冈底斯西段达若地区林子宗群典中组火山岩锆石U-Pb定年与Hf同位素

U-Pb dating and Hf isotopes of zircons from volcanic rocks in the Dianzhong Formation of Linzizong Group from the Daruo area,western Gangdese, Tibet Autonomous Region

林子宗群火山岩是对印度—欧亚大陆碰撞造山作用的响应,记录了新特提洋闭合和陆陆碰撞过程的关键信息。前人对林子宗群典中组火山岩积累了大量研究成果,但主要集中在冈底斯中、东段,而对西段的典中组火山岩研究较少,难以形成系统性的对比研究。文章对冈底斯西段达若地区典中组近底部流纹质晶屑凝灰岩进行了详实的岩相学、锆石U-Pb年代学和Hf同位素地球化学研究。结果显示,典中组近底部流纹质晶屑凝灰岩的LA-ICP-MS锆石U-Pb年龄为61.9±0.3Ma和61.1±0.6Ma,可近似代表达若地区典中组火山岩底部的成岩年龄;锆石εHf(t)值为低弱的负值,但具有变化较大的范围,介于-4.97^-1.54之间,平均-3.10,明显富集Hf同位素,二阶段Hf同位素模式年龄(TDM2)介于1083~1273Ma之间,表明岩浆主要起源于中元古界古老地壳物质,并混染了少量幔源组分。结合前人研究成果对比分析认为,印度—欧亚大陆碰撞在东西向具有穿时性,表现为中部碰撞时间早于东部和西部,同时也揭示出南冈底斯带地壳结构的差异性,即并非完全由新生地壳组成,至少在朱诺—达若—打加错一带存在古老结晶基底。

The volcanic rocks of the Linzizong Group,corresponding to the Indo-Eurasian collision orogeny,had recorded key informations of the Neotethyan closure and continental collision processes.Though numerous previous studies on volcanic rocks in the Dianzhong Formation of the Linzizong Group have obtained a large number of achievements,it is still difficult to make a systematic comparative study as the studied volcanic rocks were collected mainly from the central and eastern parts of the Gangdese belt,but limitedly from the the western Gangdese.Detailed petrography,and zircon U-Pb chronology and Hf isotopic geochemistry of the rhyolitic crystalline tuff within the bottom part of the Dianzhong Formation in the Daruo area of the western Gangdese have been conducted in this paper.The results show that the LA-ICP-MS zircon U-Pb ages of the rhyolitic crystalline tuff within the bottom part of the Dianzhong Formation are 61.9±0.3 Ma and 61.1±0.6 Ma,which can approximately represent the diagenetic age of the volcanic rock within the bottom part of the Dianzhong Formation in the Daruo area.The weak negative zirconεHf(t)values vary from-4.97 to-1.54,with an average of-3.10,showing obviously enriched Hf isotopes.The zircon two-stage Hf isotope model ages(TDM2)vary from 1083 to 1273 Ma,indicating that its magma could be mainly originated from the middle Proterozoic crustal materials with contamination of minor mantle-derived components.Based on the comparative analysis of previous research results,it is concluded that the collision between India and Eurasia continents has a feature of diachronism in east-west direction,meaning that the collisional time in the middle part of the Gangdese belt is earlier than those in the eastern and western parts of the Gangdese belt.At the same time,it is also revealed that there is the difference of crustal structure in the southern Gangdese belt,meaning that the crust of southern Gangdese belt is not entirely composed of juvenile crust,as there is an ancient crystalline basement at least in the Zhunuo-Daruo-Dajiacuo area.