Geochemistry And Chronology Of The Early Jurassic Granite In Gulian, Northern Great Xing'an Range

Northern Great Xing,an Range located in the Ergun block,closure of the Paleo-Asian Ocean,this area was influenced by closure of the Mongolia-Okhotsk Ocean and subduction of the Pacific plate from the Paleozoic to now.In this area,many tectonic event and magmation happened during the Mesozoic and huge area of volcanic formed at this time.In recent years,some people do a lot of researches about the phanerozoic eon granite in this area and some of them have different standpoints about the formation and the tectonic setting of the early Mesozoic granite.In this thesis,the early Jurassic granite samples in Gulian(Mohe county,northern Great Xing,an Range)were collected for comprehensive analysis and research.In earlier work,people have found monzogranites from early Jurassic in this area,but in my work,granodiorites and syenogranite were also found through the study of petrology.I choose two samples to do the experiment of U-Pb dating by LA-ICP-MS and the date I got shows that the age of the monzogranites is 201.8±1.3Ma and the age of the syenogranite is 200.4±1.5Ma.So,I can trust the granite formed in the early Jurassic.Tests on the main and trace elements showed these samples have higher contents of Si(65.77%~74.74%),K(3.50%~4.59%),alkali(Alk=7.19%~8.30%,AR=2.36~3.76)and Mg(0.07%~2.04%),in addition,the content of Ti is extremely low(0.17%~0.65%).These rocks can be recognized as the series of high K calc-alkaline rocks and the metaluminous rocks through geochemical characteristics of the main elements.The dates of these granite also show strong enrichment in LREEs and LILEs,depletions in HREEs and HFSEs,the negative anomalies of Ba,Nb,Sr,P,Ti are quite remarkable and the negative Eu anomaly can also be seen.According to geochemical characteristics,to my point of view,the early Jurassic granite samples in Gulian is I-type granite which formed in a subduction tectonic setting,and the primary magma for the monzonitic granite could be mainly derived from partical melting of the metagreywacke from older crust.From what has been discussed above,I think the Mongolia-Okhotsk Ocean had a southward subduction from late Triassic to Jurassic,the subduction is the main reason of the magmation during early Jurassic in Ergun block.