Analysis Of Geochronology And Tectonic Setting Of The Late Paleozoic A-type Granites In The Zhangguangcai Range Region

The Zhangguangcai Range region is situated in the central area of Heilongjiangprovince. To the east, it is separated from the Jiamusi Massif by Mudanjiang Fault; to thewest, it is bounded by the Xunke-Tieli Fault and adjacent to Songliao Basin; and to thesouth and north, it elongates to Shulan City in Jilin province and Russia over theHeilongjiang River, respectively. It is a collision orogenic belt between Lesser HingganMountains-Songnen and Jiamusi massif, and is regarded as a portion of the activecontinental margin during Hercynian movement.The Zhangguangcai Range region has been known as "giant granite province"because of the widespread distribution of the Phanerozoic granitoids. Based on theregional geological survey in the1970s, the majority of the granitoids plutons intrudedinto the Late Paleozoic strata and was overlain by the Jurassic strata, which indicated thatthe granitoids in the region formed in the Hercynian. However, with the rapiddevelopment of isotope dating techniques, the isotopic geochronology data, obtained byRb-Sr, K-Ar dating methods, etc, indicate that the majority of these granitoids formed inthe Indosinian period (250~222Ma), not the Hercynian as previously believed. In recentyears, many Indosinian granitoids, as well as some both Early and Late Paleozoicgranitoids, have been discovered in the Zhangguangcai Range region by many geologistswith LA-ICP-MS/SHRIMP dating methods.This paper, financially supported by the program named “Comparative study onmetallogenic model and series of endogenous minerals in the domestic and foreignSikhote metallogenic belt”, has discovered the Hercynian A-type granites in the study area, although most of the Hercynian granitoids belong to I-type rocks on the basis of theprevious research results.These granitoids, mainly consist of biotite monzogranite and syenogranite, haveSiO2=66.03%~77.20%, Na2O+K2O=7.75%~9.94%, A/CNK=0.91~1.15and A/NK=0.98~1.51. To sum up, they are metaluminous to weak peraluminous and alkaline rocks.All these granitoids are characterized by pronounced negative Eu anomalies and distincdepletion in Ba, Sr, P and Ti. These metaluminous-weak peraluminous alkaline A-typegranites in this paper formed at around260Ma and300Ma, i.e., the end of the LateCarboniferous to the Late Permian.On the basis of granite tectonic discrimination diagrams modified after Maniar,Pearce and Barbarin, combining with tectonic background in region, we conclude thatthese A-type granites were emplaced in the back-arc rifting regions related to thesubduction of the Paleo-Asian Oceanic plate beneath the Jiamusi Massif.