Structural,^(40)Ar/^(39)Ar Geochronological and Rheological Feature Analysis of the Guoxuepu Shear Zone:Indications for the Jitang Metamorphic Complex in the Northern Lancangjiang Zone

The Jitang metamorphic complex is key to studying the tectonic evolution of the Northern Lancangjiang zone.Through structural-lithological mapping,structural analysis and laboratory testing,the composition of the Jitang metamorphic complex was determined.The macro-and microstructural analyses of the ductile detachment shear zone(Guoxuepu ductile shear zone,2–4 km wide)between the metamorphic complex and the overlying sedimentary cap show that the shear sense of the ductile shear zones is top-to-the-southeast.The presence of various deformation features and quartz C-axis electron backscatter diffraction(EBSD)fabric analysis suggests multiple deformation events occurring at different temperatures.The average stress is 25.68 MPa,with the strain rates(έ)ranging from 9.77×10^(−14)s^(−1)to 6.52×10^(−16)s^(−1).The finite strain of the Guoxuepu ductile shear zone indicates an elongated strain pattern.The average kinematic vorticity of the Guoxuepu ductile shear zone is 0.88,implying that the shear zone is dominated by simple shear.The muscovite selected from the protomylonite samples in the Guoxuepu ductile shear zone yields a 40Ar-39Ar age of 60.09±0.38 Ma.It is suggested that,coeval with the initial Indo–Eurasian collision,the development of strike-slip faults led to a weak and unstable crust,upwelling of lower crust magma,then induced the detachment of the Jitang metamorphic complex in the Eocene.

TWO EPISODES OF MONAZITE CRYSTALLIZATION DURING METAMORPHISM AND CRUSTAL MELTING IN THE EVEREST REGION OF THE NEPALESE HIMALAYA

New monazite U\|Pb geochronological data from the Everest region suggest that 20～25Ma elapsed between the initial India—Asia collision and kyanite\|sillimanite grade metamorphism. Our results indicate a two\|phase metamorphic history, with peak Barrovian metamorphism at (32 2±0 4)Ma and a later high\|temperature, low\|pressure event (620℃, 400MPa) at (22 7±0 2)Ma.. Emplacement and crystallization of the Everest granite subsequently occurred at 20 5～21 3Ma. The monazite crystallization ages that differ by 10Ma are recorded in two structurally adjacent rocks of different lithology, which have the same post collisional p—T history.. Scanning electron microscopy reveals that the younger monazite is elaborately shaped and grew in close association with apatite at grain boundaries and triple junctions, suggesting that growth was stimulated by a change in the fluid regime. The older monazite is euhedral, is not associated with apatite, and is commonly armoured within silicate minerals. During the low\|pressure metamorphic event, the armouring protected the older monazites, and a lack of excess apatite in this sample prevented new growth. Textural relationships suggest that apatite is one of the necessary monazite\|producing reactants, and spots within monazite that are rich in Ca, Fe, Al and Si suggest that allanite acted as a preexisting rare earth element host. We propose a simplified reaction for monazite crystallization based on this evidence.

Metamorphism of the East Sector of the Southern Qinling Orogenic Belt and Its Geological Significance

The east sector of the southern Qinling belt is, lithologically, composed mainly of metapelites,quartzites, marbles and small amount of metabasites and gneisses, whose protoliths are the Silurian, Devonian andless commonly the Sinian and Upper Palaeozoic. They have been subjected at least to two epochs of metamorphism.The early epoch belongs to progressive metamorphism which is centered on high amphibolite-granulite facies in theFuping area and changed outwards into low amphibolite facies (staurolite-kyanite zone), epidote amphibolite facies(garnet zone) and greenschist facies (chlorite and biotite zones), the metamorphic age of which is about 220-260 Ma.This early-epoch metamorphism belongs to different pressure types: the rocks from greenschist to low amphibolitefacies belong to the typical medium-pressure type which shows geothermal gradients of about 17-20℃/km and wasprobably produced by a crustal thickening process related to continental collision, and the high amphibolite-granulitefacies belongs to the low-pressure type which shows geothermal gradients of about 25-38℃/km and was probablyaffected by some magmatic heats. Based on the basic characteristics of the P-T paths of the different facies calculatedfrom the garnet zonations, it can be deduced that the metamorphism of medium-pressure facies series took place dur-ing an imbricated thickening process, rather than during the uplifting process after thickening. The late-epoch meta-morphism belongs to dynamic metamorphism of greenschist facies which is overprinted on the early-epoch meta-morphic rocks and is Yanshanian or Himalayan in age, probably related to intracontinental orogeny.

High- and Ultrahigh-pressure Metamorphism and Retrogressive Textures of Gneiss in the Donghai Area——Evidence from gneisses in drillhole ZK2304

In the gneisses from the drillhole ZK2304 of the Donghai area, there have been preserved high- and ultrahigh-pressure metamorphic mineral assemblages, a series of complicated retrogressive textures and relevant metamorphic reactions. In addition to garnet, jadeititic-clinopyroxene and rutile, other peak stage (M2) minerals in some gneisses include phengite, aragonite and coesite or quartz pseudomorphs after coesite. The typical peak-stage mineral assemblages in gneisses are characterized by garnet + jadeitic-clinopyroxene + rutile + coesite, garnet + jadeitic-clinopyroxene + phengite + rutile ± coesite and garnet + jadeitic-clinopyroxene + aragonite + rutile ± coesite. The grossular content (Gro) in garnet is high and may reach 50. 1 mol%. The SiO2 content of phengite ranges from 54.37% to 54.84% with 3.54-3.57 p.f.u. Quartz pseudomorphs after coesite occur as inclusions in garnet.The gneisses of the Donghai area have been subjected to multistage recrystallization and exhibit a closewise P-T

Eclogites of the Dabie Region: Retrograde Metamorphismand Fluid Evolution

Based upon fluid effects, retrograde metamorphism of eclogites in the Dabie region can be divided into the fluid-poor, fluid-bearing and fluid-rich stages. The fluid-poor stage is marked by polymorphic inversion, recrystallization and exsolution of solid solutions, and is thought to represent eclogite-facies retrograde environments. The fluid-bearing stage is likely to have occurred at the late stage of ecologite-facies diaphthorosis and is represented by kyanite porphyroblasts, rutile, and sodic pyroxene in association with high-pressure hydrous minerals such as phengite and zoisite (clinozoisite) without significant amount of hydrous minerals such as amphibole, epidote and biotite. The fluid-rich stage might have commenced concomitantly with lower amphibolite-facies diaphthoresis and persisted all the way towards the near-surface environment. The product of this stage is characterized by plentiful hydrous and volatile-bearing phases.The dissemination-type rutile mineralizations in eclogites might have formed by preferential shearing-induced pressure solution of gangue minerals at the fluid-bearing stage. The accompanying vein rutile was precipitated from fluids of this stage after local transport and concentration, and may hence represent proximal mobilization of titanium from the eclogite. Therefore, rutile veins can be used as an exploration indicator for dissemination-type rutile deposits.

Precambrian Metamorphism of Tiya－Olokit Metamorphic Belt in Northern Balkal Region and its Metallogeny

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Extensional Tectonic Framework of Post High and Ultrahigh Pressure Metamorphism in Dabieshan,China

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Study of oxidative desulphurization process of coal with different metamorphism degrees

The oxidative desulphurization process of coal with different metamorphism degrees treated by an air-steam mixture has been studied.It has been shown that the pyrite present in black coal and anthracite is oxidized with the sulphur dioxide formation,and the process chemical mechanism does not depend on the quality of organic matter.The medium-metamorphized coal,capable of turning into a plastic state and cake in the range of investigated temperatures(350～450 ℃),is desulphurized with the greatest difficulty.The chemical mechanism dealing with the transformations of pyritic sulphur present in brown coal differs from similar processes taking place in black coal and anthracite,because FeS2 is converted with hydrogen sulphide formation at desulphurization.

THE ALTUN—NORTH QAIDAM ECLOGITE BELT IN WESTERN CHINA—ANOTHER HP-UHP METAMORPHIC BELT TRUNCATED BY LARGE SCALE STRIKE-SLIP FAULT IN CHINA

The Altun and North Qaidam Mountains at the northern margin of Qinghai\|Tibet plateau are separated by the Altyn Tagh sinistral strike\|slip fault, which is one of the largest strike\|slip fault systems in the world and was considered as the key element in the escape tectonics model for Euraisa\|India continent\|continent collision.Recently,the eclogites within quratzifeldspathic gneisses or pelitic gneisses characterized by amphibolite\|facies paragenesis were discovered in the Altun and the North Qaidam Mountains(Fig.1). They occur as lens or boundins within the Altun Group and Dakendaban Group respectively which previously were considered as metamorphic basement of Tarim block and Qaidam block. Our studies indicate that the eclogites outcrop in both the Altun and North Qaidam Mountains show similar occurrences, associated country rocks, rock and mineral assemblages, p\|T\% estimates, geochemistryand protolith feature and ages of peak metamorphism (see table) . The garnet\|omphacite\|phengite geothermobarometer gave equilibrium condition of \%p\%=2 8～3 0GPa and t =820～850℃ for the Altun eclogite and p =2 8GPa and \%t\%=730℃ for North Qaidam eclogite respectively(Fig..2). These p\|T conditions are in the coesite stability field. Moreover, Po lycrystalline quartz pseudomorphs after coesite have been identified in the Dulan area, North Qaidam Mountains (Song et al, in review). Therefore, these features suggest that both eclogites of Altun and North Qaidam Mountains probably are a same HP\|UHP metamorphic belt formed from the same of Early Paleozoic age deep subduction and collision, and subsequently displaced by the Altyn Tagh fault.The case is similar to the Dabie\|Sulu HP\|UHP metamorphic zone which was truncated by the Tanlu sinistral strike\|slip fault and splitted it into two distincts, the Dabie region and Sulu region. These correlations support an about 350～400km displacement of the Altyn Tagh sinistral strike\|slip fault (Fig.1).

Timing of metamorphism and deformation in the Swat valley, northern Pakistan:Insight into garnet-monazite HREE partitioning

New metamorphic petrology and geochronology from the Loe Sar dome in the Swat region of northern Pakistan place refined constraints on the pressure, temperature and timing of metamorphism and deformation in that part of the Himalayan orogen. Thermodynamic modelling and monazite petrochronology indicate that metamorphism in the area followed a prograde evolution from ～525 ± 25 ℃and 6 士 0.5 kbar to ～610 ± 25 ℃ and 9 士 0.5 kbar, between ca. 39 Ma and 28 Ma. Partitioning of heavy rare earth elements between garnet rims and 30-28 Ma monazite are interpreted to indicate coeval crystallization at peak conditions. Microtextural relationships indicate that garnet rim growth post-dated the development of the main foliation in the area. The regional foliation is folded about large-scale N-S trending fold axes and overprinting E-W trending folds to form km-scale domal culminations. The textural relationships observed indicate that final dome development must be younger than the 30-28 Ma monazite that grew with garnet rims post-regional foliation development, but pre-doming-related deformation. This new timing constraint helps resolve discrepancy between previous interpretations,which have alternately suggested that N-S trending regional folds must be either pre-or post-early Oligocene. Finally, when combined with existing hornblende and white mica cooling ages, these new data indicate that the study area was exhumed rapidly following peak metamorphism.

A Comparison about Metamorphism among the Oldest-Rock Units from Orogenic Belts of Dabie,Eastern Qinling and Eastern Kunlun of Central Mountain Ranges,China

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Campaign-style U-Pb titanite petrochronology:Along-strike variations in timing of metamorphism in the Himalayan metamorphic core

Present-day along-strike heterogeneities within the Himalayan orogen are seen at many scales, from variations within the deep architecture of the lithospheric mantle, to differences in geomorphologic surface processes. Here, we present an internally consistent petrochronologic dataset from the Himalayan metamorphic core(HMC), in order to document and investigate the causes of along-strike variations in its Oligocene-Miocene tectonic history. Laser ablation split-stream analysis was used to date and characterise the geochemistry of titanite from 47 calc-silicate rocks across >2000 km along the Himalaya.This combined U-Pb-REE-Zr single mineral dataset circumvents uncertainties associated with interpretations based on data compilations from different studies, mineral systems and laboratories, and allows for direct along-strike comparisons in the timing of metamorphic processes. Titanite dates range from ～30 Ma to 12 Ma, recording(re-)crystallization between 625 ℃ and 815 ℃. Titanite T-t data overlap with previously published P-T-t paths from interleaved peltic rocks, demonstrating the usefulness of titanite petrochronology for recording the metamorphic history in lithologies not traditionally used for thermobarometry. Overall, the data indicate a broad eastward-younging trend along the orogen.Disparities in the duration and timing of metamorphism within the HMC are best explained by alongstrike variations in the position of ramps on the basal detachment controlling a two-stage process of preferential ductile accretion at depth followed by the formation of later upper-crust brittle duplexes.These processes, coupled with variable erosion, resulted in the asymmetric exhumation of a younger,thicker crystalline core in the eastern Himalaya.

Paleozoic post-collisional magmatism and high-temperature granulite-facies metamorphism coupling with lithospheric delamination of the East Kunlun Orogenic Belt,NW China

Lithosphere extension and upwelling of asthenosphere at post-collisional stage of an orogenic cycle generally induce diverse magmatism and/or associated high-temperature metamorphism. Nevertheless, the intimate coexistence of post-collisional magmatic activity and high-temperature metamorphism is rare.In this contribution, a lithological assemblage composing of diverse magmatic rocks deriving from distinct magma sources and coeval high-temperature metamorphism was identified in eastern Kunlun.Petrography, ages, mineral chemistry and whole-rock geochemistry demonstrated that those intimately coexistent diverse rocks were genetically related to post-collisional extension. The garnet-bearing mafic granulites in Jinshuikou area interior of the East Kunlun Orogenic Belt are mainly composed of garnet,orthopyroxene, and plagioclase, with peak metamorphic P–T conditions of ~ 701–756 ℃and 5.6–7.0 kbar,representing a granulite-facies metamorphism at 409.7 ± 1.7 Ma. The diverse contemporaneous magmatic rocks including hornblendites, gabbros and granites yield zircon U–Pb ages of 408.6 ± 2.5 Ma,413.4 ± 4.6 Ma, and 387–407 Ma, respectively. The hornblendites show N-MORB-like REE patterns with(La/Sm)Nvalues of 0.85–0.94. They have positive zircon εHf(t) values of 0.1–4.9 and whole-rock εNd(t) values of 3.9–4.7 but relatively high(^(87)Sr/^(86)Sr)_(i)values of 0.7081 to 0.7088. These features demonstrate that the hornblendites derived from a depleted asthenospheric mantle source with minor continental crustal materials in source. As for the gabbros, they exhibit arc-like elemental signatures, low zircon εHf(t) values(-4.3 to 2.5) and variable whole-rock εNd(t) values(-4.9 to 1.2) as well as high(^(87)Sr/86 Sr)ivalues(0.7068 to 0.7126), arguing for that they were originated from partial melting of heterogeneous lithospheric mantle anteriorly metasomatized by subducted-sediment released melts. Geochemistry of the granites defines their strongly peraluminous S-type signatures. Zircons from the granites yield a large range of εHf(t) values ranging from -30.8 to -5.1, while the whole-rock samples yield consistent(^(87)Sr/86 Sr)ivalues(0.7301 to 0.7342) and negative εNd(t) values(-10.1 to -12.4). These features indicate that the S-type granites could be generated by reworking of an ancient crust. Taken together, the penecontemporaneous magmatism and metamorphic event, demonstrated the early-middle Devonian transition from crustal thickening to extensional collapse. The post-collisional mantle-derived magmas serve as an essential driving force for the high-temperature granulite-facies metamorphism and anataxis of the crust associated with formation of S-type granite. This study not only constructs a more detail Proto-Tethys evolution process of the eastern Kunlun, but also sheds new light on better understanding the intimate relationship between magmatism and metamorphism during post-collisional extensional collapse.

Diagenesis and Very Low-Grade Metamorphism of the Upper Permian Yangjiagou Formation in Eastern Changchun, China:Evidence from Clay Mineral Geothermobarometers

The metamorphic conditions of the Upper Permian Yangjiagou Formation in eastern Changchun, China, were evaluated based on the mineral assemblage, illite crystallinity, illite polytypism,the b dimension of illite, and the chemical composition of chlorite. The pelitic rocks in the Yangjiagou Formation are characterized by illite + kaolinite + chlorite ± mixed-layer chlorite/smectite and detrital quartz + plagioclase. Illite in the formation has a crystallinity of 0.38-0.55 and comprises mixed 2 M_1 and1 M_d polytypes, indicating a metamorphic temperature of >200℃. Based on the chemical composition of chlorite and the chlorite geothermometer, we estimated diagenetic to very low-grade metamorphic conditions with temperatures of 185℃～204℃. The b dimension of illite varies from 8.992 A to 9.005 A.We used a mathematical algorithm to extend Guidotti and Sassi's(1986) diagram relating illite b dimension with temperature and pressure, and used this diagram, together with illite crystallinity and chlorite chemical composition, to semi-quantitatively estimate the formation pressure at<1.2 kbar. These reveal that the Yangjiagou Formation has experienced very low-grade metamorphism.

Permo-Triassic high-pressure metamorphism in the central western Korean Peninsula, and its link to Paleo-Tethyan Ocean closure: Key issues revisited

Permo-Triassic high-pressure(HP) mafic granulites, together with the Bibong retrogressed eclogite,preserved along the central western Korean Peninsula provide important insights into the Late Permian to Triassic collisional orogeny in northeast Asia. The metamorphic pressureetemperatureetime(P-T-t)paths of these rocks, however, remain poorly constrained and even overestimated, owing to outdated geothermobarometers and inaccurate isopleth techniques. Here we evaluate the metamorphic Pe T conditions of Triassic HP mafic granulites including those in Baekdong, Sinri and Daepan and the Bibong Triassic retrogressed eclogite in the Hongseong area, and the Permo-Triassic Samgot mafic granulite in the Imjingang Belt of the central western Korean Peninsula through the application of modern phase equilibria techniques. The Baekdong and Samgot mafic granulites and the Bibong retrogressed eclogite yield a range of 12.0 -16.0 kbar and 800 -900℃, representing HP granulite facies conditions. The Sinri and Daepan granulites from the Hongseong area show relatively lower grade metamorphic conditions between HP granulite and normal granulite facies, and are characterized by sub-isothermal decompression during exhumation. The similarities in the metamorphic ages and the post-collisional igneous activity from the central western Korean Peninsula indicate that the Triassic ages represent the retrograde stage of the metamorphic Pe T paths. In contrast, the Late Permian metamorphic ages, which are older than protolith ages of the post-collisional igneous rocks, correspond to the possible prograde stage of metamorphism. The P-T-t paths presented in this paper, together with the metamorphic ages and post-orogenic igneous events reported from these areas suggest trace of the subduction, accretion and exhumation history, and indicate a tectonic linkage among the northeast Asian continents during the Paleo-Tethyan Ocean closure.

On Continent-Continent Point-Collision and Ultrahigh-Pressure Metamorphism

Up to now it is known that almost all ultrahigh-pressure (UHP) metamorphism of non-impact origin occurred in continent-continent collisional orogenic belt, as has been evidenced by many outcrops in the eastern hemisphere. UHP metamorphic rocks are represented by coesite- and diamond-bearing eclogites and eclogite facies metamorphic rocks formed at 650–800°C and 2.6–3.5 Gpa, and most of the protoliths of UHP rocks are volcanic-sedimentary sequences of continental crust. From these it may be deduced that deep subduction of continental crust may have occurred. However, UHP rocks are exposed on the surface or occur near the surface now, which implies that they have been exhumed from great depths. The mechanism of deep subduction of continental crust and subsequent exhumation has been a hot topic of the research on continental dynamics, but there are divergent views. The focus of the dispute is how deep continental crust is subducted so that UHP rocks can be formed and what mechanism causes it to be subducted to great depths and again exhumed to the shallow surface. Through an analysis of the continental process and mechanical boundary conditions of the Dabie collisional belt—an UHP metamorphic belt where the largest area of UHP rocks in the world is exposed, this paper discusses the variations of viscous stresses and average pressure in the viscous fluid caused by tectonism with rock physical properties and the contribution of the tectonic stresses to production of UHP. Calculation indicates that the anomalous stress state on the irregular boundary of a continental block may give rise to stress concentration and accumulation at local places (where the compressional stress may be 5–9 times higher than those in their surroundings). The tectonic stresses may account for 20–35% of the total UHP. So we may infer that the HP (high-pressure)-UHP rocks in the Dabie Mountains were formed at depths of 60–80 km. Thus the authors propose a new genetic model of UHP rocks—the point-collision model. This model conforms to the basic principles of the mechanics and also to the geologic records and process in the Dabie orogenic belt. It can explain why UHP rocks do not exist along the entire length of the collisional orogen but occur in some particular positions. The authors also propose that the eastern and western corners of the Himalaya collision zone are typical point-collision areas and that almost all UHP metamorphism of continental crustal rocks occurred in the two particular positions.

Mineralogical Evidence for Regional Metamorphism Overprinted by Contact Metamorphism

The Soursat metamorphic complex(SMC) in northwestern Iran is part of the Sanandaj-Sirjan metamorphic belt.The complex is composed of different metamorphic and plutonic rocks,but is dominated by metapelites composed of garnet,staurolite,kyanite,fibrolite,cordierite,and andalusite.Porphyroblasts in schists have the same fabric,and three stages of schistosity are present.The internal schistosity(Sn) inclusion trails are also offset by conjugate sets of extensional schistosity(Sn+1) and a second(Sn+2) that crenulates(Sn+1).Polyphase metamorphisms are present in the complex.Garnet,staurolite,kyanite,and fibrolite assemblage preserves conditions during the M1 metamorphic event.This assemblage yields a P-t estimate of 645±11°C and 6.5±0.5 kbar.Other samples of the central part of SMC contain cordierite and andalusite(M2) overgrowth that yields a P-t estimate of 532±33°C and 2.1±1.1 kbar.

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(M_(1)),the peak eclogite facies(M_(2)),the decompression blueschist facies(M_(3))and retrograde greenschist facies(M_(4)).Using the Grt-Omp-Phn geothermobarometer,a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658℃ 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.

Ultrahigh Pressure Metamorphism and Tectonic Evolution of Southwestern Tianshan Orogenic Belt, China: A Comprehensive Review

Recently, a huge ultrahigh-pressure(UHP) metamorphic belt of oceanic-type has been recognized in southwestern(SW) Tianshan, China. Petrological studies show that the UHP metamorphic rocks of SW Tianshan orogenic belt include mafic eclogites and blueschists, pelitic garnet phengite schists, marbles and serpentinites. The well-preserved coesite inclusions were commonly found in eclogites, garnet phengite schists and marbles. Ti-clinohumite and Ti-chondrodite have been identified in UHP metamorphic serpentinites. Based on the PT pseudosection calculation and combined U-Pb zircon dating, the P-T-t path has been outlined as four stages: cold subduction to UHP conditions before^320 Ma whose peak ultrahigh pressure is about 30 kbar at 500 ℃, heating decompression from the Pmax to the Tmax stage before 305 Ma whose peak temperature is about 600 ℃ at 22 kbar, then the early cold exhumation from amphibolite eclogite facies to epidote-amphibolite facies metamorphism before 220 Ma and the last tectonic exhumation from epidote amphibolite facies to greenschist facies metamorphism. Combining with the syn-subduction arc-like 333-326 Ma granitic rocks and 280-260 Ma S-type granites in the coeval low-pressure and high-temperature(LP-HT) metamorphic belt, the tectonic evolution of Tianshan UHP metamorphic belt during late Cambrian to early Triassic has been proposed in this paper.

Relationship between Thermal Metamorphism of Coal and the Periodic Variation Law of Geochemical Behavior of Elements:A Study on the C2 Coal Seam in the Fengfeng Mining Area of the Handan Coalfield in Hebei,China

This study highlights the response of the periodic variation of the geochemical behavior of elements to the thermal metamorphism of coal by considering the differentiation mode and differentiation degree of elements of the C2 coal seam in the Fengfeng mining area of the Handan Coalfield in Hebei,China.The periodic variation of the geochemical behavior of elements was observed to change towards a certain direction as the degree of metamorphism of a geological body increased.Based on the coincidence degree(or similarity degree)between the geochemical behavior of elements and periodic variation of elements,the 57 elements in this study were divided into two levels.The periodic variation of the geochemical behavior of first-level elements was largely synchronous with that of their first ionization energy,suggesting that only one differentiation mode exists and the elements are mainly inorganically associated.The differentiation mode of the second-level elements deviated to a certain extent from their first ionization energy;the larger the deviation,the more complicated and diverse was the differentiation mode.Among the second-level elements,the grade of coal metamorphism has evident and intuitive effects on the proportion of elements with particular structural features,such as the 4q+3 type of elements and the odd-odd elements.In addition,the distribution of elements in organic and inorganic matter within coal are subject to the structural features of the elements.The differentiation mode and differentiation intensity of elements could be characterized by the hierarchical parameter and differentiation intensity.The hierarchical parameter and differentiation intensity of certain elements showed a good positive or negative correlation with R_(max) in coal.The 57 elements in this study were quantitatively ordered according to the degree of magmatic hydrothermal fluid influence and thermal metamorphism of coal through graphs depicting the goodness of fit,correlation coefficient with R_(max),and differentiation intensity.The results of this study are consistent with the results of previous field research,illustrating the scientific significance and application value of this study on the periodic variation of the geochemical behavior of elements.