Alkaline Basalts in the Bayingou Ophiolitic Melange of Northern Tianshan Mountains:Origination from Seamounts?

Objective The Bayingou ophiolitic Tianshan Mountains of melange is located in Northerm the southern Central Asian Orogenic Belt which is the largest accretionary oroger among the European, Siberian, Tarim and North Chine cratons. The Bayingou ophiolitic melange provide a critical geological record for unraveling regional tectonic histor） and testing different tectonic models. However, previous studies were mainly concentrated on geochronology, rock combination, structural feature and geochemistry ol ophiolite, with little attention to oceanic island basalts in the Bayingou ophiolitic melange. Therefore, in this study, we focus on pillow basalts from ophiolitic melange.

The Characteristics of Yongzhu–Guomang Lake Ophiolitic Melange in Bangong-Nujiang Suture, Xizang(Tibet),China

Yongzhu–Guomang Lake ophiolitic melange exposed about 100 km with large scale and complete ophiolitic uint in Xainza County,Xizang(Tibet).It is connected with Nam Lake,Kaimeng ophiolitic mélange to the east,and

The Petrology and Geochemistry of Listwaenite in the Sartohay Ophiolitic Melange of West Junggar, Xinjiang, China

The west Junggar,located in the eastern part of Balkash-Junggar tectonic province,is a major component of the core of the Central Asian metallogenic region.This area is characterized by occurrences of ophiolitic mélanges,such as the Sartohay ophiolitic mélange in the NE and the Tangbale ophiolitic mélange in the west.As a hydrothermal alteration product of serpentinite in the Sartohay ophiolitic mélange,listwaenite lenses are gold-mineralized and crop out on surface in the ophiolitic mélange via weathering of exhumated hanging wall of fault zone.Listwaenite is mainly composed of magnesite,quartz,dolomite,and trace amounts of mariposite,chromian spinel,talc and sulfide.A vertical thermal gradient model for the hydrothermal alteration shows that serpentinite would first be transformed to talc schist,then into listwaenite as the ophiolite slices continued to rise along shear zone,with XCO2,oxygen and sulfur fugacity increase and temperature decrease.Both serpentine and magnetite were progressively destroyed during the transformation from serpentinite to talc schist,andcompletely vanished in listwaenite,while mariposite generated in weakly deformed to mylonitized listwaenite.Concentrations of most trace elements including high field strength elements and metallogenic elements,increasing from undeformed,through weakly deformed,to mylonitized listwaenite,show a positive correlation with deformation degree and content of apatite,rutile,monazite,zircon and sulfide in listwaenite.The shear zone served as pathways for percolation and accumulation of fluid and trace elements during the metasomatism from serpentinite to listwaenite.Compared to undeformed listwaenite,mylonitized listwaenite will be more favorable to be fractured and brecciated due to more intense shearing,which caused strong metasomatic reaction and then induced trace element-bearing mylonitized listwaenite.

The Petrology, Geochemistry, and Petrogenesis of E-MORB-type Mafic Rocks from the Guomangco Ophiolitic Mélange, Tibet

The Guomangco ophiolitic melange is situated in the middle part of the Shiquanhe- Yongzhu-Jiali ophiolitic melange belt （SYJMB） and possesses all the subunits of a typical Penrose- type ophiolite pseudostratigraphy. The study of the Guomangco ophiolitic melange is very important for investigating the tectonic evolution of the SYJMB. The mafic rocks of this ophiolitic melange mainly include diabases, sillite dikes, and basalts. Geochemical analysis shows that these dikes mostly have E-MORB major and trace element signatures; this is the first time that this has been observed in the SYJMB. The basalts have N-MORB and IAB affinities, and the mineral chemistry of harzburgites shows a composition similar to that of SSZ peridotites, indicating that the Guomangco ophiolitic melange probably originated in a back-arc basin. The Guomangco back-arc basin opened in the Middle Jurassic, which was caused by southward subduction of the Neo-Tethys Ocean in central Tibet. The main spreading of this back-arc basin occurred during the Late Jurassic, and the basalts were formed during this time. With the development of the back-arc basin, the subducted slab gradually retreated, and new mantle convection occurred in the mantle wedge. The recycling may have caused the metasomatized mantle to undergo a high degree of partial melting and to generate E- MORBs in the Early Cretaceous. E-MORB-type dikes probably crystallized from melts produced by about 20%-30% partial melting of a spinel mantle source, which was metasomatized by melts from low-degree partial melting of the subducted slab.

Tectonic Framework and Evolution of the Dabie Mountains in Anhui, Eastern China

The Dabie Mountains are believed to be a collisional orogenic belt between the Yangtze amd Sino-Koreancontinental plates. It is composed of the foreland fold-thrust zone, the subducting cover and basement of theYangtze continental plate, the coesite- and diamond-bearing ultra-high pressure metamorphic zone and themeta-ophiolitic melange zone in the subducting basement, the fore-arc flysch nappe and the back thrust zoneoccurring respectively on the southern and northern margins of the Sino-Korean continental plate and the in-herited basin with molassic deposits on the northern margin. When the palaeo-Dabie oceanic plate subductednorthward in the Early Palaeozoic, volcanic arc and back arc basin probably formed on the southern margin ofthe Sino-Korean continental plate. The Sm / Nd isotopic dating of the strata and eclogite which were drawn in-to the foreland fold-thrust zone indicates that the intense collision of the two continental plates took place inthe Early Mesozoic.

Relationships Between the North China Plate and the Tarim Plate

The relationships and boundary between the North China and Tarim plates have been unclear for a long time ; however, the two plates occupy a prominent position in the formation and evolution of the continental lithosphere of China. It is proposed that the Engger Us ophiolitic melange zone discovered recently north of Alaxa is a typical suture between the two plates. The ophiolitic melange zone is composed mainly of a mixture of fragments of ancient oceanic crust and sedimentary rocks of active and passive continental margins. The melange may be divided into tectonic inclusions and matrix. The suture extends northeastwards into the Republic of Mongolia and probably westwards to meet the Altun fault. With the Engger Us ophiolitic melange zone as the boundary the Alaxa area may be divided into two parts: the northern part (AN ) belongs to the Tarim plate, while the southern part (AS) the North China plate. Geological evidence shows that the two plates were amalgamated in the Late Permian or a bit later.

Permo-Carboniferous Radiolarians from the Wupata'erkan Group,Western South Tianshan, Xinjiang, China

The Wupata'erkan Group, also called Wupata'erkan Formation, distributed in the South Tianshan, Xinjiang, China, mainly consists of gray and dark gray fine-grained clastic rocks, interlayered with volcanic rocks, carbonates and cherts. Some ultra-basic rocks (blocks) punctuate the formation. The formation was variously assigned to Silurian-Middle Devonian, Silurian-Lower Devonian, and pre-Devonian, mainly based on Atrypa bodini Mansuy, Hypothyridina parallelepipedia (Brour.) and Prismatophyllum hexagonum Yoh collected from the limestone interlayers, respectively. However, radiolarian fossils obtained from 24 chert specimens of the Wupata'erkan Group, mainly include Albaillella sp. cf. A. undulata Deflandre, Albaillella sp. cf. A. paradoxa Deflandre, Albaillella cf. A. deflandrei Gourmelon, Albaillella sp. cf. A. indensis Won, Albaillella sp. cf. A. excelsa Ishiga, Kito and Imoto, Albaillella sp. and Latentifistulidae gen. et. sp. indet., are earliest Carboniferous and Late Permian. The earliest Carboniferous assemblage is characterized by Albaillella sp. cf. A. undulata Deflandre, Albaillella sp. cf. A. paradoxa Deflandre, Albaillella cf. A. deflandrei Gourmelon and Albaillella sp. cf. A. indensis Won, and the Late Permian assemblage by Albaillella sp. cf. A. excelsa Ishiga, Kito and Imoto. This new stratigraphic evidence indicates that the Wupata'erkan Group is possibly composed of rocks with different ages from Silurian to Permian, and therefore, it is probably an ophiolite melange. The discovery of Late Permian Albaillella sp. cf. A. excelsa provides more reliable evidence supporting the existence of a Permian relic ancient oceanic basin in the western part of Xinjiang South Tianshan.

Petrology, Geochemistry and Geochronology of Gabbros from the Zhongcang Ophiolitic Mélange, Central Tibet: Implications for an Intra-Oceanic Subduction Zone within the Neo-Tethys Ocean

In order to investigate the evolution of Shiquanhe-Yongzhu-Jiali ophiolitic melange belt, the gabbros from new discovered Zhongcang ophiolitic melange are studied through petrology, whole-rock geochemistry, zircon U-Pb dating and Lu-Hf isotope. The gabbros investigated in this paper contain cumulate gabbro and gabbro dike, and they have undergone greenschist-amphibolite facies metamorphism. The chondrite normalized rare earth element （REE） patterns of most of these rocks show flat types with slightly light REE （LREE） depletion and the N-MORB normalized incompatible elements diagrams indicate depletion in high field strength elements （HFSE） （Nb, Ta） and enrichment in large ion lithophile elements （LILE）. These gabbros have island arc and mid-ocean ridge basalt af- finities, suggesting that they were originated in an oceanic back arc basin. Whole rock geochemistry and high positive εNd（t） values show that these gabbros were derived from -30% partial melting of a spinel lherzolite mantle, which was enriched by interaction with slab-derived fluids and melts from sediment. U-Pb analyses of zircons from cumulate gabbro yield a weighted mean age of 114.3±1.4 Ma. Based on our data and previous studies, we propose that an intra-oceanic subduction system and back arc basin operated in the Neo-Tethy Ocean of central Tibet during Middle Jurassic and Early Creta- ceous, resembling modern active intra-oceanic subduction systems in the western Pacific.

Detrital Zircon U-Pb Geochronology from Greywackes in the Niujuanzi Ophiolitic Mélange, Beishan Area, NW China: Provenance and Tectonic Implications

The Niujuanzi ophiolitic m^lange represents the remnant of oceanic crust between the Dunhuang massif and Mingshui-Hanshan massif. Greywacke from different tectonic slices in the Niu- juanzi ophiolitic mdlange were analyzed for detrital zircon U-Pb geochronology, trace dements and whole-rock trace elements to infer their provenance and the evolution of the Niujuanzi Ocean. Sample N-76s containing Carboniferous spores has the youngest zircon age of 323 Ma, while sample N-85s without fossils has the youngest zircon age of 449 Ma. The two samples were deposited no earlier than 323 and 449 Ma, respectively. The greywackes are depleted in large ion lithophile elements, and are relatively enriched in high field strength elements. The age spectra and trace element concentrations indicate that the sediments may have been deposited near the trench. The Hf, U, and Yb contents of zircons from sample N-76s vary widely, whereas those from sample N-85s have a narrow range. Sample N-76s contains both continental and oceanic zircons, while sample N-85s contains only continental zir-cons. The sediments were derived from the continental arc and accretionary wedge. The Paleozoic oce- anic crust zircons have ages of 430-500 Ma, indicating the timing of the expansion of the Hongliuhe- Niujuanzi-Xichangjing Ocean expansion. The oldest Paleozoic continental zircon has an age of 470 Ma, suggesting that the northward subduction of the oceanic crust may have started at that time.