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 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.

Microstructures and deformation mechanisms of hornblende in Guandi complex,the Western Hills,Beijing

Multiple methods were applied to study the deformation characteristics of hornblende in Archean plagioamphibolite mylonite from the Western Hills(Beijing),including optical microscopy(OM),electron backscatter diffraction(EBSD),transmission electron microscopy(TEM),and electron probe microanalysis(EPMA).The hornblendes are σ and δ type porphyroclasts with the new-born needle shaped grains as their tails.The analysis of lattice preferred orientation(LPO) of both the porphyroclasts and the new-born grains shows that the main slip system of the deformed hornblende is(100)<001>,suggesting that the fabric characteristics of new-born grains inherit that of porphyroclasts.Sub-microstructures show the porphyroclast core is dominated by dislocation tangle,little or no dislocations in the new-born grains,and the subgrains confined by dislocations in the transition zone between porphyroclasts and new-born grains.By using plagioclase-hornblende geothermometry and hornblende geobarometry,the estimated temperature and pressure of porphyroclasts are 675.3-702.9℃ and 0.29-0.41 GPa and those of new-born grains are 614.1-679.0℃ and 0.11-0.31 GPa.The bulging recrystallization is summarized as deformation mechanisms of hornblende by the discussions of the microstructures,EBSD fabric,sub-microstructures,and the deformed temperature and pressure.