A Study On Chronology And Tectonic Environment Of The Jurassic Granites In The South Of Zhangguangcai Ridge

The Zhangguangcai Range is situated in the east of Xing Meng orogen. To the east, it is separated from the Jiamusi Massif; to the west, it is Songliao Basin; and to the south and north, it elongates to Shulan City in Jilin province and Russia over the Heilongjiang River, respectively. It is a collision orogenic belt between Lesser Hinggan Mountains-Songnen and Jiamusi massif.The Zhangguangcai tectonic belt is a collision orogenic belt between Songnen and Jiamusi massif. Because of intrusion Carboniferous- Permian strata and sedimentary contact with Mesozoic strata, they were mainly considered as Hercynian granites in the regional geological survey.However, with the rapid development of isotope dating techniques, the isotopic geochronology data indicate that the majority of these granitoids formed in the Indosinian period. The Zhangguangcai Range has been widespread distribution of the Phanerozoic granitoids.LA-ICP-MS zircon U--Pb ages isotopic dating and analysis of geochronology of granites in the south of Zhangguangcai Rang were determined in this paper for studying petrological characteristics, chronology, geochemical characteristics and tectonic environment.Shangyingbei pluton, Maoshan pluton, Laoyeling pluton, and Lafashan pluton mainly consist of Alkali feldspar granite group,have Si O2 = 67.38% ~ 78.51%, Al2O3 = 11.57% ~ 14.24%, K2 O = 3.31% ~ 6.24%, Na2 O = 2.84% ~ 4.00%, Na2 O + K2 O = 7.02% ~ 9.40%, A / CNK = 0.98- 1.09 and A / NK = 1.06- 1.55. To sum up, they are metaluminous to weak peraluminous and alkaline rocks. All these granitoids are characterized by pronounced negative Eu anomalies and distinc depletion in Ba, Sr, P and Ti.Zircon U-Pb isotopic dating is 178.9±2.7Ma( n=17, MSWD=0.27), 185.2±2.4Ma(n=16,MSWD=0.026) and 190.8±2.3Ma(n=19,MSWD=0.078),which represents the forming age of Shangyingbei pluton, Laoyeling pluton, and Lafashan pluton, respectively. The forming age of Maoshan pluton are 183.7±2.4Ma(n=20,MSWD=0.33)and182.3±3.0Ma(n=14,MSWD=0.54). These granites were early Jurassic granites.On the basis of granite petrological characteristics, geochemical characteristics, these granites are A-type granites. Combining with tectonic background in region, we conclude that these granites are A2-type granites, emplaced in the post-collision orogeny environment.