Genesis And Tectonic Setting Of Late Triassic-early Jurassic Granites In Central Jilin Province

The central region of Jilin Province,located in the southeast of the Songnen-Zhangguangcailing block lies between the north China plate and the Siberian plate,and has experienced the evolution history of the Paleo-Asian ocean tectonic regime and the Paleo-Pacific tectonic regime.Due to its complicated geological environment and special historical tectonic background,the magmatic activity in northeast China was frequent during the late Paleozoic to Mesozoic,and granitic intrusive rocks with large area were exposed and developed.Such a large number of granitic intrusive rocks have attracted extensive attention from geologists around the world.It is of great significance to clarify the formation age and genesis of granitic rocks in this area,and to discuss the evolution of Paleo-Asian Ocean tectonic regime and Paleo-Pacific tectonic regime in Northeast China.Therefore,the petrology,petrography,zircon U-Pb chronology and petrogeochemistry of the granites in the central region of Jilin Province were studied in this paper to explore their genesis and tectonic setting.According to the petrographic study,the lithology of the granitic rock samples in this study are respectively medium-coarse alkali granite,medium-grain alkali granite and fine-graine syenogranite.The zircon U-Pb result of medium-coarse alkali granite is 202.8±1.1 Ma,belonging to late Triassic,that of medium-grain alkali granite is 187.8±0.99Ma,belonging to early Jurassic and that of fine-grained syenogranite is 181.7 ± 1.1Ma,belonging to early Jurassic.Petrogeochemistry analysis shows that the late Triassic alkali feldspar granites and early Jurassic alkali feldspar granites,both of which are high in silicon and alkali,low in aluminum,magnesium and calcium,are quasi-aluminum rocks with high-potassium and calcium alkalinity and weak over aluminum rocks,belonging to type A granites and similar to type A2.The total amount of REE is high,the fractionation of LREE and HREE in alkali feldspar granite is not obvious,so ?Eu is a strong negative anomaly.The LREE and HREE fractionation of monzonite granite is not obvious,which shows a more obvious right-leaning form,SEu has a strong negative anomaly.the trace elements are all loss of large ion lithophiliac elements,like Ba and Sr and high field strong elements,like P,Ti and Nb.The study of petrography and petrogeochemistry shows that the granite magma in the late Triassic and early Jurassic should come from the partial melting of the younger basic igneous rocks in the lower crust.According to the comprehensive study,the late Triassic granites formed in in the extensional tectonic background caused by the north China plate orogenic collision of Songnen-Zhangguangcailing block after the closure of the Paleo-Asian ocean,and it is the last episode of the influence of the Paleo-Asian ocean tectonic regime.Since the early Jurassic,this area was influenced by the Paleo-Pacific tectonic domain.Previous studies on late Triassic and early Jurassic deposits in Jilin Province show that,the copper-nickel sulfide deposits are closely related to mafic and ultramafic magnatism in late Triassic and porphyry deposits are closely related to magmatism in early Jurassic.