Petrogenesis of the Olivine Gabbronorite in Pobei Complex,Northeast Tarim Plate

The Pobei complex,which is located in the northeast Tarim plate,is composed of intrusions of 5 stages.The olivine gabbronorite intrusions,which formed during the second stage of the magmatism,are mainly composed of olivine gabbronorites.In some places,there are fragmentary gabbronorites or anorthosites.All the rocks are calc-alkalic series,while the composition of clinopyroxenes shows that the parent magma is tholeiitic.The transformations of magmatic evolution and rock chemical series are attributed to contamination.Data of Sr—Nd isotopes show that the magma of the olivine gabbronorite intrusions had undergone 1.38%to 3.15%contamination of the Gudongjing Group,Changchengian System,Mesoproterozoic.The MgO value of primary magma is 7.31%, which ranges in basaltic magma.The abundance of TiO_2,Na_2O,K_2O,rare earth elements and large ion lithophile elements of the rocks is lower.Thee_(Nd)(t) values are during +2.54 and +2.81 except one sample.These characteristics suggest that the magma source is depleted continental lithospheric mantle.The magma crystallized in a stable situation,while the crystallization temperature is about 1070℃.The cumulate texture and laminar structure result from nonequilibrium fractional crystallization in some places.Low degree partial melting from continental lithospheric mantle source region results in the lack of sulfur for primary magma.At the same time,the fractional crystallization of magma was too weak.All these factors make it impossible for the olivine gabbronorite intrusions to form Cu—Ni sulfide deposit.