Recognition of Ophiolite Belt and Granulite in Northern Area of Mian－Lue， Southern Qinling， China and Their Implication

A ophiolite belt associated with the tectonic melange is recoghzed in northern area of Mian-Lue, southern Qinling. The ophiolite is considered to originate in a island arc. The occurrence of the ophiolite indicates that a paleo-ocean or finite oceanic basin existed POSt-Prot6rozoic in southern Qiuling, implying the difference of the continental basement of Southern Qinling from the Yangtze craton. The ophiolitc and themelange may mark the paleo-suture zoic between the two plates. The basic granulite is found in eastern area (Anzishan) of the ophiolite belt. The p-T path for metamorphism of the granulite demonstrates a process of continental collision.

Paleozoic and Mesozoic Basement Magmatisms of Eastern Qaidam Basin,Northern Qinghai-Tibet Plateau:LA-ICP-MS Zircon U-Pb Geochronology and its Geological Significance

The eastern margin of the Qaidam Basin lies in the key tectonic location connecting the Qinling, Qilian and East Kunlun orogens. The paper presents an investigation and analysis of the geologic structures of the area and LA-ICP MS zircon U-Pb dating of Paleozoic and Mesozoic magmatisms of granitoids in the basement of the eastern Qaidam Basin on the basis of 16 granitoid samples collected from the South Qilian Mountains, the Qaidam Basin basement and the East Kunlun Mountains. According to the results in this paper, the basement of the basin, from the northern margin of the Qaidam Basin to the East Kunlun Mountains, has experienced at least three periods of intrusive activities of granitoids since the Early Paleozoic, i.e. the magmatisms occurring in the Late Cambrian （493.1±4.9 Ma）, the Silurian （422.9±8.0 Ma-420.4±4.6 Ma） and the Late Permian-Middle Triassic （257.8±4.0 Ma-228.8＋1.5 Ma）, respectively. Among them, the Late Permian - Middle Triassic granitoids form the main components of the basement of the basin. The statistics of dated zircons in this paper shows the intrusive magmatic activities in the basement of the basin have three peak ages of 244 Ma （main）, 418 Ma, and 493 Ma respectively. The dating results reveal that the Early Paleozoic magmatism of granitoids mainly occurred on the northern margin of the Qaidam Basin and the southern margin of the Qilian Mountains, with only weak indications in the East Kunlun Mountains. However, the distribution of Permo-Triassic （P-T） granitoids occupied across the whole basement of the eastern Qaidam Basin from the southern margin of the Qilian Mountains to the East Kunlun Mountains. An integrated analysis of the age distribution of P-T granitoids in the Qaidam Basin and its surrounding mountains shows that the earliest P-T magmatism （293.6-270 Ma） occurred in the northwestern part of the basin and expanded eastwards and southwards, resulting in the P-T intrusive magmatism that ran through the whole basin basement. As the Cenozoic basement thrust system developed in the eastern Qaidam Basin, the nearly N-S-trending shortening and deformation in the basement of the basin tended to intensify from west to east, which went contrary to the distribution trend of N-S-trending shortening and deformation in the Cenozoic cover of the basin, reflecting that there was a transformation of shortening and thickening of Cenozoic crust between the eastern and western parts of the Qaidam Basin, i.e., the crustal shortening of eastern Qaidam was dominated by the basement deformation （triggered at the middle and lower crust）, whereas that of western Qaidam was mainly by folding and thrusting of the sedimentary cover （the upper crust）.

The Origin of the Black Shale Series and Bentonites from the Wufeng Formation in the Southwestern Upper Yangtze:Implications for the Convergence of the Yangtze and Cathaysia Blocks in the Late Ordovician

Early Paleozoic black organic sediments and bentonites occur widely in the craton basin within the Yangtze block and are generally believed to be genetically related to a specific tectonic setting on the cratonic boundary.However,the intimate relationship between their origins and the dynamic mechanisms are unclear,as exemplified by the genesis of the black shale series and bentonites from the Wufeng Formation during the Ordovician–Silurian transition(OST).In order to reveal the relationship between the Wufeng Formation and the convergence of the Yangtze and Cathaysia blocks(i.e.,the intracontinental Kwangsian Orogeny),two stratigraphic sections respectively in Zhaotong area(Northeast Yunnan)and Puyi area(Northwestern Guizhou)that were located in the semi-restricted inner Yangtze Sea during the OST were systematically studied,on the basis of whole-rock geochemical composition,pyriteδ^(34)S(δ^(34)Spy),total organic carbon(TOC),stable Sr isotope,pyrite framboid size distribution and zircon U-Pb age,trace elements.The evidence shows that the paleo-oceanic environment changed significantly at the turn of the early–late Katian and formed the black shale series in the Wufeng Formation.These acritarch assemblages were formed in the transition process of the Upper Yangtze Basin from passive continental margin basin to foreland basin during this interval.Based on previous research on the genetic relationship between black shale series and plate tectonic movement,a basin-mountain evolution model suitable for South China in the Late Ordovician is presented.The two bentonites in the Wufeng Formation with U-Pb ages of 445.5±0.8 Ma and 441.9±2.4 Ma primarily originated from the intermediate–acid volcanic eruption during the collision and convergence between the Yangtze and Cathaysia blocks in the Late Ordovician,the provenance region probably being located in the Jiangnan orogenic belt.Thus,we believe that the appearance of the black shale series and bentonite in the Wufeng Formation at the turn of the early–late Katian may represent the initiation of basin-mountain transformation and the Kwangsian Orogeny in South China,which provides important evidence for the collision and convergence of the Yangtze and Cathaysia blocks in the Late Ordovician.

Paleo-oceanic crust recycling in North Qinling:evidence of Pb, Nd, Sr isotopes from island arc granitoids

Recent studies indicate that the recycling of subducted oceanic crust is significant in the source of modern island arc magmas. It is reasonable, therefore, to assume that this process would have been equally important for the formation of arc magmas during the Phanerozic time. Unfortunately, studies on this respect are rare. This note discusses the source of granitic magma of the late Proterozoic-early Paleozoic North Qinling arc based on Pb, Nd, Sr isotopic data. The results provide evidence that the formation of the

Ce ANOMALY——A CHEMICAL TRACER FOR PALEO-OCEANIC REDOX VARIATION

Rare earth elements are an elemental family with special geochemical properties, and they always exist with each other in geological processes and reflect characteristically the nature of individual primitive material so that the REE composition patterns are

Geochemical Characterization of Organic Rich Black Rocks of the Niutitang Formation to Reconstruct the Paleoenvironmental Settings during Early Cambrian Period from Xiangxi Area,Western Hunan,China

The Niutitang Formation in the South China Block might be a source of hydrocarbon as it contains an enormous quantity of organic matter.Black rock of the Early Cambrian Niutitang Formation is widely distributed in the Yangtze region,but detailed geochemical understanding of it is still emerging.This research discusses the detailed geochemical characteristics of the Niutitang Formation to reconstruct the paleoenvironmental conditions,employing total organic carbon(TOC)content,major,trace,and rare earth element data.For this purpose,black rock specimens of the Niutitang Formation from two outcrop sections were utilized for geochemical characterization,and the results compared with another eight sections from the South China Block.The average total organic carbon in these sediments is significantly higher(5.80 wt.%).In the platform region,lower quantities of TOC indicate a poor potential to produce hydrocarbons.At the same time,significantly higher TOC is observed in the deep shelf and slope sediments,indicating a significant potential to produce hydrocarbons.The average Ce,Eu and Y anomalies from both Longbizui and Sancha sections studied are 0.74,0.86,1.77,1.07,and 1.19,1.30,respectively.The chemical index of alteration(CAI)throughout the Yangtze block is higher(averaging 71.32)than that of Post Archean Australian Shale(PAAS 69),indicating a moderately weathered source of the Niutitang Formation relative to PAAS.As the sediments are moderately weathered,this suggests these rocks might have been derived from felsic rocks,mainly granite-granodiorite.The normalization of REEs in the black rocks reveals a reduction of light REEs with increase in heavy REEs enrichment.Similarly,a positive Eu anomaly,negative Ce anomaly,and a moderate Y/Ho(34.61)are clues to a hybrid depositional mechanism associated with hydrothermal action and terrigenous input.These anomalies are also evidence of upwelling in the paleo-ocean and mixing of organic matter,which created anoxic bottom water during the deposition of the Niutitang Formation in the basin and upper oxic water conditions before deposition.The main controlling factors for the distribution of rare earth elements in these black rocks of the Niutitang Formation are pH,terrigenous input,source rock composition,tectonism,an upwelling mechanism,and hydrothermal activity.

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.

Paleoenvironmental implications of high-density records in Co-rich seamount crusts from the Pacific Ocean

Co-rich seamount crusts have been shown to possess great potential for providing information on paleoceanographic and paleoclimatic changes. High resolution data are essential to decipher and correctly understand such high-density records. With the development of modern microprobe techniques, detailed sampling of crusts can be performed and it is possible to retrieve detailed information about envi- ronmental changes recorded in the seamount crusts. We report here geochemical results of more than 40 elements (including all rare earth elements) of four Co-rich seamount crust samples, which were collected from seamounts in the central and western Pacific Ocean. These data were obtained with two micro-probe techniques: Electron Probe Micro Analyzer and Laser Ablation Inductively Coupled Plasma Mass Spectrometry. The chronological framework of the seamount crust samples was determined using the cos- mogenic 10Be and the Co-chronometer. Records of elemental composition, P, and Al/(Fe + Mn) and Y/Ho ratios across the sections of the four samples are used to identify paleoceanographic and paleoclimatic events over the past ~30 Ma. These data show that: (1) Al/(Fe + Mn) in the western Pacific seamount crust is a useful proxy for the assessment of changes of source materials related to the variability of the Asian monsoon; (2) P and Y/Ho can be used as proxies to infer biogenic episodes. Finally we discuss the methodology related to dating and micro-probe analysis used in crust study.

Permian Radiolarians from the A'nyemaqen Mélange Zone in the Huashixia Area of Madoi County,Qinghai Province,Western China,and Their Implications on Regional Tectonism

The Huashixia area in the Madoi County, Qinghai Province, western China is one of the most important areas for the studies of the formation and evolution of the A＇nyemaqen mélange zone due to the well exposed, relatively complete tectonostratigraphic successions. However, the timing and evolutionary process of the A＇nyemaqen paleo-ocean has long been poorly understood. Recently, we obtained abundant radiolarians from the siliceous rocks of the Maerzheng Formation of this mélange zone in Huashixia. Total 19 species in 10 radiolarian genera have been identified. They belong mainly to the Albaillellaria, Spumellaria, Entactinaria, and Latentifistularia. Four radiolarian assemblage zones are established, including： the Pseudoalbaillella rhombothoracata, Pseudoalbaillella scalprata, Ruzhencevispongus uralicus, and Follicucullus bipartitus-Follicucullus sp. cf. F. ventricosus zones. Radiolarian faunal correlations suggest a late Artinskian-early Kungurian age and late Capitanian-early Wuchiapingian age for the first two zones from the lower part and the last two zones from the upper parts of the Maerzheng Formation, respectively. All radiolarians are typical of deep water elements living in deep oceans in low latitudes during the Permian. Previously, the radiolarians of Early Carboniferous to Early Permian age and fusulinids of Early-Middle Permian age have also been reported from the A＇nyemaqen mélange successions. The combination of the newly found radiolarians with the previously reported faunal and microfloral assemblages as well as radiometric ages obtained from the same stratigraphic units indicates that the A＇nyemaqen paleo-ocean may have opened initially during the Early Carboniferous, expanded from the Early-Middle Permian, reached its acme in the early Late Permian, shrunk during the latest Permian to Early Triassic, and eventually was closed in the Middle Triassic.

Sm-Nd Isotopic Signatures and Significance of the Precambrian-Cambrian Seawater in China

Sm-Nd isotopic determination on small shelly fossils and collophanites collected from 3Precambrian-Cambrian boundary sections,namely Meishucun of Jinning in Yunnan,Maidiping of Emei inSichuan and Wushi of Kalpin in Xinjiang,shows that the seawater masses of these three remotely separatedareas were co-oceanic in the Precambrian-Cambrian transitional period,with a common seawater εNd（T） val-ue of -6.6±0.5.The εNd（T） value of the China’s seawater ranges from -5.7 to-6.6 during the periodfrom 610 Ma B.P.to 500 Ma B.P.The research suggests that there could be an "Asiaustralian Ocean" be-sides the Panthalassa Ocean and the Iapetus Ocean at that time,and the China’s seawater possibly belongedto the Asiaustralian Ocean.The Sm-Nd model age of the determined samples is about 1.8 Ga,which reflectsthe mean age of the continental source areas around the China’s seawater.