| This thesis studies geochronology and geochemistry of the Mesozoic intrusive rocks in the northeastern margin of the North China Craton,with the aim of revealing the evolution and the spatial–temporal extent of the multiple tectonic regimes.Main achievements are as follows:1.Mesozoic magmatism in the northeastern margin of the NCC can be subdivided into four stagesBased on the zircon LA-ICP-MS U-Pb dating results,the Mesozoic magmatic activity in the northeastern margin of the NCC can be subdivided into four stages,including Late Triassic(232205 Ma),Early Jurassic(189173 Ma),Late Jurassic(161156 Ma)and Early Cretaceous(125120 Ma).2.Late Triassic magmatic events in the northeastern margin of the NCC formed in extensional environments respectively related to the closure of the Paleo-Asian Ocean and the subduction and collision between the NCC and the Yangtze CratonThe Late Triassic igneous rocks in the northeastern margin of the NCC are mainly composed of adakitic granitoids and bimodal igneous rocks.The Late Triassic(220 Ma)granitoids in southern Jilin Province are geochemically similar to adakitic rocks,as indicated by their low HREE and Y contents,and relatively high Sr concentrations.Their zircon ?Hf(t)values(–17.9–7.2),Hf two-stage model ages(TDM2;23871708 Ma),apatite ?Nd(t)values(–14.4–9.2)and Nd TDM2(21681739 Ma),suggested that they were generated by partial melting of a thickened ancient lower continental crust.The Late Triassic magmatism in the southern Jilin Province formed in an extensional environment related to the collision between the NCC and YC,accompanied by the rapid exhumation of the Su–Lu ultrahigh-pressure metamorphic belt.The spatial extend of influence of the subduction and collision between the NCC and the YC is the Jiaodong-LiaonanLiaodong-Jinan area.The Late Triassic granitoids in the Yanbian area also have an affinity to adakitic rocks,which are generated by partial melting of a thickened ancient lower continental crust.Their zircon ?Hf(t)values(–15.6–7.1),Hf TDM2(19321718 Ma),suggested that they were derived by partial melting of Paleoproterozoic lower continental crust.The Yanbian Late Triassic granitoids and the Triassic alkaline rocks in the northern margin of the NCC,together with the mafic-ultramafic rocks in the southern margin of the Xing'an– Mongolian Orogenic Belt,constituting a bimodal rock association distributed along the Solonker–Xra Moron-Changchun-Yanji suture belt,indicating that they formed in an extensional environment related to the closure of the Paleo-Asian Ocean.Thus,the Paleo-Asian Ocean tectonic regime influenced both sides along the Solonker–Xra Moron–Changchun–Yanji suture belt and the north margin of the NCC.3.Early Jurassic(189–173 Ma)calc-alkaline magmatic events in the north margin of the NCC mark the onset of subduction of the Paleo-Pacific Plate beneath the EurasiaThe Early Jurassic intrusive rocks in the northeastern margin of the NCC are mainly composed of a suite of calc-alkaline intrusive rocks.The granitoids in the southern Jilin Province have high SiO2,low TiO2 contents,and low Mg# values.Combined with their zircon ?Hf(t)values(–16.7–4.2),Hf TDM2(22821491 Ma),apatite ?Nd(t)values(–9.3–6.7)and Nd TDM2(17041502 Ma),these granitoids were generated from partial melting of Paleoproterozoic–Mesoproterozoic accreted lower continental crust.Compared with the granitoids in the southern Jilin Province,the Early Jurassic intrusive rocks in the Yanbian area,located in the Xing'an–Mongolian Orogenic Belt,have relatively low SiO2 and high TiO2 contents,as well as high Mg# and ?Hf(t)values(+8.0+9.6),indicating that the primary magma was derived from partial melting of juvenile accreted lower crust.The polar variation in K2 O contents of Early Jurassic igneous rocks from continental margin to intracontinent,together with the Early Jurassic accretionary complex,reveals that the Early Jurassic magmatic events formed in an active continental margin setting and the onset of subduction of the Paleo-Pacific plate beneath the Eurasia happened in the Early Jurassic.4.Late Jurassic magmatic events in the northeastern margin of the NCC formed in a compressional environment related to oblique subduction of the Paleo-Pacific PlateThe Late Jurassic intrusive rocks in the northeastern margin of the NCC are composed mainly of a suite of adakitic rocks.Their zircon ?Hf(t)values(–19.7–17.2),Hf TDM2(24592299 Ma),apatite ?Nd(t)values(–17.6–13.9)and Nd TDM2(23882088 Ma)indicate that they were generated by partial melting of a thickened ancient lower continental crust.The Late Jurassic magmatic events in the study area formed in a compressional environment related to oblique subduction of the Paleo-Pacific Plate and a strike-slip tectonic event occurred in the Northeast Asian continental margin during the Late Jurassic to the early Early Cretaceous.5.Early Cretaceous magmatic events in the northeastern margin of the NCC formed in a backarc extensional environment related to westward subduction of the Paleo-Pacific PlateThe Early Cretaceous intrusive rocks in the northeastern margin of the NCC are composed mainly of a suite of A-type granites and bimodal igneous rocks,which geochemically belong to the high-K calc-alkaline and metaluminous–weakly peraluminous series.The Early Cretaceous alkali feldspar granites are similar to A-type granites,indicating an extensional environment.Their zircon ?Hf(t)values(–16.7–4.2)and apatite ?Nd(t)values(–9.3–6.7)suggest that they were generated by partial melting of Paleoproterozoic–Mesoproterozoic accreted lower continental crust.The coeval quartz diorites were formed via the mixing of mantle-and crust-derived melts.The bimodal magmatic events and coeval A-type granitoids indicate that the Early Cretaceous intrusive rocks in the northeastern margin of the NCC formed in a backarc extensional environment related to westward subduction of the Paleo-Pacific Plate.6.The crustal accretion of the northeastern segment of the NCC took place mainly in the Neoarchean and Paleoproterozoic,whereas the crustal accretion of the Xing'an–Mongolian Orogenic Belt occurred mainly in the Neoproterozoic.The zircon Hf TDM2(26101306 Ma)and apatite Nd TDM2(23891456 Ma)of the Mesozoic granitoids in the northeastern margin of the NCC indicate that the crustal accretion of craton area took place mainly in the Neoarchean and Paleoproterozoic and,to a lesser extent,Mesoproterozoic.The zircon Hf TDM2(718618 Ma)of the Mesozoic granitoids in the Xing'an–Mongolian Orogenic Belt indicate that the crustal accretion of the orogenic belt occurred mainly in the Neoproterozoic.The Mesozoic granitoids in the craton area originated mainly from the reworking of a Neoarchean–Paleoproterozoic accreted lower crust,whereas the Mesozoic granitoids in the orogenic belt resulted from the reworking of a Neoproterozoic accreted lower crust. |
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