Paleozoic Crustal Growth And Evolution In The South Tianshan Orogenic Belt

The accretionary orogenic belt,formed on the edge of convergent plate,is a key area to understand the growth and evolution of the crust.The Central Asian orogenic belt(CAOB)is a typical accretionary orogenic belt in the world,but whether there is a large-scale crustal growth in the process of accretionary orogeny is still a key scientific problem to be solved.The South Tianshan orogenic belt(STOB),an important part of the CAOB,was involved in the formation history of the CAOB during the Paleozoic,which provides a good window for studying the formation and evolution,the growth of continental crust and the deep structure of the CAOB.Detrital zircons from the sediments of eight rivers in the southern part of the South Tianshan orogenic belt(Weigan River,Kezil River,Kuche River,Dinar River and Ceder River),the southern margin of the Center Tianshan(Wulasitai River and Dongtasitai River)and the northern margin of the Tarim Craton(Jigedai River)are studied in this thesis.Based on the U-Pb geochronology,trace elements and Lu-Hf isotopic analysis of detrital zircons,combined with the existing data,the source of detrital zircons and the growth and evolution of Paleozoic crust in the STOB and its adjacent areas were discussed.The conclusions are summarized as follows:(1)The U-Pb ages of the detrital zircons in modern rivers from the STOB and its adjacent tectonic units were detected.The detrital zircons in rivers from the southern part of the STOB yielded U-Pb ages clustering around 310?260 Ma,460?380 Ma,840?600 Ma,2100?1800 Ma and 2500?2300 Ma.The ages of the detrital zircons from northern margin of the Tarim Craton are 320?290 Ma,460?400 Ma,820?600 Ma and 2100?1800 Ma.Compared with the southern part of the ST OB,the detrital zorcons in the rivers from the southern margin of the Central Tianshan(CTS)block have roughly similar age distributions:320?260 Ma,380?320 Ma,470?390Ma,1000?800 Ma and 2500?2200 Ma,but obviously contains 380?320 Ma age groups.(2)Provenance analysis showed that the detrital zircons in the southern rivers of the STOB are mainly from the southern part of the STOB and the northern margin of the Tarim Craton(NMTC).The CL images show that the Precambrian detrital zircons with ages of 840?600 Ma,2100?1800 Ma and 2500?2300 Ma are nearly circular in shape,with poor degree of automorphism,indicating that they underwent long-distance transportation,and the ages of these zircons are consistent with those of the Precambrian magmatic rocks in the NMTC,suggesting that they originated fro m NMTC.The Paleozoic(460?380 Ma and 310?260 Ma)detrital zircons in rivers from southern part of the STOB are mostly euhedral subhedral,with typical magmatic zoning and high Th/U ratio(mostly greater than 0.2)and obvious negative Eu anomaly and positive Ce anomaly,these evidences indicate that they belong to magmatic zircon.Moreover,the zircon?_Hf(t)values of 460?380 Ma detrital zircons range from-15.4 to+17.7,which are consistent with the zircon Hf isotopic compositions of the early Paleozoic magmatic rocks in the southern part of the STOB and the NMTC,indicating that they originated fro m the STOB and the NMTC.The310 to 260 Ma zircons are long columnar with poor roundness,which indicates that their transportation distance is limited.The age distribution characteristics of the 310to 260 Ma zircons are consistent with the ages of the late Paleozoic magmatic rocks in the STOB,and their?_Hf(t)values range from-11.1 to+15.2,which is similar to the Hf isotopic composition of late Paleozoic magmatic rocks in the STOB,indicating that the STOB provides material source for them.These zircon ages are consistent with the ages of the late Paleozoic magmatic rocks and HP/LT metamorphic rocks in the STOB,and their?_Hf(t)values range from-11.1 to+15.2,which are similar with Hf isotopic compositions of the late Paleozoic magmatites in the STOB,indicating that the STOB provides the source for these zircons.Based on the analysis of detrital zircon morphology,U-Pb age distribution of the magmatic rocks and zircon Hf isotopic composition,it is concluded that the detrital materials of Wulasitai River and Dongtaxitai River are mainly from the Baluntai area in the western segment of the CTS,and the kulumtage area in NMTC provides material source for Jigedai River sediments.(3)Based on the zircon U-Pb ages of detrital zircons in the STOB and its adjacent areas,the chronological framework of ma gmatism is established,and the Paleozoic tectonic evolution of the STOB is discussed.The 470?390 Ma zircon population from the southern margin rivers of the CTS and 460?380 Ma zircon population from river sands of the STOB probably indicate a bidirectional subduction of the South Tianshan Ocean.The absence of the 380?320 Ma zircons in the river sands from the STOB and the NMTC maybe indicate that the NMTC was in a quiet period of magmatic activity during the late Devonian to late Carboniferous,while the magmatic activity in the southern margin of the Yili-Center Tianshan block was still frequent in this period.The closure of the South Tianshan ocean and the final collision between the TC and the Yili-Center Tianshan block may have occurred during the Late Carboniferous,followed by syn-and post-collisional magmatism,represented by a large number of 310?260 Ma detrital zircons in the samples.(4)Our study reveals crustal growth and relationship with geodynamics in the STOB.Based on U-Pb geochronology and Hf isotopic characteristics,combined with crustal thickness evolution,zircon saturation crystallization temperature,magmatic rock properties and temporal and spatial distribution,it is inferred that the early Paleozoic crustal evolution of the STOB is related to the advanceing and retreating subduction of the South Tianshan ocean.During the middle Ordovician and middle Silurian(Ca.460?435 Ma),the crust of the STOB and the NMTC gradually thickened in the compression environment formed b y the advancing subduction of the South Tianshan ocean,and the ancient crustal materials gradually added to the magma formation process,showing that the zircon?_Hf(t)value decreased with the age,while the corresponding two-stage model showed an upward trend.During the middle Silurian to late Devonian(ca.435?380 Ma),the South Tianshan ocean entered the stage of retreating subduction.In the extensional environment,the crust thinned and the proportion of juvenile materials in the magmatic source area gradually increased,showing that the zircon?_Hf(t)value increased with age,and the corresponding two-stage model age decreased.The obvious difference of Hf isotopic evolution trend is shown at ca.435 Ma.It is speculated that the STOB and the NMTC underwent tectonic transformation at middle Silurian,probably from the advanceing to the retreating accretionary orogeny.(5)It is discussed that late Paleozoic crustal evolution and its dynamic mechanism in the STOB.Based on the comprehensive analysis of zircon U-Pb age,Hf isotope,zircon trace ele ments and whole rock geochemistry,it is inferred that the closure time of the South Tianshan ocean is late Carboniferous.During the late Carboniferous to early Permian(Ca.300?290 Ma),the crust of the STOB and the southern margin of the Yili-CTS block gradually thickened under the compressional environment formed by the syn-collision,resulting in a significant increase in the proportion of ancient crustal materials in the mantle-derived magma,which shows that the?_Hf(t)value of zircons decreased with age,and the corresponding two-stage model age value increased;Ca.290?265 Ma,the STOB and the southern margin of the Yili-CTS block were in a post-collision extensional environment,accompanied by crustal thinning,resulting in a gradual increase in the proportion of new biomass in the magmatite,which shows that the?_Hf(t)value of zircons increasing with the age,and the corresponding two-stage model age decreasing.(6)It is generally believed that the CAOB experienced significant crustal accretion in the Paleozoic,but the study of the STOB shows that it experienced a stage of continental crustal reworking and growth.The crust evolution of the STOB in the early Paleozoic is similar to that of other tectonic units of the CAOB(such as Yili block,CTS and North Tianshan orogenic belt),which are mainly composed of the reconstruction of the old crust and the addition of juvenile materials.However,in the late Paleozoic,the crust evolution of the STOB is mainly based on the reworking of the ancient basement rocks,and relatively limited juvenile components are added to the rocks,compared with other tectonic units of the CAOB,the STOB has no obvious crustal growth during the late Paleozoic.This dissertation contains 49 figures,10 tables and 317 references.