GEOCHEMICAL CHARACTERISTICS OF THE MESOZOIC VOLCANIC ROCKS IN ZHANHUA SAG, BOHAI BAY BASIN, EAST CHINA

Located in eastern Bohai Bay Basin， Zhanhua sag is situated on the west side of Tanlu Fault Zone， the largest fault zone in eastern China. Many works have carried out about petrologic, geochemical and chronological characteristics of the rocks in Zhanhua sag, especially the rocks located in oil reservoir. However few studies have been conducted on Mesozoic volcanic rocks developed in this sag. Here, we analyze the geochemical characteristics of Mesozoic volcanic rocks in Guxi area, in the east of Zhanhua sag via better outcrop sections, which contributes to understand geological setting of Bohai Bay Basin in Mesozoic.

The volcanic rocks’ lithologies of Zhanhua sag are mainly basaltic, tephrite, rhyolite, trachyandesites and andesite, mainly belong to alkalic series. The trace element features of these volcanic rocks are rich in LILE but lacking in HFSE. High ratio of w(Sr)/w(Y) and low content of w(Y) might suggest these samples are similar to adakite. Due to the characteristic of lacking HREE, the results for rare earth elements might suggest that volcanic rocks in Zhanhua sag came form lithospheric mantle that receives continental crust mixing. Therefore we believe that volcanic rocks in Zhanhua Sag are island-arc volcanic rocks. During the upwelling of magma, it was influenced by the collision of the Yangtse plate in Mesozoic and mixed with crust so that it has geochemical characteristics of the crust compared to typical edakite.