Geochronology And Geochemistry Of The Mesozoic Igneous Rocks In The Xing'an Massif,NE China:Constraints On The Evolution Of The Mongol-Okhotsk Tectonic Regime

This thesis studies geochronology and geochemistry of the Mesozoic igneous rocks in the Xing'an Massif of NE China,with the aim of revealing the evolution and the spatial–temporal extent of the Mongol–Okhotsk tectonic regime.LA-ICP-MS and SIMS zircon U-Pb dating results indicate that the Mesozoic magmatic activity in the Xing'an Massif can be subdivided into seven stages at Middle Triassic(242–237 Ma),Late Triassic(228–223 Ma),Early Jurassic(177–174 Ma),Middle Jurassic(168–164 Ma),Late Jurassic(158–150 Ma),the early Early Cretaceous(~141 Ma),and the late Early Cretaceous(135–132 Ma).Based on the whole-rock major and trace element data,zircon Hf isotope data,together with their rock association,spatial–temporal extent,and regional tectonic events,this thesis reveal the petrogenesis and tectonic settings of different stages of the Mesozoic igneous rocks in the Xing'an Massif,the Mesozoic evolution of the Mongol-Okhotsk tectonic regime and its influencing spatial–temporal extent.1.Middle Triassic–Early Jurassic—southward subduction of the Mongol–Okhotsk oceanic plateThe Middle Triassic igneous rocks in the Xing'an Massif comprise a suite of basaltic andesites,granodiorites,monzogranites and syenogranites.The basaltic andesites are enriched in light rare-earth elements(LREEs)and large ion lithophile elements(LILEs),and depleted in heavy rare-earth elements(HREEs)and high field-strength elements(HFSEs).The zircon ?Hf(t)values(+8.5 to+12.7)suggest that the primary magma was generated by the partial melting of a relatively depleted mantle wedge that had been metasomatized by subduction-related fluids.The granodiorites have an affinity to adakites.The zircon ?Hf(t)values(+10.7 to +12.3)suggest that the primary magma was generated by the partial melting of a juvenile thickened lower crust.The monzogranites are I-type granites.The zircon ?Hf(t)values(+10.3 to +11.8)suggest that the primary magma was generated by the partial melting of a juvenile lower crust.Taken together,the Middle Triassic igneous rocks were formed in an active continental margin setting related to the southward subduction of the Mongol–Okhotsk oceanic plate beneath the Erguna–Xing'an Massif.The Late Triassic igneous rocks in the Xing'an Massif comprise a suite of basaltic andesites,high-Mg adakitic andesites,dacites,granodiorites and monzogranites.The primary magma of the high-Mg adakitic andesites in the Handaqi area was derived from the partial melting of a young subducted oceanic crust,followed by interaction with melts derived from mantle peridotite.The Late Triassic basaltic andesites,andesites,and dacites in the Zhalantun–Moguqi area are similar to those of igneous rocks formed in the subduction zones.Their zircon ?Hf(t)values(+8.4 to +15.4)indicate that their primary magma was derived from the partial melting of a depleted mantle wedge that had been metasomatized by subduction-related fluids.The primary magma of the Late Triassic I-type granitoids was generated by the partial melting of a juvenile lower crust.The rock associations and the geochemical features indicate that they were formed in an active continental margin setting related to the southward subduction of the Mongol–Okhotsk oceanic plate beneath the Erguna–Xing'an Massif.The Early Jurassic intrusive rocks in the Xing'an Massif comprise a suite of I-type granodiorites,monzogranites and syenogranites.Their zircon ?Hf(t)values(+7.4 to +13.0)indicate that their primary magma was derived from the partial melting of a juvenile lower crust.Combined with the rock associations and distributions,it is suggested that the Early Jurassic intrusive rocks were formed in an active continental margin setting related to the southward subduction of the Mongol–Okhotsk oceanic plate beneath the Erguna–Xing'an Massif.2.Middle Jurassic—the closure of the Mongol–Okhotsk OceanThe Middle Jurassic intrusive rocks in the Xing'an Massif comprise a suite of granodiorites,muscovite-bearing monzogranites,and garnet-bearing monzogranites.The Middle Jurassic granitoids have an affinity to adakitic rocks,as evidenced by their high Sr and low Y and HREEs concentrations.Their zircon ?Hf(t)values(+3.8 to +11.7)indicate that their primary magma was derived from the partial melting of a juvenile thickened lower crust.The Middle Jurassic granitoids were formed in a compressing collisional setting,which was related to the closure of the Mongol–Okhotsk Ocean.3.Late Jurassic–early Early Cretaceous—a post-orogenic extension after the closure of the Mongol–Okhotsk OceanThe Late Jurassic intrusive rocks in the Xing'an Massif exhibit chemical affinities to A-type granites,implying an extensional environment.Their positive ?Hf(t)values(+4.7 to +9.1)indicate that the primary magma was derived from partial melting of a juvenile lower crust.Additionally,The Late Jurassic alkaline intermediate–mafic volcanic rocks of the Tamulangou Formation are within-plate potassic rocks generated by rifting,further indicating an extensional environment in the Xing'an Massif during the Late Jurassic.The early Early Cretaceous granodiorites have an affinity to adakites.They have high contents of Cr,Co,Ni and Mg# values with the ?Hf(t)values of +4.7 to +9.1.It is suggested that they were generated by partial melting of a delaminated lower crust and subsequently interacted with mantle material.The distributions of the Late Jurassic–early Early Cretaceous magmatism of NE China are limited to the west of the Songliao Basin,suggesting that they were formed in an extensional environment after the closure of the Mongol–Okhotsk Ocean.4.Late Early Cretaceous—an extensional environment related to the two tectonic regimesThe late Early Cretaceous intrusive rocks in the Xing'an Massif comprise a suite of syenogranites,granite porphyries,monzogranites,granodiorites,and they exhibit chemical affinities to A-type granites.Their positive ?Hf(t)values(+4.6 to +8.9)indicate that the primary magma was derived from partial melting of a juvenile lower crust.Combined with the coeval bimodal volcanic rocks,it is suggested that they were formed in an extensional environment related to either(i)delamination of a previously thickened region of the crust,associated with the Mongol–Okhotsk tectonic regime;(ii)back-arc extension related to the subduction of the Paleo-Pacific Plate;or(iii)the combined influence of these two tectonic regimes.5.The variations of the Mesozoic igneous rock associations and the spatial– temporal extent of the influence of the Mongol–Okhotsk tectonic regimeBased on the geochemical characteristics of the Late Paleozoic–Mesozoic igneous rocks in the Erguna and Xing'an massifs,lithostratigraphic relationship,and paleogeography reconstruction,we conclude that the temporal influence of the Mongol–Okhotsk tectonic regime in NE China spanned the period from the late Permian to the Early Cretaceous.Based on the spatial–temporal distribution of Mesozoic igneous rocks in NE China,the Mongol–Okhotsk tectonic regime extended spatially at least as far as the eastern margin of the Xing'an Massif(the west of the Songliao Basin)and the northern margin of the North China Craton.