香港世界地质公园东平洲平洲组新发现火山岩的锆石U-Pb测年、地球化学及地质意义  

Discovery of volcanic rocks in the Pingchau Formation in Tungpingchau,Hong Kong UNESCO Global Geopark:Zircon U-Pb geochronology,geochemistry and geological implications

作　　者：王璐琳[1] 刘晓鸿[1] 张志光[2] WANG Lulin;LIU Xiaohong;ZHANG Zhiguang(Institute of Nature&Culture,China University of Geosciences(Beijing),Beijing 100083,China;Chinese Academy of Geological Sciences,Beijing 100037,China)

机构地区：[1]中国地质大学(北京)自然文化研究院,北京100083 [2]中国地质科学院,北京100037

出　　处：《地学前缘》2023年第2期239-258,共20页Earth Science Frontiers

基　　金：香港特别行政区地质调查项目(AFCD/SQ/211/17/C)。

摘　　要：香港世界地质公园东平洲平洲组由于缺乏地质年代学数据,其时代归属一直存在较大争议。在该组中段下部新发现火山岩夹层,经薄片鉴定为流纹质玻屑凝灰岩,进行锆石U-Pb年代学和岩石地球化学研究,并在此基础上探讨其岩石成因和构造意义。锆石LA-ICP-MS U-Pb定年结果表明平洲组火山岩形成于早白垩世。地球化学数据显示,流纹质玻屑凝灰岩全碱含量较高(w(Na_(2)O+K_(2)O)=5.58%~9.45%),轻稀土元素分馏明显,轻稀土富集,重稀土亏损,Eu的负异常明显;微量元素呈现Rb、Ba等大离子亲石元素相对富集,高场强元素Th、U、Pb、Zr等相对富集,Sr强烈亏损,Ta、Nb相对亏损的特征。表明研究区平洲组火山岩是由大陆地壳的深熔作用形成,形成于太平洋板块俯冲引起的弧后伸展环境,是香港早白垩世火山活动最后阶段的产物,与我国东南沿海早白垩世大规模岩石圈伸展减薄和岩浆作用有关。本文的岩石地球化学和同位素年代学数据为东平洲平洲组的最新资料,为我国东南部早白垩世火山活动和构造演化提供了新的年代数据。The age of the Pingchau Formation in Hong Kong UNESCO Global Geopark has been controversial due to lack of geochronological data. In this study a new volcanic interbed was discovered in the lower part of the middle section of the Pingchau Formation, and identified as rhyolitic vitric tuff by thin section analysis. To better understand the petrology and genesis of Pingchau volcanic rocks a detailed geochronology and geochemical study was conducted through zircon U-Pb dating and major and trace element analysis. LA-ICP-MS zircon U-Pb dating of zircons from two rock samples yielded Early Cretaceous age. These rock samples have high total alkalis(w(Na_(2)O+K_(2)O)) content(5.58%-9.45%), and show obvious light rare earth element fractionation;they are enriched in light rare earth elements and depleted in heavy rare earth elements, with appearance of Eu anomalies. The rock samples are enriched in large ionic lithophile elements(e.g., Rb, Ba), high field strength elements(e.g., Th, U, Pb, Zr), extremely depleted in Sr, and relatively depleted in Ta and Nb. Combined with previous studies on Late Mesozoic volcanic belt in southeastern China, we propose that volcanic rocks of the Pingchau Formation were likely originated from partial melting of crustal rocks in back-arc extensional setting induced by subduction of the Paleo-Pacific Plate, and formed by late-stage Early Cretaceous volcanic activity in Hong Kong in association with large-scale lithospheric extension, thinning and magmatism along the southeast coast of China. The petrogeochemical and isotopic chronology data in this paper are the latest on the Pingchau Formation, and our results provide valuable new information for better understanding Early Cretaceous volcanism and tectonic evolution in southeastern China.

关 键 词：平洲组火山岩 锆石U-PB年龄 地球化学 早白垩世 香港世界地质公园 

分 类 号：P597[天文地球—地球化学] P595[天文地球—地质学]