论文部分内容阅读
藏东芒康—巴塘一线广泛发育中酸性侵入岩体,空间上沿金沙江西岸呈带状分布,以复式岩基、岩枝产出,并侵入于二叠系下统冰峰组灰岩系及金沙江带二叠系上统上段变质岩系,该区侵入岩岩性以花岗闪长岩和花岗岩为主.目前对该地区花岗岩成因、形成时代、构造意义研究程度较低.本文以巴塘县曲那西地区花岗岩体为研究对象,对该岩体LA-ICP-MS锆石U-Pb年龄、岩石地球化学特征以及岩体与金沙江缝合带的关系进行了详细研究.LA-ICP-MS锆石U-Pb年龄结果表明该岩体形成时代主要集中于晚三叠世,3个花岗岩样品形成时代分别为235.2± 1.4 Ma、229.6± 1.4 Ma、229.1±1.6 Ma.全岩岩石地球化学特征表明,区内岩体属钾玄岩、高钾钙碱性系列,富集Rb、Th、K等大离子亲石元素,明显亏损Nb、Ta、P、Zr、Ti等高场强元素.Sr-Nd同位素组成表明,巴塘岩体很可能来自于康定杂岩并且混入了少量的富集型沉积物.结合岩体地球化学特征和区域地质背景认为,该岩体形成于金沙江构造带造山后碰撞环境,进而推测金沙江洋壳的闭合至少在晚三叠世之前.“,”There are extensive intermediate-acid intrusive rocks along the border between Batang and Mangkang County,in the eastern Tibet.These rocks distributed band-like along the west bank of Jinsha River,which occur as complex batholiths and apophyses that intruded in Early Permian Bingfeng Group limestone formation and the upper position of Jinshajiang Early Permian metamorphic rocks.The intrusive rocks are mainly granite and granodiorite.So far,the genesis of these granitites,including their formation age and tectonic significance,has been poorly studied.In this paper,we chose the Batang granitoids in this region for geochemical studies and LA-ICP-MS U-Pb dating.Three granite samples were used for LA-ICP-MS U-Pb dating,the ages of the samples are 235.2±1.4 Ma,229.6±1.4 Ma and 229.1±1.6 Ma,respectively,suggesting formation ages in Late Triassic period.Geochemical studies indicate that these granites belong to shoshonite series and high K calc-alkali series,enriched in Rb,Th,K,and LILEs,but depleted in Nb,Ta,P,Zr,Ti and HFSES.Whole-rock Sr-Nd isotopic compositions show that samples were derived from the Kangding complex but mixed with a small amount of sedimentary rocks.In addition,the geochemical studies show that these granites were formed in a post-collisional tectonic extensional setting of the late age of orogeny.In combination with regional studies,we confer that the closure of Jinshajiang ocean was completed before Late Triassic.