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.

Late Mesozoic Exhumation of the Huangling Massif:Constraints on the Evolution of the Middle Yangtze River

Plate subduction leads to complex exhumation processes on continents.The Huangling Massif lies at the northern margin of the South China Block.Whether the Huangling Massif was exhumed as a watershed of the middle reaches of the Paleo-Yangtze River during the Mesozoic remains under debate.We examined the exhumation history of the Huangling Massif based on six granite bedrock samples,using apatite fission track(AFT)and apatite and zircon(U-Th)/He(AHe and ZHe)thermochronology.These samples yielded ages of 157–132 Ma(ZHe),119–106 Ma(AFT),and 114–72 Ma(AHe),respectively.Thermal modeling revealed that three phases of rapid cooling occurred during the Late Jurassic–Early Cretaceous,late Early Cretaceous,and Late Cretaceous.These exhumation processes led to the high topographic relief responsible for the emergence of the Huangling Massif.The integrated of our new data with published sedimentological records suggests that the Huangling Massif might have been the watershed of the middle reaches of the Paleo-Yangtze River since the Cretaceous.At that time,the rivers flowed westward into the Sichuan Basin and eastward into the Jianghan Basin.The subduction of the Pacific Plate beneath the Asian continent in the Mesozoic deeply influenced the geomorphic evolution of the South China Block.

The Paleoproterozoic and Neoproterozoic Carbon Cycle Promoted the Evolution of a Habitable Earth

The carbon cycle is an important process that regulates Earth's evolution.We compare two typical periods,in the Paleoproterozoic and Neoproterozoic,in which many geological events occurred.It remains an open question when modern plate tectonics started on Earth and how it has influenced the carbon cycle through time.In the Paleoproterozoic,intense weathering in a highly CO_(2)and CH_(4)rich atmosphere caused more nutritional elements to be carried into the ocean.Terrestrial input boosted high biological productivity,deposition of sediments and the formation of an altered oceanic crust,which may have promoted an increase in the oxygen content.Sediment lubrication and a decrease in mantle potential temperature made cold and deep subduction possible,which carried more carbon into the deep mantle.Carbon can be stored in the mantle as diamond and carbonated mantle rocks,being released by arc and mid-ocean ridge outgassing at widely different times.From the Paleoproterozoic through the Neoproterozoic to the Phanerozoic,the carbon cycle has promoted the evolution of a habitable Earth.

Re-Os and U-Pb Geochronology of the Erlihe Pb-Zn Deposit,Qinling Orogenic Belt,Central China,and Constraints on Its Deposit Genesis

The Erlihe Pb-Zn deposit is an important mine of the Pb-Zn metallogenic zone in the South Qinling Orogen.It has been considered a sedimentary exhalative deposit in previous investigations because the ore body occurs concordantly at the transitional location of an upright fold.Re and Os isotopic analyses for paragenetic pyrites with sphalerite and galena from the ore body have been used to determine the timing of mineralization and to trace the source of metallogenic materials.The Re-Os isotopic data of four pyrite samples construct an isochron,yielding a weighted average age of 226±17 Ma（mean square weighted deviation=1.7）,which is considered the main mineralization age.A dioritic porphyrite vein sample,showing weaker mineralization,was also dated using the SHRIMP zircon UPb isotopic method to constrain the youngest metallogenic age of the ore deposit,because it distributes along a group of tensional joints cutting not only the upright fold in the deposit field,but also the main ore bodies.The dioritic porphyrite sample yields a weighted mean ^（206）Pb/^（238）U age of 221±3 Ma,which is slightly younger than the Re-Os isotopic isochron age of the pyrites,considered as the upper age limit of the mineralization,namely the ending age of the mineralization.The Os isotopic compositions of sulfide minerals distribute within a range between Os isotopic compositions of the crust and the mantle, indicating that the ore deposit can be derived from magma-related fluid,and the metallogenic materials are most likely derived from the mixing source of the crust and the mantle.The Erlihe Pb-Zn deposit and associated dioritic porphyrite vein,important records of Qinling tectonic-magmatism-mineralization activities,were formed during the Triassic collisional orogeny processes.

^(40)Ar/^(39)Ar Dating of Xuebaoding Granite in the Songpan-Garze Orogenic Belt,Southwest China,and its Geological Significance

Thus far, our understanding of the emplacement of Xuebaoding granite and the occurrence and evolution of the Songpan-Garze Orogenic Belt has been complicated by differing age spectra results. Therefore, in this study, the ^40Ar/^39Ar and sensitive high resolution ion micro-probe （SHRIMP） U-Pb dating methods were both used and the results compared, particularly with respect to dating data for Pankou and Pukouling granites from Xuebaoding, to establish ages that are close to the real emplacements. The results of SHRIMP U-Pb dating for zircon showed a high amount of U, but a very low value for Th/U. The high U amount, coupled with characteristics of inclusions in zircons, indicates that Xuebaoding granites are not suitable for U-Pb dating. Therefore, muscovite in the same granite samples was selected for ^40Ar/^39Ar dating. The ^40Ar/^39Ar age spectrum obtained on bulk muscovite from Pukouling granite in the Xuebaoding, gave a plateau age of 200.1±1.2 Ma and an inverse isochron age of 200.6±1.2 Ma. The 4^40Ar/^39Ar age spectrum obtained on bulk muscovite from Pankou granite in the Xuebaoding gave another plateau age of 193.4±1.1 Ma and an inverse isochron age of 193.7±1.1 Ma. The ^40Ar/^36Ar intercept of 277.0±23.4 （2σ） was very close to the air ratio, indicating that no apparent excess argon contamination was present. These age dating spectra indicate that both granites were emplaced at 200.6±1.3 Ma and 193.7±1.1 Ma, respectively. Through comparison of both dating methods and their results, we can conclude that it is feasible that the muscovite in the granite bearing high U could be used for ^40Ar/^39Ar dating without extra Ar. Based on this evidence, as well as the geological characteristics of the Xuebaoding W-Sn-Be deposit and petrology of granites, it can be concluded that the material origin of the Xuebaoding W-Sn-Be deposit might partially originate from the Xuebaoding granite group emplacement at about 200 Ma. Moreover, compared with other granites and deposits distributed in various positions in the Songpan-Garze Orogenic Belt, the Xuebaoding emplacement ages further show that the main rare metal deposits and granites in peripheral regions occurred earlier than those in the inner Songpan-Garze. Therefore, ^40Ar/^39Ar dating of Xuebaoding granite will lay a solid foundation for studying the occurrence and evolution of granite and rare earth element deposits in the Songpan-Garze Orogenic Belt.

Provenance of the Permo-Carboniferous sediments in the northern Alxa and its tectonic implications for the southernmost Central Asian Orogenic Belt

As the southernmost segment of the Central Asian Orogenic Belt(CAOB),the northern Alxa orogenic belt(NAOB)connects the southeastern and southwestern segments of the CAOB.The NAOB amalgamated with the closure of the Paleo-Asian Ocean;however,the closure time of the Paleo-Asian Ocean is still on great debate.In this study,we reported new detrital zircon U-Pb geochronology and Hf-O isotopes for the Permo-Carboniferous sediments in the northern Alxa to constrain the provenance and its tectonic implications.The Permo-Carbonifereous Amushan Formation is composed of volcanic-carbonite-clastic rocks and was deposited in a shallow marine environment.Based on the zircon U-Pb geochronology,the Amushan Formation was deposited in the late Carboniferous to early Permian,but some outcrops of volcanic and clastic rocks in the Quaganqulu area were likely formed in the middle to late Permian.The integrated zircon age spectrum for the clastic rocks shows a wide range from late Archean to Paleoproterozoic,Mesoproterozoic(with a peak age at 1458 Ma),early Neoproterozoic(with peak ages of 988 Ma and 929 Ma),early Paleozoic(with a peak age at 447 Ma) and late Paleozoic(with a peak age at 294 Ma).Combined with the zircon Hf-O isotopes,the provenance was considered to be the Alxa Block,the Shalazhashan terrane and the Zhusileng-Hangwula block(and the southern Beishan orogenic belt).The multiple source regions to the south and north of the Paleo-Asian Ocean indicate the closure of this ocean before the late Carboniferous.The absence or small proportion of depositional age-approximated zircons in most samples makes their age spectra similar to extensional basins.Combined with the intra-plate volcanism,the deposits were considered to be formed in extensional settings.Accordingly,after the closure of the Paleo-Asian Ocean,the NAOB stepped into an extensional stage.

Metamorphic Evolution and Tectonic Implications of Garnet–Bearing Mica Schist in Sumdo High Pressure Belt from Eastern Lhasa Block, Tibet

The garnet-bearing mica-quartz schist of the Sumdo high pressure metamorphic beltfrom the Lhasa block is mainly composed of garnet, muscovite, albite, quartz and minor chlorite, rutile and sphene.

Deciphering the Origin of Abiotic Organic Compounds on Earth:Review and Future Prospects

The geologic production of abiotic organic compounds has been the subject of increasing scientific attention due to their use in the global carbon flux balance,by chemosynthetic biological communities,and for energy resources.Extensive analysis of methane(CH_(4))and other organics in diverse geologic settings,combined with thermodynamic modelings and laboratory simulations,have yielded insights into the distribution of specific abiotic organic molecules on Earth and the favorable conditions and pathways under which they form.This updated and comprehensive review summarizes published results of petrological,thermodynamic,and experimental investigations of possible pathways for the formation of particular species of abiotic simple hydrocarbon molecules such as CH_(4),and of complex hydrocarbon systems,e.g.,long-chain hydrocarbons and even solid carbonaceous matters,in various geologic processes,distinguished into three classes:(1)pre-to early planetary processes;(2)mantle and magmatic processes;and(3)the gas/water-rock reaction processes in low-pressure ultramafic rock and high-pressure subduction zone systems.We not only emphasize how organics are abiotically synthesized but also explore the role or changes of organics in evolutionary geological environments after synthesis,such as phase transitions or organic-mineral interactions.Correspondingly,there is an urgent need to explore the diversity of abiotic organic compounds prevailing on Earth.

Ages of the Laocheng Granitoids and Crustal Growth in the South Qinling Tectonic Domain,Central China:Zircon U-Pb and Lu-Hf Isotopic Constraints

The Laocheng granitoid pluton is located in the South Qinling tectonic domain of the Qinling orogenic belt,southern Shaanxi Province,and consists chiefly of quartz diorite,granodiorite and monzogranite.A LA-ICP-MS zircon U-Pb isotopic dating,in conjunction with cathodoluminescence images,reveals that the quartz diorite and granodiorite were emplaced from 220 Ma to 216 Ma,while the monzogranite was emplaced at～210 Ma.In-situ zircon Hf isotopic analyses show that theε_（Hf）（t） values of the quartz diorite and granodiorite range from-8.1 to ＋1.3,and single-stage Hf model ages from 809 Ma to 1171 Ma,while theε_（Hf）（t）values of the monzogranite are-14.5 to ＋16.7 and single-stage Hf model ages from 189 Ma to 1424 Ma.These Hf isotopic features reveal that the quartz diorite, granodiorite and monzogranite were formed from the mixing of the magmas derived from partial melting of the depleted mantle and the lower continent crustal materials,and there were two stages of continental crust growth during the Neoproterozoic（～800 Ma）and Indosinian（～210 Ma）eras, respectively,in the south Qinling tectonic domain of the Qinling orogrnic belt,Central China.

Xiba Granitic Pluton in the Qinling Orogenic Belt, Central China: Its Petrogenesis and Tectonic Implications

Xiba granitic pluton is located in South Qinling tectonic domain of the Qinling orogenic belt and consists mainly of granodiorite and monzogranite with significant number of microgranular quartz dioritic enclaves. SHRIMP zircon U-Pb isotopic dating reveals that the quartz dioritic enclaves formed at 214±3 Ma, which is similar to the age of their host monzogranite （218±1 Ma）. The granitoids belong to high-K calc-alkaline series, and are characterized by enriched LILEs relative to HFSEs with negative Nb, Ta and Ti anomalies, and right-declined REE patterns with （La/Yb） N ratios ranging from 15.83 to 26.47 and δEu values from 0.78 to 1.22 （mean= 0.97）. Most of these samples from Xiba granitic pluton exhibit εNd（t） values of 8.79 to 5.38, depleted mantle Nd model ages （T DM ） between 1.1 Ga and 1.7 Ga, and initial Sr isotopic ratios （ 87 Sr/ 86 Sr） i from 0.7061 to 0.7082, indicating a possible Meso-to Paleoproterozoic lower crust source region, with exception of samples XB01-2-1 and XB10-1 displaying higher （ 87 Sr/ 86 Sr） i values of 0.779 and 0.735, respectively, which suggests a contamination of the upper crustal materials. Quartz dioritic enclaves are interpreted as the result of rapid crystallization fractionation during the parent magmatic emplacement, as evidenced by similar age, texture, geochemical, and Sr-Nd isotopic features with their host rocks. Characteristics of the petrological and geochemical data reveal that the parent magma of Xiba granitoids was produced by a magma mingling process. The upwelling asthenosphere caused a high heat flow and the mafic magma was underplated into the bottom of the lower continent crust, which caused the partial melting of the lower continent crustal materials. This geodynamic process generated the mixing parent magma between mafic magma from depleted mantle and felsic magma derived from the lower continent crust. Integrated petrogenesis and tectonic discrimination with regional tectonic evolution of the Qinling orogen, it is suggested that the granitoids are most likely products in a post-collision tectonic setting.

Hamatophyton from the Late Devonian of Anhui Province,South China and Evolution of Sphenophyllales

从 Chaohu 区域的上面的泥盆纪(Famennian ) Wutong 形成收集的 Hamatophyton verticillatum 的保存得很好的标本，安徽省，华南，在三份订单和多重部门显示更完全的肥沃的轴。比较显示 Hamatophyton 可能没有掌状的计划吃了无菌的叶子但是有稀罕部门的钩状的线性的。我们建议 Sphenophyllales 的七个权威的人物：(1 ) 完全 whorled 侧面的机关；(2 ) 无菌的叶子；(3 ) 球果 i；(4 ) “ sporangiophores ”或有 reflexed 的梗付小费给适用的 sporangia；(5 ) 三肋骨或四肋骨的主要木部；(6 ) 主要木部的总督成熟；并且(7 ) 第二等的木部。Sphenophyllales 可能基于 whorled 的类似从 Iridopteridales 发源侧面的机关， ribbed 主要木部和外部 protoxylem 搁浅。在到 Sphenophyllales 的从 Iridopteridales 的转变，词法变化部分包含 whorled 到完全 whorled 侧面的机关，到叶子的无菌的最终的附器，和到“ sporangiophores ”的肥沃的最终的附器有苞的 /stalks；解剖修正包括第二等的木部的主要木部，和存在的配置和成熟。

A New Lycopsid, Zhenglia radiata gen. et sp. nov., from the Lower Devonian Posongchong Formation of Southeastern Yunnan, China, and Its Evolutionary Significance

A new plant, Zhenglia radiata gen. et sp. nov., from the Posongchong Formation （Early Devonian, Pragian age） of the Wenshan District of southeastern Yunnan Province, China is a small herbaceous lycopsid. The aerial branches divide into foliar and fertile stems. The arrangement of the leaf bases on the stem shows lepidodendroid-like phyllotaxy. Possibly the leaf base bears a ligulate pit. The microphyll consists of a swollen, decurrent base, and an entire, linear lamina. The fertile axis is composed of sporophylls arranged helically, forming a compact area similar to cone-like structure. Each sporophyll consists of a widened lateral extension and a distal lamina. The ovoid-elongate sporangia are attached adaxially on the surface of sporophylls. Based on morphological similarities, especially the undivided microphylls, the style of phyllotaxy, the form of the cone-like structures and sporangia, the new lycopsid Zhenglia radiata is placed in the order Protolepidodendrales and a possible evolutionary relationship with the arborescent lycopsids of the Late Devonian and Carboniferous is considered.

Quantitative Study of the Morphological Variations among Dzieduszyckia Populations from the Devonian Rongxian Formation,South China and their Influencing Factors

The rhynchonellide brachiopod Dzieduszyckia Siemiradzki is widely distributed in the Lower Famennian strata,and exhibits a wide range of morphological variations among populations.1272 autochthonous Devonian Rongxian Dzieduszyckia fossils were collected from three populations in South China,namely the Dazhai,Dalong,and Longmi sections.A series of quantitative morphological analyses were conducted to understand the morphological variation among populations and identify the triggers responsible for such variations.The results show that the Dazhai population differs significantly in morphospace from the Dalong and Longmi populations,while the latter two exhibit more similar morphological features,suggesting that geographic distribution may play a key role in affecting Dzieduszyckia morphology.The Longmi population shows a distinct morphospace from the Dazhai population and an overlapped morphospace with the Dalong population,indicating that habitat may not be the controlling factor.Lastly,the Dazhai and Dalong populations share some morphological similarities,but the Dazhai and Longmi populations occupy completely different morphospaces,indicating that evolution had a greater impact on phenotypic divergence than habitat.Our findings suggest that geographic distribution is the predominant factor influencing the morphological variation of Dzieduszyckia,while evolution and living habitat are less significant,the former of which might play a more important role.

Redox evolution of western Tianshan subduction zone and its effect on deep carbon cycle

Knowing the phase relations of carbon-bearing phases at high-pressure(HP) and high-temperature(HT) condition is essential for understanding the deep carbon cycle in the subduction zones.In particular,the phase relation of carbon-bearing phases is also strongly influenced by redox condition of subduction zones,which is poorly explored.Here we summarized the phase relations of carbon-bearing phases(calcite,aragonite,dolomite,magnesite,graphite,hydrocarbon) in HP metamorphic rocks(marble,metapelite,eclogite) from the Western Tianshan subduction zone and high-pressure experiments.During prograde progress of subduction,carbonates in altered oceanic crust change from Ca-carbonate(calcite) to Ca,Mg-carbonate(dolomite),then finally to Mgcarbonate(magnesite) via Mg-Ca cation exchange reaction between silicate and carbonate,while calcite in sedimentary calcareous ooze on oceanic crust directly transfers to high-pressure aragonite in marble or amorphous CaCO3 in subduction zones.Redox evolution also plays a significant effect on the carbon speciation in the Western Tianshan subduction zone.The prograde oxygen fugacity of the Western Tianshan subduction zone was constrained by mineral assemblage of garnet-omphacite from FMQ-1.9 to FMQ-2.5 at its metamorphic peak(maximum P-T) conditions.In comparison with redox conditions of other subduction zones,Western Tianshan has the lowest oxygen fugacity.Graphite and light hydrocarbon inclusions were ubiqutously identified in Western Tianshan HP metamorphic rocks and speculated to be formed from reduction of Fe-carbonate at low redox condition,which is also confirmed by high-pressure experimental simulation.Based on petrological observation and high-pressure simulation,a polarized redox model of reducing slab but oxidizing mantle wedge in subduction zone is proposed,and its effect on deep carbon cycle in subduction zones is further discussed.

Structural features and deformational ages of the northern Dabashan thrust belt

A large-scale pop-up structure occurs at the front of the northern Dabashan thrust belt （NDTB）, bound by the NNE-dipping Chengkou fault to the south, and the SSW-dipping Gaoqiao fault to the north. The pop-up structure shows different features along its strike as a direct reflection of the intensity of tectonic＂ activity. To the northwest, the structure is characterized by a two-directional thrust system forming a positive flower-like structure. In contrast, the southeastern part is composed of the vertical Chengkou fault and a series of N-directed backthrusts, showing a semi-flower-like structure. We present results from Ar-Ar dating of syntectonic microthermal metamorphic sericite which show that the Chengkou fault experienced intense deformation during the mid-Mesozoic Yanshanian epoch （about 143.3 Ma）, causing rapid uplift and thrusting of the northern Dabashan thrust belt. During the propagation of this thrust, a series of backthrusts formed because of the obstruction from the frontier of Dabashan thrust belt, leading to the development of the pop-up structure.

Differential Evolution of High-Pressure and Ultrahigh-Pressure Metapelites from Habutengsu, Chinese Western Tianshan: Phase Equilibria Modelling and ^(40)Ar/^(39)Ar Geochronology

Metapelite is one of the predominant rock types in the high-pressure–ultrahigh-pressure（HP–UHP） metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during metamorphism are poorly understood. In this study, we present comparative petrological studies and 40^Ar/39 ^Ar geochronology of HP and UHP pelitic schist exposed along the Habutengsu valley. The schist mainly comprises quartz, white mica, garnet, albite and bluish amphibole. In the Mn O–Na2O–Ca O–K2O–Fe O–Mg O–Al2O3–Si O2–H2O（Mn NCKFMASH） system, P–T pseudosections were constructed using THERMOCALC 333 for two representative pelitic schists. The results demonstrate that there was a break in the peak metamorphic pressures in the Habutengsu area. The northern schist has experienced UHP metamorphism, consistent with the presence of coesite in the same section, while the southern one formed at lower pressures that stabilized the quartz. This result supports the previous finding of a metamorphic gradient through the HP–UHP metamorphic belt of the Chinese western Tianshan by the authors. Additionally, phengite in the northern schist was modelled as having a Si content of 3.55–3.70（a.p.f.u.） at the peak stage, a value much higher than that of oriented matrix phengite（Si content 3.32–3.38 a.p.f.u.）. This indicates that the phengite flakes in the UHP schist were subjected to recrystallization during exhumation, which is consistent with the presence of phengite aggregates surrounding garnet porphyroblast. The 40^Ar/39^ Ar age spectra of white mica（dominantly phengite） from the two schists exhibit similar plateau ages of ca. 315 Ma, which is interpreted as the timing of a tectonometamorphic event that occurred during the exhumation of the HP–UHP metamorphic belt of the Chinese western Tianshan.

Paleoproterozoic crustal evolution of the Hengshan-Wutai—Fuping region,North China Craton

An arguable point regarding the Neoarchean and Paleoproterozoic crustal evolution of the North China Craton（NCC）is whether the tectonic setting in the central belt during the mid-Paleoproterozoic（2.35-2.0 Ga）was dominated by an extensional regime or an oceanic subduction-arc regime.A review of the midPaleoproterozoic magmatism and sedimentation for the Hengshan-Wutai-Fuping region suggests that a back-arc extension regime was dominant in this region.This conclusion is consistent with the observation that the 2.35-2.0 Ga magmatism shows a typical bimodal distribution where the mafic rocks mostly have arc affinities and the acidic rocks mainly comprise highly-fractioned calc-alkaline to alkaline（or A-type）granites,and that this magmatism was coeval with development of extensional basins characteristic of transgressive sequences with volcanic interlayers such as in the Hutuo Group.Although the final amalgamation of the NCC was believed to occur at ～1.85 Ga,recent zircon U-Pb age dating for mica schist in the Wutai Group suggests a collisional event may have occurred at ～1.95 Ga.The metamorphic ages of ～1.85 Ga,obtained mostly from the high-grade rocks using the zircon U-Pb approach,most probably indicate uplifting and cooling of these high-grade terranes.This is because（i）phase modeling suggests that newly-grown zircon grains in highgrade rocks with a melt phase cannot date the age of peak pressure and temperature stages,but the age of melt crystallization in cooling stages;（ii）the metamorphic P-T paths with isobaric cooling under 6-7 kb for the Hengshan and Fuping granulites suggest their prolonged stay in the middle-lower crust;and（iii）the obtained metamorphic age data show a continuous distribution from 1.95 to 1.80 Ga.Thus,an alternative tectonic scenario for the Hengshan-Wutai-Fuping region involves：（i）formation of a proto-NCC at ～2.5 Ga;（ii）back-arc extension during 2.35-2.0 Ga resulting in bimodal magmatism and sedimentation in rifting basins on an Archean basement;（iii）a crustal thickening event in the extended region resulting in a kyanitetype metamorphism at ～1.95 Ga,and（iv） uplifting and cooling of the thickened crust from 1.93 to 1.80 Ga.

Geology and mineralization of the Bayan Obo supergiant carbonatite-type REE-Nb-Fe deposit in Inner Mongolia, China: A review

The Bayan Obo supergiant carbonatite-related rare-earth-element-niobium-iron(REE-Nb-Fe) endogenetic deposit(thereafter as the Bayan Obo deposit), located at 150 km north of Baotou City in the Inner Mongolia Autonomous Region, is the largest rare-earth element(REE) resource in the world. Tectonically,this deposit is situated on the northern margin of the North China Craton and adjacent to the Xing’anMongolian orogenic belt to the south. The main strata within the mining area include the Neoarchean Se’ertengshan Group and the Mesoproterozoic Bayan Obo Group. Generally, the rare earth, niobium, and iron mineralization within the deposit are intrinsically related to the dolomite carbonatites and the extensive alteration of the country rocks caused by the carbonatite magma intrusion. The alteration of country rocks can be categorized into three types: contact metasomatism(anti-skarn and skarn alteration), fenitization,and hornfelsic alternation. As indicated by previous studies and summarized in this review, the multielement mineralization at Bayan Obo is closely associated with the metasomatic replacement of siliceous country rocks by carbonatite magmatic-hydrothermal fluids. The metasomatic process is comparable to the conventional skarnification that formed due to the intrusion of intermediate-acid magmatic rocks into limestone strata. However, the migration pattern of Si O2, Ca O, and Mg O in this novel metasomatic process is opposite to the skarn alteration. Accordingly, this review delineates, for the first time, an antiskarn metallogenic model for the Bayan Obo deposit, revealing the enigmatic relationship between the carbonatite magmatic-hydrothermal processes and the related iron and rare earth mineralization.Moreover, this study also contributes to a better understanding of the REE-Nd-Fe metallogenetic processes and the related fluorite mineralization at the Bayan Obo deposit.

SHRIMP zircon U-Pb dating of a small tonalite intrusion in metamorphic belt of Chinese Altai orogen and its geological implication

Small granitic intrusions occur in the progressive metamorphic belts of the Chinese Altai orogen.SHRIMP U-Pb analyses were performed on zircons separated from a tonalite intrusion and yielded a concordant age of 409.6±3.7 Ma(MSWD=0.93),restricting the emplacement and crystallization of the tonalite intrusion to the Early Devonian.Combined with the existing geological data of the contemporary large granitic plutons in the Chinese Altai orogen,the tonalite is considered to be formed together with other granitic rocks in a continental arc setting.The tonalite intrusion has consistent foliation with its country rocks,indicating the strain resulting in the regional deformation should be very strong during or after the early Devonian.

Three-dimensional discrete element numerical simulation of Paleogene salt structures in the western Kuqa foreland thrust belt

Taking the Paleogene salt strata in the west of Kuqa foreland thrust belt as study object, the deformation features of salt structure in the compression direction and perpendicular to the compression direction were examined to find out the control factors and formation mechanisms of the salt structures. By using the three-dimensional discrete element numerical simulation method, the formation mechanisms of typical salt structures of western Kuqa foreland thrust belt in Keshen and Dabei work areas were comprehensively analyzed. The simulation results show that the salt deformation in Keshen and Dabei work areas is of forward spread type, with deformation concentrated in the piedmont zone;the salt deformation is affected by the early uplift near the compression end, pre-existing basement faults, synsedimentary process and the initial salt depocenter;in the direction perpendicular to the compression direction, salt rocks near the compression end have strong lateral mobility with the velocity component moving towards the middle part, and the closer to the middle, the larger the velocity will be, so that salt rocks will aggregate towards the middle and deform intensely, forming complex folds and separation of salt structures from salt source, and local outcrop with thrust faults. Compared with 2 D simulation, 3 D simulation can analyze salt structures in the principal stress direction and direction perpendicular to the principal stress, give us a full view of the formation mechanisms of salt structures, and guide the exploration of oil and gas reservoirs related to salt structures.