广东莲花山花岗岩体锆石U-Pb年龄、地球化学特征及地质意义  

Zircon U-Pb Ages and Geochemical Constraints of the Lianhuashan Granitoids in Guangdong Province and Their Geological Implications

作　　者：杨航 辛宇佳 李建华[1] 张培星 YANG Hang;XIN Yu-jia;LI Jian-hua;ZHANG Pei-xing(Institute of Geomechanics,Chinese Academy of Geological Sciences,Beijing 100081;Department of Geology,Northwest University,Xi’an,Shaanxi 710069)

机构地区：[1]中国地质科学院地质力学研究所,北京100081 [2]西北大学地质学系,陕西西安710069

出　　处：《地球学报》2022年第2期211-223,I0002-I0011,共23页Acta Geoscientica Sinica

基　　金：国家自然科学基金项目(编号:41822205;42072239);中国地质科学院基本科研业务费项目(编号:DZLXJK202005);中国地质调查局项目(编号:DD20190306)联合资助。

摘　　要：广东莲花山岩体位于莲花山断裂带北部,由规模巨大的侏罗—白垩纪花岗岩组成。本文对莲花山岩体中部进行了系统的岩石学、地球化学、锆石U-Pb同位素和Lu-Hf同位素研究,获得片麻状花岗岩、细粒黑云母花岗岩和细-中粒黑云母花岗岩的锆石^(206)Pb/^(238)U年龄分别为(142.5±1.5)Ma(MWSD=3.5,N=30)、(138.9±0.6)Ma(MWSD=1.4,N=30)和(145.5±0.7)Ma(MWSD=1.2,N=28),表明岩体为晚侏罗至早白垩世岩浆活动的产物。地球化学特征显示岩体为偏铝质-弱过铝质(A/CNK=0.97~1.1)、富碱(K_(2)O+Na_(2)O=6.1 wt%~8.1 wt%)、富钾(K_(2)O/Na_(2)O=1.4~1.8),富集Rb、Th、U、K、Pb,亏损Ba、Ta、Nb、Sr、P、Ti,与壳源岩浆特征类似。岩体SiO_(2)含量差异较大(69.5 wt%~80.1 wt%),高硅样品明显经历一定程度的结晶分异,属于分异的I型花岗岩。所有样品锆石ε_(Hf)(t)值均为负值(–4.5~–2.0),在年龄-ε_(Hf)(t)图中,均落入球粒陨石演化线和华夏基底演化线之间,暗示源岩主要为古老壳源基底物质。在此基础上,结合区域构造-岩浆记录,本文认为莲花山花岗岩体的形成与古太平洋板片俯冲后撤(roll back)诱发的弧后扩张作用相关。The Lianhuashan granitoids,located in the north of the Lianhuashan fault zone,are composed of massive Jurassic–Cretaceous granites.This paper presents a synthetical petrologic,geochemical and geochronological study and Lu-Hf isotopic analysis of the granitoids.LA-ICP-MS zircon U-Pb dating yielded ages of 146~139 Ma,indicating that the granitoids were mainly generated in Late Jurassic to Early Cretaceous period.The granitoids are metaluminous to weakly peraluminous(A/CNK=0.97~1.1),and have the data K_(2)O+Na_(2)O=6.1 wt%~8.1 wt%and K_(2)O/Na_(2)O=1.4~1.8.They are all depleted in Ba,Ta,Nb,Sr,P and Ti,and enriched in Rb,Th,U,K and Pb.All these features point to a major crustal source.The Lianhuashan granitoids show large variation in the SiO_(2) content(69.5 wt%~80.1 wt%),and the high SiO_(2) samples underwent strong fractional crystallization,exhibiting characteristics of highly fractionated I-type granites.All zircons from these samples show negative ε_(Hf)(t)values from–4.5 to–2.0,recording a predominant contribution of partial melting of ancient crustal rocks.However,on the ages versus ε_(Hf)(t)diagram,all the samples are plotted on the field between the evolution curve of chondrite and Cathaysia basement,indicating minor mantle contributions to the source of the Lianhuashan granitoids.Integrating regional tectono-magmatic records,the authors infer that the Lianhuashan granitoids were formed in a back-arc extension environment,which was associated with roll-back of the subducting Paleo-Pacific plate in the Late Jurassic to Early Cretaceous period.

关 键 词：早白垩世 莲花山岩体 I型花岗岩 地壳伸展 锆石U-PB LA-ICP-MS LU-HF同位素 

分 类 号：P597.3[天文地球—地球化学] P588.121[天文地球—地质学]