The East Kunlun Orogenic Belt of the northern Tibetan Plateau can be divided by the Qimantagh–Xiangride, Aqikekulehu–Kunzhong and Muztagh–Buqingshan sutures into, from north to south, the Northern Qimantagh, Central Kunlun and Southern Kunlun belts. The Yazidaban ophiolitic mélange, located in the westernmost region of the Qimantagh–Xiangride suture between the Northern Qimantagh and Central Kunlun belts, consists predominantly of serpentinite, basalt, diabase and andesite. The serpentinites are characterized by low total rare earth element (ΣREE) concentrations and depletion of mid-REEs, showing an ophiolitic ultramafic affinity. The basalts have low ΣREE concentrations, a slight enrichment in light REEs, depletion of large ion lithophile elements and insignificant fractionation of high field strength elements, which are attributed to an incompatible element-enriched mid-ocean ridge basalt. Both diabases and andesites are characterized by high ΣREE, strong enrichment in light REEs, depletion of Nb, Ta, P and Ti, and significant fractionation of high field strength elements. These geochemical characteristics indicate that the diabase and andesite were mostly generated in a subduction-related setting. Igneous zircons from the diabase yielded a 206 Pb/ 238 U age of 421.5 ± 2.2 Ma (MSWD = 0.44), representing the formation time of the subduction-related rocks. Together with the regional geology, these constraints on the ultramafic rock, basalt, andesite and diabase suggest that the Yazidaban ophiolitic mélange represents the remnants of oceanic crust from a back-arc basin and the associated subduction-related magmatic rocks. Supplementary material: Details of U–Th–Pb laser ablation inductively coupled plasma mass spectrometry data of zircons from the diabase, and major and trace element compositions for the samples from the Qimantagh mélange, East Kunlun Orogen are available at https://doi.org/10.6084/m9.figshare.c.4283219
The Qijiagou section on the southern margin of the Junggar Basin preserves thick and extensive Carboniferous carbonate rocks, and is the type section of the Upper Carboniferous in the Junggar Basin. Here, we studied the lithological assemblages and brachiopod fossils of the Upper Carboniferous Qijiagou Formation on the southern margin of the Junggar Basin. Outcrop observations indicate laterally variable thicknesses of this unit, comprising thick-bedded, fossiliferous, channelised, sandy, and calcareous limestone. Petrographic observations revealed four microfacies, indicating deposition of the formation in diverse depositional environments, including lagoonal to delta-dominated middle shelf settings. In addition, we identified 15 species of brachiopod fossils in 11 genera which belong to Spiriferida Waagen, Productida Sarycheva et Sokolskaya, and Strophomenida Öpik. Thus, based on the characteristics of the lithological assemblage and brachiopod fossils, we infer that the Qijiagou area had a relatively warm environment in the middle and late Late Carboniferous. This included a turbulent and shallow water body with two regressions which occurred in a carbonate platform sedimentary environment.
Vertebrate fossils are abundant in sandy mudstones of the Cretaceous Donggou Formation in the Sikeshu area, southern margin of the Junggar Basin, Xinjiang. Here, a few pycnodont remains from the Sikeshu area are described. Isolated elements are identified as a fragment of vomer and a fragment of prearticular referred to Xinjiangodus gyrodoides. These specimens are very small, corresponding to small pycnodont individuals. The larger specimens found in the Late Cretaceous in the United Kingdom, Spain, Morocco, and the United States were mostly from marine sediments, whereas the small specimens described here lived in continental paleoenvironments. The fossils are buried in situ, and their preservation methods involve carbonaceous pressing film and carbonate mineralization, of which carbonate mineralization is the main method. This research result fills the gap in Cretaceous Stenodon data in the Junggar Basin and provides important fossil data for the study of Cretaceous Stenodon in the Junggar Basin and even in Central Asia. This study provides a new perspective for understanding the paleoecology of inland lakes in western China at the end of the Cretaceous.
Gastropod fossils, mostly of small individuals, are abundant in the Palaeogene strata in the Sikeshu Depression on the southern margin of the Junggar Basin. The main groups are bithyniids and other prosobranchs. We studied the taxonomy and taphonomy of the gastropod assemblages of the Ziniquanzi Formation in the Sikeshu Depression. The gastropods inhabited muddy-bottom and shallow-water environments in a river floodplain and riverside setting. The results of this study are useful for recognising the Cretaceous–Palaeogene boundary, and demonstrate that the lake ecosystem had recovered from the end-Cretaceous mass extinction.
Large numbers of algae fossils are present in the oil shale of the Chang 7 Member of the Triassic Yanchang Formation in the Tongchuan area, which is located on the southern margin of the Ordos Basin. They are characterised by their high abundance, low differentiation, and monotonous species. Here, we report an example of Botryococcus. They have different shapes, which are mostly spherical but can also be also beaded and clumped. The massive fossils can be divided into spherical, heart-shaped, and three-lobed shapes. The fossils are mostly orange when observed under a single-polarisation microscope. When observed under reflected fluorescence, most of the fossils are yellow and light yellow–brown. Most of the Botryococcus fossils are beautifully preserved and developed along the horizontal bedding of mudstone, which is a typical in-situ burial. Botryococcus may be one of the important parent materials for the formation of the source rocks and crude oil in the Triassic Yanchang Formation in the Ordos Basin. It is concluded that the late Triassic sedimentary period, when Botryococcus prospered in the Tongchuan area, coincided with the expansion of the lake basin and gradually reached the maximum flooding surface of the lake. The climate was warm, suitable, and rainfall was abundant.
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