Discovery of a Miocene Mafic Dyke from the Western Hills of Beijing and its Geological Implications

The present study is the first report of a Miocene marie dyke from the Dahuichang,in the Western Hills of Beijing.The dyke cats the fossil-dated Changxindian Formation of Eocene sequences and yields K-Ar ages of 14-15 Ma.The dyke iS fine-grained diabase and has 49.84%-50.81%SiO2 and 3.56-3.97% NazO+K2O,high TiO2(1.65%-1.93%)and MgO(7.36%-9.85%),and low K20(<1.22%) contents,with Na20>K2O and slightly varied magnesium numbers(Mg#=55.54-62.74).In trace elements geochemistry,the dyke is very similar to the Miocene basalts from Jining and Hanuoba.The enrichment of light rare earth elements([La/Yb]N=5.03--6.12)and large ion lithophile elements (LILEs),no negative Eu anomafies,relatively high Cr(265-326 ppm)and Ni(155-262 ppm),and almost constant V concentrations (194-213 ppm)reveal that the composition close to the Primary basaltic magma from an enriched.mantle source.with little crustaI contamination and fractional crystallization.The basaltic magma was possibly derived from the upwelling asthenosphere mantle beneath eastern China during the Miocene lithospheric thinning.