Breakup And Closure Evolution Progress Of The North Altyn Ocean From Neoproterozoic To Early Paleozoic

The Northern Altyn ophiolite tectonic mixtite belt (or Hongliugou-Lapeiquan ophiolite tectonic mixtite belt) goes along the northern margin of the Altyn Mountains. This belt consists of mafic-ultramafic rocks, cumulate rocks, sheeted dikes, pillow basalts and cherts, which crops out as tectonics rock slices. It was preliminary division in the 1990's; and now has obtained the approval of most geologists. Qiashikansayi Hollow section is one of the most typical Ophiolite Tectonic Mixtite sections in this belt. Through to analyzing of bimodal volcanics rock slice and Early Paleozoic ophiolite Tectonic Mixtite section, combination with regain research results, This selected topic studies the evolution progress of the North Altyn Ocean from Neoproterozoic to Early Paleozoic.Bimodal volcanics rock slice is located at the south entrance of Qiashikansayi Hollow, which is about lkm long and consists of meta basalts and altered rhyolites. Based on geochemistry data, meta basalts derive from enriched mantle, suffer from contamination of crust, and are formed in a continental rift setting; Altered rhyolites are similar to the composition of A-type granites, and the result by partial melt of lower crust resulting from the underplating of mantle-derived magma. Zircon LA-ICP-MS dating of rhyolites yields a U-Pb age of 749.8±4.6Ma, representing the time of crystallization of the bimodal volcanics, and providing time evidence for the initial breakup of North Altyn Ocean. Together with regional geological published report, there are similar time of initial breakup betwwen North Altyn Ocean and North Qilian Ocean. We suggest that this volcanic action is relative closely with breakup of Rodinia Supercontinent.Early Paleozoic ophiolite section in this paper is located at the south of Qiashikansayi Hollow and about 5km long, which is bounded in the south by Beiketan-Hongliuquan high-pressure metamorphic belt and in the south by Jurassic polymictic breccia. We can make out basalt rock slice, deep-sea chert rock slice, ultramafic rock rock slice, and carbonate rock slice. Those rock slices are separated from Middle-Upper Ordovician Lapeiquan Group sedimentary rocks by faults. The section includes two basalt rock slices:the north basalt rock slice and the south basalt rock slice. LA-ICP-MS Zircon U-Pb dating of basalt of the north basalt slice gives the crystallizing age of 437±14Ma, which is similar to the age (late Ordovician) of basalt from the south slice, shows that oceanic crust of North Altyn Ocean still formed in Ordovician Silurian. Ages of metamorphic zircons suggests that Qiashikansayi basalts underwent two stage metamorphisms (403±13Ma and 261±12Ma). Based on geochemistry data, parent magma of basalts goes through clinopyroxene fractionation, and suffers from slight hybridization of crust materials. Both of basalt rock slices derive from spinal-bearing mantle, the source of basalts from the north rock slice is plausible spinal Iherzolite mantle, and that from the south one is plausible-depleted Iherzolite mantle; moreover, the former are formed by higher partial meting. All basalts are tholeiite of E-MORB type; LILE enrichment relative to HFSE, Th/Ta (1.58-4.27), the zircons of Proterozoic age, especially for the similar HFSE (TiO2, Zr and Y) content to present back-arc basin basalt.Through isolating and identificating Radiolaria from Cherts, which are collected from Deep-sea chert rock slices of this section and the north of Hongliuquan. We discover Middle-Late Ordovician radiolarias, including Protoceratoikiscum sp. cf. P. chinocrystallum Goto, Umeda and Ishiga, Inanibigutta sp.,Haplentactinia? sp., Inanigutta unica (Nazarov), and Inanigutta aksakensis (Nazarov). This is the first age restriction of radiolaria about this ophiolite mixtite belt.Middle-Upper Ordovician Lapeiquan Group clastic rocks mainly consist of lithic sandstone and lithic quartz sandstone in this section, which closely coexist with various tectonics rock slices. Composition of rock debris is not single, one class is volcanic rock debris, which come from mafic, intermediate and acidic volcanic rocks; another one derive from sedimentary rock, including pelite and chert, those reveal the source of clastic rocks is various. The study of detrital composition and analysis on heavy minerals show that the source of clastic rocks has sedimentary rocks, metamorphic rocks and igneous rocks. Tectonic discrimination diagrams of major elements and trace elements show that clastic rocks are formed in a continental island arc tectonic environment. Accordingly, we presume that clastic rocks come from the source of continental island arc, which includes basement and cover.Together with regional geological published report, we point out that there are three Characteristics of convergence process about the North Altyn Ocean. Firstly, subduction-related intermediate-acid magmatic rocks distribute widely in the north and south of the North Altyn ophiolite tectonic mixtite belt, some of them invade directly metamorphic basement Archean-Lower Proterozoic, indicates that there are bilateral subduction of the North Altyn Ocean. On the basis of geochronology date including subduction-related magmatic rocks, eclogites and clastic rocks, this ocean starts bilateral subduction during Middle Cambrian and may continue to early stage of early Ordovician. Secondly, U-Pb age of subduction-related metagranitic intrusions in the south of this belt lies between 482-443Ma, which shows that the North Altyn Ocean subductes toward the south during Ordovician; and that of subduction-related magmatic rocks in the north of this belt lies between 503-480Ma, metagranitic intrusions are not in males aged 480 to 440Ma, those with the age smaller than 440Ma all are fromed in syn-collisional and post-collisional environment, basalts from north of Beiketan-Hongliuquan high-pressure metamorphic belt are formed in back-arc basin seamount environment. We suggest that there may be a back-arc basin caused by the northward subduction of the North Altyn Ocean, Middle-Late Ordovician radiolarias and Late Ordovician gabbro (449.5±10.9Ma) indicate that the back-arc basin continues from Early Ordovician to Late Ordovician. Thirdly, we discover Middle-Late Ordovician radiolarias from cherts and obtaine Zircon LA-ICP-MS U-Pb age of 437±14Ma from basalts in Qiashikansayi Hollow, indicating that the North Altyn residual Oceanic crust may last to Ordovician Silurian. Early Ordovician-Early Silurian magmatic rocks are composed of subduction type and syn- collision type, which indicates the tectonic environment of this ocean transfer from subduction to collision. Another main period of magmatic activity is late Silurian- Early Devonian, many rocks (including basalts, magmatic rocks and sedimentary rocks) undergo dynamic metamorphism, and thus, we presume that those magmatic rocks are formed in post- collisional environment in this period.On the basis of geochronological framework of the Northern Altyn ophiolite tectonic mixtite belt, together with magmatic event, metamorphism and depositional feature in each period, it is concluded that the evolution progress of the North Altyn Ocean can be divided into 6 stages from early Neoproterozoic to early Paleozoic. Qb2-Nh:initial breakup of this ocean; Z-∈1: ocean spreading;∈2-O1:early stage of subduction; O1-O3:late stage of subduction; O3-D2: tectonic environment transfer from subduction to collision; S3-D1:post-collisional orogenic period.