FEATURES AND ORIGIN OF MESOZOIC BIMODAL VOLCANIC ROCKS IN FUJIAN PROVINCE

Mesozoic bimodal volcanic rocks in Fujian province occur in the Early Jurassic Fankeng Formation and Early Cretaceous Shimaoshan Group. In section, the two lithostratigraphic units are marked by rhyolitic rocks overlying basalts directly. The bimodal volcanic rocks of the two epochs are both developed in volcanic depressions （basins） in a downfaulted zone and were formed in an intraplate extensional environment on active continental margins. Basalts originated by 10%～20% of partial melting of the upper mantle; whereas rhyolitic magma was derived by 15%～23% of partial melting of the lower crust. Appreciable amounts of mantle materials were introduced into the rhyolitic magma to form the mantlecrust mixture.