Late Paleozoic Tectonic Evolution And Magmatism Of Southwest Tianshan, Northwest China

The Central Asian Orogenic Belt(CAOB;also named the Altaids or Altaid collage)is sandwiched between the European,Siberian blocks to the north and the Tarim and North China blocks to the south(present-day coordinates).It is one of the largest orogenic belts in the world and preserves important evidence for accretionary orogenesis and Phanerozoic continental growth.The Chinese Southwest Tianshan is situated along the southwest margin of the CAOB and records the process of final collision between the Tarim Block and the Siberian Block.Thus,its composition and tectonic history provide important insights into the amalgamation of Eurasia and Phanerozoic continental growth of the CAOB.The Chinese Southwest Tianshan Orogenic Belt is composed mainly of Precambrian metamorphic strata,early Paleozoic sedimentary strata of the active continental margin,and a late Paleozoic accretionary wedge and ophiolite belt,which record the history of Precambrian paleo-continental nucleus formation,early Paleozoic oceanic crust subduction,and late Paleozoic collisional orogenesis,respectively.Correlation of regional geological mapping has shown that the Paleozoic strata are widely developed in the Southwest Tianshan.I investigated these Paleozoic strata by integrated approach involving field structure mapping,detrital zircon LA-ICP-MS U-Pb geochronology,major and trace element geochemistry,In-situ zircon Lu-Hf isotope geochronology,and mineral electron probe analysis.The establishment of the regional chronostratigraphic framework,tectonic evolution,as well as the magmatism processes has allowed the reconstruction of the Late Paleozoic Southwest Tianshan Orogenic belt tectonic evolution.The field scale structure section analysis was used to develop a tectonostratigraphic framework of the Late Paleozoic tectonic evolution process in the Southwest Tianshan Orogenic belt.The geometric and kinematic characteristics of the Biedieli and Tuoshigan structural sections shows that the Late Paleozoic tectonic deformation style is dominated by brittle-plastic deformation,and the structure framework controls on Meso-Cenozoic deformation patterns in space and time.The relative age of the undeformed diabase indicates that the brittle-plastic deformation occurred before the Permian,and the Meso-Cenozoic intracontinental uplift and exhumation process mainly brittle deformation domain.These new zircon U–Pb age and in-situ Lu–Hf isotopic data,in conjunction with published data,provide quantitative constraints on the stratigraphic ages and sediment sources of the Paleozoic strata of Southwest Tianshan and enable the construction of a tectonic model for the Tianshan Orogenic Belt.Based on the detrital zircon LA–ICP–MS U–Pb geochronology and Lu–Hf isotopes of the Wulagen metasedimentary rocks from the Wuqia area,we have redefined the geochronological framework of the wulagen metasedimentary rocks.The Wulagen metasedimentary rocks are composed mainly of metamorphic sandstones and mica schists,with a maximum depositional age of late Paleozoic(Early Devonian or later)rather than the Precambrian period,and the Kyrgyzstan North Tianshan area represents the major source region to the Wulagen metasedimentary rocks.The detrital zircon U–Pb age spectra for the Wulagen metasedimentary rocks show four major age groupings at 2700–2000Ma,2000–1500Ma,1000–700Ma,and 600–400Ma(with main age peaks at 2543 Ma,1814Ma,830 Ma,and 460 Ma,respectively).The early Paleozoic(~460Ma)age peak is consistent with early Paleozoic subduction-related magmatism in the southern CAOB.The zircon age population of 600–400Ma(~460Ma),characterized by negative ?Hf(t)values(-15.0 to-0.6)and Mesoproterozoic two-stage model ages,suggests that the early Paleozoic magmatic events involved melting of ancient crust and played an important role in crustal evolution in the southern CAOB.The detrital zircon LA–ICP–MS U–Pb geochronology feature within the Biedieli area provide insight into the Paleozoic sedimentation history and chronostratigraphic framework.The detrital zircon U–Pb age spectra for the Paleozoic Biedieli sedimentary strata show five major age peaks at 440Ma?640Ma?810Ma?1960Ma and 2360 Ma.The early Paleozoic(~430Ma-474Ma)age peak is consistent with early Paleozoic subduction-related magmatism in the southern CAOB.The Chinese Central Tianshan Terrane area represents the major source region to the Paleozoic Biedieli sedimentary rocks.The Biedieli sedimentary rocks of the Wushi area deposited on the active continental margin backgroud,which is formed with the tectonic setting of closure of the Late Paleozoic Southwest Tianshan Ocean crust.Detailed petrography,geochronology,geochemistry analysis was conducted on the Baleigong ophiolite,Guofanggonglu basalt and the Baleigong granite to construct the magmatism during the tectonic evolution of the Late Paleozoic collision orogenic process of Southwest Tianshan.The Guofanggonglu basalt represents residual oceanic basin tectonic background during the closure of the Southwest Tianshan oceanic crust.The Baleigong granite(281-282Ma)represents postcollisional magmatism in the Southwest Tianshan Orogenic Belt,constrains the timing of collision in the southwestern Tianshan,and indicates that the Southwest Tianshan Ocean closed prior to the Permian.Together these data provide important insights into the genetic links between regional-scale tectonics deformation and crustal-scale magma evolution,and relatively accurate controls the collision time of Southwest Tianshan orogen belt.