【研究目的】柳东地区位于右江盆地东缘，近年来该地区早三叠世石炮组中首次发现一套火山岩夹层，为研究盆地东缘的构造演化特征提供了重要依据。【研究方法】本文以新发现的柳东地区火山岩为研究对象，通过LAICP-MS锆石U-Pb年龄及Hf同位素分析，研究右江盆地东缘火山岩的形成时代及岩浆源区特征。【研究结果】结果表明，火山岩的岩性为凝灰岩-凝灰质熔岩-凝灰质火山角砾岩组合，指示了典型的火山岩特征。其2件凝灰岩样品中锆石LA-ICP-MS U-Pb加权平均年龄分别为（244.0±1.9） Ma和（247.1±0.9） Ma，锆石微量元素（U、Th、Hf、Nb）表现出大陆岩浆弧属性特征。锆石Hf同位素结果显示εHf （t）为-13.2~-1.2，二阶段模式年龄（TDMC）为1.35~2.06 Ga，其岩浆源区主要为地壳物质，平均地壳存留年龄为1.7~2.06 Ga。【结论】该火山岩夹层与其所出露的地层时代基本相符，属于早三叠世末期—中三叠世早期火山喷发产物，火山岩形成于火山弧-同碰撞的构造环境，代表了弧后盆地的沉积环境，与印支期古特提斯洋的闭合相关。该成果为古特提斯构造域的演化研究提供了新的基础资料，具有重要意义。
This paper is the result of geological survey engineering.
[Objective]Eastern Liuzhou is located in the eastern margin of Youjiang Basin. In recent years, a set of volcanic rock interlayer has been found in the Early Triassic Shipao Formation in the eastern Liuzhou for the first time, which provides an important basis for studying the tectonic evolution of the eastern margin of the Youjiang Basin.
[Methods]Taking the newly discovered volcanic rocks in Eastern Liuzhou as the research object, this paper investigates the formation age and magmatic source characteristics of the volcanic rocks in the eastern margin of the Youjiang Basin through LA-ICP-MS zircon U-Pb age and Hf isotope analysis.
[Results]The results show that the volcanic rocks are composed of tuff, tuff lava, and tuffaceous volcanic breccia, showing typical characteristics of volcanic rock. LA-ICP-MS zircon U-Pb age dating results show that the weighted mean ages of zircon in the two tuff samples are (244.0±1.9) Ma and (247.1±0.9) Ma respectively. The trace elements (U, Th, Hf, Nb) compositions of zircon show the characteristics of continental magma arcs-derived zircons. The zircon have εHf(t) values of-13.2--1.2, with second-stage model ages (TDMC) of 1.35-2.06 Ga, suggesting that their magma source is mainly crustal materials, with average crustal residual age of 1.7-2.06 Ga.
[Conclusions]The timing volcanic rock interlayer is basically consistent with the stratigraphic age of the Early Triassic Shipao Formation, and are the product of volcanic eruption during the late Early Triassic to the early Middle Triassic. The volcanic rocks were formed in volcanic arc-related, syn-collisional tectonic setting, which is the representative of the sedimentary environment of back-arc basin, and is related to the closure of the paleo-Tethys during the Indosinian period. This outcome provides new and crucial information for a better understanding the tectonic evolution of the paleo-Tethys.
Highlights: The volcanic rocks in eastern Liuzhou are the products of volcanic eruption during the late Early Triassic to the early Middle Triassic, and were formed in volcanic arc-related, syn-collisional setting related to the closure of the Paleo-Tethys.
魏春夏,陆刚,杨锋,李巍,钟锋运,金靖杰,潘艺文,梁力杰,白晓. 广西柳东地区火山岩锆石U-Pb-Hf同位素特征及其对岩浆源区的限定[J]. 中国地质, 2022, 49(4): 1309-1322.
WEI Chunxia, LU Gang, YANG Feng, LI Wei, ZHONG Fengyun, JIN Jingjie, PAN Yiwen, LIANG Lijie, BAI Xiao. Zircon U-Pb ages and Hf isotopic compositions of volcanic rocks in Eastern Liuzhou, Guangxi: Constraint for source characteristics of magmas[J]. Geology in China, 2022, 49(4): 1309-1322(in Chinese with English abstract).