Geochronology,petrogenesis And Tectonic Setting Of The Mesozoic Intrusive Rocks In The South Hunchun Area

The south Hunchun area is located in the eastern part of Jilin provience,junction of China,Russia and Korea,and was considered to be part of the orogenic collage between the North China Craton in the south and the Khanka Block in the northeast.The special tectonic position makes the south Hunchun area a key to understanding the relationship between the Paleo-Pacific tectonic regime and the older Paleo-Asian oceanic regime,especially the transition between the regimes during the Mesozoic.Previous workers suggested that the Changchun–Yanji suture marks the eastern extension of the Solonker–Xar Moron suture and that it defines the terminal closure of the Palaeo-Asian Ocean in the end-Permian or even earlier.However,based on the subduction complexes in the Hulan,Seluohe,and Qinglongcun areas along the Changchun–Yanji suture,Zhou and Wilde(2013)argued that the Changchun–Yanji suture marks the southern margin of the Jilin–Heilongjiang high-pressure metamorphic belt due to the subduction of the Jiamusi–Khanka Block beneath the North China Craton.If this model stands,the Yanbian area is unrelated to the CAOB.Therefore,whether the Changchun–Yanji suture is the eastward extension of the Solonker–Xar Moron–Changchun suture or justmarks the southern margin of the Jilin–Heilongjiang high-pressuremetamorphic belt is still debated.During the last decade,the impact and process of the Paleo-Pacific oceanic plate to eastern China has arose many geologists‘ attention.However,it is still unclear when this subduction began.This thesis mainly studies on the rock association,geochronology and geochemistry of the Early Mesozoic intrusive rocks in the south Hunchun,Yanbian area,with the aim of revealing the Early Mesozoic tectonic evolutionary history and its tectonic affinities of Yanbian area.The main achievements are as follows:(1)Mesozoic magmatism in the south Hunchun area can be subdivided into thtee stages: Early–Middle Triassic(249–237 Ma),Late Triassic(224–206 Ma),and Early Jurassic(200–187 Ma).Early to Middle Triassic(249–237 Ma)intrusive rocks consist mainly of tonalite,along with several bodies of diorite,granodiorite and monzogranite,and most are located in the southern part of the Banshi town.Late Triassic(224–206 Ma)intrusive rocks include gabbro,dioritic xenoliths,and monzogranite.Exposures of these intrusive rocks are sparse,and most are found at Xiaopanling and southern pare of Jingxing town.Early Jurassic intrusive rocks(205–160 Ma)are widespread in the south Hunchun area,and they consist mainly of diorite,quartz diorite,tonalite,granodiorite,monzogranites,and syenogranites.Granodiorite most are located in the Jingxin town,while monzogranites,and syenogranites most are located in the Quanhe area.(2)The Early–Middle Triassic(249–237 Ma)adakitic granite were produced by the partial melting of subducted oceanic slabs,and the melts were contaminated by mantle peridotite during their ascent,whereas the coeval non-adakitic diorite and monzogranite were most likely derived from partial melting of crustal material.The remarkably high zircon Hf isotopic signature(?Hf(t)= +9.4 – +18.9),the enrichment in LILE,LREEs,and the depletion in HFSE suggest that these 224 Ma gabbros were derived from the partial melting of depleted mantle modified by subduction-related fluids.The 212 Ma monzogranite was most likely derived from juvenile material mixed with old crustal material as evidenced by their high SiO2,low MgO,and Cr concentrations,and variable ?Hf(t)values(–4.6 to +10.0).Except for the 197 Ma tonalites with affinity to the high silica adakites(HSAs),the overall geochemical evolution of Early Jurassic(200–187 Ma)rocks was consistent with fractional crystallization from quartz diorite,granodiorite,and monzogranite to syenogranite.(3)The primary magma for Early Mesozoic intrusive originated by partial melting of juvenile crustal material,juvenile mantle derived components(such as underplated basaltic magma,Fig.12b),ancient recycled crustal components,and the sedimentary components are partly governing the generation of the Mesozoic intrusive rocks.The syn-subduction lateral continental growth as well as the post-collisional vertical continental growth by accretion of mantle-derived material played an important role in the Phanerozoic continental growth of the CAOB.(4)The identification of collision-related high Sr/Y granitoids from the south Hunchun area reveals the Paleo-Asian oceanic system ended in the Late Permian.Late Triassic intrusive rocks formed in a post-collisional extensional environment related to the final closure of the Paleo-Asian Ocean.Early Jurassic intrusive rocks in the study area mark the beginning of the circum-Pacific tectonic regime.(5)The Changchun–Yanji suture is the eastward extension of the Solonker–Xar Moron–Changchun suture not just marks the southern margin of the Jilin–Heilongjiang high-pressure metamorphic belt.A regional comparison shows that the Yanbian area,the Tumangang belt and the Kwanmo massif of the Korean Peninsula have the same geological evolutionary history.Both of them show great affinity with the CAOB.