Tectonic Implications Of Angular Unconformity Of Upper Trassic And Permain Granites In The Eastern Jiamusi Massif

In recent years,the tectonic evolution of the Jiamusi Massif has been highly regarded,especially since the late Paleozoic. The discovery of Permian magmatic arc has provided an important evidence for us to cognize the tectonic evolution of the Jiamusi Massif since the late Paleozoic. During the research on field geological investigation, we have found that the angular unconformity of upper Trassic and Permain granites.Later,we put emphasis study on it’s petrology, geochemistry and isotope chronology.The results of LA-ICP-MS U-Pb dating show that the Fangshan granite was formed in Early Permain, zircon age range from 277 Ma to 278 Ma. The geochemical characteristics show that ω(SiO2),ω(Na2O),ω(K2O) values of the granitic rocks are 70.72%-74.32%,3.23%-4.06%,3.40%-4.56% respectively.These rocks also have ω(K2O)/ω(Na2O) values of 1.05-1.33, and A/CNK values of 1.09-1.25,and rich in lithophile elements(Rb and K), depleted in high field strength elements(Nb,Ta,P,Zr and Ti). These geochemical features suggest that the Fangshan granitic rocks are slightly peraluminous and are of highly fractionated I-type granite. The geochemical characteristics of the Fangshan granitic rocks are same to the granitic rocks from the eastern part of the Jiamusi Massif(Meizuo,Qingshan,Chushan,Chaihe,Shichang and Jingshan),these geochemical characteristics represent a feature of volcanic arc granite(VAG).Indicating that the eastern margin of the Jiamusi massif was active continental margin during the early Permain.Nanshuangyashan Group is a series of interactive marine and terrestrial deposit, U-Pb ages of detrital zircons from Nanshuangyashan Group are mainly focus on two peaks:~273Ma and ~500Ma, secondly are Precambian age, as well as age of youngest detrital zircon is 217 ± 7 Ma. Two peak ages of ~273Ma and ~500Ma are ages of two important magmatic events occurred in Jiamusi massif. The former is formation age of Permian magmatic arc, and the latter is Early Paleozoic advanced metamorphism, and show that material source of Nanshuangyashan Group is mainly from Jiamusi massif.Combine with the fossil evidence and the youngest detrital zircon age(217± 7 Ma) from Nanshuangyashan Group,we deduce than the sedimentary age of Nanshuangyashan Group is later period of the Late Triassic.The discovery of the angular unconformity of upper Trassic and Permain granites has provided new evidences for us to cognize the tectonic evolution of the Jiamusi Massif since the late Paleozoic. The eastern margin of the Jiamusi massif was active continental margin during the early Permain.then,from early Triassic to middle Triassic,it is the time of uplifting and denudation,which is marked by loss of upper Triassic and middle Triassic.During late Triassic, interactive marine and terrestrial sedimentary are developed, the eastern margin of the Jiamusi massif convert into passive continental margin.