甘孜一理塘蛇绿混杂岩带中段奥陶纪变质石英砂岩碎屑锆石同位素年代学及其地质意义

ISOTOPIC CHRONOLOGY OF DETRITE ZIRCONS FROM THE ORDOVICIAN META MORPHIC QUARTZ SANDSTONE IN THE MIDDLE PART OF THE GANZI-LITANG OPHIOLITIC MELANGE BELT AND ITS GEOI OGICAL SIG-NIFICANCE

甘孜一理塘蛇绿混杂岩带经历了原特提斯洋到古特提斯洋的演化,本次研究通过对甘孜一理塘蛇绿混杂岩带中段奥陶纪变质石英砂岩的岩石学、年代学等进行研究,探讨其形成时代、物质来源及其构造背景,为恢复甘孜一理塘蛇绿混杂岩带的组成及演化提供新的数据。18件样品的粒度分析结果显示:大部分样品曲线具“推移+跳跃+悬浮”三个次总体的粒度特征,碎屑粒度中细砂为主,平均粒度Φ值为2.25,频率分布以正偏为主,具有较高的结构成熟度;结合岩石学特征、基本层序、沉积构造,判别该变质石英砂岩形成于滨岸高能浅水环境。175颗碎屑锆石Th,U丰度分别为22×10^(-6)~823×10^(-6)、46×10^(-6)~1119×10^(-6),Th/U比值范围0.07~3.83,绝大多数测点Th/U比值大于0.1,Th,U含量具较好的正相关性,阴极发光(CL)图像显示大部分锆石颗粒具有清晰的振荡环带状结构;年龄值范围在494~3524Ma,可划分为5个聚集区间:494~587Ma,710~1198Ma,1622~1969Ma,2300~2524Ma,2845~3524Ma;对第一组年龄中的38个测点进行微量元素特征分析,显示这些锆石均为岩浆成因,38个测点中最年轻的锆石年龄为495Ma。年龄频谱对比显示该变质石英砂岩同时接受了来自扬子地块西缘和拉萨地块的物源。综合分析认为,该变砂岩属于构造混杂在甘孜一理塘蛇绿混杂岩带内的外来岩块,其沉积年龄不早于奥陶纪,属于原特提斯洋的陆缘沉积产物。

The Ganzi-Litang ophiolitic melange belt has experienced the evolution from Proto-Tethys Ocean to the Palaeo-Tethys Ocean.This paper mainly studies the petrology and chronology of the Ordovician metamorphic quartz sandstone in the middle part of the Ganzi-Litang ophiolitic melange belt,and discusses its formation age,material source and tectonic setting,aiming to provide new data for the recovery of the composition and evolution of the Ganzi-Litang ophiolitic melange belt.The results of particle size analysis of i8 samples show that most of the sample curves have three sub-general particle size characteristics,namely,"propelling,jumping and suspension".The clastic particle size is mainly fine grained sand,with an average particle size of 2.25@.The frequency distribution is mainly positive deviation,with a high structure maturity.Combined with petrologic characteristics,basic sequence and sedimentary structure,it is proved that the metamorphic quartz sandstone was formed in coastal high-energy shallow water environment.The Th and U contents of the 175 detrital zircons range from 22×10^(-6)to 823×10^(-6)and 46×10^(-6)to 1119×10^(-6),respectively,with Th/U ratios ranging from 0.07 to 3.83.The Th/U ratios of most of the test sites are greater than O.1,indicating a good positive correlation between Th and U contents.Cathodeluminescence(CL)images show that most of the zircon particles have clear oscillating ring structure with the age ranging from 494 to 3524 Ma,which can be divided into five aggregation intervals.They are 494 to587Ma,710 to1198Ma,1622 to1969Ma,2300 to2524 Ma,and2845to3524Ma.The char acteristics of trace elements at 38 dating sites in the first group zircon particles show that these zircons are all magmatic in origin,with the youngest zircon dating of 495 Ma.Age spectrum correlation shows that the metamorphic quartz sandstone has received provenance from both the western margin of the Yangtze Block and the Lhasa Block.Through comprehensive analysis,it is considered that the metamorphic sandstone belongs to the exotic rocks in the Ganzi-Litang ophiolitic melange belt and is the continental margin deposition product of the Proto-Tethys Ocean,with a sedimentary age not earlier than the Ordovician.