甘肃北山四顶黑山镁铁、超镁铁质岩地球化学特征及其地质意义

GEOCHEMICAL FEATURES AND GEOLOGICAL SIGNIFICANCE OF MAFIC AND ULTRA-MAFIC ROCK IN SIDINGHEISHAN OF BEISHAN REGION, GANSU PROVINCE

四顶黑山镁铁、超镁铁质岩分布于黄山东—镜儿泉铜镍硫化物矿床北偏东约100km处,位于甘、新交界处。主要由橄榄岩、角闪橄榄岩、辉橄岩、辉石岩、辉长岩、角闪辉长岩、橄榄辉长岩等组成。研究认为该区镁铁、超镁铁质岩是同一岩浆源在不同阶段分异的产物。超镁铁质岩属玄武质科马提岩,镁铁质岩为拉斑玄武岩;镁铁质岩的TiO2含量<1.2%,Nb含量<2.0×10-6,Nb/Lapm=0.25～0.50,Th/Lapm=0.25～0.46,Ti/Ti*=0.37～0.88。镁铁质岩的εNd(t)值普遍大于+3,说明其起源于软流圈地幔,为富集地幔向亏损地幔过渡类型。在原始地幔标准化曲线上镁铁质岩、超镁铁质岩均相对富集大离子亲石元素Rb、Ba、K、U,且出现Nb、Ti的负异常。表明华力西中期哈萨克斯坦板块出现俯冲—增生的岛弧体制,在该体制下岩浆沿断裂多期次脱离地幔而上侵,为古亚洲洋闭合之前俯冲增生的产物,并非形成于碰撞后伸展环境。

Mafic and ultra-mafic rock in Sidingheishan is located in the area about 100Km norm-west to east Huanshan-Jingerquan copper-nickel sulfide deposit where is the boundary between Gansu and Xinjiang Province. It is composed of peridotite, cortlandtite, gabbro, pyroxenolite, bojite, olivine-gabbro. The study sup- ports that marie and uhra-mafic rock were the production differentiated from the same magma source in deferent stage. The ultra-marie rock is basaltic komatiite with TIO2〈1.2%, Nb〈2.0×10^-6,Nb/Lapm=0-25-0.50, Th/Lapm= 0.25-0.46,Ti/Ti＊=0.37-0.88, and the marie rock is tholeiite. The values of eNd （t）within marie rock most are more than ＋3, this reveals that the rock came from asthenospheric mantle which is the transition type from depleted to enriched mantle. On the standardized curve of primary mantle, it is showed that mafic and uha-mafic rock enriched the lithophile elements of big ion, such as Rb, Ba, K, U, and the negative anomalies of Nb and Ti. The paper support that the rock were the production of subduetion and accretion before the closure of Paleo- asian ocean, not of extensional process after collision.