塔里木板块东北部坡北岩体内橄榄辉长苏长岩岩石成因

摘要

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 L 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 TiO2, Na2Ot K2O, rare earth elements and large ion lithophile elements of the rocks is lower. The s^(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 1070X1, 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 oiivine gabbronorite intrusions to form Cu—Ni sulfide deposit.%坡北岩体位于塔里木板块东北部,由5个阶段的岩浆作用形成,其中,第二阶段岩浆作用形成的侵入体岩石类型单一,主要为橄榄辉长苏长岩,局部有零星的辉长苏长岩和斜长岩.岩石的SiO2-FeO/MgO图解显示其化学组成为钙碱性系列,但单斜辉石的Al2O3-SiO2及n(Si)-n(AlⅣ)图解表明母岩浆为拉斑玄武质岩浆.岩浆演化方向与岩石化学系列的转化是同化混染作用的结果,侵入体遭受了长城系古硐井岩群1.38％ ～3.15％的混染.原生岩浆为MgO含量约7.31％的玄武质岩浆.岩石的TiO2、Na2O、K2O、稀土元素、大离子亲石元素丰度均很低；除一件样品外,其余岩石的εNd(t)值为+2.54～ +2.81,这些特征表明岩浆源区属亏损型大陆岩石圈地幔.岩浆在结晶过程中处于物理化学条件相对稳定的近平衡状态,局部不平衡分离结晶形成堆晶结构和纹层状构造,岩浆结晶温度约为1070℃.由于岩浆源区熔融程度低,导致原生岩浆贫硫,且岩浆在演化过程中没有经历充分的分离结晶作用,这些因素使得橄榄辉长苏长岩侵入体不具备硫达到过饱和进而发生铜镍硫化物熔离成矿的潜力.