新疆北山清白山花岗岩体成因及构造环境分析

Genesis and Tectonic Environment of Qingbaishan Granite in Beishan,Xinjiang

清白山花岗岩体出露于新疆北山地区,为厘清该岩体的成因、形成时代以及构造环境,以探讨新疆北山地区大地构造演化的制约因素,对其开展了LA-ICP-MS锆石U-Pb同位素定年和地球化学特征分析。结果显示,岩体锆石206Pb/238 U同位素加权平均年龄为(408.4±2.0)Ma(n=15,MSWD=0.47),为早中泥盆世。花岗岩属高钾钙碱性过铝质高分异的“S”型花岗岩:SiO2含量75.27%~75.78%,TiO2含量0.005%~0.01%,Al2O3含量13.97%~14.41%,MgO含量0.07%~0.09%,K2O+Na2O含量8.00%~8.42%,K2O/Na2O比值为1.08~1.45,铝饱和指数A/CNK为1.11~1.21;轻、重稀土元素分馏作用不明显(La/Yb)N=3.42~4.57,具明显四分组效应,铕负异常(δEu为0.11~0.16)强烈,富集Rb,U,K,P等元素,亏损Th,Sr,Zr,Ti等元素,总体表现出高硅、富铝、富钾、低钛、低Sr、Yb的特征。结合区域地质资料及前人研究成果,本文认为清白山花岗岩体形成于俯冲消减的末期板块碰撞环境,暗示北山地区洋盆在早泥盆世已处于俯冲消减的末期,至中泥盆世洋盆闭合,此时洋盆已开始进入主体碰撞阶段。

Qingbaishan granite body exposed in Beishan area,Xinjiang.In order to clarify the genesis,formation age and tectonic setting of the granite body and explore constraints on tectonic evolution in the region.In this paper,LA-ICP-MS zircon U-Pb isotopic dating and geochemical characteristics were analyzed.The results show that the zircon 206 Pb/238 U isotope-weighted mean age is 408.4±2.0 Ma(n=15,MSWD=0.47),which is at the turn of the Devonian.Granite is a high-potassium,calcium-alkali-aluminum,highly differentiated S-type granite:SiO2 content is between 75.27%and 75.78%,TiO2 content is 0.005%to 0.01%,Al2O3 content is between 13.97%and 14.41%,and MgO content is 0.07%~0.09%,K2O+Na2O content is 8.00%~8.42%,K2O/Na2O ratio is 1.08~1.45,aluminum saturation index A/CNK is 1.11~1.21;light and heavy rare earth element fractionation is not obvious(La/Yb)N=3.42~4.57,with obvious four-group effect,negative anomaly(δEu is 0.11~0.16)strong,enriching Rb,U,K,P and other elements,loss of Th,Sr,Zr,Ti and other elements,the overall performance High silicon,aluminum rich,potassium rich,low titanium,low Sr,Yb.Combined with regional geological data and previous research results,this paper believes that the Qingbaishan lead-zinc mining area is formed in the late plate collision environment of subduction and subsidence,suggesting that the Beishan ocean basin was closed after the Early Devonian and began to enter the main collision stage at this time.