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

作　　者：ZHANG Zhicheng HAN Baofu WU Tairan 

机构地区：[1]Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing 100871, China

出　　处：《Acta Geologica Sinica(English Edition)》2009年第3期640-647,共8页地质学报（英文版）

基　　金：funded by the National NaturalScience Foundation of China(40672130);the Instrumental Analysis Fund of Peking University

摘　　要：The present study is the first report of a Miocene mafic dyke from the Dahuichang, in the Western Hills of Beijing. The dyke cuts 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% Na2O＋K2O, high TiO2 （1.65%-1.93%） and MgO （7.36%-9.85%）, and low K2O （〈1.22%） contents, with Na2O〉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 anomalies, 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 crustal contamination and fractional crystallization. The basaltic magma was possibly derived from the upwelling asthenosphere mantle beneath eastern China during the Miocene lithospheric thinning.The present study is the first report of a Miocene mafic dyke from the Dahuichang, in the Western Hills of Beijing. The dyke cuts 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% Na2O＋K2O, high TiO2 （1.65%-1.93%） and MgO （7.36%-9.85%）, and low K2O （〈1.22%） contents, with Na2O〉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 anomalies, 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 crustal contamination and fractional crystallization. The basaltic magma was possibly derived from the upwelling asthenosphere mantle beneath eastern China during the Miocene lithospheric thinning.

关 键 词：MIOCENE mafic dyke Dahuichang Western Hills of Beijing 

分 类 号：P534.621[天文地球—古生物学与地层学] S759.92[天文地球—地质学]