Zircon U-Pb Geochronology of Volcanic Rocks from Gaojiayu Formation, Liaohe Group, Liaoning Province and Its Geological Significance

There are a large number of meta-sedimentary-volcanic strata in the Paleoproterozoic Liaoji belt between Longgang block and Langlin block,which is called Liaohe Group. There has been a debate on the sedimentary time of the Liaohe Group and the relationship among its inner groups. In order to solve these problems,the zircon U-Pb LA-ICP-MS geochronology of the volcanic rocks from the Gaojiayu Formation in the eastern Liaoning Province is studied. The dating results show that the magmatic activities of the Gaojiayu Formation can be divided into four stages:2 179-2 145 Ma,2 079 Ma,1 985 Ma and 1 928 Ma. Combining with the published chronological data,the volcanic activities in the Liaohe Group are mainly concentrated in three stages:the early magmatic activity in the first stage (2 210-2 140 Ma) is mainly acid volcanic rock,showing bimodal volcanic rock assemblage in the late stage; the second stage (2 100-2 050 Ma) magmatism is dominated by basic magmatism; the third stage (2 000-1 960 Ma) magmatism also shows bimodal volcanic rock assemblage. The sedimentary time of the Liaohe Group is limited to 2 229-1 928 Ma by the age of the volcanic rocks,and the deposition lasts at least 300 Ma. The chronological results show the ages of the of volcanic rocks in the Langzishan Formation,Lieryu Formation,Gaojiayu Formation and Dashiqiao Formation of Liaohe Group are alternant,and there is no regular upper-lower orderly relationship. In combination with the fact that the formations of Liaohe Group in the field mostly contact in the form of ductile shear zone,it is considered that the rock groups of Liaohe Group are not orderly sedimentary from the bottom to the top,but "disordered as a whole and orderly as a part".