Geochronology And Geochemical Characteristics Of The Jurassic Granite From Xinlitun Area In Northern Great Xing’an Range And Their Geological Implications

The Great Xing’an Range are located in the eastern section of the Central Asian Orogenic Belt,the tectonic magmatic activity is strong,and the Phanerozoic granites are very developed.Studying the diagenetic age and tectonic setting of these granites is of great significance for exploring the tectonic-magmatic evolution history of northeast China.This paper applies petrography,chronology and rock geochemistry to discuss the diagenetic age,petrogenesis,magma source area and tectonic setting of the Jurassic granite in Xinlitun area in the northern part of the Great Xing’an Range,and mainly obtains the following understanding:Zircon U-Pb isotope dating results show that the diagenetic age of the Xinlitun medium-fine grained monzogranite and the fine grained granodiorite is 187.2±2.3 Ma and 192.4±1.6 Ma,the diagenetic age is Early Jurassic;The diagenetic age of medium-fine grained granodiorite is149.9±1.7 Ma,and the diagenetic age is Late Jurassic.The results of rock geochemical research show that the Early Jurassic medium-fine grained monzonitic granite and fine grained granodiorite are characterized by rich silicon,potassium,high alkali,poor magnesium,low phosphorus,titanium,relatively be enriched in Rb,Th,Ba,K,Sr elements,relatively be depleted in Ta,Nb,P,Ti elements,no Eu and Ce anomalies,P₂O₅ and Al₂O₃ content decreases with the increase of SiO₂,belonging to moderately fractionated I-type granite.The magma originates from a partially melted source area with residual amphibole phases in the lower crust.Late Jurassic medium-fine grained granodiorite is characterized be enriched in silicon,aluminum,alkali,be depleted in Mg,Y,Yb,and high Sr.A/CNK ranges from 1.01 to 1.30.It is a high potassium calc alkaline calc alkaline series rock,relatively be enriched in Rb,K,Ba,La,Sr,Ce elements,relatively be depleted in Ta,P,Ti,Nb,Yb,Y,Lu elements,weak positive europium anomaly,belonging to moderately fractionated I-type granite,and has the geochemical characteristics of"C"type adakite.The magma originates from the melting source area of the lower crust.Through the discussion of tectonic background,this paper believes that the Early Jurassic medium-fine grained monzonitic granite and fine grained granodiorite in Xinlitun were formed in the volcanic island-arc granite of the active continental margin,which is closely related to the southern subduction of Mongol-Okhotsk Ocean.The Late Jurassic medium-fine grained granodiorite was formed in a post collisional uplift tectonic environment,closely related to the closure of the Mongol-Okhotsk Ocean suture zone.