江南中段慈化地区新元古代高镁安山岩的厘定及其构造意义  

Geochronology and Geochemistry of Cihua Neoproterozoic High-Mg Andesites in Jiangnan Orogen and Their Tectonic Implications

作　　者：张玉芝[1,2] 王岳军[2,3] 郭小飞[1,2] 甘成势 邢晓婉[1,2] 宋菁菁[1,2] 

机构地区：[1]中国科学院广州地球化学研究所同位素地球化学国家重点实验室,广东广州510640 [2]中山大学地球科学与地质工程学院,广东广州510275 [3]广东省地质过程与矿产资源探查重点实验室,广东广州510275

出　　处：《地球科学（中国地质大学学报）》2015年第11期1781-1795,共15页Earth Science-Journal of China University of Geosciences

基　　金：国家重点基础研究发展计划(973计划)项目(No.2014CB440901);国家自然科学基金项目(Nos.41402165;41372198;41190073)

摘　　要：江南造山带被普遍认为是扬子与华夏陆块在新元古代的拼合带,其拼合机制及精细时代却一直备受争议.在江南造山带中段湘赣交界慈化地区识别出新元古代火山岩,并对其进行了锆石U-Pb年代学及主微量地球化学研究.该火山岩发育于冷家溪群地层中,其锆石LA-ICP-MS年代学测试得到了832±12Ma的206Pb/238 U加权平均年龄(n=16,MSWD=0.12),代表其喷发年龄.主量元素结果显示,SiO2和MgO含量分别为57.67%~61.33%和3.51%~4.29%,Mg#为52~57,高于正常弧火山岩,属高镁安山岩.微量元素富集轻稀土元素和大离子亲石元素,亏损高场强元素,其Nb-Ta、Ti亏损,具"弧型"地球化学特征,可能来源于受板片熔体/流体或者俯冲再循环沉积物交代的难熔地幔源区.上述资料表明,慈化高镁安山岩是江南造山带中段楔形地幔源区受消减组分交代作用的产物,暗示此时江南造山带中段仍在消减,扬子和华夏陆块尚未完全拼合.结合前人研究成果,江南造山带不同区段的闭合时间可能存在差异.It is generally accepted that the Jiangnan orogenic belt led to the amalgamation of the Yangtze with Cathaysia blocks during the Neoproterozoic period.However,its tectonic evolution and amalgamation timing are still in debate.This paper presents a set of new geochronological and geochemical data for the newly-identified Neoproterozoic high-Mg volcanics from Cihua（Jiangxi）.LA-ICP-MS zircon U-Pb dating of the representative sample yields a weighted mean age of 832±12Ma（n=16,MSWD=0.12）,representing the eruption age of the andesite.Geochemical results indicate that they can be classified as highMg andesites with SiO2 ranging from 57.67%to 61.33%,MgO from 3.51%to 4.29% and Mg-number from 52 to 57.Their chondrite-normalized REE patterns exhibit a left-sloping pattern with enriched LREEs relative to HREEs.On the primitive mantle-normalized multi-element patterns,these samples have strong enrichment in LILE and depletion in HFSE with marked negative Nb-Ta,Ti and positive Th anomalies,similar to those of the typical arc volcanics.The generation of the volcanic rocks might have been attributed to an interaction of the subducted melt/fluid or sediment with the overlying refractory mantle,suggesting the central Jiangnan orogen was still on the subduction till^832Ma.It is concluded that the Jiangnan orogen consistsof several branches with distinct amalgamation evolution.

关 键 词：慈化高镁安山岩 锆石U-PB定年 新元古代俯冲作用 江南中段 地球化学. 

分 类 号：P59[天文地球—地球化学]