Genetic Significances and Composition Characteristics of Olivine from Pobei Mafic-Ultramafic Complex,Xinjiang

The composition of olivine can directly give the messages of parental magma composition,magmatic fractional crystallization,and sulfide segregation.Electron microprobe analyses show that all olivine grains in the Pobei complex are chrysolite and their chemical compositions are closely related to the types of host rocks.The Fo values of the olivines in olivine-gabbro,troctolite or olivine-clinopyroxenite,wehrlite or peridotite are in the ranges of 74 to 81,75 to 82.7,and 83.6 to 86.3,respectively,indicating that peridotite was crystallized earlier than olivine-gabbro.Ni contents of the olivines vary from 308×10-6 to 1762×10-6.The MgO content of the parent magma,which contains the highest Fo olivine,was estimated to be 14.38%,indicating that the parent magma of the Pobei complex is a high-MgO basalt.Modeling results of olivine fractional crystallization and sulfide segregation show that ~25% olivine in the parental magma of the Pobei complex was fractionated and crystallized.A slight sulfide segregation(~0.2%) occurred during the period of olivine fractional crystallization proceeding from 2% to 4% suggests that the metallogenic potential of the Pobei complex is limited.