The tectonic evolution of the Paleo‐Asian Ocean between the North China Block (NCB) and the Mongolia Block (MOB) is a contentious issue, and geodynamic models remain speculative. In an effort to puzzle out this controversy, a paleomagnetic study was carried out on the Silurian to Permian formations in central‐eastern Inner Mongolia (China). More than 680 sedimentary and volcanic samples were collected from 86 sites. We have established titanium‐poor magnetite and hematite as the principal magnetic carriers. Anisotropy of the magnetic susceptibility measurements demonstrate negligible deformation of the majority of study rocks with sedimentary fabrics. From primary magnetizations, a Late Devonian and a Permian pole are calculated for Inner Mongolia Block (IMB) at λ = 46.8°N, φ = 349.1°E, dp = 14.6°, dm = 27.3° with N = 3 and λ = 48.7°N, φ = 3.7°E, dp = 5.2°, dm = 9.1° with N = 6, respectively. Two stages of secondary magnetization are also identified probably due to Early Permian and Early Cretaceous magmatic events. As preliminary results, the comparison of our new paleomagnetic poles with available data from NCB, MOB, and Siberia indicates that (1) the paleolatitudes of IMB, NCB, and MOB are consistent between Late Devonian and Permian, suggesting pre‐Late Devonian closure of the Paleo‐Asian Ocean and further evaluation of these three blocks as a single entity and (2) post‐Permian intracontinental deformation was significant and characterized by block rotations, which are due to strike‐slip faulting within the welded NCB‐IMB‐MOB block.
It is hotly debated about the final closural time and position of the Paleo-Asian Ocean. Some geologists advocated the “Solonker” suture marks the final closural zone in Permian, whereas others insist in middle Paleozoic. Our three study areas, the Hongqi, the Ondor Sum and the Mandula is essential and important to solve those controversies. The litho-tectonic units recognized in the Hongqi-Ondor Sum area include the Hongqi-Ondor Sum melange belt, the Bainaimiao arc belt, North China Craton and post-orogenic unconformably sedimentary rocks. The Hongqi-Ondor Sum melange belt experienced two phase ductile deformation and one phase ductile-brittle deformation. D1 is responsible for the regional greenschist foliation S1, elongated mineral lineation L1, and intrafolial fold F1. The kinematic criteria indicates a top-to-the-NW shearing sense. D2 is characterized by various sized of unsymmetrical folds with nearly NE axis corresponding to the NW thrust shearing. D3 formed the regional framework in the Hongqi and the Ondor Sum areas. The Mandula area contains olistostrome sediments, turbiditic sediments and volcano-sedimentary rocks. Detrital zircon grains in sedimentary samples argue the Mandula study area received the southern Bainaimiao arc materials and coeval Permian volcanic erupting materials nearby. The sediments and volcanic rocks in Mandula area subject a nearly NW-SE or N-S compressional shortening. The geological data support that an Early Paleozoic subduction and collsioan, Late Palezoic rifting and rift closure model. The so called “Solonker” ophiolitic fragments indeed are olistostrome. Typical ophiolite components are not observed in the Mandula area.
At the northern margin of the North China Block (NCB), the Xilamulun Fault (XMF) is a key belt to decipher the tectonic evolution of Central-Eastern Asia, as it records the Paleozoic final closure of the Paleo-Asian Ocean, and localizes a Late Triassic intracontinental deformation. In this study, structural analysis, 40Ar–39Ar dating, and paleomagnetic studies were performed to investigate the kinematics of the XMF and to further discuss its Triassic geodynamic significance in the Central-Eastern Asia framework after the Paleozoic Central Asian Orogenic evolution. The structural analyses reveal two phases of ductile deformation. The first one (D1), which displays N-verging and E–W trending folds, is related to the Early Paleozoic collisional event between the NCB and the Songliao–Hunshandake Block (SHB). The second phase (D2) displays a high-angle foliation and a pervasive sub-horizontal E–W stretching lineation with kinematic criteria indicative of dextral strike–slip shearing. The 40Ar–39Ar dating on mylonitic granite places the main shearing event around 227–209 Ma. This D2 shearing is coeval with that of the dextral strike–slip Bayan Obo–Chifeng Fault (BCF) and the Chicheng–Fengning–Longhua Fault to the south, which together constitute a dextral shearing fault system on the northern margin of the NCB during the Late Triassic. The paleomagnetic study performed on the Middle Permian Guangxingyuan pluton, located between the XMF and BCF, documents a local clockwise rotation of this pluton with respect to the NCB and SHB. Our multidisciplinary study suggests an NNW–SSE shortening and strike–slip shearing dominated tectonic setting on the northern margin of the NCB during the Late Triassic. Combining the contemporaneous dextral strike–slip movements of the XMF and BCF in northern China and the sinistral strike–slip movement of East Gobi Fault (EGF) in southeastern Mongolia with the large-scale tectonic framework, a Late Triassic NNW–SSE shortening-eastward extrusion tectonic model for Central-Eastern Asia is firstly proposed. The NNW–SSE shortening results in the eastward extrusion of the continental wedge bounded by the BCF and EGF, which is accommodated by the different kinematic patterns of the southern (XMF and BCF) and northwestern (EGF) bounding faults. This shortening-extrusion tectonic framework is tentatively interpreted as the result of the far field forces associated with three Late Triassic lithosphere-scale convergences in East Asia: i) northward intracontinental subduction between the NCB and South China Block, ii) collision of the Qiangtang Block with the Qaidam Block, and iii) southward subduction of the Mongol–Okhotsk Ocean beneath the Mongolia Block.
Abstract This study illustrates the clay mineralogy and sedimentary geochemistry of the Red River and its major tributaries and distributaries in northern Vietnam and shows how these methods can be used to unravel grain size, provenance, hydraulic-sorting, and chemical weathering effects. All sand samples are SiO2-rich and consequently depleted in most chemical elements (but Sn and Pb) relative to the upper continental crust (UCC). The order of element mobility indicated by αAlE values, which estimate the degree of depletion in mobile element E relative to the UCC standard, is Ca ≥ Na > Sr > Mg > Ba ≥ K ≥ Rb. In mud fractions, SiO2 decreases, and other elements consequently increase. The grain size-dependent intrasample chemical variability of fluvial sediments reflects the grain size distribution of detrital minerals, which is strictly controlled in turn by the settling-equivalence principle. The 87Sr/86Sr ratio in Red River sands varies widely between 0.716 and 0.748, and εNd ranges from −8.5 to −13.8. The negative εNd values and high 87Sr/86Sr ratios point at a significant contribution from Precambrian crystalline basement, directly or through recycling of Triassic siliciclastic strata. Clay-mineral assemblages, dominated by illite and smectite with subordinate kaolinite and minor chlorite, suggest largely physical erosion in the upper catchment and stronger weathering in the monsoon-drenched lower catchment. Extremely intense weathering is demonstrated by a Quaternary soil sample from the Red River valley in northernmost Vietnam, which is a pure quartzose sand yielding a tourmaline-dominated heavy-mineral suite and a kaolinite-dominated clay-mineral assemblage. In the humid landscapes of northern Vietnam, no detrital mineral, excepting quartz, muscovite, tourmaline, prismatic sillimanite, anatase, and zircon, can resist even shallow early pedogenesis.
Global glaciation, oxidation event and eukaryotic expansion and diversification in the Neoproterozoic period are marked events that characterize the early evolution of the Earth, but how the interactions occurred among these events is not well understood. The organic matters preserved in the black shales of the Datangpo Formation (Cryogenian period) are sensitive to redox conditions, and thus its accumulation and preservation offer beneficial clues to unravel the early evolutional history of the Earth. This study presents new chemostratigraphic data of iron component, TOC content, sulfur isotope of pyrite, carbon and oxygen isotopes of carbonaceous shale of the Datangpo Formation (Cryogenian period) in the Datangpo section, South China. The analyzed results imply abundant nutrients existing in the ocean in the Early Cryogenian. The nutrients, such as phosphorus, resulting from neighbor volcanic eruptions, provided nutrients that enabled microbes to flourish during the Cryogenian interglacial gap. Iron components and sulfur isotopes indicated anoxic, euxinic deep water environments for the black shales in the lower portion of the Datangpo Formation. The anoxic setting was good for the preservation of organic matter, but terrigenous materials inputs, as revealed by the high Al2O3 contents, diluted the organic carbon content (TOC). Cited as : Wang, C., Shi, G. Redox condition and organic carbon accumulation mechanism in the Cryogenian Nanhua Basin, South China: Insights from iron chemistry and sulfur, carbon, oxygen isotopes of the Datangpo Formation. Advances in Geo-Energy Research, 2019, 3(1): 67-75, doi: 10.26804/ager.2019.01.05
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