| The northern margin of Tarim Craton and the South Tianshan(NMTC-STS)region occupies the transitional zone between Tarim Craton and Central Asian Orogenic Belt(CAOB),and is therefore of significant importance for understanding the prolonged geological history of them.However,there still exist controversies surrounding the subduction polarity and time of closure of the South Tianshan Ocean as well as the architecture and growth of continental crust in the Tarim and SW CAOB.This thesis focuses on several Paleozoic igneous intrusions from the NMTC-STS region.In addition to detailed field and petrographic observations,we report results of LA-ICP-MS zircon U-Pb ages,MC-ICP-MS zircon Hf isotopic compositions and whole-rock major and trace elements in this thesis,aiming not only to reveal the petrogenesis of these rocks,but also delineate the Paleozoic tectonic evolution and the architecture and growth of continental crust of the SW CAOB and northern Tarim.In this thesis,the following findings are obtained.1)The spatio-temporal evolution of Paleozoic magmatism in the NMTCSTS is summarized.Based on newly acquired and previously published zircon U-Pb ages,the Paleozoic igneous activity can be temporally subdivided into three stages,that is,Late Ordovician,Middle Silurian to Middle Devonian and Late Carboniferous to Middle Permian.The Late Ordovician magmatism is represented by few very small intrusions,whereas the later two magmatic activities resulted in some plutonic rocks,most of which are of granitoid,occupying a large area of the region.2)We discuss the petrogenesis and tectonic implications of intrusive rocks from the NMTC-STS.The studied Middle Paleozoic rocks include gabbroic,dioritic and granitic intrusions.The gabbroic intrusion was emplaced around the boundary between Late Silurian and Early Devonian(424±4 Ma and 416±4 Ma).Gabbroic samples show high MgO contents and Mg# values(49 to 53),with high positive ?Hf(t)values(+4.0~+11.4),and are characterized by enrichment in large ion lithophile elements(LILEs)and depletion in high field-strength elements(HFSEs),implying that the parental magma was derived from a depleted mantle source previously metasomatized by slab-derived fluids.The dioritic intrusion was emplaced in the Early Devonian(415±2 Ma)and is geochemically and isotopically(?Hf(t)=+3.0~+9.4)comparable to the gabbroic rocks,suggesting that the parental magma was a result of fractional crystallization of the depleted mantle-derived basaltic magma.The granitic intrusions were formed during the Early to Middle Devonian.Samples from granitic intrusions are consistently of high-K calc-alkaline and weakly peraluminous series.They were dominantly sourced from juvenile deep crustal rocks,or from mixtures of juvenile and ancient crustal materials.Despite the rock types,the studied rocks consistently exhibit geochemical features comparable to that of Andean-type continental arc rocks,implying that the northern margin of Tarim Craton was an active continental margin during Middle Paleozoic times.The Late Paleozoic igneous rocks are predominately composed of granitoids,and A-,I-and S-type granites were formed within the same magmatic event.The Ku'erchu II granitic rocks(311±4 Ma)are of A-type in genesis,indicating an extensional tectonic setting.The depth of magma sources for early-stage granites,e.g.,Ku'erchu II,Qigexing and Mangqisu plutons,emplaced from 311~302 Ma increased with time(revealed by whole-rocks geochemistry,e.g.,Sr concentration of the three plutons increased from 91.9 through 192.8 to 592.5 ppm),plausibly suggesting a crustal thickening process.On the contrary,the late-stage granites(300~290 Ma)are characterized low Sr(58.6~67.4 ppm)and high Y(15.9~19.4ppm)contents,implying that thinning of the crust occurred after the lithosphere delamination.Subsequently,resulted from the ongoing underplating of mantle-derived magmas,metasedimentary rocks at shallow crustal levels were locally partially remelted,leading to the emplacement of garnet-bearing,S-type granites(~283 Ma).3)The architecture and growth of continental crust of the SW CAOB and northern Tarim as well as the southern boundary of the SW CAOB is imaged by the Hf isotope mapping.The deep continental crust beneath the NMTC-STS consists mainly of Precambrian rocks,with a small amount of juvenile materials(e.g.,in Hulashan area).Compared with Paleozoic granitoids from the Quruqtagh area(?Hf(t)=-23.4~+2.5,TDM2 =1244~2845 Ma),ones from the Hulashan area show much “juvenile” Hf-inzircon isotopic compositions(?Hf(t)=-8.68~+13.2,TDM2=543~1854Ma),showing the significant difference between the two areas in terms of the architecture and growth of deep continental crust.Therefore,the Xinger fault likely represent the boundary between the SW CAOB and Tarim Craton.4)A new tectonic evolution model is proposed by this study.During 460~430 Ma,the southward subduction of the South Tianshan Ocean led to the collision/accretion of microcontinent blocks(e.g.Hulashan block)within the South Tianshan Ocean to the NMTC along the present Xinger Fault.Subsequently,during 430~370 Ma,a new continental arc system evolved along the northern Tarim margin(including the ancient northern Tarim Craton and newly accreted microcontinent blocks),as represented by gabbros,diorites and granites exposed in Hulashan,Quruqtagh,and other areas of the NMTC and southern margin of the South Tianshan Accretionary Complex.During the second-stage subduction South Tianshan Ocean,which ultimately resulted in collisions of Tarim Craton and SW CAOB,the north-dipping subduction is inferred,and the northern Tarim margin is thought to be a passive margin,since no any Late Devonian and Early Carboniferous continental arc-type magmatism has been recognized so far.The 320 to 270 Ma granitoids from the South Tianshan and northern Tarim Craton were plausibly emplaced in a post-collisional setting.The age distribution of Late Paleozoic felsic plutons from the South Tianshan display an irregular migration of post-collisional magmatism from the East to the West,implying a “scissors-like” collisional process. |
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