Abstract:

Petrological and geochemical characteristics of the Xijianshan (21.0±0.65) Ma and Dongxiaoshan (5.46±0.32) Ma Cenozoic basalts, i.e., two typical volcanic bodies in the Yitong volcanic cluster, is studied in detail in this paper. It is suggested that volcanic rocks from both the Xijianshan and Dongxiaoshan belong to OIB type and have a common source region from the asthenosphere. They saw low degree of crustal contamination and fractional crystallization during magmatic stage. The Xijianshan volcanic rocks do not contain mantle-derived xenoliths, consist mainly of olivine basalt, are high in SiO₂, Al₂O₃ and Na₂O content, and belong to sodium alkaline rocks. The magmas responsible for their formation were derived from 70km to 100 km underneath the crust. The basanites from the Dongxiaoshan contain mantle-derived xenoliths, being high in  MgO, TiO₂,  CaO and K₂O contents, can be classified into potassium peralkaline rocks. The depth of their original magma is over 100 km. The present authors suggest that it is the depth of their magma derivation that resulted in their geochemical difference between the Xijianshan and Dongxiaoshan volcanic rocks.

Key words: Cenozoic basalt, the original depth of magma, geochemistry, Yitong