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-com pressive orogenic event; and the interm ediate-acid sequence intrusions usually contains TTG assemblage with some gabbroic inclusions,indicating that the crust is composed of gabbroic rocks.Therefore,the rock-forming mechanismis that,under the decompression conditions of continental litho sphere extending,the peridotite from mantle were partially melted to form picritic magma with a Mg value of 0.831.The magm a 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 int ruded into the upper crust to form rocks during the uplifting orogenic movement.