Crustal contamination and magmatic evolution of the Bijiashan basic-ultrabasic intrusion belt in the Beishan Terrane, northeastern Tarim Craton (NW China)

The Bijiashan intrusion belt, one of the basic-ultrabasic intrusion belts in the Beishan Terrane, northeastern Tarim Craton, is composed of the Hongshishan, Hongshishanxi, Xuanwoling, Bijiashandong, and Bijiashan intrusions. These intrusions are mainly composed of dunite, troctolite, olivine gabbro, and gabbro. Magma evolution of the intrusions was strongly controlled by fractional crystallization, and the crystallization sequence was mainly olivineplagioclasepyroxene. This is evidenced by whole-rock MgO contents that are positively correlated with TFe2O3 and negatively correlated with Al2O3, CaO, and Na2O and an increase in total rare-earth element and trace elements contents from ultrabasic to basic rocks. Most of the basic-ultrabasic rocks are enriched in large ion lithophile elements (Cs, Rb, Sr, and U) and depleted of high field strength elements (Zr, Hf, Nb, Ta, and Ti). The variable and relatively low Ce/Pb (0.17-7.3) and Nb/U (0.13-10.1) ratios, variable Th/Yb (0.02-4.1) ratio, initial Sr-87/Sr-86 ratios (0.7033 to 0.7094) and epsilon(Nd) (t) (7.54 to -4.25), suggest less than 6% crustal contamination. Based on the maximum Fo value of olivine (Fo90), whole-rock average composition and olivine-liquid compositional relationships, the parental magma is of high-Mg basaltic magma containing 13.4wt.% MgO. According to rare-earth element contents and Sr-Nd isotopic compositions, the parental magma was originated from partial melting of depleted sub-continental lithospheric mantle.