PETROGENESIS OF POTASSIC ALKALINE VOLCANISM AND PLUTTONISM IN A PROTEROZOIC RIFT TROUGH NEAR BEIJING

Controlled by the Middle Proterozoic incipient rifting and extensional faulting,in the Miyun-Pinggu area of the eastern part of Beijing , potassic alkaline basaltic and phonolitic volcanic rocksof the Dahongyu Formation in the rift area are spatially and temporally associated with rapakivigranites and gabbro-diabase（anorthosite） intrusions. They display similarities to within-plate al-kaline basalt and A-type granite with respect to isotopic and geochemical characteristics, The bi- modal mafic and sialic magmas were derived by a low degree of fusion of the upper mantle andlower crust.