Abstract

Volcanic rocks of the Tam Danh Formation form a short differentiated series of basalt-andesite, which consist mainly of basalt and andesitobasalt, and lesser, andesite. In chemical content, sodium is richer than kalium (sodic series) with usually medium content of kalium, and they belong to the tholeiitic series. They are quite rich in the LREE and have high content of Rb, Th, Nb and low content of Ta, P, Zr, Hf, Ti. According to geochemical characteristics, they differ from basalts of the Bằng Giang Formation that could be derived from two different magma sources.