PETROGENESIS OF POTASSIC ALKALINE VOLCANICS ASSOCIATED WITH RAPAKIVI GRANITES IN THE PROTEROZOIC RIFT OF BEIJING, CHINA

A suite of potassic alkaline volcanites is spatially and temporally as sociated with rapakivi granites which are related to Proterozoic (1.85 to 1.40 Ga) rifting in the Beijing area. The volcanites are spread al ong an EW-trending 150 km long sea basin rift and the rapakivi granite s were emplaced along a major EW-fault in an adjacent uplifted region. The alkaline magmatism is comparable to the middle Proterozoic anorog enic magmatism in North America and Fennoscandia and is related to inc ipient breakup of the craton they intrude. The volcanic rocks and the rapakivi granites show within plate affinities. They also have similar overall REE distribution patterns that are enriched in LREE. The rapa kivi granites show larger negative Eu anomalies than the volcanites. T he data suggest that the rapakivi granites were crystallised from a mo re evolved magma than the volcanic rocks.