Stage division and genesis discussion of basaltic volcanism during the coneforming stage of Tianchi volcano in Changbaishan region

During the cone-forming stage of Tianchi volcano, activity of basaltic volcanism was frequent. In this paper, based on field geological survey and analysis of chronology and geochemistry, the authors determined active stages of the basaltic volcanism, and investigated magmatic source and evolution. The results show that the activity of the basaltic volcanism forming scoria cones or mini valley basalt can be divided into 2 stages:Laofangzixiaoshan period (about 0.87-0.54 Ma) belonging to alkaline basaltic volcanism and Laohudong period (about 0.34-0.1 Ma) belonging to alkaline and tholeiitic volcanism. Alkaline basalt is rich in Al, Ti, K and P, and poor in Mg, and tholeiite is rich in Mg, Fe and Ca, and poor in Na. The two types of basalt have similar characteristics in REEs, trace elements and Eu anomalies, exhibiting obvious right-declined REE patterns, slightly positive Eu anomalies, enrichment of Ba, K, Pb, P and Ti, and depletion in Th, U and Sr. Nevertheless, in contrast to alkaline basalt, content of REEs and trace elements, and fractional degree of HREE and LREE are lower in tholeiite. The basalt formed during the coneforming stage of Tianchi volcano was derived from evolutional magma, and was identical with the transitional position of uppermantle and lower-crust, which had characteristics of common fountain, and had the same or similar evolution process with significant crystallization differentiation and faint crustal contamination. Compositions of the evolutional magma were controlled by multi-stages crystallization differentiation and recirculation during early crystallization.