The Sedimentary Timing, Source Provenance And Tectonic Setting Of Paleozoic Clastic Sedimentary Rocks In The Altai Orogen, Xinjiang

The Chinese Altai orogen is situated in the northern Xijiang Uygur Autonomous, between the Sayan and associated belts to the north and the Junggar basin to the south, extending northwestwards for more than 2000 km. The Chinese Altai orogen is a part of the Central Asian Orogenic Belt (or Altaids), which is one of the largest Phanerozoic accretionary type orogens in the world, with both lateral and vertical accretion of juvenile materials contributing to the crustal growth. As for the Chinese Altai orogen, recent studies have shown that the continental vertical accretion is an important aspect of the crustal growth in this orogen. However, controversies on its tectonic evolution and timing of its basement still exist. The traditional viewpoints proposed that the basement of the Chinese Altai formed in Precambrian and the Open-closing model was the probable model for the geological evolution of this orogen.In northwestern and southern Chinese Altai orogen, thick Palaeozoic clastic sedimentary rocks are extensively distributed. These rocks mainly consist of sedimentary rocks and metasedimentary rocks, such as sandstone, siltstone, pelite, schist and phyllite, which were intrude by widely distributed Paleozoic granites and cut by complex fault system. As for the deposition time and tectonic setting of the clastic sedimentary rocks, most studies suggested that the clastic sedimentary rocks in northwestern Altai formed in Sinian to Early Cambrian and deposited on a passive continental margin, while the clastic sedimentary rocks in the south margin of the Chinese Altai formed in Early to Middle Devonian and deposited in rift-related tectonic setting on a passive continental margin.The clastic sedimentary rocks of the Habahe Group in Northwest Altai are different to those rocks of the Kangbutiebao Formation and the Altay Formation in South Altai in the major element contents, but similar in their REE and trace element contents. In comparison with the sandstones and hornblende-bearing schists of the Habahe Group (SiO₂= 68-77 wt%, Al₂O₃= 9-14 wt%, Fe₂O₃= 2.3-5.7 wt%, MgO= 0.7-4.6 wt%, CaO= 0.3-3.5 wt%, Na₂O= 0.7-3.1 wt%; K₂O= 0.3-4.0 wt%), the Habahe pelites and biotite-bearing schists possess slightly higher Al₂O₃ (15-19 wt%), Fe₂O₃ (6.4-11.5 wt%) and MgO (3.3-5.0 wt%) and lower SiO₂ (58-64 wt%) contents. However, all clastic sedimentary rocks from the Habahe Group show similar REE patterns with distinct LREE enrichments (La_N/Yb_N= 3-13) and negative Eu anomalies (Eu/Eu*= 0.43-0.80).In the south margin of the Chinese Altai, the chlorite-bearing schists from the Kangbutiebao Formation have similar compositions of the major elements (SiO₂= 61-64 wt%, Al₂O₃= 15-18 wt%, Fe₂O₃= 7.5-8.0 wt%, MgO= 3.3-3.7 wt%) to the Habahe pelites and biotite-bearing schists, while the mica-bearing schists and phyllites (SiO₂= 63-77 wt%, Al₂O₃= 11-17 wt%, Fe₂O₃= 3.5-6.2 wt%, MgO 0.8-3.4 wt%) are similar to the Habahe sandstones and hornblende-bearing schists. The Kangbutiebao clastic sedimentary rocks show similar ranges of the HFSEs (e.g. Nb, Ta, Zr, Hf, Y) and some LILEs (e.g. Sc, Th, U) contents to those of Post-Archaean Australian Shale (PASS), with lower Rb and Sr contents. Except for few mica-bearing schist, most Kangbutiebao clastic sedimentary rocks display similar REE patterns to Habahe clastic sedimentary rocks, with distinct LREE enrichments (La_N/Yb_N= 2.88-8.24) and negative Eu anomalies (Eu/Eu*= 0.45- 0.89) too. The geochemical characteristics of clastic sedimentary rocks from the Altai Group are consistent with those of the Habahe sandstones and hornblende-bearing schists. The Altai clastic sedimentary rocks posssess high SiO₂ (72-79 wt%) and low Al₂O₃(10-13 wt%), Fe₂O₃ (3.4-5.5 wt%), MgO (2.3-2.9 wt%) contents, also show LREE enrichments (La_N/Yb_N= 4.29-9.90) and negative Eu anomalies (Eu/Eu*= 0.46-0.65).The detrital zircons separated from these rocks have oscillatory compositional zoning, possess high Th/U ratios, and are enhedral to subhedral, showing short transportation from an igneous provenance. Their predominant ²⁰⁶Pb/²³⁸U ages were between～460 and～540 Ma, mostly with positiveεHf values. Except for this predominant group, a small population of Proterozoic and few Archean zircons also existed. Moreover, a concordant detrital zircon with Archean ²⁰⁷Pb/²⁰⁶Pb age (3087±20 Ma) had been discovered, which confirmed the involvement of Archean material in the source.Based on our geochemical data of the Palaeozoic clastic rocks in the Chinese Altai, U-Pb ages and Hf isotope data for the detrital zircons from these rocks, the following major conclusions can be drawn:(1) The clastic sedimentary rocks in northwestern and southern Chinese Altai orogen have similar characteristics of petrology and geochemistry. Their source materials experienced relatively weak chemical weathering and simple sedimentary recycling, and were dominated by intermediate to acidic arc sources. These rocks probably deposited on active continental margins, instead of passive continental margins proposed by previous reserchers.(2) The Habahe Group was deposited in the Middle Ordovician or later, not in the Precambrian. The deposition age of the Kangbutiebao Formation was later than the Early Silurian, which is consistent with the upper Silurian to lower Devonian age assigned to this formation. The clastic sedimentary rocks in Tarlang and southern Halasu from Altay Formation deposited before Early Devonian (470 Ma to 412 Ma), probably indicating that the clastic sedimentary rocks from this formation have different deposition ages in different areas.(3) The detrital zircons separated from these clastic sedimentary rocks was predominated by Cambrian- Early Ordovician magmatic zircons with enhedral to subhedral shape and positiveεHf values, suggesting similar sources mainly derived from Cambrian to Early Ordovician magmatic arcs. These clastic sedimentary rocks were different sedimentary products in same tectonic settings. (4) The source materials of these rocks were dominated by Early Palaeozoic clastic sediments, with subordinate Proterozoic and minor Archean crustal materials. These characteristics are different to the composition of clastic sedimentary rocks from North China Plate, indicating close affinities to Siberian Plate.(5) The migmatite from the Habahe Group and the garnet-sillimanite gneiss from the Altay Formation both underwent strong metamorphism in the Devonian. Together with the Permian regional metamorphism, these metamorphism events show a multiple thermal history for the Chinese Altai.(6) The evidences were not enough to prove the existence of Precambrian basement in the Chinese Altai orogen, inviting further detailed study. The clastic sedimentary rocks, such as the Habahe Group, Kangbutiebao Formation and Altay Formation, deposited after Middle Ordovician and could not represent the basement of the Chinese Altai orogen.