Era And Genesis Of The Mesozoic Granitic Rocks In The Southern Segment Of Zhangguangcailing,and Its Tectonic Background

In this thesis,taking Mesozoic granitic rocks in the southern segment of Zhangguangcailing as the object of the study,the author reclassified the formation age of granitic rocks in this area through the chronology research of zircon SHRIMP U-Pb of granitic rocks and the geochemistry study of major elements and micro elements of the whole rock.The author established the chronological structure of granitic rocks and discussed the formation,causes and tectonic setting of granitic rocks of different eras in the research area.Combined with previous researches,this thesis discussed the evolutionary history of Mesozoic magmatic structure in the southern segment of Zhangguangcailing.The time of final collision-assemblage of SongnenZhangguangcailing and Jiamusi blocks and the time of subduction of the Paleo-Pacific plate westward are limited.In this thesis,the SHRIMP zircon U-Pb isotope dating of the Mesozoic granitic rocks in the southern segment of Zhangguangcailing area is carried out.The zircons of granitic rocks are all self-shaped and semi-self-shaped columns.The cathode luminescence image shows that the zircon has a typical magma oscillation growth zone,and Th/U ratio>0.3,suggesting that the zircon measured are magma genesis,and the age obtained is the crystallization age of granitic rocks.The dating results show that the formation time of acid intrusive rocks in the study area is between 168Ma and 190.3Ma.According to the geochemical characteristics,petrogenesis and tectonic setting of the Mesozoic intrusive rocks in the Zhangguang cailing area,the Mesozoic magmatism in this area is divided into three stages,namely Early Jurassic,Late Early Jurassic and Middle Jurassic.The rock combination in the early Early Jurassic is medium-fine-grained diorite,medium-fine granodiorite and plaque-like medium-grain granodiorite,which belongs to high-potassium calc-alkaline I,the granite series.It formed in the tectonic setting of the active continental margin,indicating that the Palco-Pacific plate has subducted westward in the early Early Jurassic.The rock combination of the late Early Jurassic is a monzonitic granite and an alkali-long granite,belonging to the high-calcium-calcium-alkaline A2 type granite series,which is formed in the extensional tectonic setting and represents a stretch in the subduction process.The effect confirmed that the Songnen-Zhangguangcailing plate and the Jiamusi plate had not been assembled during this period,and the collision time between the Songnen-Zhangguangcailing plate and the Jiamusi plate was further limited to 170 Ma.The rock combination of the Middle Jurassic is a granodiorite and a monzonitic granite.It is a series of high-potassium-calcium-alkaline I-type granites formed on the collisional tectonic setting which is consistent with the closure time of the Mudanjiang River.The granitic magmatism in this period is related to the collision between the Songnen-Zhangguangcailing plate and the Jiamusi plate,and it is confirmed that the Songnen-Zhangguangcailing plate collides with the Jiamusi plate and the flattening has begun at 170.3 Ma.The early Jurassic granitic rocks have a symbiotic relationship in space.The geochemical characteristics of the rocks from the early Early Jurassic to the late Early Jurassic are obviously different.The content of SiO2 and K2O increased,while the content of other elements including Al2O3,Na2O,FeOT,MgO,TiO2,CaO and P2O5 gradually decreased,and the magma differentiation index(DI),ongitudinal prime number(FL),iron and magnesium index(MF),alkalinity rate(AR),total alkali content(ALK)and K/N value gradually increased.The magma crystallization differentiation is getting higher and higher,and the diagenetic temperature pressure is gradually decreasing,indicating that the magma evolves from the medium to the acidic direction.The rock source gradually transits from the mantle source to the shell source,the rock genesis transits from ?-type granite to A2 granite and the tectonic setting transits from the subduction function of the ancient Pacific to the extension function.The Middle Jurassic intrusive rocks are closely symbiotic in space,and they all belong to high-calcium-calciumalkali-aluminum supersaturated series in the chemical composition of rocks.Medium-finegrained granodiorite,porphyritic medium-fine-grained monzonite and porphyritic mediumcoarse monzonite all originate from the same source zone and are formed in the ?-type granite environment,and the temperature and pressure are gradually increased.The fine-granite alkali granite is obviously different from other types of granite in this period in terms of color,mineral composition,inclusion content and petrochemistry.The magma-derived crust-mantle mixed source region is formed in the A2 granite environment.The contact boundary between the mafic inclusions and the host fine-grain granodiorite is clear.The geochemical characteristics of the rocks are different,and the formation temperature and pressure of the inclusions are higher than the host,which indicate that the mafic inclusions and the host fine-grain granodiorite are formed in different magmatic stages.Through the chronology and geochemistry of the intrusive rocks in the southern segment of Zhangguangcailing,this thesis clarified that the Paleo-Pacific plate has subducted to the west in the early Early Jurassic period,and Songnen-Zhangguangcailing block and Jiamusi block finally collided and combined at 170Ma-160Ma.There are 85 figures,28 tables and 309 references in this dissertation.