Characteristics of sulfur species and their implications in Lower Cambrian black shales from southern margin of Yangtze Platform

There are different sulfur forms in the black shales from the Early Cambrian of the Yangtze platform. With its emphasis on pyrite and organosulfur, this paper discusses their distribution and formation. The research shows that sulfur phases take regular variations laterally as well as vertically in the research areas. In western researched profile with high terrigenous supply at the time it formed, there exists a larger amount of pyrite and less organosulfur, and pyrite amount declines while organosulfur content increases upwards along the profile. This black shale profile is characterized by relatively light sulfur isotope composition with evolution trend of becoming heavier both for pyrite and organosulfur from bottom to top along the profile. Opposite situation occurs in eastern profiles which were located farther away from terrigenous land. Here pyrite amount obviously decreases and organic matter has combined more sulfur, although these two kinds of sulfur species take similar trend in content variation along profiles to that for western profile. At the same time more³⁴S is accumulated in sulfur species of black shale samples from eastern profile, and sulfur isotope composition gradually turns lighter from bottom to top. In combination with other information of iron, organic carbon contents and petrographic features, it can be established that sea-level change, supply of terrigenous matters, tectonic background and natures of paleoceanic chemistry have exerted great influence on the distribution of sulfur species in these black shales.     

Geochemistry of black shale at the bottom of the Lower Cambrian in Tarim Basin and its significance for lithosphere evolution

At the bottom of the Lower Cambrian, there widely develops a suit of rock association com-posed of black shales interbedded with thin silicalite in the India, northern Pakistan, Iran, southern France, England, northern Oman, the former USSR, southern Kaza…     

Cambrian-Ordovician sequence stratigraphy on the northern Tarim Platform and its correlation with Yangtze Platform and North China Platform

The Cambrian and Ordovician on the northern Tarim Platform are mainly composed of carbonates. On the basis of detailed outcrop analysis, the sequence stratigraphic system of the Cambrian-Ordovician in the northern Tarim Platform is outlined in this paper. Altogether 35 third-order sequences, 12 supersequences, 4 supersequence sets and 2 megasequences are recognized. The characteristics of the major sequence boundaries have been documented with an integrated examination of outcrop, seismic and borehole data, and the ages of these sequence boundaries have been calibrated through the combination of sequence stratigraphy with biostratigraphy. It is discovered that there is good correlation of the sequence stratigraphy of the Cambrian-Ordovician among Tarim, Yangtze and North China platforms. This may illustrate that the development of the Cambrian-Ordovician carbonate sequences in these three platforms is mainly controlled by regional or global sea level changes. This forms the theoretical basis for the construction of high-resolution chronostratigraphic system of the Cambrian-Ordovician in the three platforms in China.     

Organic carbon isotopes of the Sinian and Early Cambrian black shales on Yangtze Platform, China

Organic matter of the Sinian and early Cambrian black shales on the Yangtze Platform belongs to the light carhon group of isotopes with the δ¹³C values from - 27% to - 35 %, which are lower than those of the contempomneously deposited carbonates and phosphorites. A carbon isotope-stratified paleooceanographic model caused by upwelling is proposed, which can be used not only to interpret the characteristies of organic carbon isotopic compositions of the black shales, but also to interpret the paleogeographic difference in the organic carbon isotope compositions of various types of sedimentary rocks. Project supported by the National Natural Science Foundation of China (Grant No. 49472114) and the Open Laboratory of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences.     

Trace elements and stable isotopic geochemistry of an Early Cambrian chert-phosphorite unit from the lower Yurtus Formation of the Sugetbrak section in the Tarim Basin

A chert-phosphorite unit from the Sugetbrak section in the Tarim Basin was analyzed for rare earth elements (Ce, Eu), redox sensitive proxies, and carbon isotopic compositions (δ¹³C_carb and δ ¹³C_org) in the lower Yurtus Formation of the Early Cambrian period. Redox sensitive element ratios (Th/U, V/Cr, Ni/Co, and V/Sc) were employed to determine the palaeoenvironmental conditions during this period. The ratios indicated that the depositional environment of the chert-phosphorite-black shale unit ranged from suboxic to anoxic. Negative Ce and positive Eu anomalies in the chert-phosphorite assemblages of the studied Yurtus Formation indicated the existence of a redox-stratified ocean, similar to that of South China. Overturn or upward expansion of the deep water-mass probably reached the shallow marine zone after the formation of the Yurtus phosphorites. The characteristics of the negative Ce anomaly may be due to phosphoritic inheritance from the Ce-depleted signature of the overlying water column. Subsequent hydrothermal inputs and reduced detrital supplies of the deep water caused by the upwelling affected certain redox sensitive elements in the sedimentary basin. δ ¹³C_carb and δ¹³C_org negative excursions in the Yurtus chert-phosphorite unit may be related to a transgression phase when episodic basinal upwelling moved ¹²C- and P-rich waters from the pelagic basin floor to the continental shelf. Although carbon isotopic compositions in the Yurtus chert-phosphorite assemblages may have suffered from diagenetic alteration, they can be used to probe diagenetic conditions. Multi-proxy geochemical studies indicated that the δ ¹³C_carb values of the Yurtus chert-phosphorite assemblages might be considered reflections of a predominantly suboxic environment that was subsequently affected by hydrothermal inputs due to the upwelling.     

贵州遵义下寒武统黑色页岩镍、钼多金属矿床的形态硫特征及成矿模式

黄家湾矿是贵州遵义地区寒武系底部新发现的黑色页岩镍钼多金属元素矿床。根据形态硫测试结果,文章分析了黄家湾下寒武统牛蹄塘组黑色页岩剖面和矿石中不同形态硫（硫酸盐硫、黄铁矿硫、有机硫）的特征,推测镍、钼金属富集层处还原性硫的来源。     

Hydrogen isotope composition of natural gases from the Tarim Basin and its indication of depositional environments of the source rocks

By measuring carbon and hydrogen isotope compositions for C1, C2 and C3 of 74 gas samples, natural gases from the Tarim Basin can be divided into six groups on the basis of their origins: (1) coal-type gas derived from coal measures; (2) coal-type gas generated from the T-J lacustrine mudstones; (3) oil-type gas derived from the Cambrian and low Ordovician marine source rocks; (4) oil-type gas from the source rocks deposited in the marine-transitional facies; (5) mixing gas between gas derived from the Carboniferous transitional source rocks and the Mesozoic humic gas, and (6) mixing gases of thermal genetic gas and little deep gas in the Southwest depression of the Tarim Basin. The δ D values of methane in natural gases originating from different type kerogens are affected by both palaeo-environments of the source rock formation (kerogen types) and thermal maturity, with sedimentary environment (kerogen type) as the main controlling factor. Under the similar thermal maturity, the hydrogen isotope composition of methane is more enriched in deuterium in marine environments than lacustrine one. With the increase of thermal maturity and the increase of carbon atomic numbers of gaseous alkanes, the hydrogen isotopes become enriched in deuterium. The δ D values of ethane and propane (δ D2, δ D3) are controlled mainly by thermal maturity and to a lesser degree by sedimentary environment of the source rock formation. The partial reversal of hydrogen isotopes for gaseous alkanes would be related to the microbial oxidation, mixing of sapropelic and humic gases and / or mixing of gases from similar kerogen sources with various thermal maturities. In the oil-type gas, the sulfate reduction reaction would result in the reversed order of δ D1 and δ D2 (e.g. δ D1>δ D2).     

Basalt and tonalite from the Amami Plateau, northern West Philippine Basin: New Early Cretaceous ages and geochemical results, and their petrologic and tectonic implications

Abstract Basalts and tonalites dredged from the Amami Plateau in the northern West Philippine Basin have the geochemical characteristics of intraoceanic island arc rocks: low ⁸⁷Sr/⁸⁶Sr (0.70297–0.70310), intermediate ¹⁴³Nd/¹⁴⁴Nd (0.51288–0.51292), moderate light rare earth element (LREE) enrichment (La/Yb = 4.1–6.6) and high La/Nb (1.4–4.3). The incremental heating of hornblende from tonalites yielded well‐defined plateaus and ⁴⁰Ar/³⁹Ar isochron ages of 115.8 ± 0.5 Ma and 117.0 ± 1.1 Ma, while plagioclase yielded disturbed Ar release patterns, with ages ranging from 70 to 112 Ma. Taken together, these results show that the Amami Plateau was formed by subduction‐related magmatism in the Early Cretaceous period, earlier than indicated by prior K/Ar results. The results support tectonic models in which the West Philippine Basin was opened within a complex of Jurassic–Paleocene island arc terranes, which are now scattered in the northern West Philippine Basin, the Philippine Islands and Halmahera. The Amami Plateau tonalites and basalts have higher Sr/Y and lower Y and ⁸⁷Sr/⁸⁶Sr compared with younger tonalitic rocks from the northern Kyushu–Palau Ridge and the Tanzawa complex, which were formed by the subduction of the Pacific Plate beneath the Philippine Sea Plate. Based on the geochemical characteristics of the basalts, the Early Cretaceous subduction zone that formed the Amami Plateau may have been the site of slab melting, which suggests that a younger and hotter plate was being subducted at that time. However, the Amami tonalites were probably formed from basaltic magma by fractional crystallization or by partial melting of basaltic arc crust, rather than by melting of the subducted slab.     

Rhenium-osmium isotope constraints on the age and source of the platinum mineralization in the Lower Cambrian black rock series of Hunan-Guizhou provinces,China

The bottom of the Lower Cambrian series is an important bed typical of boundary event. The bed had been enriched with many useful elements such as Ni, Mo, Cu, Pb, Zn, Au, Ag, Ru, Rh, Pd, Os, Ir and Pt, many rare and dispersed elements such as Cd, Se, Tl, …