribute mainly as bands along the margins of orogenic belts,and their occurrences within cratons are very limited.Precambrian blueschists,of which the oldest ones are about 800-700 Ma in age,are rare;most of the exposed blueschist terranes are of post-Paleozoic metamorphic age.As diagnostic evidence of ancient subduction zones,blueschist plays an important role in understanding plate tectonics.Blueschist-eclogite transition at cold subduction zones involves dehydration reactions and fluid release,which are of great importance in facilitating slab-mantle wedge water and element recycling,generating arc magmatism,and triggering intermediate-depth earthquakes in the subducting slab.Metamorphic P-T paths of blueschists and associated rocks provide key information on constraining the onset of the subduction initiation and subsequent geodynamic evolution.As a cold geothermal indicator,the emergence of blueschist offers robust evidence for the start of modern plate tectonics on the Earth.Blueschist-facies metamorphism still represents an important research direction in metamorphic geology,which requires further investigations on determining the beginning of plate tectonics,constraining phase-transition processes,and constructing a global blueschist database.

High-Pressure Eclogite-Blueschist Metamorphic Belt and Closure of Paleo-Tethys Ocean in Central Qiangtang, Qinghai-Tibet Plateau

The high-pressure metamorphic belt （HPMB） of eclogite-blueschist in Central Qiangtang （羌塘） lies in the Longmu Co （龙木错）-Shuanghu （双湖） suture zone. To the west, the HPMB extends 500 km from Hongjishan （红脊山） to Caiduochaka （才多茶卡）, east of Shuanghu; to the east it extends to Baqing （巴青） and Jitang （吉塘） in Qamdo （昌都）, and then bends southward to Yunnan （云南） Province. Including the Lancangjiang （澜沧江） blueschist belt, the entire HPMB is about 2 000 km long. In Central Qiangtang, the belt is mainly composed of blueschist and eclogite, whereas in West Yunnan it contains only blueschist. The Baqing-Jitang segment is dominated by garnet phengite schist. 40Ar-39Ar dating of glaucophane and phengite from the blueschists yielded plateau ages ranging from 223 to 215 Ma, whereas SHRIMP U-Pb dating of zircon from the eclogites gives metamorphic ages of 243-217 Ma. The calculated metamorphic conditions for the blueschists are 410-460 ℃ and 0.67-0.75 GPa, and for the eclogites, 〈500℃ and 1.56-2.35 GPa. The metamorphic ages suggest that the Longmu Co-Shuanghu suture closed in the Late Triassic. The region south of the Longmu Co-Shuanghu- Lancang suture consists of the pan-African basement overlain by Condwana sedimentary and metasedimentary rocks, whereas the region north of the suture is dominated by the Jinning （晋宁） basement and Yangtze sedimentary and metasedimentary rocks. The Qiangtang HPMB marks the closure of the paleo-Tethys Ocean.

Early Cambrian high pressure/low temperature metamorphism in the southeastern Tarim craton in response to circum-Gondwana cold subduction

The circum-Gondwana subduction initiated by the early Cambrian has been suggested to reflect the establishment of the modern plate tectonics.The metamorphic rocks with low thermobaric(T/P)ratios indicative of cold subduction in the present tectonic regime have not been well investigated.To better understand the circum-Gondwana subduction and to test its possible link with the emergence of the modern plate tectonics,this study focused on blueschist-facies metamorphic rocks in the Altyn Tagh of the southeastern Tarim craton.Mineral assemblage and chemistry,phase equilibrium modelling,and quartz-in-garnet Raman elastic geobarometry reveal that the zoisite blueschist and glaucophane(Gln)-bearing quartz schist in northern Altyn Tagh were metamorphosed to lawsonite to epidote blueschistfacies at 520–545℃ and 16–19 kbar.It reflects high-pressure(HP)/low temperature(LT)metamorphism with low T/P ratios of<300℃/GPa and thermal gradients of<10℃/km.These blueschist-facies metamorphic rocks underwent rapid decompression starting at P-T conditions of<495℃ and<9.6 kbar during exhumation.Ar-Ar geochronology records paragonite Ar-Ar plateau ages of 520–506 Ma for the zoisite blueschist samples and phengite Ar-Ar plateau ages of 522–516 Ma for the Gln-bearing quartz schist samples,suggesting that the peak HP/LT metamorphism occurred prior to ca.522 Ma.Based on new results and available data from the major Gondwana blocks,cold subduction was suggested to profoundly operate along circum-Gondwana in the early Cambrian after the amalgamation of Gondwana.The extensive circum-Gondwana subduction represents the earliest global cold subduction in Earth’s history associated with the establishment of the modern plate tectonics,as directly recorded by the studied early Cambrian blueschist-facies metamorphic rocks and a dramatic drop in the mean T/P of metamorphism since the early Paleozoic.

Study and implication of the high-pressure felsic granulite in the Qinling complex of East Qinling

There is a typical assemblage of garnet + kyanite + microperthite + quartz + rutile in high-pressure (HP) felsic granulite of Qinling complex in Songshugou area. East Qinling. The HP granulite was formed at 800 -900℃ and 1.3 -1.6GPa and has experienced two stages of retrograde metamorphism at 600- 650 ℃, 0.8-1.0GPa and 500-600℃, 0.3-0.6GPa, forming two retrograde metamorphic assemblages of margarite + plagiodase (PlI)+quartz and sillimanite + biotite + plagioclase(PlII) + microdine+quartz, respectively. They construct a two-stage clockwise P-T path which shows down-pressure cooling in both early and late stage.

Discovery and study of high-pressure basic granulites in Songshugou area of Shangnan, East Qinling

1 Geological occurrence The ultramafic rocks in the Songshugou area of the North Qinling are structurally emplaced in middle-late Proterozoic amphibolites (fig. 1) which are regarded as an external rootless ophiolite sheet with emplacement age of (983±140) Ma. The lenticular diopsidegarnet-amphibolites, named garnet-amphibolte by others, are exposed in the amphibolits. The high-pressure basic granulites discussed in this note were found in the lenticular garnet-

40Ar-39Ar Geochronology and Tectonic Implications of the Blueschist from Northwestern Qiangtang, Northern Tibet, Western China

Blueschist exposed in the northwestern Qiangtang terrane, northern Tibet, western China （84 30＇ E, 34024＇ N）, provides new constraints on the tectonic evolution of Qiangtang as well as northern Tibet. The blueschist represented by lawsonite- and glaucophane-bearing assemblages equilibrated at 375-400 C and 11 kbar. 4Ar-39Ar analysis on mineral separate from one blueschist sample yielded a well-defined plateau age of 242 Ma. Geochemical studies show the blueschist is metamorphosed within-plate basalts. The high pressure-low temperature blueschist indicates a Triassic event of lithosphere subduction, and clearly represents an extension of the central Qiangtang metamorphic belt, and defines an in situ suture between eastern and western Qiangtang.

Compressional origin of the Aegean Orogeny,Greece

The Aegean Sea area is thought to be an actively extending back-arc region,north of the present day Hellenic volcanic arc and north-dipping subduction zone in the Eastern Mediterranean.The area shows extensive normal faulting,ductile‘extensional’shear zones and extensional S-C fabrics throughout the islands that have previously been related to regional Aegean extension associated with slab rollback on the Hellenic Subduction Zone.In this paper,we question this interpretation,and suggest the Cenozoic geodynamic evolution of the Aegean region is associated with a Late Cretaceous-Eocene NE-dipping subduction zone that was responsible for continentcontinent collision between Eurasia and Adria-Apulia/Cyclades.Exhumation of eclogite and blueschist facies rocks in the Cyclades and kyanite-sillimanite grade gneisses in the Naxos core complex have pressures that are far greater than could be accounted for purely by lithospheric extension and isostatic uplift.We identify four stages of crustal shortening that affected the region prior to regional lithospheric extension,herein called the Aegean Orogeny.This orogeny followed a classic Wilson cycle from early ophiolite obduction(ca.74 Ma)onto a previously passive continental margin,to attempted crustal subduction with HP eclogite and blueschist facies metamorphism(ca.54-45 Ma),through crustal thickening and regional kyanite-sillimanite grade Barroviantype metamorphism(ca.22-14 Ma),to orogenic collapse(<14 Ma).At least three periods of‘extensional’fabrics relate to:(1)Exhumation of blueschists and eclogite facies rocks showing tight-isoclinal folds and top-NE,base-SW fabrics,recording return flow along a subduction channel in a compressional tectonic setting(ca.50-35 Ma).(2)Extensional fabrics within the core complexes formed by exhumation of kyanite-and sillimanite gneisses showing thrust-related fabrics at the base and‘extensional’fabrics along the top(ca.18.5-14 Ma).(3)Regional ductile-brittle‘extensional’fabrics and low-angle normal faulting related to the North Cycladic Detachment(NCD)and the South(West)Cycladic Detachment(WCD)during regional extension along the flanks of a major NW-SE anticlinal fold along the middle of the Cyclades.Major low-angle normal faults and ductile shear zones show symmetry about the area,with the NE chain of islands(Andros,Tinos,Mykonos,Ikaria)exposing the NE-dipping NCD with consistent top-NE ductil e fabrics along 200 km of strike.In contrast,from the Greek mainland(Attica)along the SE chain of islands(Kea,Kythnos,Serifos)a SW-dipping low-angle normal fault and ductile shear zone,the WCD is inferred for at least 100 km along strike.Islands in the middle of the Cyclades show deeper structural levels including kyanite-and sillimanite-grade metamorphic core complexes(Naxos,Paros)as well as Variscan basement rocks(Naxos,Ios).The overall structure is an~100 km wavelength NW-SE trending dome with low-angle extensional faults along each flank,dipping away from the anticline axis to the NE and SW.Many individual islands show post-extensional large-scale folding of the low-angle normal faults around the domes(Naxos,Paros,Ios,Sifnos)indicating a post-Miocene late phase of E-W shortening.

Sapphirine-Bearing Pelitic Granulite from Ailaoshan Orogen,West Yunnan,China: Metamorphic Conditions and Tectonic Setting

To reveal the petrological characteristics, metamorphic evolution histon and tectonic setting of the pelitic granulites from Ailaoshan Orogen, Uest Yunnan, China, a comprehensive study in mineral chemistry, petrogeochemistry and geochronology studies is presented in this paper. Two metamorphic stages of the granulites can be established:(1) the peak metamorphism recorded by the mineral assemblage of garnet, kyanite, K-feidspar and rutile, and the initial retrograde metamorphism shown by the mineral assemblage of garnet, sillimanite, sapphirine, spinel, K-feldspar, plagioclase and biotite;(2) the superim-posed metamorphism recorded by the mineral assemblage of biotite, muscovite, plagioclase, quartz and ilmenite. Zircon LA-ICP-MS U-Pb dating indicates that the protolith of the granulite was deposited after 337 Ma. The initial retrograde metamorphism occurred at P-T conditions of 8.6-12 kbar at 850-920℃ estimated by mineral assemblages, the low pressure limit of kyanite stability and GBPQ geothermobarometer in Indosinian (about 235 Ma), and the late superimposed metamorphism occurred at the P-T condition of 3.5-3.9 kbar at 572-576℃ estimated by GBPQ geothermobarometer since 33Ma. The first stage was related to the amalgamation of the South China and Indochina blocks during the Triassic, and the second stage was possibly related with the large scale sinistral slip-shearing since the Oligocene. It is inferred that the upper continental crust was suhducted/underthrusted to the lower continental crust (deeper than 30 km) and underwent granulite-facies metamorphism and then quickly exhumed to the middle-upper crust (10-12 km) and initial retrograde metamorphism occurred due to the collision of the Indochina and South China blocks during Indosinian, which was followed by superimposition of the second stage of metamorphism since the Oligocene.

Geological,Structural and Mineralogical Approach to Investigate the Evolution of Low-and very Low-Grade Metamorphic Units from the Intra-Pontide Suture Zone,Central Pontides,Turkey

A multidisciplinary approach combining multiscale geological-structural analyses(from field to microscale) and clay mineral transformations(clay mineralogy, illite and chlorite "crystallinity" and b cell dimension(b_(0)) of K-white mica) was adopted to unravel the tectono-metamorphic evolution of low-and very low-grade tectono-metamorphic units from the Intra-Pontide suture zone in northern Turkey. The mineralogical study allowed to better evaluate the structural stacking outlined during the geological mapping, leading to distinguish three tectono-metamorphic units, two in epizone(Daday and Emirkoy units) and one in anchizone(Cifter Unit) metamorphic conditions. The mesostructural and microstructural analyses suggest a polyphase tectonic-metamorphic evolution. The different macroscopic features observed between the two units characterized by the same epizone metamorphism, can be justified by the evidence that these metamorphic conditions were acquired during the last stages of exhumation in the Daday Unit, while they constrained the metamorphic-peak conditions in the Emirkoy Unit. Contemporary analyses and comparison between structural and mineralogical data have thus proven to be a powerful tool to investigate the low-grade and very low-grade metamorphic environments, allowing at the same time to solve the apparent contradictions deriving from the mineralogical study and to significantly improve the detail of the geological mapping in the field.

Recognition and implication of eclogite in the western Altun Mountains, Xinjiang

SINCE high-pressure metapelitic rocks have been discovered in the Beiketan area of eastern Al-tun Mountains,a set of eclogite has been recognized in south of Jianggesayi,western Altun.The eclogite,associated with quartz-garnet-clinopyroxenite and metamorphosed retrogradelyto garnet-amphibolite partially,is exposed in the Proterozoic meso- to hypometamorphic rocksand can be traced eastward to northwest of Mangya asbestos deposit,for about 200

Exhumation of deeply subducted crust:Review and outlook

The discovery of ultrahigh-pressure(UHP)index minerals such as coesite and micro-diamond in supracrustal rocks indicate that a large amount of crustal rocks were subducted to mantle depths and subsequently returned to the Earth’s surface.However,exhumation mechanism of the deeply subducted crustal rocks has been a hot topic in geodynamic community and remains controversial.Previous studies have been performed by petrologic observations,analogue experiments and numerical simulations for exhumation processes and mechanisms of deeply subducted crustal rocks,and a series of important results have been achieved.In this paper,we summerize in detail recent progresses and existing problems on types of subduction zones,exhumation processes and mechanisms of deeply subducted crustal rocks,and related factors.Based on these,three forthcoming prospects are presented:(1)The geological processes within the exhuming crust;(2)the interaction between the exhuming crust and the surface process;(3)the interaction between the exhuming crust and the mantle.

Zircon Ages of Metasedimentary Rocks in the Wuwamen Ophiolitic Mélange,Chinese South Tianshan:Implications for the Paleozoic Subduction-Accretion in the Southern Central Asian Orogenic Belt

High-temperature and high-pressure(high-grade)metamorphic complexes of variable ages are common in the Central Asian orogenic belt(CAOB),and their precise geochronology and origin are essential to unravel the orogenic architecture and crust-production rate of the CAOB.Hereby it is essential to differentiate between pre-orogenic Precambrian basement and Paleozoic arc-accretionary complexes.This study provides precise in situ zircon U-Pb ages for the metasedimentary rocks in the Wuwamen ophiolitic mélange,which is traditionally thought to represent the pre-orogenic basement of the southwestern CAOB.A meta-sandstone from the meta-flysch sequence revealed a widespread ca.1.8 Ga high-grade metamorphic overprint similar to that of the underlying orthogneisses and,thus,was interpreted to represent pre-orogenic basement fragments that occur as tectonic blocks in the Paleozoic ophiolitic mélange.In contrast,a schist from the mélange matrix is characterized by a ca.333 Ma high-grade metamorphic overprint,indicating that the northward subduction of the South Tianshan Ocean was still active at this time.The thrust-imbricated high-grade metamorphic rocks of contrasting origins in an accretionary complex have important implications for the understanding the accretionary history and crustal growth of the CAOB.Furthermore,the strongly deformed ophiolitic mélange was intruded by an undeformed granite dyke with an emplacement age younger than ca.294 Ma,thus providing a minimum age limit for the final closure of the South Tianshan Ocean.

Evolution of pore structure in organic shale with typeⅢkerogen and high kaolinite content in Ningwu Basin

Special deposition environment makes organic-rich shales in Ningwu Basin have typeⅢkerogen and high kaolinite content,which are also famous as the kaolinite ore.The specific composition of shale in Ningwu Basin can change the pore structure and thus,influence the shale gas storage and transport.This study focuses on the pore structure and its evolution in shales with typeⅢkerogen and high kaolinite content.In this study,14 Upper Paleozoic shale samples,whose total organic matter contents(TOC)range from 0.39%to 5.91%and maturities(represented by vitrinite reflectance)range from 1.22%to 2.06%,were collected.Scanning electron microscopy(SEM),high-pressure mercury injection,and low-tempera-ture N2 adsorption experiments were used to analyze pore structure of these shale samples.Results show that when the TOC content is smaller than 1.4%,the kaolinite content decreases linearly and quartz content increases linearly with increasing the TOC content.In contrast,when TOC content is>1.4%,the clay content tends to increase with increasing TOC.Based on the SEM images,organic pores and clay pores were identified in shale samples with typeⅢkerogen and high kaolinite content.During the maturation process,the kaolinite content decreases and illite content increases with increasing the vitrinite reflectance.At the same time,the pore volume and pore surface area both increase with increasing the vitrinite reflectance,and it may be because more organic pores and clay pores in the illite were generated during the maturation process.This study can provide further understandings of shale gas accumulation in shale with typeⅢkerogen and high kaolinite content.