Late Paleozoic Sedimentary Characteristics And Its Tectonic Evolution In Northern Alax Area, Inner Mongolia

The Alxa area occupies a key central position, which is fringed by the Siberian plate in the north, by the Tarim plate in the west and by the North China plate in the south. Alxa is a very important position in the Inner Mongolia to study the closing time and tectonic evolution of the middle part of Southern Central Asian Orogenic Belt(CAOB). Previous studies on the crustal evolution and structural unit division within the Alxa region, including petrological, chronological and geochemical research on ophiolites and related granite, providing an excellent basis for further study. However, despite numerous studies, studies on late Paleozoic sediments in this area are still opening and the closing and evolutionary mode of the Paleo-Asian Ocean remains ambiguous. In view of the above problems, based on sedimentological analyses, paleontological studies, geochemical studies, detrital zircon geochronological studies, along with regional geological survey and previous research results, the Paleozoic sedimentary-tectonic evolution of the Alxa were well established to provide fundamental data for studying the Paleozoic sedimentary-structural evolution of the CAOB.In the Alxa area, three fault belts could be divided from north to south, including the Yagan fault belt, Engelwusu fault belt(ophiolite belt) and Chaganchulu fault belt(ophiolite belt). Based on these fault belts, the north of the Alxa can be divided into four tectonic subunits with different geological characteristics: the Yagan tectonic zone(YZ) which is out of the study area, the Zhusileng-Hangwula tectonic zone(ZHZ) which contains a complete stratigraphic sequence spanning the Neoproterozoic to the Triassic, the Shalazhashan tectonic zone(SZ) which contains the ’Amushan’ and ’Benbatu’ formations, and the Nuoergong-Langshan tectonic zone(NLZ) which is characterized by extensively outcropped Precambrian basement rocks and Permian granitoids.In the ZHZ, the detailed studies of fossils and detrital zircon ages(420±3Ma、419.6±3.6Ma) on the Yuanbaoshan Formation indicate its Early Silurian to Middle Devonian age, instead of Early Silurian. Using the zircon ages(284 Ma) in volcanic, the age of the Fangshankou Formation has been redefined as the Late Permian. According to the fossils and detrital zircon ages(299Ma), the age of the Shuangputang Formation is determined to Early Permian. There are two relatively complete stratigraphic sequences established in the ZHZ. The first sequence from the bottom to the top includes Xishuangyingshan(∈2-O1x), Bandingtaolega(S1b)and Yuanbaoshan formations(S1-D2y), indicating an evolutionary process from archipelagic ocean to active continental margin. The Shuangputang(P1sb) and Fangshankou(P1f) formations form the second sequence which represents a deposit system of back-arc sedimentary basin. Based on sedimentological analyses, geochemical studies, detrital zircon geochronological studies and previous research results and the eastern tectonic evolution of the Xing’an-Mongolia orogenic belt, a conclusion has been made that the Hangwula area experienced two opening and closing process of the Paleozoic ocean, the first time of the ocean closed in the Devonian- Carboniferous, the second took place after the middle Permian.The stratigraphic sequence of the ’Amushan’ and ’Benbatu’ formations in the SZ have been redefined, among which the ’Amushan’ Formation represent a sediment type of the back-arc basin. Its limestone on the bottom formed in Late Carboniferous-early of Early Permian, volcanic rocks- sandstone unit formed in the late Early Permian, or continued into the middle Permian. According to the analysis of sedimentary and geochemical characteristics and the Early Permian radiolarian fossils collected in the Chaganchulu ophiolite belt, combined with previous studies of age(the gabbro age in ophiolite belt: 275 ± 3 Ma, Wuliji collision-type granites age: 250.8 ± 2.0Ma), it has been considered that the closing time of the Chaganchulu Ocean is between 275 Ma and 250 Ma, namely the ophiolite belt formed in late Permian. The ’Benbatu’ Formation formed in the early Late Carboniferous, earlier than the ’Amushan’ Formation. Its sedimentary sequence represents a process of stable continental margin to an active continental margin.The age of the Suji volcanic rocks has been recognized as middle Permian, according to the rhyolite zircon age data(265.6Ma and 267.2 Ma) in the NLZ. Based on the geochemical characteristics of volcanic and information of tectonic evolution, the Suji volcanic rocks are indicated to form in a continental arc environment.Finally, based on a detailed study of the Paleozoic stratigraphy and sedimentary geology, sedimentary and tectonic evolution progress of the study area is divided into five stages:① Middle Cambrian- Early Ordovician, the Huhetaoergai intra-oceanic arc and the Shalazhasha island arc were gradually formed. Between the Huhetaoergai intra-oceanic arc and the Tarim Plate, the Hangwula area was an archipelagic ocean with a relatively stable tectonic environment. ② Early Silurian- Middle Devonian, the subduction of the ocean between the Tarim Plate and the Huhetaoergai intra-oceanic arc produced an active continental margin in the south of the intra-oceanic arc. ③ Late Devonian- Early Carboniferous, the ocean in the Hangwula area closed. ④ Late Carboniferous- Early Permian, active continental margin in the north of SZ and back-arc basin in the Chaganchulu were gradually formed,consisting of a typical trench- arc- basin system. By the subduction of the ocean located in the north of Huhetaoergai, there was a back-arc basin formed in Hangwula area. ⑤ Middle- Late Permian,oceans in the study area began subduction. At the late Permian, the ocean in the Chaganchulu was closed. After the middle Permian, the ocean in the Hangwula was closed and in late Late Permian ocean in the Engeerwusu closed.