Petrogenesis of Indosinian Granitoids in Middle-Segment of South Qinling Tectonic Belt:Constraints from Sr-Nd Isotopic Systematics

South Qinling Tectonic Belt（SQTB）is located between the Shangzhou-Danfeng and Mianxian-Lueyang sutures.There are a lot of early Mesozoic granitoid plutons in its middle segment, comprising the Dongjiangkou-Zhashui granitoid plutons at the northeast,Huayang-Wulong-Laocheng granitoid plutons at the central part,Xiba granitoid pluton at the northwest and Guangtoushan-Liuba granitoid plutons at the southwest.These Indonisian granitoids contain a mass of various scale mafic enclaves,which show sometimes clear boundaries and sometimes transitional boundaries with their host granitoids.These granitoids also exhibit metaluminous to peraluminous series,commonly higher Mg# and a wide range of petrochemistry from low-K tholeiite series,through mid-K and high-K calc-alkaline series to shoshonite series and predominated samples are attributed to mid-K and high-K calc-alkaline series.Detailed analyses in Sr-Nd isotopic systematics and petrochemistry reveal that there may be regionally initial granitoid magma of the Indonisian granitoid plutons,comprising Dongjiangkou-Zhashui,Huayang-Wulong-Laocheng,Xiba,and Guangtoushan-Liuba granitoid plutons,which were produced by hybrids of magmas in various degrees,and the initial magmas were derived from both the mantle and the lower continental crust（LCC）sources in the SQTB.The initial granitoid magma further did the magma hybrid with the magmas from the LCC,crystallization fractionation,and assimilation with upper crustal materials during their emplacement to produce these granitoid plutons in the SQTB.These magmatism processes are most likely to occur under continent marginal arc and syn-collision to post-collision tectonic backgrounds.

Geochemistry,Geochronology,Sr-Nd Isotopic Compositions of Jiang Tso Ophiolite in the Middle Segment of the Bangong- Nujiang Suture Zone and Their Geological Significance

The Jiang Tso ophiolite, situated in the middle segment of the Bangong- Nujiang Suture Zone, is a part of the easternmost Qieli Lake ophiolite subzone and is close to the south of Pung Lake ophiolite. The rock association of Jiang Tso ophiolite is relatively complete and is mainly composed of metamorphic peridotite, gabbro and diabase. Comparing with N-MORB, the ophiolite is high in Mg and low in Ti, K, Na, P, and is depleted in Nb, Ta, Hf, Th and enriched in Rb, Sr and Ba. Geochemical characteristics of the Jiang Tso ophiolite indicate it is of a supra-subduction zone type formed in the spreading ridge of back arc basin. The SHRIMP U-Pb dating of zircons from the gabbro yielded a weighted average age of 188.1±4.1 Ma （MSWD=1.4）, indicating the Jiang Tso ophiolite was formed in the late stage of early Jurassic. The Sr, Nd isotopic compositions show that the Tethyan mantle domain is the depleted mantle （DM）, with enriched mantle domain II （EMII）. They have the same Sr, Nd isotopic composition with the India Ocean MORB type.

Petrogenesis and Tectonic Implications of A-type Granites in Zhaheba in the East Junggar Region of Xinjiang,China:Evidence from Geochronology,Geochemistry and Sr-Nd Isotopic Compositions

The magma source,petrogenesis,tectonic setting and geochronology of the late Paleozoic A-type granites widely exposed in the Zhaheba area,East Junggar,have thus far not been well-constrained.A better understanding of these issues will help to reveal the magmatic processes and continental growth of Central Asia.The A-type granites in Zhaheba include the Ashutasi alkaline granites and the Yuyitasi syenogranites,which were emplaced at 321.5±4.8 Ma and 321.7±0.6 Ma,respectively.The major rock-forming minerals are orthoclase,perthite,arfvedsonite and quartz,which exhibit the following principal geochemical characteristics of A2-type granites.(1)Their REE distribution curves each exhibit a‘V’-shaped pattern and a marked depletion in Eu.They are rich in large-ion lithophile elements Rb,Th and U as well as high-field-strength elements Nb,Ta,Zr and Hf,but significantly depleted in Ba,Sr,P and Ti.(2)Their(^(87)Sr/^(86)Sr)i values(0.7021-0.7041),εNd(t)values(4.57-5.16)and REE distribution patterns are in basic agreement with those of the Kalamaili A-type granite belt in East Junggar.The T DM2 values of the alkaline granites and syenogranites range from 661 to 709 Ma.The A-type granites may be the products of upwelling asthenosphere-triggered partial melting of immature lower crust.The alkaline granites were late-stage products of crystallization and differentiation.Compared to the syenogranites,the alkaline granites are significantly lower in K_(2)O,Na_(2)O,Al_(2)O 3,FeO,MgO and CaO,but significantly higher in incompatible elements(e.g.,SiO_(2),Rb,and Sr).The magmatic crystallization temperatures of the syenogranites and alkaline granites are 874℃ and 819℃,respectively.As their age gradually decreases(peak ages:322 Ma and 307 Ma,respectively),there is a gradual decrease in the T_(DM2)of the A-type granites and a gradual increase in theεNd(t)value from the Ulungur belt to the Kalamaili belt in East Junggar.The study of A-type granites is therefore one of the keys to understanding the laws and mechanisms of crustal accretion during the Phanerozoic period,as well as also being of great significance for understanding the Paleozoic accretion.

Genesis of A-type Granites in the East Junggar, Xinjiang and Growth of Continental Crust——Evidence from Geochronological and Sr-Nd Isotopic Compositions

The magma source,petrogenesis,tectonic setting and its geochronology of the Late Paleozoic A-type granites,which widely exposed in Zhaheba area,East Junggar,have not been well constrained so far(Fig.1 a,b).A better understanding of above issues will help us to reveal the magmatic processes and the continental growth of Central Asia(Xiao et al.,2009).

Mafic Dyke Records of Paleoproterozoic Mantle Plume Activity in the Karelian Craton: U-Pb Baddeleyite/Zircon Geochronology and Sr-Nd Isotopic Data

Mafic dykes preserved important information on mantle melting regimes in the early Earth history.Despite the fact that a large volume of geochronological data for mafic dykes was recently received,several important issues

Late Paleozoic Mantle Source Nature of Tianshan Orogen, Northwest China: Evidence from the Geochemistry, Zircon U-Pb Dating, Hf and Whole Rock Sr-Nd-Pb Isotopes of the Mafic Dykes

The Tianshan Orogen(TO)is one of the largest typical accretionary orogenic belts in the world.Of which,the late Paleozoic was a critical era to understand the tectonic and geodynamic transition from accretion to collision.However,the late Paleozoic tectonic evolutionary history,especially for the time of the ocean-continent transition,is still debated although the origin and tectonic settings for the Paleozoic volcanic,felsic igneous magmatism in TO and reginal geology have been done in the last decades.In contrast,the researches on the mafic dykes in TO was not systematically carried out till now.Reginal-scale mafic dykes are commonly regarded as the products created in a extensional setting,and used to identify the major tectonic events such as rifting and continental break-up and further trace the mantle natures and geodynamic mechanism(Halls,1982;Bleeker and Ernst,2006;Li et al.,2008;Ernst et al.,2010;Srivastava,2011;Hou,2012;Peng,2015;Peng et al.,2019).There are widespread late Paleozoic mafic dykes beside the huge of intermediate-acid igneous rocks in the TO,being an idea object to reveal the extensional events,tectonic evolution and the mantle nature and geodynamic processes.We present the ICPMS in situ zircon U–Pb dating,Lu-Hf and whole-rock Sr-Nd isotopes as well as the geochemistry data for these mafic dykes to better constraint their petrogenesis and mantle nature.New zircon U-Pb dating for 12 samples from the representative basic dykes and basalts yield three distinct stages of^332 Ma,316–302 Ma and 288–282 Ma,respectively.In which,the first stage of mafic dykes is mainly occurred in both East Tianshan Orogen(ETO)and West Tianshan Orogen(WTO),and composed of dolerite with minor basalts.The second stage of mafic dyke also can be found in both ETO and WTO.However,in contrast to the first stage of mafic dykes,they have relatively variable rock types from the dolerite/or gabbros to gabbroic diorite.The third stage of mafic dykes are slightly intermediate in composition,and chiefly consist of andesitic-basaltic dolerite with some diorites.They are widely developed not only in both ETO and WTO,but also in the Beishan area to the east of the ETO,indicating a large-scale mafic magmatism in Tianshan and adjacent areas.

Geochemistry of trace elements and Sr-Nd isotopes of foraminifera shell from the Okinawa Trough

Trace elemental associations and Sr-Nd isotopic compositions are of important to recognition of biogenic material from mixed marine sediments. The foraminifera shell from the Okinawa Trough strongly enriches Sr,P,Mn and Ba, enriches Li,U,Th,Sc,Co,Cu,Pb,Zn,Cr,Rb,Y,Sb and light rare earth elements,slightly enriches V,Ga,Zr,Nb,Cd and middle rare earth elements,is short of Mo,In,Sn,Cs,Hf,Ta,W,Ti,Bi and heavy rare earth elements. The mechanism of elemental enrichment in forminifera is the concentrations of trace elements in sea water and selective absorption of trace elements during foraminifera living, as well as the geochemical affinity between major elements and trace elements.The REE (rare earth elements)partition pattern of foraminifera shell of the Okinawa Trough shows enrichment of middle rare earth elements with slightly negative Ce anomaly,which are different from those of foraminifera of the Pacific Ocean.The Sr,Nd isotopic ratios of the Okinawa Trough foraminifera are 0 709 769 and 0 512 162,respectively, which are different not only from those of oceanic water, but also from those of river water of China's Mainland, the former is slightly higher than those of oceanic water,but much lower than those of river water;the latter is slightly lower than those of oceanic water,but higher than those of river water,demonstrating that the Okinawa Trough sea water has been influenced by river water of China's Mainland.

Zircon U-Pb Chronology and Whole-rock Sr-Nd Isotope Compositions of Granite Porphyry in the Kaladaban Area of the Northern Altyn

Objective The Altyn Tagh marks the northern margin of the Qinghai-Tibet Plateau and lies between the Tarim block to the north and the Qaidam block,Qilian Orogen,and Kunlun orogenic belt to the south.The Altyn Tagh region contains ophiolite,high-to ultrahigh-pressure metamorphic rocks,and igneous rocks.Previous research has virified the occurrence of continental rifting,subduction,slab roll-back,and collision between the Tarim block and Proto-Tethys oceanic plate.Moreover,Kaladaban volcanic rocks are mainly distributed in the north Altyn region.Studies of the magmatic evolution of this region have proposed that Altyn oceanic plate was subducted during the Ordovician(Han et al.,2012;Wang et al.,2017).However,the specific timing and other aspects of the subduction are debated,and an investigation of granite porphyry in the Kaladaban area would improve our understanding of this subduction event.In this study,we present new U-Pb zircon dating result and Sr-Nd isotope composition data for granite porphyry from the North Altyn region.The objective is to constrain the timing of subduction of the North Altyn oceanic plate and establish the petrogenesis and magma source of the granite porphyry.

Early Cretaceous to Middle Eocene Magmatic Evolution of Eastern Pontides:Zircon U-Pb Ages and Hf Isotopes,and Geochemical and Sr-Nd Isotopic Constraints from Multiphase Granitoids,NE Turkey

The Eastern Pontides orogenic belt(EPOB)represents a significant segment of the AlpineHimalayan orogenic belt that evolved from the Paleozoic to Cenozoic periods.Here we report new zircon U-Pb ages,together with Lu-Hf isotopes,and whole-rock geochemical and Sr-Nd isotopic analyses of plutonic rocks from EPOB,northeastern Turkey.Our aim is to interpret magmatic evolution in which the granitoids formed.Zircon U-Pb dating of six samples yielded crystallization ages of~134,~82,~39 Ma,respectively.They show a wide range of87Sr/86Sr((i))(0.7039–0.7109),andεNd(t)values varying from-9 to+4.6,yielding model ages(TDM)from 520 to 1623 Ma,suggesting a heterogeneous magma source.Dated zircons show exlusively positiveεHf(t)values(+12.4 to+1.4),yielding model ages(TDM)from 352 to 1059 Ma,implying that they are most likely derived from a juvenile lower crust rather than the mature continental crust.In this study,we suggested that the northward subduction of the Neo-Tethyan oceanic slab began from the Early Cretaceous and resulted in the Late Cretaceous magmatism.Moreover,the Middle Eocene magmatism in the EPOB was related to the collision of the Anatolide Taurid Platform(ATP)with the Pontides.

Sr-Nd isotopic compositions of the Changjiang sediments: Implications for tracing sediment sources

The suspended particulate and fine-grained floodplain sediments were collected from the main stream and tributaries of the Changjiang River for Sr-Nd isotopic measurements. The εNd(0) values gradually decrease downstream from -10.8 on average in the upper reaches to -12.3 in the lower reaches, whereas the 87Sr/86Sr ratios increase correspondingly, averaging 0.721899 and 0.725826 respectively in the upper and middle-lower reaches. The compositional variations primarily reflect the complex con- trols of provenance rocks, chemical weathering, and sediment characters between different catchments, among which the abnormal Sr-Nd isotopic compositions of the Yalong, Fujiang, Tuojiang and Yuanjiang rivers indicate the sediment provenance contributions from the Emeishan Basalt in the upper reaches and the old metamorphic and siliceous rocks in the middle-lower reaches. The Sr-Nd isotopic ratios of the Changjiang sediments can better reflect the average composition of weathered continental crust compared to other major rivers in the world because of the unique source rock types in the Changjiang drainage basin. The recognition of the Sr-Nd isotopic systematics of the Changjiang sediments will contribute to our understanding of the Changjiang evolution history and continental weathering processes during the Cenozoic, and also to reconstructing the paleoenvironmental changes in East China and the marginal seas.

Eclogites from the Northern Dabie Mountains, eastern China: Geochemical characteristics, Sr-Nd isotopic compositions and tectonic implications

The petrologic geochemical and Sr-Nd isotopic compositions of the eclogites from the mafic-ultramafic rock belt (MUMRB) in the Northern Dabie Mountains indicate that: (1) the protoliths of most of eclogites are tholeiitic basalt and a few may be gabbro, and most of them produced from the Yangtze subducted continental crust (lower crust and formed during the deep subduction) and a part may be from paleo-oceanic relics between the Yangtze and North China continental plates; (2) their positive Nb anomalies and related trace element characteristics show that they did not form in the island-arc setting; (3) the metamorphosed MUMRB with eclogite and meta-peridotite blocks along the southern part of the Mozitan-Xiaotian fault zone may represent the suture zone produced during the collision between the Yangtze and North China continental plates, which included the Yangtze subducted continental crust and paleo-oceanic relics.

Geochemical research on C-O and Sr-Nd isotopes of mantle-derived rocks from Shandong Province, China

This paper presents systematic studies on the C-O and Sr-Nd isotopic compositions for Cretaceous Badou carbonatites, Fangcheng basalts, and Jiaodong lamprophyres and Paleozoic Mengyin kimberlites in Shandong Province, China. Paleozoic kimberlites have normal and uniform C-O isotopic compositions with δ13C and δ18O in the range of -4.8‰--7.6‰ and +9.9‰-+13.2‰, respectively. However, Cretaceous three different types of mantle-derived rocks have quite different C-O isotopic compositions, indicating that the mantle sources are probably partially contaminated with organic carbon-bearing crustal materials. These Cretaceous rocks show uniform and EMII-like Sr-Nd isotopic compositions and also indicate that the mantle sources were affected by recycled crustal materials. Comparative studies of C-O and Sr-Nd isotopes reveal that the lithospheric mantle beneath the eastern North China Craton had different isotope characteristics in the Paleozoic, the early Cretaceous, and the Tertiary time. This demonstrates that the lithospheric mantle beneath the region underwent at least twice reconstructions since the Paleozoic. Available data imply that the first reconstruction mainly happened during the Triassic-Jurassic time with gradual changes and the second in the Cretaceous with abrupt changes. Results also show that the early Cretaceous (especially at 120-130 Ma) was perhaps the key period leading to the dramatic change of the Mesozoic geodynamics on the eastern North China Craton.

A Fast Separation Method for Isotope Analysis Based on Compressed Nitrogen Gas and Ion-Exchange Chromatography Technique--A Case Study of Sr-Nd Isotope Measurement

High-purity N2 was used to increase the mobile phase flow rate during ion purification of ion-exchange resin. This was performed to improve the efficiency of isotope separation and puri- fication, and to meet the efficiency requirements of rapid multiple-collector-inductively coupled plasma mass spectrometry （MC-ICPMS） analysis. For Cu isotope separation, our results indicated that at a gas flow rate 〉60 mL/min, the separation chromatographic peaks broadened and the re-covery rate decreased to 〈99.2%. On the other hand, no significant change in the Cu peaks was ob- served at a gas flow rate of 20 mL/min and the recovery rate was determined to be 〉99.9%. The Cu isotope ratio, measured by the standard-sample bracketing method, agreed with reference data within a±2 SD error range. The separation time was reduced from the traditional 10 h （without N2） to 4 h （with N2）, indicating that the efficiency was more than doubled. Moreover, Sr and Nd isotope separation in AGV-2 （US Geological Survey andesite standard sample） accelerated with a 20 mL/min gas flow, demonstrating that with the passage of N2, the purified liquid comprised Rb/Sr and Sm/Nd ratios of 〈0.000 049 and 〈0.000 001 5, respectively. This indicated an effective separation of Rb from Sr and Sm from Nd. MC-ICPMS could therefore be applied to accurately determine Sr and Nd isotope ratios. The results afforded were consistent with the reference data within a±2 SD error range and the total separation time was shortened from 2 d to 〈10 h.

Isotopic feature and uranium dating of the volcanic rocks in the Okinawa Trough

Volcanic rocks from the northern and middle Okinawa Trough were dated by ura- nium-series dating method. Differential fractions using magnetic procedure were designed to separate samples. New report on the ages and isotopic data of rocks in the northern trough (especially black pumice) was discussed. Based on the uranium dates and Sr-Nd isotopic ratio, magmatic evolution process of the Okinawa Trough was noted. Firstly, there have been wide silicic volcanic activities in the Okinawa Trough from late Pleistocene to present, and the volcanic rocks can be divided into three subgroups. Secondly, magma generally came from PREMA source area under the Okinawa Trough. Magmatic evolution in the northern trough was similar to the middle, but different to the south. Finally, volcanic activities indicated that opening of the southern Okinawa Trough did not happen due to the collision between Luson Arc and Eurasian Plate until the early Pleistocene.

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.

REE and Sr-Nd Isotope Geochemistry for Yixian Fluorite Deposit,Western Liaoning Province,China,and Its Geological Implications

Up to now, there were no systematic studies of geochemistry and isotopic age for the Yixian （义县） fluorite deposit, western Liaoning （辽宁） Province, China. Based on the analysis of metallogenic geological setting, we studied the REE, Rb-Sr and Sm-Nd isotopes. The chondrite-normalized REE patterns of fluorite are characterized by moderate LREE depletion （LREE/HREE=0.95-3.57, （La/Yb）N=0.08-2.84） and enrichment of Sr （146×10^-6-596×10^-6） and moderately positive Eu anomalies （δEu=1.10-1.34）, which are similar to those of the host Mesoproterozoic carbonate rocks. The fluorite display （^87Sr/^86Sr）t=-0.708 5, （^143Nd/^144Nd）t=-0.511 785, and δNd（t）=-12.8, which are similar to those of the host Mesoproterozoic carbonate rocks and volcanic rocks of Middle Jurassic Lanqi （蓝旗） Formation. The REE and Sr-Nd isotope geochemistry suggest that the source of the ore-forming material may be the volcanic rocks of Lanqi Formation and host carbonate rocks. The Sm-Nd isochron age of 154±14 Ma （MSWD=0.23） indicates that the Yixian fluorite mineralization nearly corresponds to the period of Lanqi Formation. Based on the integrated geological and geochemical studies, coupled with previous studies, we suggest that Yixian fluorite deposit formed in the extension setting of postcoilisional stage and may be attributed to the partial melting of ancient basaltic rocks in the lower crust induced by underplating of basic magma and to the reaction between the F-rich ore-forming fluids and the host carbonate rocks.

Petrogenesis and tectonic implications of Early Cretaceous andesitic-dacitic rocks,western Qinling(Central China):Geochronological and geochemical constraints

40Ar/39Ar and zircon U-Pb geochronological and whole-rock geochemical analyses for the Laozanggou intermediate-acidic volcanic rocks from the western Qinling orogenic belt,Central China,constrain their petrogenesis and the nature of the Late Mesozoic lithospheric mantle.These volcanic rocks yield hornblende or whole-rock 40Ar/39Ar plateau ages of 128.3-129.7 Ma and zircon U-Pb age of131.3±1.3 Ma.They exhibit SiO2 of 56.86-66.86 wt.%,K2 O of 0.99-2.46 wt.%and MgO of 1.03-4.47 wt.%,with Mg#of 42-56.They are characterized by arc-like geochemical signatures with significant enrichment in LILE and LREE and depletion in HFSE.All the samples have enriched Sr-Nd isotopic compositions with initial 87Sr/86Sr ratios ranging from 0.7112 to 0.7149 andεNd(t)values from 10.2 to 6.3.Such geochemical signatures suggest that these volcanic rocks were derived from enriched lithospherederived magma followed by the assimilation and fractional crystallization(AFC)process.The generation of the enriched lithospheric mantle is likely related to the modification of sediment-derived fluid in response to the Triassic subduction/collision event in Qinling orogenic belt.The early Cretaceous detachment of the lithospheric root provides a reasonable mechanism for understanding the petrogenesis of the Laozanggou volcanic sequence in the western Qinling orogenic belt.

Origin of the Neoproterozoic Baijianshan Banded Iron Formation at the Southeastern Margin of the Tarim Block in NW China:Implication for an Extremely Reducing Ocean

The Neoproterozoic banded iron formations(BIFs)were closely associated with the“Snowball Earth”during the breakup of the Rodinia,thus they played an important role in our understanding of the atmospheric and oceanic oxygen levels during this period.In this contribution,the Neoproterozoic(ca.737 Ma)Baijianshan BIF at Southeast Tarim,northwestern China was identified.Magnetite is the dominated iron-species,which occurs as the lamina interbedded with chert.The BIF contains low concentrations of trace elements,and is depleted in light rare earth elements(LREEs)based on comparison with the Post-Archean Australian Shale(PAAS).In addition,the BIF exhibits slightly positive La-Eu anomalies,negligible Ce anomalies,insignificant Y anomalies,chondritic Y/Ho ratios(23-32),and slightly chondritic initial ε_(Nd)(t=737 Ma)values(−0.45 to 1.46,averaging 0.37).All these features indicate that the precipitation of Baijianshan BIF was closely related to the submarine low-T hydrothermal fluids with little detrital contribution.Moreover,the Baijianshan BIF is characterized by the significant enrichment of heavy Fe isotopes,with δ^(57)Fe_(IRMM-014) values ranging from 1.78‰ to 3.05‰,revealing the partial oxidation of Fe^(2+) into Fe^(3+) during the precipitation of this BIF.Our data suggest that the formation of Baijianshan BIF was closely associated with a significantly reducing ocean,which most likely was isolated from the oxidized atmosphere by a local ice sheet.This Neoproterozoic Baijianshan ocean has the initial oxygen levels as low as,or even lower than that of Archean and Paleoproterozoic oceans.

Geochronology and Ore Genesis of the Niujuan-Yingfang Pb-Zn-Ag Deposit in Fengning,Northern North China Craton:Constraints from Fluid Inclusions,H-O-S Isotopes and Fluorite Sr-Nd Isotopes

The Niujuan-Yingfang Pb-Zn-Ag deposit in northern North China Craton(NCC)is hosted at the contact zone between Permian biotite monzogranite and Hongqiyingzi Group migmatitic gneiss.The orebodies are structurally controlled by NE-trending F1 fault.Mineralization can be divided into three stages:(1)siliceous-chlorite-pyrite stage,(2)quartz-Ag-base metal stage,and(3)fluorite-calcite stage.Four types of fluid inclusions were identified,including:(1)liquid-rich aqueous inclusions,(2)vapor-rich inclusions,(3)liquid-rich,solid-bearing inclusions,and(4)CO2-bearing inclusions.Mi-crothermometric measurements reveal that from stage I to III,the homogenization temperatures range from 317 to 262℃,from 297 to 192℃,and from 248 to 151℃,respectively,and the fluid salinities are in the ranges from 1.1 wt.%to 6.5 wt.%,1.2 wt.%to 6.0 wt.%and 0.7 wt.%to 4.0 wt.%NaCl equiva-lents,respectively.Fluid boiling and cooling are the two important mechanisms for ore precipitation according to microthermometric data,and fluid-rock interaction is also indispensable.Laser Raman spectroscopic analyses indicate the fluid system of the deposit is composed of CO2-NaCl-H2O±N2.Me-tallogenic fluorites yielded a Sm-Nd isochron age of 158±35 Ma.Theδ34SV-CDt values of sulfides range from-1.3‰ to 6.3‰,suggesting that the sulfur may be inherited from the basement metamorphic ig-neous rocks.Hydrogen and oxygen isotopic compositions of quartz indicate a metamorphic origin for the ore-forming fluid,and the proportion of meteoric water increased during the ore-forming processes.Sr-Nd isotopes of fluorites show a crustal source for the ore-forming fluid,with primary metamorphic fluid mixed with meteoric water during ascent to lower crustal levels.Combined with the geological,metallogenic epoch,fluid inclusions,H-O-S and Sr-Nd isotopes characteristics of the deposit,we suggest that the Niujuan-Yingfang deposit belongs to the medium-low temperature hydrothermal vein-type Pb-Zn-Ag polymetallic deposit,with ore-forming fluids dominantly originated from metamorphic fluids.

Evolution of Asian drying since 30 Ma revealed by clay minerals record in the West Pacific and its tectonic-climatic forcing

As the second largest dust source on the globe,the tectonic and climatic evolution of continental Asia has an important impact on regional and global climate change.The West Pacific is the main sediment sink for eolian dust transported eastward from the Asian interior.Clay minerals,as the major fine-grained weathering products of continental rocks,can be readily transported by wind or currents over long distances and thus have been widely used in the reconstruction of paleoclimate and weathering history.However,the overall evolutionary tendency and response mechanism of clay mineral records over large spatial and long timescales across Asia remain unclear.Here,two continuous and high-resolution clay mineral records since 30 Ma were reconstructed from sediments at Deep Sea Drilling Program(DSDP)Sites 292 and 296 in the Philippine Sea.Clay minerals and Sr-Nd isotope compositions were used to constrain provenance and reconstruct the drying history of the dust source region since the Oligocene.The results show that the clay-sized detrital sediments in the Philippine Sea are a mixture of Asian eolian dust and volcanic materials from the West Pacific arcs.Based on the clay mineral compositions and eolian flux,we reveal that the Asian interior has been continuously drying since the late Oligocene and that stepwise enhanced aridification occurred at approximately 20,14,7,and 3 Ma.Compared with other regions of the world,the relative contents of illite and chlorite increased more dramatically in Asia during the late Cenozoic,and the inconsistency became more obvious at approximately 20 Ma.Since 3 Ma,illite and chlorite have increased consistently across the globe.Combined with the Asian tectonic and climatic history,we suggest that the increase in illite and chlorite from Asia between 20 and 3 Ma was mainly in response to the uplift of the Himalayan-Tibetan Plateau,whereas after 3 Ma,it was primarily controlled by global cooling driven by the expansion of the Arctic ice sheet.