知识与财富的链接
塔里木北缘皮羌地区发育大量闪长质-花岗质岩脉以及哈拉峻花岗岩Ⅰ号和Ⅱ号岩体,这些花岗质岩脉和岩体与赋存大型钒钛磁铁矿矿床的皮羌辉长质岩体空间上共生,推测其成因与塔里木地幔柱岩浆上涌有关。闪长质-花岗质岩脉出露在皮羌辉长质岩体的东缘、北缘及南缘,而哈拉峻花岗岩Ⅰ号和Ⅱ号岩体主要出露在辉长质岩体的西部。花岗质岩脉具有高硅(SiO2=64%~74%)、富碱(Na2O+K2O =8%~10%)、富集Rb、Th、U等大离子亲石元素和Nb、Ta、Zr、Hf等高场强元素,(Ga/Al)×104变化于2.85~3.85之间,具有明显Eu、Ba、Sr负异常等特征,类似A型花岗岩。闪长质岩脉具有低硅(SiO2=53%~59%)、低全碱含量(Na2O+K2O=5%~8%)、以及Eu和Ba正异常。哈拉峻花岗岩Ⅰ号和Ⅱ号岩体的地球化学特征与花岗质岩脉相似。闪长质-花岗质岩脉、哈拉峻花岗岩Ⅰ号和Ⅱ号岩体的εNd(t)值范围在-2.9~-0.2之间,与皮羌辉长质岩体的εNd(t)值相近(-1.1~2.1),表明他们来源于类似的地幔源区。我们认为闪长质-花岗质岩脉、哈拉峻花岗岩Ⅰ号和Ⅱ号岩体主要由底侵的镁铁质岩浆分异出的中酸性熔体结晶分异并伴随不同程度的地壳混染形成,皮羌辉长质岩体是镁铁质岩浆浅部侵位的产物。因此~280Ma大量幔源岩浆底侵是塔里木大火成岩省中A型花岗岩形成的必要前提。
The Early Permian dioritic and granitic dikes and Halajun granitic plutons I and II from the Piqiang region in the northern Tarim Block are spatially associated with the giant Piqiang Fe-Ti-V oxide-bearing gabbroic intrusion. They are considered to be genetically related to the magma upwelling of the Tarim mantle plume. Dioritic-granitic dikes occur in the eastern, northern and southern margin of the Piqiang gabbroic intrusion, and the Halajun granitic plutons I and II mainly outcrop to the west of the intrusion. The granitic dikes have typical A-type granitic affinity. They have 64% to 74% SiO2 and 8% to 10% Na2O+K2O. They are enriched in large ion lithophile elements (Rb, Th and U) and high field strength elements (Nb, Ta, Zr and Hf), but have very negative Eu, Ba and Sr anomalies. The rocks have high (Ga/Al)×104 ratios of 2.85 to 3.85. The dioritic dikes have 53% to 59% SiO2 and 5% to 8% Na2O+K2O.
They have trace element patterns similar to the granitic dikes. The Halajun granitic plutons I and II have compositions similar to the granitic dikes. They both have εNd(t) values ranging from -2.9 to -0.2, comparable to those of the Piqiang gabbroic intrusion (εNd(t)=-1.1~2.1), indicating that they have been derived from the same mantle reservoir. We propose that the dioritic-granitic dikes, Halajun granitic plutonsⅠ and Ⅱ were predominantly generated by the emplacement of intermediate-felsic melts from differentiation of the newly underplated mafic magma ponding at the bottom of the lower crust, and subsequent fractional crystallization and assimilation to variable extent, whereas the Piqiang gabbroic intrusion are likely the product of emplacement of mafic magma at a shallow level. Hence large volume of underplating mafic magmas is the prerequisite for the formation of the coeval mafic-ultramafic intrusion and A-type granitic plutons of the Tarim large igneous province.
