青海岗察花岗岩类地球化学特征、构造环境及成因

GEOCHEMICAL CHARACTERISTICS,TECTONIC ENVIRONMENT AND PETROGENESIS OF GANGCHA GRANITOIDS IN QINGHAI

为探讨青海岗察花岗岩类的成因机制,通过岩相学、主量元素、微量元素地球化学特征研究,岩体由辉长辉绿岩、辉石闪长岩、紫苏辉石闪长岩、石英闪长岩、花岗闪长岩、二长花岗岩等组成。岩石里特曼指数δ=1.37-2.36,属钙碱性系列;铝饱和指数（A/CNK）=0.765-1.018,属准铝质-弱过铝质岩石。在ACF图显示岗察岩体主要属I-S型花岗岩。稀土总量48.11×10^-6-249.35×10^-6,轻重稀土比值4.12-10.72,LaN/YbN比值3.30-12.85,稀土元素分配曲线显示轻稀土元素曲线向右倾斜,重稀土元素呈平缓型,说明轻稀土元素分馏明显,重稀土元素分馏不明显,显示岩浆源区的残留矿物含有角闪石。δEu=0.34-1.21,铕元素具负异常,说明源区在部分熔融过程中有斜长石的残留。微量元素蛛网图显示Rb具有明显富集,K,Th含量较高,Ba相对亏损,Ce,Hf,Zr,Sm,Y,Yb等明显低于洋脊花岗岩,具有后碰撞花岗岩特征。岗察岩体侵入时间上发生在陆陆碰撞期后的陆内叠覆造山阶段,其构造环境属后碰撞陆内造山叠覆-底侵构造环境。其成因是中三叠世,大陆碰撞地壳加厚的地质背景下,岩浆底侵-混合作用形成,为中央造山带印支期构造-岩浆事件的产物。

The lithological and geochemical characteristics of major elements and trace elements of the Gangcha granitoids in Qinghai Province are studied so as to discuss its origin mechanism.Gangcha intrusive rocks consist of gabbro-diabase,augite diorite,hypersthene diorite,quartz diorite,granodiorite,monzogranite and so on.It is showed that the Rock Rittmann indexσ=1.37-2.36,characteristic of calc-alkaline series;and the aluminum saturation index（A/CNK）=0.765-1.018,characteristic of metaluminous-weakly peraluminous rocks.ACF diagram indicates that the intrusion belongs to I-S type granite.Total rare-earth elements ranges from 48.11×10^-6 to249.35×10^-6,LREE/HREE=4.12-10.72,LaN/YbN=3.30-12.85,and REE distribution curve displays a right tilt of the light rare-earth elements and a flat type of the heavy ones,suggesting that the residue of magma source area contains amphibole minerals.The analysis shows that Eu displays negative abnormal of 0.34-1.21,indicating that plagioclase is refractory residue left in partial melting process.Spider diagram of trace elements displays a significant positive Rbanomaly,a slight negative Ba-anomaly,K and Th enrichment and the value of Ce,Hf,Zr,Sm,Y,Yb are lower than the ridge granitoids obviously,a characteristics of post-collisional granites.Gangcha intrusion occurred in the orogenic stage after continental collision period,formed in the intracontinental orogenic folding-underplating environment.Therefore,the Gangcha intrusion is experienced underplating and magma-mixing accompanied by crustal thickening during continental collision in the middle Triassic.It is a product of tectonic and magmatic processes in the central orogenic belt.