青海南部二叠纪火山岩Sr、Nd、Pb同位素特征及地质意义

对青海南部治多-杂多一带二叠纪火山岩进行了全岩Sr、Nd、Pb同位素分析, 样品的I_Sr=0.70333~0.70417, ε_Nd(t) =4.1~5.5, 206Pb/204Pb=17.547~17.986, 207Pb/204Pb=15.476~15.584, 208Pb/204Pb=37.778~37.976.结合岩石学、地球化学研究表明, 该套火山岩为OIB型, 形成于板内伸展拉张的裂谷环境, 属于青藏高原及邻区晚古生代裂谷系统, 原始岩浆可能来自亏损的古软流圈, 裂解机制与古地幔柱活动有关.样品铅同位素的V₁=31.39~46.78, V₂=22.38~42.45, 落入扬子铅同位素省区域, 表明早二叠世青海南部地区具亲扬子性, 揭示出昌都地块在晚古生代属于特提斯区, 为泛华夏陆块群或扬子板块的一个组成部分. 

Sr, Nd and Pb Isotopic Geochemistry of Permian Volcanic Rocks from Southern Qinghai and Their Geological Significance

Isotopic ratios of Sr, Nd and Pb were analyzed for the Permian volcanic rocks in the Zhiduo and Zaduo areas of southern Qinghai which is located at the Qamdo block. The analytical results of the sample rocks are as follows: I_Sr = 0.703 33 -0.704 17, ε_Nd(t) =4.1-5.5, 206Pb/204Pb=17.547-17.986, 207Pb/204Pb=15.476-15.584, 208Pb/204Pb= 37.778-37.976. Combined with lithological and geochemical research findings, the above results indicate that the volcanic rock is of OIB type and came into formation in the stretching rift tectonic environment. It falls into the Late Palaeozoic rift system of the Qinghai-Tibet Plateau and its adjacent areas. The primitive magma might have originated from the deplected asthenosphere and the continental breakup is related to the activity of mantle plume. With the V₁ and V₂ of the lead isotopic ratios ranging from 31.39 to 46.78, 22.38 to 42.45 respectively, the lead isotope of the rock samples can be labeled the Yangtze lead isotopes, which indicates that the Qamdo block belonged to Tethyan realm during Late Palaeozoic and was a part of the Pan-Cathaysian continental group or Yangtze Plate.