Geochemical Characters and Petrogenesis of the Bijiashandong Basic-Ultrabasic Intrusion,NE Tarim Plate

The Bijiashandong basic-ultrabasic intrusion,located in the Beishan area in Xinjiang,northeastern Tarim plate,is the most eastern intrusion of Bijiashan belts.The intrusion is oval-shaped and outcrops about 5km2.The main kinds of the rocks are feldspathic wehrlite,olivine gabbro,olivine-bearing gabbro,gabbro and diabase.The rocks containing olivine are tholeiitic series and the gabbros are calc-alkalic series.Petrography,petrochemistry and geochemistry characters indicate the fractional crystallization of olivine,pyroxene and plagioclase in magma evolution.Nd,Sr,Pb isotopic compositions and lithogeochemical characteristics imply the exchange of material between the magma and the wall-rocks.It is proved that the primitive magma of Bijiashandong intrusion was high-Mg tholeiitic magma(MgO=11.2%) and derived from an OIB-like mantle source,which are similar to other basic-ultrabasic intrusions of Permain in the northeastern Tarim plate,indicating that these intrusions were mainly derived from an OIB-like mantle source and could be related to mantle plume.