COUPLING RELATIONSHIPS BETWEEN OROGENS AND BASINS,LOWER CRUST AND UPPER CRUST IN THE QINGHAI—TIBET PLATEAU

Uplift\|related geological and geophysical data available from regional geological mapping , detailed structural, tectonic, granitic, volcanic, metamorphic , geochronological studies, deep seismic reflection, wide\|angle seismic experiment, seismic tomography, broadband seismic network, and magnetotelluric sounding of key areas of the Qinghai—Tibet plateau are radically different from models of plate subduction or collision. Key geological features include: (1) obvio us time difference between plate collision and uplift of the plateau; (2) developments of intracrustal low\|velocity layers , low resistivity layers and discontinuous subhorizontal reflectors; (3) similar results between the rate and time of uplift of the Qinghai\|Tibet plateau and the time and rate of subsidence of its surrounding basins; (4) subhorizontal detachments and metamorphic core complexes occurred in Himalayan and Longmenshan; (5) weak deformation of late Cenozoic sediments and development of major steeply to gently dipping normal faults in the central part of the plateau; (6) discovery of high pressure and ultrahigh pressure metamorphic rocks in Nanbajiawa; (7) young volcanic rocks distributed from the northern plateau to the southern plateau; (8) nearly concordant processes among thrusting , mountain building, horizontal extension and intrusion of leucogranite in Himalayans.

A missing link of the Proto-Tethys Ocean between the Qinling and Qilian orogens,China:Insights from geochronology and structural geology

The Qinling-Qilian connection zone(QQCZ)is a key area to reveal the relationship and to make a link of the North Qinling and the North Qilian orogens,China.Here we present U-Pb dating data of detrital zircons from four sedimentary/metasedimentary rocks in the QQCZ and the southwestern North China Block(NCB)and detailed regional structural data.Three episodes of fold deformation(D1,D2 and D3)are distinguished in the QQCZ,with the former two occurred during the early Paleozoic.The D1 deformation is mainly characterized by regionally penetrative schistosity and some residual rootless intrafolial folds due to the intensive superpositions by the subsequent D2 and D3 deformations.The D2 deformation characterized by tight folds,associated axial plane foliations and crenulation lineations indicates a stress field characterized by NNE-SSW-directed compression,which may be induced by the collision between the NCB and the southern blocks.The D3 deformation which might occur during the Mesozoic is marked by upright open folds and kink bands.The similarity of the detrital zircon age spectra of the Huluhe Group in the North Qilian Orogen and the Erlangping Group in the North Qinling Orogen suggests that the two groups have similar provenance,which may indicate that the North Qilian Orogen corresponded to the North Qinling Orogen in a regional tectonic framework.In addition,the remarkable age peak at^435 Ma of the detrital zircon age spectrum of the Duanjiaxia Formation in the southwestern NCB indicates that this formation obtained the provenance of the North Qilian and North Qinling orogens,which may be generated by the collage of the southwestern NCB and the QQCZ during the Late Ordovician-Early Silurian.Based on structural,detrital zircon and metamorphic data,we suggest that the North Qilian and North Qinling orogens underwent similar evolution during the early Paleozoic due to the closure of the North Qilian and the Kuanping oceans which located at the northern boundary of the Proto-Tethys Ocean.

Early Paleozoic accretionary orogens along the Western Gondwana margin

Early Paleozoic accretionary orogens dominated the Western Gondwana margin and were characterized by nearly continuous subduction associated with crustal extension and back-arc basin development.The southwestern margin is represented by Famatinian and Pampean basement realms exposed in South America,both related to the protracted Paleozoic evolution of the Terra Australis Orogen,whereas the northwestern margin is mainly recorded in Cadomian domains of Europe and adjacent regions.However,no clear relationships between these regions were so far established.Based on a compilation and reevaluation of geological,paleomagnetic,petrological,geochronological and isotopic evidence,this contribution focuses on crustal-scale tectonic and geodynamic processes occurring in Western Gondwana accretionary orogens,aiming at disentangling their common Early Paleozoic evolution.Data show that accretionary orogens were dominated by high-temperature/lowpressure metamorphism and relatively high geothermal gradients,resulting from the development of extended/hyperextended margins and bulk transtensional deformation.In this sense,retreating-mode accretionary orogens characterized the Early Paleozoic Gondwana margin,though short-lived pulses of compression/transpression also occurred.The existence of retreating subduction zones favoured mantle-derived magmatism and mixing with relatively young(meta)sedimentary sources in a thin continental crust.Crustal reworking of previous forearc sequences due to trenchward arc migration thus took place through assimilation and anatexis in the arc/back-arc regions.Therefore,retreating-mode accretionary orogens were the locus of Early Paleozoic crustal growth in Western Gondwana,intimately associated with major flare-up events,such as those related to the Cadomian and Famatian arcs.Slab roll back,probably resulting from decreasing convergence rates and plate velocities after Gondwana assembly,was a key factor for orogen-scale geodynamic processes.Coupled with synchronous oblique subduction and crustal-scale dextral deformation,slab roll back might trigger toroidal mantle flow,thus accounting for bulk dextral transtension,back-arc extension/transtension and a large-scale anticlockwise rotation of Gondwana mainland.

EXTENSION OF THE CATHAYSIAN DIWA-TYPE OROGENSAND CONTINENTAL MARGIN SPREADING IN SOUTHEAST CHINA

A dynamic mechanism, "lower lithospheric delamination and extension of the Cathaysian Diwa-type orogens" , has been put forward to explain the formation of the Cenozoic continental-margin-spreading-belt in Southeast China. The processes of formation and develop-ment for the belt are from the stage of the Cathaysian Diwa-type orogens at which compression and thickening of curstobodies prevailed, to the stage of the basin-and-range-type structures at which the late orogenic crustal extension occurred, and then to the stage of the epicontinental Diwa-type rifts at which the lithospheric extension took place.

Genesis of LCT Pegmatites during Early Paleozoic Orogeny of the North Qinling Orogenic Belt,China:Emplacement Conditions and Structural Control

The Guanpo pegmatite field in the North Qinling orogenic belt(NQB),China,hosts the most abundant LCT pegmatites.However,their emplacement conditions and structural control remain unexplored.In this contribution,we investigated it combining pegmatite orientation measurement with oxygen isotope geothermometry and fluid inclusion study.The orientations of type A1 pegmatites(P_(f)<σ_(2))are predominantly influenced by P-and T-fractures due to simple shearing in Shiziping dextral thrust shear zone during D_(2)deformation,whereas type A2 pegmatites(contemporaneous with D_(4))are governed by hydraulic fractures aligned with S_(0)and S_(0+1)stemming from fluid pressure(P_(f)<σ_(2)).Additionally,type B pegmatites(P_(f)≤σ_(2))exhibit orientations shaped by en echelon extensional fractures in local ductile shear zones(contemporaneous with D_(3)).The albite-quartz oxygen isotope geothermometry and microthermometric analysis of fluid inclusions in elbaites from the latest pegmatites(including types B and A2)suggest that the crystallization P-T for late magmatic and hydrothermal stages are 527.5-559.2℃,320℃,3.1-3.6 kbar and 2.0 kbar,respectively.Our observations along with previous studies suggest that the genesis of the LCT pegmatites was a long-term,multi-stage event during early Paleozoic orogeny(including the collision stage)of the NQB,and was facilitated by various local fractures.

The Weixi High-silica Granitoids in the Central Sanjiang Orogenic Belt,Southwest China:Implications for Growth of the Continental Crust

High-silica granitoids record the formation and evolution of the continental crust.A new intrusive complex has been recognized among silicic volcanic rocks of the Weixi arc,Southwest China.The intrusions consist of granites,granitic porphyries,and granodiorites.Zircon U-Pb age data indicate that the Weixi granitoids formed at 248-240 Ma and were coeval with silicic volcanic rocks of the Weixi arc.The Weixi granitoids are enriched in Rb,Th,and U,depleted in Ba,Sr,Nb,Ta,and Ti,and have high light/heavy rare earth element ratios and slightly negative Eu anomalies.The Weixi granitoids have negative ε_(Nd)(t)values(-9.8 to-7.8)and negative zircon ε_(Hf)(t)values(-12.02 to-5.11).The geochemical and isotopic features suggest the Weixi granitoids were derived by partial melting of ancient crustal material.The Weixi granitoids and silicic volcanic rocks were derived from the same magma by crystal accumulation and melt extraction,respectively,and they record the formation of a continental arc in the central Sanjiang orogenic belt.

Early Neoproterozoic Granite-Gneisses of the Junggar Alataw(Southeastern Kazakhstan):Age,Petrogenesis and Tectonic Implications

The combined petrographic,petrological,geochemical and geochronological study of the Neoproterozoic gneisses of the Sarychabyn and Baskan complexes of the Junggar Alataw of South Kazakhstan elucidate the Precambrian tectonic evolution of the Aktau–Yili terrane.It is one of the largest Precambrian crustal blocks in the western Central Asian orogenic belt.The U-Pb single-grain zircon ages indicate that granite-gneisses formed from the same source and crystallised in the early Neoproterozoic ca.930–920 Ma.The chemical composition of gneisses corresponds to A2-type granites.The whole-rock Nd isotopic characteristics(εNd(t)=−4.9 to−1.0 and TNd(DM-2st)=1.9 to 1.7 Ga)indicate the involvement of Paleoproterozoic crustal rocks in magma generation.Early Neoproterozoic ca.930–920 Ma A-type granitoids in the Aktau–Yili terrane of South and Central Kazakhstan might reflect within-plate magmatism adjacent to the collisional belt or a local extension setting in back-arc areas of the continental arc.

Enhancing Geological Understanding and Identifying Gold Anomalies in the Ailaoshan Orogen

The Ailaoshan Orogen in the southeastern Tibet Plateau,situated between the Yangtze and Simao blocks,underwent a complex structural,magmatic,and metamorphic evolution resulting in different tectonic subzones with varying structural lineaments and elemental concentrations.These elements can conceal or reduce anomalies due to the mutual effect between different anomaly areas.Dividing the whole zone into subzones based on tectonic settings,ore cluster areas,or sample catchment basins(Scb),geochemical and structural anomalies associated with gold(Au)mineralization have been identified utilizing mean plus twice standard deviations(Mean+2STD),factor analysis(FA),concentration-area(CA)modeling of stream sediment geochemical data,and lineament density in both the Ailaoshan Orogen and the individual subzones.The FA in the divided 98 Scbs with 6 Scbs containing Au deposits can roughly ascertain unknown rock types,identify specific element associations of known rocks and discern the porphyry or skarn-type Au mineralization.Compared with methods of Mean+2STD and C-A model of data in the whole orogen,which mistake the anomalies as background or act the background as anomalies,the combined methods of FA and C-A in the separate subzones or Scbs works well in regional metallogenic potential analysis.Mapping of lineament densities with a 10-km circle diameter is not suitable to locate Au deposits because of the delineated large areas of medium-high lineament density.In contrast,the use of circle diameters of 1.3 km or 1.7 km in the ore cluster scale delineates areas with a higher concentration of lineament density,consistent with the locations of known Au deposits.By analyzing the map of faults and Au anomalies,two potential prospecting targets,Scbs 1 and 63 with a sandstone as a potential host rock for Au,have been identified in the Ailaoshan Orogen.The use of combined methods in the divided subzones proved to be more effective in improving geological understanding and identifying mineralization anomalies associated with Au,rather than analyzing the entire large area.

Geochronology and Geochemistry of the Xingxingxia Triassic A-type Granites in Central Tianshan,NW China:Petrogenesis and Tectonic Implications

The Triassic granitoids in Central Tianshan play a key role in determining the petrogenesis and tectonic evolution on the southern margin of the Central Asian orogenic belt.In this study,we present SHRIMP zircon U-Pb ages,Hf isotopic and geochemical data on the Xingxingxia biotite granite,amazonite granite and granitic pegmatite in Central Tianshan,NW China.Zircon U-Pb dating yielded formation ages of 242 Ma for the biotite granite and 240 Ma for the amazonite granite.These granitoid rocks have high K_(2)O with low MgO and CaO contents.They are enriched in Nb,Ta,Hf and Y,while being depleted in Ba and Sr,showing flat HREE patterns and negative Eu anomalies.They have typical A-type granite geochemical signatures with high Ga/A_(1)(8–13)and TFeO/(TFeO+MgO)ratios,showing an A_(2) affinity for biotite granite and an A_(1) affinity for amazonite granite and granitic pegmatite.Zircon ε_(Hf)(t)values of the granitoids are 0.45–2.66,with Hf model ages of 0.99–1.17 Ga.This suggests that these A-type granites originated from partial melting of the lower crust.We propose that Xingxingxia Triassic A-type granites formed under lithospheric extension from post-orogenic to anorogenic intraplate settings and NE-trending regional strike-slip fault-controlled magma emplacement in the upper crust.

Tectonic Evolution and Lithospheric Structure of the Beishan Orogen:Insights from Magnetotelluric Studies

The Beishan orogen,located in the central segment of the Tianshan–Solonker suture within the southern Central Asian Orogenic Belt(CAOB),is crucial for understanding the accretionary processes and continental growth in Central Asia.This orogen developed through the episodic amalgamation and accretion of continental margin arcs,island arcs,ophiolites,and accretionary wedges,undergoing a complex process of accretion and evolution.Since the Phanerozoic,the Beishan orogen has experienced multiple phases of magmatic and collision events.The intricate distribution of magmatic arc rocks has obscured the complete basement traces,and the spatial superposition of multiple magmatic arc phases has complicated the study of its evolutionary history.

Remnant Paleoproterozoic Subduction or Lithospheric Drip Initiation at the Yilgarn Craton Margin:Constraints from P-wave Tomography

The Paleoproterozoic was a critical time in whether modern-style plate tectonics had become globally dominant(e.g.,Wan et al.,2020).The Capricorn Orogen witnessed the assembly of the Pilbara and Yilgarn Cratons and an exotic microcontinent,the Glenburgh Terrane,to form the West Australia Craton(WAC)through two collisional orogenic events,the 2215–2145 Ma Ophthalmian and 2005–1950 Ma Glenburgh Orogenies(Johnson et al.,2013;Fig.1).Compared to other Proterozoic orogenic belts in Australia,the Capricorn Orogen preserves‘complete'opposing continental margin successions,together with intervening arc fragments associated with oceanic closure and foreland basins associated with collisional loading(Cawood et al.,2009).

Provenance of Conglomerate and Sandstone from Early Permian Shoushangou Formation in Xi Ujimqin,Inner Mongolia:Implications for Understanding Paleo-Asian Ocean Subduction

During the Late Carboniferous to Early Permian,a rift was formed by post-collisional extension after ocean closure or an island arc-related basin formed by Paleo-Asian Ocean(PAO)subduction in the Xi Ujimqin area.Nevertheless,the closure time of the PAO is still under debate.Thus,to identify the origin of the PAO,the geochemistry and U-Pb age of zircons were analyzed for the extra-large deep marine,polymict clastic boulders and sandstones in the Shoushangou Formation within the basin.The analyses revealed magmatic activity and tectonic evolution.The conglomerates include megaclasts of granite(298.8±9.1 Ma)and granodiorite porphyry(297.1±3.1 Ma),which were deposited by muddy debris flow.Results of this study demonstrated that the boulders of granitoids have the geochemistry of typical I-type granite,characterized by low Zr+Nb+Ce+Y and low Ga/Al values.The granitoid boulders were formed in island arc setting,indicating the presence of arc magmatism in the area that is composed of the Late Carboniferous to Early Permian subduction-related granitoid in southern Xi Ujimqin.Multiple diagrams for determining sedimentary provenance using major and trace elements indicate that Shoushangou sediments originated from continental island arc-related felsic rocks.Detrital zircon U-Pb age cluster of 330–280 Ma was obtained,indicating input from granite,ophiolite,Xilin Gol complex,and Carboniferous sources to the south.The basin was geographically developed behind the arc during the Early Permian period because the outcropped intrusive rocks in the Late Carboniferous to Early Permian form a volcanic arc.The comprehensive analyses of source areas suggest that Shoushangou sediments developed in a backarc basin in response to the northward subduction of the PAO.The backarc basin and intrusive rocks,in addition to previously published Late Carboniferous to Early Permian magmatic rocks of arc unit in Xilin Gol,confirm the presence of an Early Permian trencharc-basin system in the region,represented by the Baolidao arc and Xi Ujimqin backarc basin.This study highlights the importance and potential of combined geochemical and geochronological studies of conglomerates and sandstone for reconstructing the geodynamic setting of a basin.

Apatite Fission-Track Thermochronology in the Tamusu Area,Bayingobi Basin,NW China,and its Geological Significance

The Bayingobi basin is located in the middle of Central Asia Orogenic Belt,at the intersection of Paleo-Asian Ocean and Tethys Ocean,as well as the junction of multiple tectonic plates.This unique tectonic setting underpins the basin's intricate history of tectonic activity.To unravel the multifaceted tectono-thermal evolution within the southwestern region of the basin and to elucidate the implications of sandstone-hosted uranium mineralization,granitic and clastic rock samples were collected from the Zongnai Mts.uplift and Yingejing depression,and apatite fission track(AFT)dating and thermal history simulation analysis were performed.AFT dating findings reveal that the apparent ages of all samples fall within the range of 244 Ma to 112 Ma.In particular,the bedrock of the Zongnai Mts.and Jurassic detrital apatite fission tracks have undergone complete annealing,capturing the uplift-cooling age.Meanwhile,the AFT ages of Cretaceous detrital rocks are either equivalent to or notably exceed the age of sedimentary strata,signifying the cooling age of the provenance.A comprehensive examination of AFT ages and palaeocurrent direction analyses suggests that the Cretaceous source in the Tamusu area predominantly originated from the central and southern sectors of the Zongnai Mts.uplift.However,at a certain juncture during the Late Early Cretaceous,the Cretaceous provenance expanded to include the northern part of the Zongnai Mts.uplift.Based on the results of thermal history simulations and previous studies,it is considered that the Tamusu area has undergone four distinct tectonic uplift events since the Late Paleozoic.The first is the Late Permian to Early Triassic(260-240 Ma),which is associated with the closure of the Paleo-Asian Ocean and the accretionary orogeny within the Alxa region.The second uplift event took place in the Early Jurassic(190-175 Ma)and corresponded to intraplate orogeny following the closure of the Paleo-Asian Ocean.The third uplift event is the Late Jurassic to Early Cretaceous(160-120 Ma),which is linked to the East Asia's position as the convergence center of multiple tectonic plates during this period.The fourth uplift event is linked to the Late Early Cretaceous(112-100 Ma),driven either by the westward subduction of the eastern Pacific plate or the mantle upwelling resulting from the Bangong-Nujiang oceanic lithosphere subduction and slab break-off.The primary stress orientation for the first three tectonic uplift phases approximated a nearly SN direction,while the fourth stage featured a principal stress direction of NW.The fourth tectonic uplift event of the Late Early Cretaceous and basaltic eruption thermal event during this period likely exerted a significant influence on the formation of the Tamusu sandstone-hosted uranium deposit.

Petrogenesis and tectonic implications of the Silurian adakitic granitoids in the eastern segment of the Qilian Orogenic Belt,Northwest China

Geodynamic mechanism responsible for the generation of Silurian granitoids and the tectonic evolution of the Qilian orogenic belt remains controversial. In this study, we report the results of zircon U–Pb age, and systematic whole-rock geochemical data for the Haoquangou and Liujiaxia granitoids within the North Qilian orogenic belt and the Qilian Block, respectively, to constrain their petrogenesis, and the Silurian tectonic evolution of the Qilian orogenic belt. Zircon U–Pb ages indicate that the Haoquangou and Liujiaxia intrusions were emplaced at423 ± 3 Ma and 432 ± 4 Ma, respectively. The Haoquangou granodiorites are calc-alkaline, while the Liujiaxia granites belong to the high-K calc-alkaline series.Both are peraluminous in composition and have relatively depleted Nd isotopic [ε_(Nd)(t) =(-3.9 – + 0.6)] characteristics compared with regional basement rocks, implying their derivation from a juvenile lower crust. They show adakitic geochemical characteristics and were generated by partial melting of thickened lower continental crust. Postcollisional extensional regime related to lithospheric delamination was the most likely geodynamic mechanism for the generation of the Haoquangou granodiorite, while the Liujiaxia granites were generated in a compressive setting during continental collision between the Qaidam and Qilian blocks.

Early Devonian Post-collisional Granitic Magmatism in the North Qilian Orogenic Belt,Western China:Insights into Lithospheric Delamination and Orogenic Collapse

Post-collisional magmatism contains important clues for understanding the reworking and growth of continental crust,as well as lithospheric delamination and orogenic collapse.Early Devonian magmatism has been identified in the North Qilian Orogenic Belt(NQOB).This paper reports an integrated study of petrology,whole-rock geochemistry,Sm-Nd isotope and zircon U-Pb dating,as well as Lu-Hf isotopic data,for two Early Devonian intrusive plutons.The Yongchang and Chijin granites yield zircon U-Pb ages of 394-407 Ma and 414 Ma,respectively.Both of them are characterized by weakly peraluminous to metaluminous without typical aluminium-rich minerals,LREE-enriched patterns with negative Eu anomalies and a negative correlation between P_(2)O_(5) and SiO_(2) contents,consistent with geochemical features of I-type granitoids.Zircons from the studied granites display negative to weak positive ε_(Hf)(t)values(−5.7 to 2.1),which agree well with those of negative ε_(Nd)(t)values(−6.4 to−2.9)for the whole-rock samples,indicating that they were derived from the partial melting of Mesoproterozoic crust.Furthermore,low Sr/Y ratios(1.13-21.28)and high zircon saturation temperatures(745℃ to 839℃,with the majority being>800℃)demonstrated a relatively shallow depth level below the garnet stability field and an additional heat source.Taken together,the Early Devonian granitic magmatism could have been produced by the partial melting of ancient crustal materials heated by mantle-derived magmas at high-temperature and low-pressure conditions during postcollisional extensional collapse.The data obtained in this study,when viewed in conjunction with previous studies,provides more information about the tectonic processes that followed the closure of the North Qilian Ocean.The tectonic transition from continental collision to post-collisional delamination could be constrained to~430 Ma,which is provided by the sudden decrease of Sr/Y and La/Yb ratios and an increase in zircon ε_(Hf)(t)values for granitoids.A two-stage tectonic evolution model from continental collision to post-collisional extensional collapse for the NQOB includes(a)continental collision and crustal thickening during ca.455-430 Ma,characterized by granulite-facies metamorphism and widespread low-Mg adakitic magmatism;(b)post-collisional delamination of thickened continental crust and extensional collapse of orogen during ca.430-390 Ma,provided by coeval high-Mg adakitic magmatism,A-type granites and I-type granitoids with low Sr-Y ratios.

The Geology, Fluid Inclusions, and O-S Isotopes of the Mibei Gold Deposit, Hunan Province, Southern China

The Mibei gold deposit,located in the southwestern part of the Xuefengshan uplift zone,the middle section of the Jiangnan orogenic belt in southern China,has estimated gold resources of approximately seven tons.This deposit is primarily a quartz vein-type gold deposit,with ore bodies occurring mainly within Neoproterozoic metasediments.The main metallic minerals in the ore are pyrite,chalcopyrite,and arsenopyrite.In this study,the petrography and microthermometry of ore-forming fluid inclusions,oxygen isotopes of gold-bearing quartz,and sulfur isotopes of goldbearing sulfides and arsenopyrite were analyzed.Three types of fluid inclusions were identified:type Ⅰa three-phase inclusions comprising vapor and two phases of liquids(V_(CO_(2))+L_(CO_(2))+L_(H2O)),type Ⅰb two-phase liquids(L_(CO_(2))+L_(H2O)),typeⅡ two-phase vapor-rich inclusions(V/V+L> 50%),and type Ⅲ pure liquid inclusions.Type Ⅰ inclusions were heated uniformly to the liquid phase,type Ⅱ inclusions were heated uniformly to the gas phase,and type Ⅲ inclusions were heated without change.In general,the temperature range of homogenization to liquid phase of fluid inclusions in the Mibei gold deposit is 204-227℃.The salinity of the inclusion ranges from 4.6 to 12.2 wt% NaCl equiv.The δ~(18)O_(SMOW) of gold-bearing quartz varies from 16.9‰ to 17.5‰.The δ~(18)O_(H2O) of gold-bearing quartz are varied from 6.5‰ to 7.5‰.The δ~(34)S values of gold-bearing pyrite range from 1.7‰ to 6.8‰.The δ~(34)S values of gold-bearing arsenopy rite range from 5.6%o to 5.9‰.Theδ~(34)S values of pyrite from wall rocks slate range from 6.4‰ to 11.6‰.This evidence implies that the ore-forming fluids of the Mibei gold deposit originated from magmatic-hydrothermal processes,mixing with minor S from the surrounding metasediments.Combined with the evolution of the Jiangnan orogenic belt,due to the magmatic and tectonic activities of the Xuefengshan uplift during the Caledonian period,the fault seal mechanism controlled the ore-forming process.Overall,the Mibei gold deposit is more akin to a magmatic-hydrothermal gold deposit.

Early Cretaceous Metasomatized Lithospheric Mantle beneath the Central Jiangnan Orogen in South China:Geochemical and Sr-Nd Isotope Evidence from the Tuanshanbei Dolerite

It is well established that Cretaceous magmatism in the South China Block(SCB)is related to the Paleo-Pacific subduction.However,the starting time and the associated deep crust-mantle processes are still debatable.Mafic dike swarms carry important information on the deep earth(including mantle)geodynamics and geochemical evolution.In the Jiangnan Orogen(South China).there is no information on whether the Mesozoic magmatic activities in this region are also directly related to the Pacific subduction or not.In this study,we present detailed zircon U-Pb geochronological,wholerock element and Sr-Nd isotope data for Early Cretaceous Tuanshanbei dolerite dikes,and provide new constraints on the condition of the lithospheric mantle and mantle dynamics of the SCB during that time.LA-ICP-MS zircon U-Pb dating suggests that this dolerite erupted in the Early Cretaceous(~145 Ma).All samples have alkaline geochemical affinities with K_(2)O+Na_(2)O=3.11-4.04 wt%,K_(2)O/Na_(2)O=0.50-0.72,and Mg^(#)=62.24-65.13.They are enriched in LILE but depleted in HFSE with higher initial^(87)Sr/^(86)Sr ratio(0.706896-0.714743)and lower ε_(Nd)(t)(-2.61 to-1.67).They have high Nb/U,Nb/La,La/Sm and Rb/Sr,and low La/Nb,La/Ta,Ce/Pb,Ba/Rb,Tb/Yb and Gd/Yb ratios.Such geochemical signatures suggest that the fractional crystallization is obvious but crustal contamination play a negligible role during magmatic evolution.Tuanshanbei dolerite were most likely derived from low-degree(2%-5%)partial melting of a phlogopite-bearing mantle material consisted of~85% spinel peridotite and~15% garnet peridotite previously metasomatized by asthenospherederived fluids/melts with minor subduction-derived fluids/melts.Slab-rollback generally lead to the upwelling of the hot asthenosphere.The upwelling of asthenosphere consuming the lithospheric mantle by thermo-mechanical-chemical erosion.The lithospheric mantle may have partially melted due to the heating by the upwelling asthenosphere and lithospheric extension.It is inferred that the Tuanshanbei dolerite might be associated with the initial slab rollback and corresponding lithospheric extension occurred potentially at ca.145 Ma.

Tectonic framework of Qilian orogen: reveal from an aeromagnetic anomaly feature

The Qilian Orogenic belt is one of the typical orogenic belts globally and a natural laboratory for studying plate tectonics.Many researchers have studied the ophiolite and high pressure and ultra-high pressure metamorphic rocks in the Qilian orogen and obtained valuable achievements.However,a hot debate exists on the basement property,the distribution of ophiolite,and the boundaries of tectonic units.Large-scale high-precision aeromagnetic surveys have recently been conducted in the Qilian Orogenic belt and adjacent areas.In this study,we are trying to analysis the tectonic framework of the Qilian Orogen using 1:500,000 aeromagnetic data.The results provide geophysical perspectives for studying the structural framework and deformation of this area.According to the aeromagnetic∆T anomaly map,the central and Southern Qilian have the same magnetic anomaly feature that noticeably differs from the North Qilian Orogenic belt and the Qaidam Block.This result indicates that the central and Southern Qilian have a unified magnetic basement and differ from the North Qilian orogenic belt and Qaidam Block.The map shows the distribution of ophiolite in the North Qilian orogenic belt.Linear magnetic anomalies represent the ophiolites because the mafic–ultramafic rocks usually have high magnetic susceptibility.The ophiolite belts are continuously distributed in the western part of North Qilian orogenic belt and have a large scale.However,the scale of the ophiolite belt and the outcropping of mafic–ultramafic rocks reduces when they pass through Qilian County to the east.The results indicate differences in the evolution process between the eastern and western parts of North Qilian,with Qilian County as the transition zone.This study also systematically defines the geophysical boundaries of the Qaidam Block,Qilian Block,North Qilian Orogenic belt,and Alxa block.It is proposed that the sinistral displacement of the Altun Fault is adjusted and absorbed by the series of NE-trending faults in the Qilian orogen and merge into the Longshoushan–Gushi Fault.The extension of the North Qilian Orogenic belt is strengthened by the neotectonics movement along the shearing direction,which separated the North Qilian Orogenic belt into several segments and formed a series of northeast-trending faults.

Origin of the Yueguang gold deposit in Xinhua, Hunan Province, South China: insights from fl uid inclusion and hydrogen–oxygen stable isotope analysis

The Yueguang gold deposit is located in Fengjia,Xinhua County,Hunan Province,South China.It represents a recently discovered small-scale gold deposit situated in the southwestern region of the Jiangnan Orogenic Belt,west of the Baimashan granitic batholith.In order to discern the characteristics of the ore-formingfluids,the underlying mineralization processes,and establish a foundation for the origin of the Yueguang gold depositfluid inclusion micro-thermometry,as well as quartz hydrogen and oxygen isotope analysis,have been carried out on samples obtained from various stages of mineralization.The hydrothermal miner-alization stages within the Yueguang gold deposit can be categorized into three stages:(i)the barren,pre-ore quartz-pyrite stage(Stage Ⅰ),the quartz-pyrite-gold stage(Stage Ⅱ),and the post-ore quartz-carbonate stage(Stage Ⅲ),with the second stage being the main mineralization stage.Thefluid inclusions identified in samples from the main min-eralization stage can predominantly be described with the NaCl–H_(2)O and CO_(2)–NaCl–H_(2)O systems.These inclusions display homogenization temperatures ranging from 158.8 to 334.9℃,and thefluid salinity ranges from 0.3%to 4.0%(wt.%NaCl equiv.).Laser Raman spectroscopy analysis of individual inclusions further reveals the presence of gas-phases such as CO_(2),CH_(4),and N_(2).Isotopic analysis indicatesδ^(18)Ofluid values ranging from 3.95 to 6.7‰ and δDH_(2)O values ranging from-71.9 to-55.7‰.These results indi-cate that the ore-formingfluid of the Yueguang gold deposit belongs to metamorphic hydrothermalfluids of middle-low temperature and low salinity.In the process of ore formation,gold is transported in the form of Au(HS)2-complexes,with gold deposition being driven byfluid immiscibility.Therefore,the Yueguang gold deposit is categorized as an orogenic gold deposit dominated by metamorphic hydrother-malfluid.It may become a new target for gold exploration in the Baimashan region,central Hunan Province.

Two stages of subsidence and its formation mechanisms in Mid-Late Triassic Ordos Basin,NW China

Based on a large number of newly added deep well data in recent years,the subsidence of the Ordos Basin in the Mid-Late Triassic is systematically studied,and it is proposed that the Ordos Basin experienced two important subsidence events during this depositional period.Through contrastive analysis of the two stages of tectonic subsidence,including stratigraphic characteristics,lithology combination,location of catchment area and sedimentary evolution,it is proposed that both of them are responses to the Indosinian Qinling tectonic activity on the edge of the craton basin.The early subsidence occurred in the Chang 10 Member was featured by high amplitude,large debris supply and fast deposition rate,with coarse debris filling and rapid subsidence accompanied by rapid accumulation,resulting in strata thickness increasing from northeast to southwest in wedge-shape.The subsidence center was located in Huanxian–Zhenyuan–Qingyang–Zhengning areas of southwestern basin with the strata thickness of 800–1300 m.The subsidence center deviating from the depocenter developed multiple catchment areas,until then,unified lake basin has not been formed yet.Under the combined action of subsidence and Carnian heavy rainfall event during the deposition period of Chang 7 Member,a large deep-water depression was formed with slow deposition rate,and the subsidence center coincided with the depocenter basically in the Mahuangshan–Huachi–Huangling areas.The deep-water sediments were 120–320 m thick in the subsidence center,characterized by fine grain.There are differences in the mechanism between the two stages of subsidence.The early one was the response to the northward subduction of the MianLüe Ocean and intense depression under compression in Qinling during Mid-Triassic.The later subsidence is controlled by the weak extensional tectonic environment of the post-collision stage during Late Triassic.