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.

Petrology,geochemistry and source characteristics of the Burpala alkaline massif,North Baikal

The Burpala alkaline massif contains rocks with more than 50 minerals rich in Zr,Nb,Ti,Th,Be and rare earth elements（REE）.The rocks vary in composition from shonkinite,melanocratic syenite,nepheline and alkali syenites to alaskite and alkali granite and contain up to 10%LILE and HSFE,3.6%of REE and varying amounts of other trace elements（4%Zr,0.5%Y,0.5%Nb,0.5%Th and 0.1%U）.Geological and geochemical data suggest that all the rocks in the Burpala massif were derived from alkaline magma enriched in rare earth elements.The extreme products of magma fractionation are REE rich pegmatites,apatite-fiuorite bearing rocks and carbonatites.The Sr and Nd isotope data suggest that the source of primary melt is enriched mantle（EM-Ⅱ）.We correlate the massif to mantle plume impact on the active margin of the Siberian continent.

Carbonate reservoirs modified by magmatic intrusions in the Bachu area,Tarim Basin,NW China

Oil and gas exploration in carbonate rocks was extremely successful in recent years in the Ordovician in Tarim Basin.NW China.Here,we investigate the carbonate reservoirs in the Bachu area of the Tarim Basin through petrological and geochemical studies combined with oil and gas exploration data.Geochemical analysis included the major,trace,and rare earth elements;fluid inclusion thermometry;clay mineral characterization;and carbon and oxygen isotopes of the carbonate rocks.Homogenization temperatures of the fluid inclusions of Well He-3 in the Bachu area indicate three groups,60-80 ℃,90-130 ℃,and 140-170 ℃,and suggest that the carbonate rocks experienced modification due to heating events.The porosity in the reservoir is defined by fractures and secondary pores,and there is a notable increase in the porosity of the carbonate reservoirs in proximity to magmatic intrusion,particularly approximately 8-10 m from the intrusive rocks.The development of secondary pores was controlled by lithofacies and corrosion by various fluids.We identify supercritical fluids with high density（138.12-143.97 mg/cm3） in the Bachu area.The negative correlations of δ13C（-2.76‰ to-0.97‰） and δ18O（-7.91‰ to-5.07‰） suggest that the carbonate rocks in the study area were modified by high-salinity hydrothermal fluid.The formation of clay minerals,such as illite and montmorillonite,caused a decrease in porosity.Our study demonstrates the effect of magmatic intrusions in modifying the reservoir characteristics of carbonate rocks and has important implications for oil and gas exploration.

Zircon U-Pb Age, Geochemistry and Geological Implication of the 255 Ma Alkali-Rich Dykes from Ulungur Area, North Xinjiang

Alkali-rich dykes of the Late Permian in Ulungur （乌伦古） area are representative prod- ucts of granitic magmatism in the evolution of the Paleozoic orogenic belt in the East Junggar （准噶尔）, North Xinjiang （新疆）. We selected two representative samples for geochemical analysis （major and trace elements）, and twenty-two zircon grains for zircon dating. Isoplot （ver3.0） was used to calculate isotopic age and make concordia diagrams. This study shows that the trachy porphyry dykes, featuring low concentration of A1203, CaO, MgO and high alkali contents, are metaluminous alkaline rock and belong to A-type granitoids. The dykes have low concentration of the REE and incompatible elements, and the REE patterns show clear negative Eu anomalies （6Eu=0.74-0.58）, enriched LREE （LREE/HREE〉4, （La/Yb）N=5.97-4.63） and undifferentiated HREEs. Similar normalized REE and in- compatible element patterns are also showed in the dykes from Yemaquan （野马泉） and granites from Ulungur, suggesting that they are possibly originated from the same source and formed in the same tectonic environment, but the trachy porphyry dykes are more evolved. The age of the trachy porphyry dykes is 255.3 - 2.4 Ma, which is probably the crystallization time of the trachy porphyry. The dykes formed in late-orogenic phase of post-collision process or within-plate environment, which suggested that the trachy porphyry dykes possibly crystallized in the transition period during which the tectonic setting changed from post-collision to with- in-plate environment. So we consider that the age when the post-collision ended and the crustal cratonization begun in the East Junggar is 255 Ma, Late Permian.