桂东南早三叠世火山通道的发现及岩石成因  

The discovery and petrogenesis of Early Triassic volcanic vent in Southeastern Guangxi

作　　者：农军年 孙明行 郭尚宇 李昌明 韦守东 覃初礼 赵子宁 周辉 张鹏 NONG Junnian;SUN Minghang;GUO Shangyu;LI Changming;WEI Shoudong;QIN Chuli;ZHAO Zining;ZHOU Hui;ZHANG Peng(China—ASEAN Geological Cooperation Center,Nanning,530023;Guangxi Institute of Geological Survey,Nanning,530023;Guangxi Mineral Resources and Reserves Evaluation Centre,Nanning,530023)

机构地区：[1]中国—东盟地学合作中心(南宁),南宁530023 [2]广西壮族自治区地质调查院,南宁530023 [3]广西矿产资源储量评审中心,南宁530023

出　　处：《地质论评》2022年第3期1089-1105,共17页Geological Review

基　　金：中国地质调查局地质调查项目(编号:DD20160035-11);广西科技厅项目(编号:桂科AD20297091)的成果。

摘　　要：位于扬子地块与华夏地块拼合带西南段的桂东南容县下罗杏发现早三叠世火山通道,对剖析华南印支期区域构造演化具有重要的地质意义。该火山通道出露主要岩性为隐爆角砾岩、流纹岩、粗面安山玢岩、霏细斑岩等,柱状节理发育,为超浅成相—溢流相。流纹岩LA-ICP-MS锆石U-Pb年代学分析表明,加权平均值为249.9±1.6 Ma(MSWD=1.9),代表了火山喷出年龄,属早三叠世(印支期)岩浆活动产物。岩石具有钾玄岩—高钾钙碱性系列,较低的TiO_(2)含量、较高的K_(2)O/Na_(2)O值、明显亏损Ta、Nb、P、Ti等高场强元素,显示出弧火山岩的特征。锆石的ε_(Hf)(t)值为-14.50~-6.61,对应的单阶段Hf模式年龄(T_(DM))为1.17~1.49 Ga,二阶段Hf模式年龄(T_(2DM))为1.70~2.20 Ga。综合地球化学、锆石Hf同位素组成特征及区域地质资料,认为下罗杏火山岩,为地壳减压松弛的构造背景下,地幔或软流圈物质上涌,诱发地壳物质发生高温熔融作用的形成,其构造—岩浆热事件的动力应该来源于古太平洋向华南板块的俯冲。Located at southwestern convergence belt of Yangtze and Cathaysia Massifs near Xialuoxing village,Rongxian county, southeastern Guangxi Zhuang Autonomous Region, the Early Triassic volcanic vent ( pipe) ishighly significant in studying the Indosinian regional tectonic evolution of South China. The vent consists of a suiteof cryptoexplosion breccia, rhyolite, trachyandesite porphyrite, felsites porphyry, with columnar joints, and showscharacteristics of super-hypabyssal intrusion and effusive facies. The LA-ICP-MS zircon U-Pb dating of rhyolitefrom the vent gives a weighted average age of (249. 9 ± 1. 6) Ma (MSWD = 1. 9), which represents theeruption age of volcanic rocks and suggests the production of Early Triassic magmatism. Geochemically, theintrusive rocks belong to shoshonite to high potassium-calc alkaline series, with low TiO_(2)content, high K_(2)O / Na_(2)Oratio, obvious depletion of high field strength elements such as Ta,Nb,P,Ti, and are typical of the arc volcanicrocks. Zircons of these rocks have ε_(Hf)(t) values of -14. 50 to -6. 61, with corresponding single-stage Hf modeage (T_(DM) ) 1. 17 ~ 1. 49 Ga and two-stage model age (T_(2DM) ) ranged from 1. 70 to 2. 20 Ga. Comprehensive dataanalysis on the geochemistry, Hf isotopic characteristics of zircon and regional geology suggest that the Xialuoxingvolcanic rocks were formed by melting of the Paleoproterozoic meta-sedimentary rocks in an extensional tectonicsetting. Heat input associated with the upwelling of asthenosphere mantle might have played an important role incrustal melting and crystallization of the granitoids. Geodynamically, the formation of the Xialuoxing volcanic rockswere related to the subduction of the Paleo-Pacific plate.

关 键 词：火山通道 早三叠世 LA-ICP-MS 锆石U-PB定年 HF同位素 地球化学 

分 类 号：P581[天文地球—岩石学] P534.51[天文地球—地质学]