| Tarim cratonis one of the blocks early Cambrian black shale series exposed in Xinjiang, China, which has many similarities with Yangtze cratonin stratigraphic sequence, lithologic mineral combination, paleontology etc.Many thorough studies have been finished on the early Cambrian black shale series inYangtze craton on sequence stratigraphy, biostratigraphy, lithostratigraphy, mineralogy, paleoecology and petrogeochemistry, element geochemistry, organic geochemistry, ore depositgeochemistry, metallogeny and ore mineralogy etc. Actually,research and geological prospecting achievepsents on the early Cambrian black shale seriesmainly concentrated in Keping area, northwest Tarim, and geological prospecting achievements most generated in Beishan area.However, research on the early Cambrian black shale seriesin Kuruktag-Beishan region, east-central Tarim, is weak. This thesis discusses the storage space of the black shale series, source (area) of petrogenic and metallogenic, deposition time, sedimentary paleogeography and climatic environment, lithologic and mineral association,occurrence and enrichment of vanadium and so on, through sedimentology, paleontology and geochemistry in Xishanbulake formation (Shuangyingshan formation), the bottom of the lower Cambrian,Kuruktag-Beishan region.The following understandings have been made:(1) The source (area) of the early Cambrian black shale series:the transgressive directions of Tarim Basin are from west to east and from north to south in the Early Cambrian, and the cross bedding and paleocurrent analysis of Neopro-terozoic strata shows that the source may be from the northTarim. That means the source (area) of the early Cambrian black shale series may come from Tarim carbonate platform of the western Tarim basin,central Tianshan platform and Fandishan archicontinent of the northern Tarim basin.U-Pb isotopic geochronology, trace elements and eigenvalue of its detrital zircons show that the source(area) may also come from the Paleoproterozoic and Neoproterozoic intermediate acid rocks and (ultra)maficrocks. Qieganbulake and Pingtaishan areas dominated by the intermediate acid rocks, and Moheershan area contains both of them. Of course, it will be better to concern the oceanic basin uplift, submarine exhalation and marine organisms as a source too. The metallogenic elements mainly came from weathering and denudation products of (ultra) mafic rocks and brine compounds came from deep fault connecting with magma chamber.(2) The storage space of the Early Cambrian black shale series:the rupture of Tarim cratonin late Archean formed broad depression; Tarim movement lead to the overall folding inversion of the Tarim basement and the formation of anticlinorium, which also formed deep faults, steep dipping basement faults at north and south sides and internal part of northeastern uplift belt margin of Kuruktag.These isolated environment provide a possible paleo geographic space for formation of the black shale series. At the same time, the open of paleo Asian Ocean in Neoproterozoic led to many geological events, such as the subduction of the South Tianshan ocean,the Junggar ocean and the Tarim plate which generate a local tension, and then developed rift faulted basin, back arc margin basin and a series of multi island ocean and graben-horst structure.Mostly, the isolated enclosed environment around multi island ocean and deep-bathyal stranded faulted basins formed by the paleo-uplift may also provide ancient geographical space for deposition of the black shale series.(3) The ancient geographical environment of the Early Cambrian black shale series:the element geochemical characteristics of volcanic rocks in the Xishanbulake formation show that they formed in the continental rift setting. Based on lithological, petrologpcal, mineralogical, elements and paleontological characteristics, the northeastern margin uplift belt in Kuruktag came from the South Tianshan-North Tianshan ocean characterized by the gradually deepened sea water from west to the east and the north to the south. This strata shows a regular change of siliceous and phosphorous content decrease sand mud and ash composition increased from the oldest to the youngest in time, and siliceous content reduces and argillaceous content increased from west to east in pace. The sedimentary environment is gradually changed from continental shelf facies to basin facies from Kuluketage area to the Beishan area. The black shale series in Qieganbulake and Pingtaishan areas formed in reducing environment of the deep ocean basins, and the black shale series in the Moheershan and Dapingliang areas formed in shallow water environment of continental margin. And the sediments-seawater interface during sedimentary period just near the oxidation-reduction interface. The black shale series has the double sedimentary characteristics of normal seawater and hot water.(4) The deposition time of the Early Cambrian black shale series:Zircon U-Pb age of the tuff in the interlayer of the black shale series at the bottom of Xishanbulake formation in Kuluketage areais 521.6±7Ma. and the shell fossils (Spherical fossil. Gen.et Sp.indet. B) is found in phosphorite rock in the bottom of Xishanbulake formation which show a deposition age equivalent to the age of the first small shell fossils belt in Meishucun formation in Yangtze craton. The Shuangyingshan formation in Beishan area found the Bergeroniellus-Serrodiscus-Calodiscus-Subeia fossil assemblage which have been found in lower Longwangmiao formation. That means the Beishan area Early Cambrian black shale series have an equivalent sedimentary age to late Longwangmiao period. Therefore, the sedimentary age of Xishanbulake and Shuangyingshan formation are early Cambrian.(5) The lithology and mineral assemblages of the Early Cambrian black shale series:Based on means of rock-mineral identification, X-ray powder diffraction and scanning electron microscope analysis, petrology characteristics of the Early Cambrian black shale series behave as "silicon, phosphorus and volcanic rockin the upper; clay, sand and dust in the lower", and present reverse graded structure, grain size become coarse and color becomes shallow gradually, horizontal bedding changes from extremely developed to undeveloped from below to above. The lithologic association is mainly carbonaceous siltstone,siliceous rocks(chert rocks), carbonaceous slate, dark gray mudstone, argillaceous siltstone,fine sandstone, tuff, stone coal, phosphorite rocks, etc. Minerals are mainly quartz, illite (mica), pyrite, vanadium-bearing muscovite, vanadium tourmaline, vanadium garnet, apatite, zircon, titanite, barite, gypsum, sphalerite, calcite, potash feldspar, and alunite etc.(6) Sedimentary formation and favorable ore-bearing lithology and facies structure of the Early Cambrian black shale series:According to the lithological association, elements and their combinations from these section:The silicalite-shale formation is the sedimentary formation types of the early Cambrian black shale series, which can be further split into the vanadium-silver-molybdenum-uranium-phosphorus bearing formation(Kuluktage type) and the vanadium-zinc-cobalt-molybdenum-uranium-phosphorus bearing formation(Beishan type).The carbon,silicon,mud,phosphatic bearing rock types are the most favourable lithological association to mineralization, and the carbon, mud bearing rock typesare second. The types of C and D is ore-bearing facies structure in the Kuluktage area, i.e.The Siliceous rocks and carbon powder sandstone interbed is the most favourable to mineralization, and the higher this kind of sedimentary microfacies occurrence frequency, the better ore potentiality.the type of B is ore-bearing facies structure, in the Beishan area,i.e.The oregrade positively correlated with thickness of the ore-bearing carbonaceous slate.(7) Based on carbon and oxygen isotope, sporopollen fossil and CIA analysis. During the deposition periodfrom Gaskiersice age cap carbonates to the early Cambrianblack shale series, at the platform,slope-shelf and basin faces the of Tarim basin exist carbon isotope change gradient,δ13Closs from west to east and from down to up.The carbon may be derived from marine carbonate,biogenic organic carbon and deep source,while organic carbon is thought to be from fungus and algae marine plankton, and which is vanadium-bearing.the early Cambrian black shale series was formed in the warm,wet climate conditions, in the Kuluktage area. On the contrary,it was formed in the cold,dry conditions in the Beishan area, the paleotemperature presents the east-high and west-low feature.(8) The biomarker of the Early Cambrian black shale series:The n-alkanes are composed of nC14-nC29, with nC17 and nC18 as the main peaks, and the peak pattern is characteristic with single peak distributionin gas chromatography(GC).Mean value of nC22-/nC23- ratios and OEP mean value of the black charcoal shale respectively is 3.30 and 1.07; Mean value of nC22-/nC23+ ratios and OEP mean value of the vanadium-bearing black charcoal shaler espectively is 4.30 and 1.17.Data shows the predominance of light hydrocarbons and no obvious odd-even carbon number predominance in the n-alkanes,and organic carbon which The organic matteris mainly belong to sapropel type and its main sourceis thought to be the fungus and algae marine plankton. R0 of the black charcoal shale is 1.23(mean value), R0 of the vanadium-bearing black charcoal shale is 0.87(mean value), that indicates the organic matter is mature to high-mature,belong to oil generation zone, and which isn’t conducive to vanadium enrichment.Pr/Ph of the black charcoal shale of is 0.85(mean value), Pr/Ph of the vanadium-bearing black charcoal shale is 0.82(mean value), show that terpanes abundant and strong reduction environment. The color index of sporopollen is tan to brown, Ro values range 0.84-1.26, paleotemperature of the strata experience range 105～142℃, higher than diagenetic temperature.(9) Based on the analysis of mathematical statistics, the obvious negative correlation between V and Si of the rocks shows that siliceous components aboundare not conducive to the enrichment of vanadium.According to cluster in gspectral pattern, V and organic carbon content(TOC) clustered into one group, which indicators TOC plays a major role in the initial enrichment process of vanadium. Vmainly occurs in Vanadium mica, V-Fe oxides, and vanadate in the study area,which is as isomorphism hosting in the lattice of hydromica or Metal mineral crystal.In conclusion, the rupture of Tarim craton in late Archean and the open of the paleo-Asian Ocean in Neoproterozoic, which provide both ancient geographical space for black shale series sedimentation and metallogenic material. After Gaskiers, ocean became warmed, ascending current frequency,the sea was abundant nutrients,vanadium-bearing phytoplankton blooming, i.e. initial enrichment of vanadium and other metal elements was beginning. A global transgression in early Cambrian, Kuluktge-Beishan area is below the normal wave base of oxygen-deficient environment, vanadium with dead marine organis mssank into bottom of the sea, provide sufficient conditions for enrichment of vanadium.Genesis of the early Cambrian black shale series should be attributed to regional background factors, biological outbreak led to biological productivity increaseing, and organic matter was degraded by bacteria, also related to the sea-level change. |
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