Grenvillian orogeny in the Southern Cathaysia Block: Constraints from U-Pb ages and Lu-Hf isotopes in zircon from metamorphic basement

Metamorphic basement rocks in the Cathaysia Block are composed mainly of meta-sediments with different ages. New zircon U-Pb geochronological results from the meta-sedimentary rocks exposed in the Zengcheng and Hezi areas, southern Cathaysia Block, show that they consist dominantly of early Neoproterozoic （1.0-0.9 Ga） materials with minor Paleo- to Mesoproterozoic and late Neoproterozoic （0.8-0.6 Ga） components, suggesting that the detritus mostly come from a Grenvillian orogen. The youngest detrital zircon ages place a constraint on the deposition time of these sediments in Late Neoproterozoic. Zircon Hf isotopic compositions indicate that the Grenvillian zircons were derived from the reworking of Mesoproterozoic arc magmatic rocks and Paleoproterozoic continental crust, implying an arc-continent collisional setting. Single-peak age spectra and the presence of abundant euhedral Grenvillian zircons suggest that the sedimentary provenance is not far away from the sample location. Thus, the Grenvillian orogen probably preexisted along the southern margin of the Cathaysia Block, or very close to the south. Similarity in the ages of Grenvillian orogeny and the influence of the assembly of Gondwana in South China with India and East Antarctic are discussed, with suggestion that South China was more likely linked with the India-East Antarctica continents in Early Neoproterozoic rather than between western Laurentia and eastern Australia.     

Origin of ore-forming fluid in the Piaotang tungsten deposit in Jiangxi Province: Evidence from helium and argon isotopes

The Piaotang tungsten deposit in Jiangxi Province is one of the most important tungsten deposits in the South China Tungsten Metallogenic Province.In this study,we selected wolframite and chalcopyrite from the main mineralizing stage of the Piaotang tungsten deposit,and analyzed the helium and argon isotopic compositions in fluid inclusions of these minerals.The results show a 3He/4He ratio of 0.17 to 0.86 R/Ra,which fall within the range between continental crust(0.01―0.05 R/Ra) and air-saturated water(1 R...     

Finding of ancient materials in Cathaysia and implication for the formation of Precambrian crust

U-Pb dating for fifty-six detrltsl zircons from a paragneiss in Nanxiong area, northern Guangdong Province, Indicates that the latest Neoproterozoic sediments in Cathaysia hinterland are composed of numerous Grenvillian and Necerchaean clasUc materials, as well as some Mesoproterozolc detritus. Minor Paleoarchaean （3.76 Ga） and Mesoarchaean （3.0-3.2 Ga） zircons, which are the oldest zircons In South China, also are firstly found in the sediments, suggesting that the Cathaysia Block may contsln very old materials. The Hf isotope compositions of thirty-seven zircons reveal that these clastlc materials have different origins. Minor zircons crystslllzed from magma generated from relatively juvenile crust, while the parental magma of most zircons was derived from ancient crust. Integration of U-Pb dating and Hf Isotope analysis of these zircons suggests that the generation of juvenile crust in the Cathaysia block mainly occurred at 2.5-2.6 Ga. Mesoarchaean （3.0-3.3 Ga）, late Paleoproterozolc （-1.8 Ga） and Paleoarchaean （-3.7 Ga） may also be important episodes of crustal growth. Grenvllllan magmatism is extremely Intense, but it mainly involved recycling of ancient crustal components with little formation of Juvenile crust. The marked presence of -2.1 Ga Hf model ages and the absence of the zircons with crystsllizatlon ages at -2.1 Ga suggest that the parental magma of many zircons was probably derived from the mixed source consisting of Neoarchaean and late Paleoproterozoic materlals.     

Re-Os isotope dating of pyrite from the footwall mineralization zone of the Xinqiao deposit,Tongling, Anhui Province: Geochronological evidence for submarine exhalative sedimentation

The Xinqiao deposit is located in the Tongling area,Anhui Province.This deposit is dominantly composed of stratiform sulfide orebodies,which are restricted to the transitional zone from clastic rocks to carbonates at the base of the Late Carboniferous Huanglong Formation.There are discordant veinlets or networks of mineralization in the footwall rocks below the stratiform sulfide orebodies.The underlying discordant mineralization zones coexist with conformable stratiform sulfide orebodies,which are considered to be the most representative features of massive sulfide deposits formed by submarine exhalative sedimentation.This study reports a pyrite 187Re-187Os isotopic age of 319±13 Ma(MSWD=16) for the footwall mineralization zone.This isochron age is consistent with the interval of strata hosting the conformable stratiform sulfide orebody,and provides direct geochronological evidence for Carboniferous exhalative sedimentary mineralization.     

Geochemical characteristics of pore water in shallow sediments from Shenhu area of South China Sea and their significance for gas hydrate occurrence

The Shenhu area is one of the promise target areas for marine gas hydrate exploration in the northern margin of the South China Sea.Pore water samples were collected from two piston cores (8.75 and 8.52 m) in site HS-A and site HS-B in the Shenhu area,and their major anion and cation contents and trace element contents have been analyzed in this study.Cl- concentrations in pore waters are similar to the seawater value and no systematic change along depth profiles has been found for the Cl- content in both s...     

Glycerol ether biomarkers and their carbon isotopic compositions in a cold seep carbonate chimney from the Shenhu area,northern South China Sea

At modern cold seeps,the anaerobic oxidation of methane(AOM)is the dominant pathway for methane consumption in marine sediments.AOM,which is mediated by a consortium of methane oxidizing archaea and sulfate reducing bacteria,is proposed to be responsible for authigenic carbonate formation.A methane-derived carbonate chimney was collected from the Shenhu area, northern South China Sea.The membrane lipids and their very low carbon isotopic compositions(?115‰to?104‰)in the Shenhu chimney suggest the presence of an AOM process.Three specific archaeal and bacterial biomarkers were detected,including Ar,DAGE 1f,and monocyclic MDGD.Their strongly depleted??13C values(?115‰to?104‰),which are lower than those of the normal marine lipids in sediments,reveal biogenic methane as their origin.The carbonate deposits exhibiting a chimney structure indicate that a vigorous methane-rich fluid expulsion may have occurred at the seafloor.We propose that the decomposition of gas hydrates at depth is the likely cause of seepage and cold seep carbonate formation in the Shenhu area.     

Rare earth elements and carbon isotope geochemistry of the Doushantuo Formation in South China: Implication for middle Ediacaran shallow marine redox conditions

The middle Ediacaran Shuram excursion, the largest negative δ 13 C carb excursion in Earth history, has been interpreted as indirect evidence for episodic oxidation and remineralization of deep ocean DOC (dissolved organic carbon). It has been hypothesized that such oxidation event may have occurred when anoxic DOC-laden deep water was brought to shallow shelves during oceanic upwelling, which is expected to cause localized anoxia in shallow environments. To test this prediction, we systematically analyzed rare earth elements (REE) and δ 13 C carb of the upper Doushantuo Formation carbonates in the Yangtze Gorges area of South China, which were deposited in an inner shelf environment and record a large negative δ 13 C carb excursion correlated to the Shuram event. The REE data show a significant positive shift in Ce/Ce* values, synchronous with a pronounced negative δ 13 C carb shift. This positive Ce/Ce* shift is interpreted to represent an oceanic anoxia event in shallow shelf environments, which may have been caused by the upwelling or impingement of oxygen-depleted and 12 C-enriched deep water onto shelves. This anoxia event coincides with a sharp decline in the abundance and diversity of Ediacaran acanthomorphic acritarchs, raising the possibility that these two geobiological events may be causally related.     

Hf isotopic composition of zircons from the Huashan-Guposhan intrusive complex and their mafic enclaves in northeastern Guangxi: Implication for petrogenesis

High precision zircon U-Pb dating indicates that main intrusive bodies (Tong'an,Niumiao,Huashan,Lisong),and a mafic microgranular enclave in the Huashan-Guposhan complex were formed at 160-163 Ma.The εHf(t) values of zircons from the Huashan granite vary from -2.8 to +0.3 and those from the Lisong granite vary from -2.3 to +0.3,which are obviously different with those values (+2.6 to +7.4) of the mafic enclaves from the Lisong granite.These Hf isotopic data indicate that the mafic enclaves and host granites...