Acta Geologica Sinica ‐ English Edition最新文献

The Late Permian was marked by a series of important geological events and widespread organic-rich black shale depositions, acting as important unconventional hydrocarbon source rocks. However, the mechanism of organic matter (OM) enrichment throughout this period is still controversial. Based on geochemical data, the marine redox conditions, paleogeographic and hydrographic environment, primary productivity, volcanism, and terrigenous input during the Late Permian in the Lower Yangtze region have been studied from the Putaoling section, Chaohu, to provide new insights into OM accumulation. Five Phases are distinguished based on the TOC and environmental variations. In Phase I, anoxic conditions driven by water restriction enhanced OM preservation. In Phase II, euxinic and cycling hydrological environments were the two most substantial controlling factors for the massive OM deposition. During Phase III, intensified terrestrial input potentially diluted the OM in sediment and the presence of oxygen in bottom water weakened the preservation condition. Phase IV was characterized by a relatively higher abundance of mercury (Hg) and TOC (peak at 16.98 wt%), indicating that enhanced volcanism potentially stimulated higher productivity and a euxinic environment. In Phase V, extremely lean OM was preserved as a result of terrestrial dilutions and decreasing primary productivity. Phases I, II and IV are characterized as the most prominent OM-rich zones due to the effective interactions of the controlling factors, namely paleogeographic, hydrographic environment, volcanism, and redox conditions.

There are two factors, source composition and magmatic differentiation, potentially controlling W-Sn mineralization. Which one is more important is widely debated and may need to be determined for each individual deposit. The Xitian granite batholith located in South China is a natural laboratory for investigating the above problem. It consists essentially of two separate components, formed in the Triassic at ca. 226 Ma and Jurassic at ca. 152 Ma, respectively. The Triassic and Jurassic rocks are both composed of porphyritic and fine-grained phases. The latter resulted from highly-differentiated porphyritic ones but they have similar textural characteristics and mineral assemblages, indicating that they reached a similar degree of crystal fractionation. Although both fine-grained phases are highly differentiated with elevated rare metal contents, economic W–Sn mineralization is rare in the Triassic granitoids and this can be attributed to less fertile source materials than their Jurassic counterparts, with a slightly more enriched isotopic signature and whole-rock ε_{Nd(226 Ma)} of –10.4 to –9.2 (2σ = 0.2) compared with ε_{Nd(152 Ma)} of –9.2 to -8.2 (2σ = 0.2) for the Jurassic rocks. The initial W-Sn enrichment was derived from the metasedimentary rocks and strongly enhanced by reworking of the continental crust, culminating in the Jurassic.

The Linxia Basin is characterized by an abundance of Cenozoic sediments, that contain exceptionally rich fossil resources. Equids are abundant in the Linxia Basin, the fossil record of equids in this region including 16 species that represent 10 genera. In comparison to other classic late Cenozoic areas in China, the Linxia Basin stands out, because the fauna and chronological data accompanying Linxia equids render them remarkably useful for biostratigraphy. The anchitheriines in the region, such as Anchitherium and Sinohippus, represent early equids that appeared in the late stages of the middle and late Miocene, respectively. Among the equines, most species of Chinese hipparions have been identified in the Linxia Basin and some species of the genera Hipparion and Hippotherium have FAD records for China. Furthermore, Equus eisenmannae is one of the earliest known species of Equus in the Old World and is well-represented at the Longdan locality. Some species with precise geohistorical distributions can serve as standards for high-resolution chronological units within this framework. Located at the eastern margin of the Tibetan Plateau and subject to considerable uplift, the Linxia Basin has served as a biogeographic transition area for equids throughout the late Cenozoic.