甘肃北山地区花岗岩类地球化学特征及大地构造意义

GEOCHEMICAL CHARACTERISTICS AND TECTONIC SIGNIFICANCE OF THE GRANITOIDS IN BEISHAN AREA, GANSU PROVINCE

本文利用常量元素、稀土元素和锶、钕同位素等研究了甘肃北山地区晋宁、加里东和海西期花岗岩类的地球化学特点。由老到新，岩石中SiO2呈逐渐增加、Al2O3呈逐步递减的趋势。岩石属钙碱性系列，且K2O、LILE和LREE相对富集，Sr、PTi、Y和Yb含量相对较低。微量元素和同位素地球化学特征显示：晋宁期花岗岩为老地壳部分熔融的产物（S型），加里东期主要由源自上地幔的岩浆分离结晶而成（I型），海西期则以较成熟地壳部分熔融而形成的S型花岗岩为主，有些具M（？）型性质。它们的成岩环境表现为同构造碰撞和火山弧型，暗示了华北同塔里木、哈萨克斯坦地块间由汇聚到进一步碰撞、焊接的大地构造演化过程。

Beishan Area is located in northwestern Gansu Province where North China,Tarim and Kazakhstan blocks (NCB, TAB, KSB) conjoined. The granitoids in this area with isotopic ages from 1 756 Ma to 352 Ma are chiefly formed in Jinningnian, Caledonian, and Hercynian orogenies. Geochemically, the granites show the charcteristics of calc-alkaline magma series, and derived from partiabelting of elder crust, fractional melting of Inaic magma, partial melting of coninental crust. Most of thern are enriched in K20, LREE, LILE, and depleted in Sr, P,T, Y and Yb. STh content(wt% ) increased in the rocks while Al2O3 decreaSe with time.Isotopic features of Sr and Nd indicate that had evolved from elder continental crust, dePletive upper manle to the continenal crust with high rnaurity. The depletbe upper Inande rnay reate to an arc syStem in which the caedonian granites formed, The petmpnetic environmnt of the granites reflected the collishal setting and welding of the three blocks, NCB, TAB and KSB. In comination of analysis for the regional geological setting, the colisional granites imPly that the three blocks colided together by sutural zones with occurrence of eclogite, ophiolite, S-, I - and M(? ) - type granites.