查看更多>>摘要：The West Qinling Orogen(WQO)is located in the western part of the Qinling Orogen and in the transition zone of Qilian Orogen,Songpan-Garzc Orogen and Yangtze Block,and also the key position of Triassic collision orogenic event.The study of the Early Triassic strata in the WQO is contributed to analyze the closure process of the paleo-Tethys.We conducted LA-ICP-MS U-Pb dating studies on detrital zircons to determine the provenance,depositional age,and tectonic setting of the Early Triassic Longwuhe Formation in the Lintan area of the WQO.The results show that the majori-ty of the detrital zircons in the Longwuhe Formation are mainly magmatic origin and have characteris-tic of crust source zircon.The lowest limit of sedimentation of the Longwuhe Formation is constrained to the Early Triassic,with the youngest detrital zircon age of 253±3 Ma.The ages can be divided into five age groups:3 346-1 636 Ma,with two peak ages of ca.2 495 and ca.1 885 Ma;1 585-1 010 Ma,with a peak age at ca.1 084 Ma;992-554 Ma,with a peak age at ca.939 Ma;521-421 Ma,with a peak age at ca.445 Ma;418-253 Ma,with a peak age at ca.280 Ma.Apparently,the sources of the Long-wuhe Formation include the northern margin of the WQO,the Qilian Orogen(QLO)and the base-ment of the southern margin of the North China Block(NCB),of which the ancient basement of the southern margin of the NCB is the main source area of the Longwuhe Formation.Combined with pre-vious studies,we propose that the Longwuhe Formation was formed in a fore-arc basin,which is relat-ed to the closure of the A'nyemaqen-Mianlüe Ocean from the Early Permian to Early-Middle Triassic due to the northward subduction-collision of the Yangtze Block(YZB).This also indicates that the A'nyemaqen-Mianlüe Ocean has flat subduction characteristics.
查看更多>>摘要：Obtaining geological and landform dislocation features,as well as the measured strati-graphic activity age,provides direct evidence to evaluate fault activity,which is more difficult to do in areas with low tectonic activity,such as eastern and central China.A detailed investigation of the fault activity,trenching,drilling joint geological profile,geological survey,and chronological analysis were used to obtain the spatial geometry,fault kinematics,and activity chronology of the Jiulong fault.The conclusions are as follows:(1)The Jiulong fault was a fracture zone composed of four branch faults,with a width of around 30-40 m and good extendibility,while the maximum surface rupture length was 373 m.(2)The Jiulong fault has many strata dislocations,and the dislocation distance decreased from bottom to top,demonstrating synsedimentary structure characteristics,with a maximum stratigraphic dislocation distance of 18.2 m.(3)Preliminary analysis suggested the Jiulong fault as a secondary frac-ture of the Xiangfan-Guangji fault zone and provided evidence of the southeastward extension of the Xiangfan-Guangji fault.A preliminary hypothesis purported the Xiangfan-Guangji fault as the seismo-genic fault of the Ms 5.0 magnitude earthquake in 1911.(4)According to OSL and ESR dating analy-ses,the upper breakpoint of the Jiulong fault cuts into the Late Pleistocene Xingang Formation(Qp3x)strata,and the latest active age of the Jiulong fault was 57.6 ka.The chronology analysis confirmed an active fault from the Late Pleistocene and identified a weak tectonic in Jiujiang Province,which repre-sents the largest active fault outcrop uncovered in the area so far.This study provides evidence and re-search materials for the evaluation of fault activity and seismic stability in this region.
查看更多>>摘要：Detrital U-bearing minerals(e.g.,zircon,apatite)U-Pb ages with specific trace-element geochemistry,are frequently used in provenance analyses.In this study,we focus on the Yarlung River drainage in South Tibet,characterized by two distinct lithologic units:The Gangdese batholith to the north(mainly granitoids)and the Tethyan Himalaya(mainly sedimentary rocks)to the south,which plays a crucial role in the erosion of the Tibetan Plateau.To constrain the provenance of the Yarlung River Basin,we performed trace-element and U-Pb age analyses of detrital apatite from the river sands of the Yarlung River and its tributaries.Our findings indicate that the detrital apatite U-Pb age pat-terns of the north tributaries exhibit main peaks at approximately 40 and 60 Ma,consistent with the corresponding U-Pb age patterns of detrital zircon published.Further,their trace element casts fall mainly in the Type I granite region,also indicating the Gangdese arc-dominated source.However,those of the south tributaries(～60-20 Ma)exhibit a different age distribution from the detrital zircon U-Pb groups(～110-150,～500,and 1 100 Ma),suggesting that the detailed apatite U-Pb signals can provide excellent constraints on the provenance of igneous and metamorphic rock sources but less so for sedi-mentary rock sources.Combined with previous detrital zircon data in the study area,our detrital apa-tite information can highlight young metamorphic events from a complex background(i.e.,Niyang and Nianchu rivers),which offers additional constraints on the provenance of the Yarlung River Basin.Generally,a combination of geochemistry and geochronology of multi-detrital heavy minerals,such as zircon and apatite,can provide powerful tools for provenance analysis.
查看更多>>摘要：Abundant mafic-felsic intrusions distributed in the Altyn Orogen record orogenic histo-ries related to Proto-Tethys and Paleo-Tethys evolution.Zircon U-Pb dating of the intrusive rocks in the eastern Altyn Orogen identifies at least three major tectono-magmatic episodes,yielding ages of～426,～376-373 and～269-254 Ma.The first two emplacement episodes correspond to the post-collisional magmatism in the Altyn Orogen.The～426 Ma granitoids possess adakitic characteristics coupled with enriched isotopes,suggesting that they originated from partial melting of thickened lower continental crust induced by upwelling asthenospheric mantle after slab break-off of the South Altyn Ocean Plate.Next,the～376-373 Ma mafic-intermediate rocks and coeval granitoids represent a large thermal event that involved mantle melting with induced new juvenile lower continental crust melting in a post-collisional extensional setting.Finally,the～254 Ma diabase dykes intruded into the～269 Ma granit-oids,which were related to the widespread Late Paleozoic magmatism resulting from Paleo-Tethys Ocean subduction.Post-collisional magmatism in the Altyn Orogen significantly enhances understand-ing of the tectono-magmatic evolution in the northern Tibetan Plateau.The penetrative influence of Paleo-Tethys Ocean subduction was more extensive than previously thought.
查看更多>>摘要：Seismic anisotropy originating within the continental crust is commonly used to deter-mine the deformation and kinematic flow within active orogens and is attributed to regionally oriented mica or hornblende grains.However,naturally deformed rocks usually contain compositional layers(e.g.,parallel compositional banding).It is necessary to understand how both varying mineral contents and differing intensities of compositional layering influence the seismic properties of the deep crust.In this study,we analyzed the seismic response of migmatitic amphibolite with compositional banding structures.We present the microstructures,fabrics,calculated seismic velocities,and seismic anisotro-pies of mylonitic amphibolite from a horizontal shear layer preserved within the Ailao Shan-Red River shear zone,southwestern Yunnan,China.The investigated sample is characterized by pronounced cen-timeter-scale compositional banding.The microstructures and fabrics suggest that migmatitic amphib-olite rocks within deep crust may delineate regions of deformation-assisted,channelized,reactive,po-rous melt flow.The origin of compositional banding in the studied migmatitic amphibolite is attributed primarily to partial melting together with some horizontal shearing deformation.The microfabrics and structures investigated in this study are considered to be typical for the base of active horizontal shear layers in the deep crust of southeastern Tibet.Seismic responses are modeled by using crystal preferred orientations for minerals of the migmatitic amphibolite by applying the Voigt-Reuss-Hill homogeniza-tion method.Calculated P-wave and S-wave velocities are largely consistent in the various layers of the migmatite.However,seismic anisotropies of P-wave(AVp)and S-wave(AVs)are higher in the melano-somes(AVp=5.6％,AVs-6.83％)thanthose in the leucosomes and the whole rock(AVp=4.2％-4.6％,AVs=3.1％-3.2％).In addition,there is pronounced,S-wave splitting oblique to the foliation plane in the migmatitic amphibolite.The multiple parallel compositional layers generate marked variation in the geometry of the seismic anisotropy(VsI polarization)in the whole rock.Combined with the mac-roscale geographical orientation of fabrics in the Ailao Shan-Red River shear zone,these compositional banding effects are inferred to generate significant variations in the magnitude and orientation of seis-mic anisotropy,especially for shear-wave anisotropy(AVs)in the deep crust.Hence,our data suggest that layering of various origins(e.g.,shear layers,partial-melting layers,and compositional layers)rep-resents a new potential source of anisotropy within the deep crust.
查看更多>>摘要：The Wutonggou iron deposit is located in the well-known iron metallogenic belt in the eastern Tianshan,NW China,and has been regarded as a sedimentary iron deposit.Although hydro-thermal overprinting could play indispensable roles in the formation of high-grade iron ores in sedi-mentary iron deposits,previous studies mainly focused on sedimentary-related iron mineralization,while the nature and contribution of hydrothermal fluids are poorly constrained.Accordingly,an inte-grated study of ore geology,H-O-C isotopes and 40Ar-39Ar dating,is conducted on the Wutonggou de-posit,in order to reveal the features,source,and timing of hydrothermal mineralization.The studied deposit includes two mining sections namely the Jianshan and Wutonggou.The δ18O values of early magnetite from the Jianshan section range from+3.0‰ to+5.8‰ that nearly consistent with classic magmatic magnetite,while increase to 6.3‰-8.0‰ in the late stage.Quartz from the two sections shows comparable H-O isotopic compositions and identical fractionation trends,and is plotted in or pe-riphery to the primary magmatic water area.Calcites from the two sections are broadly similar in car-bon and oxygen isotopic compositions,and siderite from the Wutonggou section is plotted in the same region.Thus,comparable stable isotopic compositions and evolution trends indicate similar magmatic fluids contributed hydrothermal iron mineralization in the two mining sections.Moreover,water-rock interactions of varying degrees generated distinct mineralization styles in the Jianshan and Wutonggou sections,and caused the isotopic fractionation in late stages.Biotite extracted from a hydrothermal sid-erite ore yielded a 40Ar-39Ar plateau age of 299.5±2.0 Ma,indicates the timing of hydrothermal iron mineralization is corresponding to the emplacement of vicinity granitoids.Taken together,the hydro-thermal mineralization in the Wutonggou iron deposit was the product of remobilization and upgrad-ing of early sedimentary iron ores,and ore-forming fluids were most probably originated from regional granitic magmatism.
查看更多>>摘要：Radon(Rn)and helium(He)gases from uranium decay form distinct anomalies related to buried uranium deposits.In order to trace the geochemical anomalous sources from the volcanic-related uranium deposits in deeply buried areas,systematical Rn contents and He isotope ratios were analyzed from the Daguanchang uranium deposit.The soil gas Rn concentrations above the deep urani-um are ten times higher than those in barren areas,indicating that instantaneous Rn content measure-ments can be used to detect deeply buried uranium.The helium isotope ratios(3He/4He)of the unminer-alized samples from the mineralized drill hole(ZK1)are relatively lower and uniform compared to those of the samples from no-mineral drill hole(ZK2).However,the Th and U contents of the drill core samples from ZK1 are slightly lower than those of the samples from ZK2,indicating that the lower 3He/4He ratios in ZK1 are most likely due to the addition of 4He from underlying uranium intervals.The differences in the instantaneous Rn contents are consistent with the variations in the He isotope ra-tios of the drill core samples.These results demonstrate that soil gas Rn and 3He/4He ratios are useful tracers and can indicate the existence of deeply buried volcanic-related hydrothermal uranium ores.
查看更多>>摘要：The podiform chromitites in the Luobusha ophiolite have been thought to experience a very deep formation,but the maximum depth is still an open issue.Here,we have investigated the structural stability of natural magnesiochromite using the synchrotron-based powder X-ray diffraction and diamond anvil cells up to 48.6 GPa and 2 450 K.The results have shown that spinel-type magnesio-chromite first decomposes into corundum-type'Cr2O3'+B1-type'MgO'at 11-14 GPa and 1 250-1 450 K,then modified ludwigite(mLd)-type'Mg2Cr,O5'+corundum-type'Cr2O3'at 14.3-20.5 GPa and 1 300-2 000 K,and finally CaTi2O4-type phase at 24.5 GPa.During the quenching procession from high-temperature-pressure conditions,the mLd-type phase appeared again and was kept at ambient conditions.We also obtained the isothermal equation states of spinel-type and CaTi2O4-type phases,re-vealing the composition effect on their elasticities.Based on the updated results,we propose chromitites could not experience pressure exceeding～14.3 GPa(approximate maximum depth～400 km)in the subduction-recycling genesis model.
查看更多>>摘要：The three-dimensional(3D)morphology,anatomy,and in-situ chemical composition anal-ysis of fossils are crucial for systematic paleontology and determining their phylogenetic positions.Scanning electron microscopy(SEM)coupled with energy-dispersive X-ray spectroscopy(EDS),offers valuable structural and chemical information for the analysis of fossils.However,its primary limitation is the restriction to two-dimensional surface data,which limits the exploration of fossils'3D complexi-ties.Conversely,3D X-ray microscopy(3D-XRM),also known as a novel form of micro-computed to-mography(micro-CT)facilitates the non-destructive 3D reconstruction of fossil specimens.Neverthe-less,it lacks the capability to provide in-situ compositional data.Acknowledging the constraints inher-ent in these individual techniques,and in response to the evolving requirements of paleontological re-search,this study introduces an integrated approach that combines 3D-XRM with EDS-coupled fo-cused ion beam scanning electron microscopy(FIB-SEM).This innovative strategy is designed to syner-gize the advantages of both techniques,thereby addressing challenges that conventional methods can-not.It enables the rapid identification of regions of interest(ROI)within fossil specimens at microme-ter resolution.Subsequently,this method collects detailed data on both 3D structures and chemical compositions at the nanometer scale for the identified ROI.This integrated approach represents a sig-nificant advancement in paleontological and geological research methodologies,promising to meet the increasing demands of these fields.
查看更多>>摘要：Burgess Shale-type deposits provide a wealth of information on the early evolution of ani-mals.Questions that are central to understanding the exceptional preservation of these biotas and the paleocnvironments they inhabited may be obscured by the post-depositional alteration due to metamor-phism at depth and weathering near the Earth's surface.Among over 50 Cambrian BST biotas,the Chengjiang and Qingjiang deposits are well known for their richness of soft-bodied taxa,fidelity of preservation,and Early Cambrian Age.While alteration via weathering has been well-investigated,the thermal maturity of the units bearing the two biotas has not yet been elucidated.Here we investigate peak metamorphic temperatures of the two deposits using two independent methods.Paleogeotempera-ture gradient analyses demonstrate that the most fossiliferous sections of the Chengjiang were buried at a maximum depth of～8 500 m in the Early Triassic,corresponding to～300 ℃,while the type area of the Qingjiang biota was buried at a maximum depth of～8 700 m in the Early Jurassic,corresponding to～240 ℃.Raman geothermometer analyses of fossil carbonaceous material demonstrate that peak temperatures varied across localities with different burial depth.The two productive sections of the Chengjiang biota were thermally altered at a peak temperature of approximately 300 ℃,and the main locality of the Qingjiang biota experienced a peak temperature of 238±22 ℃.These results from two independent methods are concordant.Among BST deposits for which thermal maturity has been docu-mented,the Qingjiang biota is the least thermally mature,and therefore holds promise for enriching our understanding of BST deposits.
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