一种新构造类型的含铜镍矿化基性-超基性杂岩体

甘肃北山地区的怪石山含铜镍硫化物矿化基性-超基性杂岩岩体,为一个岩浆底侵-隆升造山过程侵入体的构造类型典型实例。主要依据是:它与平头山-砂井序列的中酸性侵入岩类共同构成一种壳幔岩浆混合型的"双峰式"火成构造组合;在岩体发育时段(410～350Ma或泥盆—石炭纪)内,区内缺少相应时代的地层沉积和火山作用,表明发生在非挤压造山事件中,相对处于一种伸展裂解的动力学环境;所见中酸性序列侵入岩多具TTG组合和含有辉长质岩石包体,揭示下地壳主要由辉长质岩石构成等。依此提出岩体的成岩机制是:在一种大陆岩石圈伸展减压条件下,地幔橄榄岩发生部分熔融,形成Mg#值为0.831的苦橄质岩浆,并上侵到地壳或壳幔边界,进而引起辉长质的下地壳产生局部熔融,形成一套TTG组合的混合岩浆,尔后以岩浆底侵方式随张扭帚状构造系发育和隆升造山过程入侵上地壳而成。

A New Structural Type of Basic-Ultrabasic Complex with Ni-CU Mineralization

Our study shows that the Guaishishan basic-ultrabasic complex with Ni-Cu mineralization in Beishan area,Gansu,is a typical example of intrusion bodies related with the magma under-plating and uplifting orogenic movement.The main evidences are that ①the complex and intermediate-acid intrusions of the Pintoushan Sand-drain sequence form the double-peak igneous structural assemblage mixed with crust-mantle magma;②Within the developing period （410-350Ma） of rock bodies,neither sedimentary strata nor volcanism can be seen in this area,showing an extensional and cracking dynamics environment during the non-compressive orogenic event;and③the intermediate-acid sequence intrusions usually contains TTG assemblage with some gabbroic inclusions,indicating that the crust is composed of gabbroic rocks.Therefore,the rock-forming mechanism is that,under the decompression conditions of continental lithosphere extending,the peridotite from mantle were partially melted to form picritic magma with a Mg value of 0.831.The magma intruded upward to crust or crust-mantle boundary and caused partial melting of the gabbroic in lower crust to form mixed magma with TTG assemblage.Subsequently,the magma developed in terms of magma under-plating according to tense-shearing brush structure series and then intruded into the upper crust to form rocks during the uplifting orogenic movement.