Search

Late Paleozoic pillow basalts are well preserved in the Youjiang Basin of SW China. In this study, we conducted geochemical and Sr−Nd isotopic study on the Late Devonian and Late Carboniferous basalts in Napo area, Youjiang Basin. The analyzed samples comprise moderate SiO2, TiO2, Mg#, and REE contents, with slightly positive or negative Eu anomalies, similar to the chemical nature of OIB (Oceanic island basalt). All basalts show low (143Nd/144Nd)i and (87Sr/86Sr)i values, close to EMI-type (I-type Enriched Mantle) magma source. Late Carboniferous basalts display higher (87Sr/86Sr)i, Nb/Th, and lower εNd(t), Zr/Nb values than those of the Late Devonian basalts, implying progressively decreased enrichment components in the magma source processes. Such evolution trend reflects a continuous lithospheric thinning event, supporting a progressive rifting setting of the Youjiang Basin along the southwestern margin of the Yangtze Block in Late Paleozoic. Combined with regional geological background, the Youjiang Basin is viewed as a continental rift basin related to the opening of the Paleo-Tethys Ocean in Late Devonian and became a passive continental margin basin in Carboniferous. The Paleo-Tethys branch ocean probably extended further to the southern Youjiang Basin, likely corresponding to the Song Ma suture zone. [ABSTRACT FROM AUTHOR]

The southwest basin is a key to study the origin and development of the South China Sea (SCS). We do not know much about its boundaries, geological history, and the formation of its sea floor because it has a complex and highly regional structural background, notable sediment activity, and yet few floor rocks. Here a granodiorite sample was collected from the southern margin of the southwest basin of the South China Sea. The results indicated that the Ar-Ar ages of biotites in the sample are 110.3±0.5 Ma, suggesting that they were products of magmatic intrusion during the Early Cretaceous period. The sample's geochemistry showed it had high SiO, KO, and AlO but low TiO levels. Tectonic discriminant diagrams suggested that the sample might represent extrusion-related magmatism, either in an arc or forearc setting in the SCS area and that the sample mainly belonged to the syncollision type, whose formation was related to orogenies. The sample may be part of the main rock that made up the boundaries of rift system. The process of tension cracking was similar to the development of the Red Sea, in which the rifting and sagging occurred in the continental crust. The southwest basin may not be an original ocean, but a rift developed through finite extension on continental crust basement. The oceanic crust came into being when the width and depth of the rift valley reached a certain scale. The granodiorite sample we collected provides a means of determining the boundary of the southwest basin and the clues that may help researchers expand relevant models. It constitutes an important datum regarding the analysis of the formation and development of the SCS. [ABSTRACT FROM AUTHOR]

The article focuses on a study which examined the paleoclimate inference from the evolution of Palaeophycus and Beaconites and trace fossil assemblages in Triassic in Jiyuan, Central China. Information on formations in the Triassic succession is given. According to the study, sharp scratch marks on the wall of the Palaeophycus indicate a firm substrate while Beaconites suggests a dry environment during the Early Triassic. It notes the dominance of vertical assemblages in Late Triassic.

The Lower Carboniferous Jiujialu Formation bauxite deposits of northern Guizhou Province, China, are a Kazakhstan subtype of karst bauxite deposits. The provenance of the Jiujialu Formation has long been debated, with uncertainty about the formation of the bauxite deposits. Here we report new geochemical data that indicate the affinity between the Lower-Middle Ordovician clastic rocks and argillaceous dolostones and the overlying Carboniferous bauxite deposits, all of which are characterized by high Al2O3, K2O, and ΣREE contents, flat post-Archean Australian shale (PAAS)-normalized REE patterns, and uniform immobile element ratios (TiO2/Al2O3, Nb/TiO2, and Zr/TiO2). Their similar detrital zircon age distributions further indicate the link between the bauxite deposits and the clastic rocks and argillaceous dolostones. Zircon age spectra of clastic rocks of the Lower Silurian Hanchiatien Formation in northern Guizhou match those of the bauxite deposits, with a maximum age peak at ∼980 Ma and other secondary age peaks, suggesting these clastic rocks may represent the provenance of the bauxite deposits. The youngest detrital zircons (∼445 Ma) occur only in the bauxite deposits and are probably sourced from K-bentonite beds of the Ordovician-Silurian transition. Our analyses indicate that the source materials of the bauxite deposits in the Jiujialu Formation are of mixed provenance: Lower-Middle Ordovician aluminosilicate rocks and argillaceous dolostones of the underlying strata, and Lower Silurian clastic rocks and K-bentonite from adjacent areas. A comparison of Early Carboniferous bauxitic provenances in northern and central Guizhou indicates that paleotopography was the major factor controlling the provenance of these bauxite deposits. [ABSTRACT FROM AUTHOR]