Basic features and genesis of the early-middle Cambrian epoch volcanics in the Xiangmaoshan district of western part of northern Qilian

At Xiangmaoshan,alkaline basalt series with mark of rich Na and low K,Mg are the main of ∈₁-∈₂ basic volcanic rock.REE is lower and belongs to the type of enrichment LREE.Distribution pattern of REE is right dip smooth curve,and there is appreciably Eu positive anormaly.Trace element content is close to the abundance of midocean ridge intergrade basalt and intraplate tholeiite,and magma derived from the enriched source of mantle.So,this volcanic rock series was formed in rift micro-ocean basin tectonic setting of badly extension mechanism,then,the spreading velocity of ocean basin was 1-2 cm/a,belongs to the growth initial stage.