LI Hong-liang1,2，LI Guang-ming1,2*，DING Jun1,2，LIU Hong2，HUANG Han-Xiao2
(1. College of Earth Science, Chengdu University of Technology, Chengdu 610059, China;
2. Chengdu Center, China Geological Survey, Chengdu 610081, China)
Abstract: 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 zircon εHf(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.
Keywords: Zircon U-Pb dating; Hf isotopes; Dianzhong Formation; tuff; Gangdese
ACTA MINERALOGICA SINICA Vol. 39, No.3, 2019, page 334
