黔西南晚侏罗世二长花岗斑岩的厘定及其年代学和地球化学研究  

A study of geochemistry and LA-ICP-MS U-Pb zircon dating of the late Jurassic Luokun monzonite granite-porphyry intrusion in the southwestern Guizhou

作　　者：杨恩林 周明忠[1] 张宏伟[1] 曾昭光 陆建宝[2] 周克林 YANG En-lin;ZHOU Ming-zhong;ZHANG Hong-wei;ZENG Zhao-guang;LU Jian-bao;ZHOU Ke-lin(School of Geography and Environmental Science,Guizhou Normal University,Guiyang 550025,China;117 Geology Brigade,Guizhou Bureau of Geology and Mineral Resources,Guiyang 550018,China)

机构地区：[1]贵州师范大学地理与环境科学学院,贵阳550025 [2]贵州省地质矿产勘查开发局117地质大队,贵阳550018

出　　处：《矿物岩石地球化学通报》2023年第3期508-520,共13页Bulletin of Mineralogy, Petrology and Geochemistry

基　　金：黔科合基础研究项目[2017-1406];贵州师范大学博士科研项目(GZNUD[2018]19);贵州省地勘局项目(黔地矿科合[2016]28号)。

摘　　要：近期在燕山期岩浆活动缺乏的黔西南地区发现了罗悃二长花岗斑岩,为厘定其形成年龄,明确黔西南地区燕山期岩浆岩活动机制,本文对其进行了激光剥蚀电感耦合等离子质谱仪(LA-ICP-MS)锆石U-Pb定年,Hf同位素和主、微量元素分析。结果表明,18颗柱状锆石年龄为(157.8±0.4)Ma(MSWD=1.4),代表二长花岗斑岩的侵位年代为晚侏罗世;锆石的176Hf/177Hf值(0.282390~0.282478)和εHf(t)(均值为-8.17)表明二长花岗斑岩的岩浆源区主要以富集地幔为主;岩石的SiO_(2)含量为65.91%,CaO为1.44%,Al_(2)O_(3)为15.67%,MgO为0.57%,TFe_(2)O_(3)为4.23%,K_(2)O为4.19%,Na_(2)O为5.72%,σ为3.76~4.98,A/CNK为0.93~0.96,显示富碱、准铝质的特征,应为I型花岗岩类;岩石的∑REE(323.57×10^(-6)~369.50×10^(-6))含量中等,LaN/YbN(7.84~9.30)显示为轻重稀土分馏,δEu(0.79~0.83)略微铕负异常,总体表现出右倾稀土配分曲线;明显富集Rb、Ba、Th、K、Nd,而强烈亏损Nb、Ta、P、Ti等元素。元素和同位素特征表明,罗悃二长花岗斑岩是富集岩石圈地幔部分熔融的产物,岩浆经历了分离结晶演化过程,并受到部分地壳物质的混染,形成于板内伸展环境。罗悃二长花岗斑岩晚侏罗世成岩时代的厘定,说明该时期黔西南地区处于岩石圈伸展-减薄阶段。There is lack of reported Yanshanian magmatic activity in the southwestern Guizhou.Recently,the Luokun monzonitic granite porphyry in the southwestern Guizhou has been found.In this paper,zircon U-Pb dating,Hf isotope analysis,and analysis of major and trace elements of the Luokun monzonitic granite porphyry have been carried out,to determine its formation age and to clarify the activity mechanism of Yanshanian magmatic rocks in the area.The LA-ICPMS U-Pb analyses of 18 zircon grains of the Luokun porphyry gave a weighted mean 206Pb/238U age of(157.8±0.4)Ma(MSWD=1.4),which is interpreted as the emplacement age of this intrusion.Lu-Hf isotopic analyses of those zircon grains gave 176Hf/177Hf ratios ranging from 0.282390 to 0.282478 and the averageεHf(t)value of-8.17,suggesting that its parental magma was mainly derived from the enriched mantle.The Luokun monzonitic granite porphyry samples have average contents of major elements of 65.91%for SiO_(2),1.44%for CaO,15.67%for Al2O3,0.57%for MgO,4.23%for TFe_(2)O_(3),4.19%of K_(2)O,and 5.72%for Na_(2)O,withσvalues of 3.76~4.98,A/CNK ratios of 0.93~0.96,and rich alkali and meta-aluminous features.Thus,the Luokun monzonitic granite porphyry belongs to the I-type granite.Its samples have moderate REE contents of 323.57×10^(-6)~369.50×10^(-6),relatively weak negative Eu anomalies(δEu values of 0.79~0.83),the differentiation between LREE and HREE(LaN/YbN values of 7.84~9.30),and slightly right inclined REE patterns.They are obviously enriched in Rb、Ba、Th、K、Nd,but depleted in Nb、Ta、P、Ti significantly.Geochemical and isotopic data indicate that the Luokun monzonitic granite porphyry was formed in the intraplate extensional setting by the magma,derived from the partial melting of enriched lithospheric mantle,through the evolution process of fractional crystallization and contamination of part of crustal materials.The determination of diagenetic age of Late Jurassic for the Luokun monzonitic granite porphyry indicates that the southwestern Guizhou was in the

关 键 词：黔西南 二长花岗斑岩 晚侏罗世 

分 类 号：P597[天文地球—地球化学]