Age, Geochemistry And Geological Significance Of Late Triassic-early Jurassic Intrusive Rocks In The Southeast Region Of Tieli City, Heilongjiang Province

The southeast region of Tieli City, Heilongjiang Province is located at the metallogenic belt of Zhangguangcai-Lesser Xing’an ranges, which is the intersection of Siberian plate,North China plate, and Pacific plate. Being affected by the violent plate activity and magmatism since Mesozoic era, large scaled petrometallogeny occurred in this area. The formation time of the granitoids in the southeast region of Tieli City is documented as Variscan; however, it was actually formed in the Mesozoic era. Therefore, a systematic study on the Mesozoic granitoids in this region is highly significant.In this report, the petrogenesis and age of the Mesozoic intrusive rock in this area was confirmed through LA-IPC-MS zircon U-Pb dating, and geochemical analysis. It was found that the isotopic ages of the Late Triassic-Early Jurassic intrusive rocks, including medium grained-coarse grained syenogranite, medium grained-fine grained granodiorite,and medium grained monzogranite, are 192-230 Ma, 183 Ma and 197 Ma, respectively.The geochemical characteristics of these granites show that the contents of SiO2(50.84%~ 77.3%), K2O(2.3 6% ~ 3.49%),MgO(0.28 % ~ 3.13%), MnO(0.09 % ~ 0.27%),CaO(0.60 % ~ 7.12 %) and P2O5(0.04 % ~ 1.10 %) are relatively low, and A/CNK = 0.69~ 1.25,A/NK = 1.10 ~2.17, u = 0.1~1.09. It can be seen from the Chondrite standardized REE diagrams that the light rare earth elements are more abundant than the heavy rare earth elements in the Mesozoic granites, which shows a obvious negative Eu anomalies. It was also found that LILEs such as Rb and K are enriched, and HFSEs such as Nb, Ta, P and Ti are depleted from the primitive mantle standardized trace element spider diagram.The four Mesozoic intrusive plutons in the region are all aluminum-weakly peraluminoushigh potassium calc-alkaline I-type granite, of which their magmas are originated from partial melting of lower crustal material. Fractional crystallization occurred during the granitic magma evolution. The Mesozoic granite in the area lies in the superimposition region of the post-collision granite defined by Pearce.