新疆北山地区中坡山北镁铁质岩体岩石地球化学与岩石成因

Lithogeochemistry and petrogenesis of Zhongposhanbei mafic rock body,at Beishan region,Xinjiang.

中坡山北镁铁质岩体位于新疆北山裂谷带的中带,岩体形态为相互联通的岩盆状,出露面积约180km^2。由角闪辉长岩、橄榄辉石岩、橄榄辉长岩、苏长辉长岩和斜长岩组成,岩相带呈同心环状展布,相互间呈渐变过渡关系。锆石U-Pb法同位素年龄为274+4Ma。岩相学、岩石化学、造岩矿物晶体化学和稀土元素地球化学特征均显示了非常发育的分离结晶作用。橄榄辉石岩和橄榄辉长岩主要由早期结晶相聚集而成,橄榄石是最初的液相线相,单斜辉石是数量最多的分离结晶相。斜长岩由残余岩浆结晶而成。母岩浆应该是高镁的拉斑玄武岩浆。Nd、Sr、Pb同位素组成和岩石地球化学特征充分证明了岩浆与围岩之间的物质交换。同化混染作用明显地改变了侵入岩的同位素组成和大离子亲石元素丰度。除斜长岩外,各种岩石的TiO_2、Na：O、K_2O、大离子亲石元素和稀土元素丰度均很低,受同化混染作用影响较小的样品的ε_Nd(t)=+6．80,这些特征证明其岩浆源区属亏损型地幔。FeO~*和SiO_2含量证明,熔融作用开始时,源岩为二辉橄榄岩；当熔融作用持续到一定程度时,源岩物质转化为方辉橄榄岩。稀土元素地球化学证明,熔融作用发生于尖晶石稳定域内。综合各方面要素可以证明,中坡山北岩体是塔里木板块二叠纪期间第四种类型地幔源区部分熔融的产物。

Zhongposhanbei rock body lies in the middle belt of Beishan rift zone in Xinjiang and covers an area of 180km^2 in shape of interconnected lopolith, It consists of hornblende gabbros, divine pyroxenites, divine gabbros, noritegabbros and anorthosites, these lithofacies belts extend in concentric ring form, and each other gradual transition. Zircon U-Pb age is 274 ± 4Ma. Evidences for greatly developmental fractional crystallization comes from the petrography, petrochemistry, crystal chemistry of petrogenetic minerals and REEs geochemistry. Olivine pyroxenites and divine gabbros are mainly formed by assembly of early crystallized phases; divine is original liquidus phase, and elinopyrixene is mainly fractional crystallized phase in quantity. Formation of anorthosites are originate from crystallization of residual magma. Primary magma formed these rocks should be high-Mg tholeiitie magma. Nd, Sr, Pb isotopic compositions and lithogeochemical characteristics fully imply the exchange of material between magma and wallrock. Contamination have changed evidently the isotopic compositions and LILE abundances of the intrusive rocks. All kinds of rocks have very low TiO2, Na2O, K2O contents and LILE, REEs abundances except anorthosites. A divine gabbro sample has εNd （t） = ＋ 6.8, it is infleneed less by contamination. These characteristics indicate that their magmatic source belong to the depleted mantle. FeO ＊ and SiO2 content also prove that main rock of magmatic source is lherzolite when melting just begin, then the source rock has changed into harzburgite when the function maintain to the certain degree. REEs geochemical Characteristics suggest that melting occur in the spinel stable field. It is proved by various pertinent data that Zhongposhanbei rock body is the product of partial melting of the fourth type mantle source region in Permian epoch in Tarim plate.