甘肃北山小西弓南早泥盆世-晚志留世糜棱岩化石英闪长岩体年代学、地球化学特征及其地质意义

摘要

In order to deepen the understanding about the tectonic evolution of Dunhuang land mass, and to understand the tectonic deformation history of the alteration rock type gold depositin Xiaoxigong shear zone before the mineralization, the mylonitized quartz diorite body from the south side of Xiaoxigong gold deposit have been selected to carry out the LA-ICP-MS zircon U-Pb chronology and geochemical studes.In this paper, 24 zircons have been analyzed totally, their Th and U contents range from 46.5 × 10-6 to 266.4 × 10-6 and from 196.3 × 10-6 to 496.3 × 10-6, respectively, their Th/U ratios vary from 0.24 to 0.53, and their LA-ICP-MS U-Pb ages are (411.3 ± 5.8)Ma (MSWD =2.7), with the formation age of early Devonian.The SiO2 and TiO2 contents of these mylonitized quartz diorites vary from 57.88% to 58.26% and from 0.81% to 0.86% respectively, their σ values range from 1.38 to 1.56, their alkali indexes(NK/A) change from 0.4 to 0.41, showing the features of calc-alkaline rocks.Their aluminum saturations(A/CNK) vary from 0.95 to 0.98, belonging to the subaluminous rock series, with I-type granite characteristics.These mylonitized quartz diorites have weak Eu anomalies, their ΣREE range from 73.85× 10-6 to 160.37 × 10-6, with moderate low ΣREE content.Their ΣLREE/ΣHREE ratios change from 8.3 to 11.25, with the relative loss of HREE.The contents of Ba, Sr and Zr are higher, while the contents of Rb, Th and U are lower.The tectonic setting for these mylonitized quartz diorites is the volcanic arc environment, which is considered as the response of the Dunhuang block the subducted and collised with the Beishan orogenic belt during the Early Paleozoic to Early Late Mesozoic.The obtained dating data suggests that the subduction collision was lasted to the Early Devonian in this area.%为深化对敦煌地块构造演化的认识,了解小西弓剪切带蚀变岩型金矿成矿前的构造变形历史,选取位于小西弓金矿南侧的糜棱岩化石英闪长岩体进行了LA-ICP-MS锆石U-Pb年代学、地球化学研究.共分析测试了24个点,锆石的Th含量为46.5×10-6~266.4×10-6,U含量为196.3×10-6~496.3×10-6,Th/U值为0.24~0.53,取得LA-ICP-MS锆石U-Pb年龄值为(411.3±5.8)Ma(MSWD =2.7),形成时代为早泥盆世.SiO2含量为57.88%~58.26%,TiO2含量为0.81%~0.86%,σ值在1.38~1.56之间变化,碱指数NK/A为0.4~0.41,属钙碱性岩石,铝饱和度A/CNK值0.95~0.98,属次铝质岩系,具I型花岗岩特征.弱负Eu异常,∑REE为73.85×10-6~160.37×10-6,中等偏低含量,∑LREE/∑HREE为8.3~11.25,HREE相对亏损.微量元素Ba、Sr、Zr含量较高,Rb、Th、U含量较低.构造环境为火山弧环境,认为是敦煌地块早古生代-晚古生代早期对北山造山带俯冲、碰撞过程的响应.精确定年值的获得,进一步确认该地区俯冲碰撞阶段延续到了早泥盆世.