| The Carboniferous to Neogene strata in the South Tianshan are well developed, mainly consisting of clastic rocks and carbonate rocks. Based on field geological survey and microfacies analysis, this study systematically differentiates the lithofacies of the Carbonifereous to Neogene strata, indepth study has been made on sedimentary systems. In addition, our research also focuses on detrital component, whole rock geochemistry, single-grain mineral chemistry and detrital zircon U-Pb geochronology. This study elaborates the provenance of the clastic rocks of the Carboniferous to Neogene and its tectonic attributes for a better understanding of the collision, uplifting and erosion history of the South Tianshan.Clastic rocks, Carbonate rocks, gypsum and coal, four types of rocks developed in Carboniferous to Neogene strata in the study area. Carbonate rocks include calcirudite, oolitic limestone (with terrigenous clasts), foraminifer limestone, bioclastic grainstone, bioclastic packstone, bioclastic wackestone, plaster limestone and reef limestone. Clastic rocks include cobble conglomerate, pebbly conglomerate, granule conglomerate, quartz sandstone (greywacke), lithic quartz sandstone (greywacke), lithic sandstone (greywacke), feldspar lithic sandstone (greywacke), siltstone and mudstone. That is17types of lithofacies in all.Sedimentary systems show that the Carboniferous to Neogene in the South Tianshan and the southern South Tianshan mainly consists of nine sedimentary system types, including clastic littoral sedimentary system, carbonate platform sedimentary system, carbonate slope sedimentary system, continental slope sedimentary system, basin sedimentary system, alluvial fan sedimentary system, river sedimentary system, delta sedimentary system and lake sedimentary system, and can be subdivided into fourteen sedimentary facies. Marine sedimentary systems mainly developed in the Carboniferous to middle Permian strata. Continental sedimentary systems developed in the middle Permian to Neogene strata.Analysis of petrology and whole rock geochemistry of detrital rocks shows that the Carboniferous and Triassic sandstones have a low compositional, textural maturity and high matrix content. The unstable component of sandstones increases from the Carboniferous to the Triassic. The Carboniferous and Permian have a similar characteristic of REE abundences, which is obviously higher than that in the Triassic. The fractionation degree of light and heavy rare earth elements and the ratios of La/Th and La/Y descend from the Carboniferous to the Triassic and the Th/U ratio increases, which implicate the increase of recycled sediments. According to the distinction of detrital composition, major elements, rare earth elements and trace elements, the provenance tectonic settings of the Carboniferous were continental island arcs, active continental margin and passive continental margin, and the Permian’s was continental island arcs and the provenance of the Triassic was more similar to continental island arcs and ancient sedimentary geological bodies. The Carboniferous has a more complex provenance compared with the Permian and Triassic provenances. The Carboniferous in The South Tianshan had bidirectional sources. This provenance characteristics are of typical foreland basin. Therefore, the collision between Tarim and Yili-Central Tianshan should occur before Carboniferous. The South Tianshan area began the evolutional stage of foreland basin already in Carboniferous.The characteristics of detrital heavy mineral and detrital zircon U-Pb geochronology indicate that the source rocks of the Carboniferous are mostly composed of early Paleozoic medium-high grade metamorphic rocks, granite, mafic-ultramafic volcanic rocks from the central Tianshan and South Tianshan, and a little Proterozoic basement rocks from Tarim. The source rocks of the Permian mostly consist of metasedimentary rocks and granite from the Tianshan. The basic volcanic rocks from Tarim are the secondary source of the Permian. The Triassic sediments are mainly derived from low-grade to high-grade meta-sedimentary rocks, granitoid, arc basalts and subduction related peridotite from the Tianshan. The detrital component of the upper Triassic derived from high-grade meta-sediments and granitoid is much more than that of the lower Triassic. The source rocks of the Jurassic comprise middle-high grade metamorphic rocks, granitoid and basic-ultrabasic rocks from the Tianshan. The Neogene sediments are derived from low-grade metamorphic rocks, granite and high-ultrahigh pressure metamorphic rocks, lacking of middle-high grade metamorphic rocks. Comparing with the garnets from eclogite in the Southwest Tianshan, the detrital garnet compositions of the Carboniferous accord with that of the ecologite, which suggest that the eclogite in the South Tianshan shear zone has been exhumed at surface yet at Carboniferous time. Some of the detrital chrome spinels in the Carboniferous to Triassic sediments are sourced from oceanic island basalts, which provide sedimentary evidence for the archipelagic orogenisis of the South Tianshan.The collision time of the South Tianshan Orogen could be limited to the end of early Devonian to middle Devonian. This conclusion is based on the following five reason.(1) The youngest group of zircon age in the upper Carboniferous Ayilihe Formation is404-542Ma.(2) The provenance of detrital rocks of the upper Carboniferous Ayilihe Formation had the tectonic settings of continental island arcs, active continental margin and passive continental margin, showing bidirectional sources.(3) The Devonian showed a typical regressive sedimentary sequence and the switching time was in the end of early Devonian.(4) The South Tianshan area was in the evolutional stage of foreland basin in Carboniferous and the collisional orogenisis occurred before Carboniferous.(5) The lower Carboniferous unconformably overlaps on terranes of different ages from Proterozoic eon to early Devonian, which is a regional unconformity.According to sedimentary characteristics and provenance analysis, a uplifting-exhumation model was constructed to illustrate the Carboniferous to Neogene uplifting and denudation history of the Tianshan. The preliminary uplifting of the South Tianshan happened in Carboniferous, the denudation of the upper Paleozoic occurred in Triassic. In early Jurassic, the South Tianshan had been large-scale exhumed and the high pressure metamorphic rocks in the South Tianshan shear zone was an important source of the eary Jurassic sediments in Kuqa depression. The South Tianshan was uplifted slightly from middle Jurassic to early Cretaceous and the lower Paleozoic and Devonian in this area became the main source of elastics for the Kuqa depression. The high pressure metamorphic belt of the South Tianshan and the alkaline granite belt of the southern central Tianshan became the primary source of elastics for the Kuqa depression from early Cretaceous to Eocene because of the deplanation of the South Tianshan. Resulting from the far field effect by collision between the Indian and the Eurasian pates, the South Tianshan was uplifted rapidly and blocked the elastics supply of the high pressure metamorphic belt, the lower Paleozoic was the main provenance of sediments in Kuqa depression from late Eocene to Pliocene. However, a lot of detrital garnets, derived from high pressure metamorphic rocks, was found in the western South Tianshan. This suggests that the western South Tianshan had not been extensive uplifted and there was a discrepant uplifting history in different part of the South Tianshan. |
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