FEATURES AND DISTRIBUTION OF VOLCANIC ROCK ASSOCI ATIONS OF VARIOUS AGES IN THE BEISHAN AREA, GANSU

On the basis of a large amount of data combined with field work in some ore districts and petrochemical work, the authors have compiled a map of volcanic rock associations and their distribution in the Beishan area, summarized the features of volcanic rock associations of various ages and their distribution in the Beishan area. It is suggested that the volcanic rocks in most horizons belong to the successively differentiated rock association of normal K Na type and dominantly calc alkaline, intermediate to acid compositions, while basic and bimodal volcanic rock associations of the tholeiite series are only developed in Proterozoic, Middle Ordovician, Upper Ordovician and Lower Permian strata. The alkaline series is scanty and no alkaline rocks （σ＞9.0） occur in the study area. Intermediate acid rocks of the potassic type may appear in some ore districts such as Baishantang, Gongpoquan and Langwashan. The features of volcanic rocks reflect the tectonic evolution history of multiple "openings" and "closings" in the Beishan.