Cenozoic Volcanic Activity And Its Tectonic Significance In The Tumen River Field, Changbai Mountains Area, Jinlin Province, Northeast China

The volcanic area of the Changbai Mountains is located in the east of Jilin province, Northeast China. It is one of very important areas of volcanic rocks outcropped in eastern China that has the biggest area and the most dangerous of possibility of potential eruption. The Cenozoic basaltic volcanic rocks of the Tumen river field are distributed on the east of the Tianchi crater of the Changbai Mountains. All of the basaltic volcanics in the Tumen river field (TRF) are basaltic rocks, which erupted in a flooding way and outcropped along the Tumen river or its upriver stream.The volcanics in TRF are obviously different from shield-forming basaltic volcanics of the Tianchi volcano in characters of volcanic geology, petrology and volcanic chronology. The volcanics in TRF are mainly tholeiitic rock series, whereas the shield-forming basaltic basalts are mainly trachybasalt in the Tianchi volcano. the time spans of the volcanic activity of the TRF and the shield-forming basaltic basalts are 5.5~0.19Ma and 2.0~1.2Ma, respectively. The volcanics outcropped in Junjianshan area are a part of volcanic rocks in the TRF. It is confirmed that the basaltic rocks of Junjianshan can not be treated as the shield-forming basaltic volcanics of the Tianchi volcano.The most of the volcanics in TRF is tholeiite. Besides,there is also a few other types of basaltic rocks such as basaltic trachyandesite,alkaline basalt,trachybasalt and basaltic andesite. Studying of petrology show that the basaltic rocks in TRF belong to the rocks of sodic tholeiite series, and the magmas experienced three stages of evolution. The first process is that crystallization differentiation took place in the pocket of magmas in upper mantle chambers, when the tholeiitic parental magmas generated a series of evolutionary magmas which are mainly tholiite with a few other kinds of basaltic rocks. The second process is that assimilation of different degrees of the material occurred in the crust with digestion of different kinds of minor elements in varied magmas during the magma ascending and going through the crust. Some evidence indicates that the main site of assimilation of magmas is in the upper crust. The third process is characterized by crystallization of some degree when the magmas arising and erupted on the surface. After the eruption some rocks located in middle or lower stratohorizon of thick zones of rocks flowage generated big phenocryst of plagioclase. In a brief, the process of magmas...