玉树北部二叠纪镁铁质岩的成因及其对大陆弧后盆地形成的指示  

Petrogenesis of Permian mafic rocks from the northern Yushu area and implications for the formation of a continental back-arc basin

作　　者：刘彬[1] 蔡昊龙 马昌前[2,3] 吴栾 程琦 李福林 孙洋[1] LUU Bin;CAI Hao-long;MA Chang-qian;WU Luan;CHENG Qi;LI Fu-lin;SUN Yang(School of Geosciences,Yangtze University,Wuhan 430100,China;State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan 430074,China;Faculty of Earth Sciences,China University of Geosciences,Wuhan 430074,China;Wuhan Center,China Geological Survey(Central South China Innovation Center for Geosciences),Wuhan 430205,China)

机构地区：[1]长江大学地球科学学院,湖北武汉430100 [2]中国地质大学地质过程与矿产资源国家重点实验室,湖北武汉430074 [3]中国地质大学地球科学学院,湖北武汉430074 [4]中国地质调查局武汉地质调查中心(中南地质科技创新中心),湖北武汉430205

出　　处：《岩石矿物学杂志》2024年第2期344-359,共16页Acta Petrologica et Mineralogica

基　　金：国家自然科学基金项目(42130309,41972066,41502050);中国地质调查局项目(12120115026901,1212011121270)。

摘　　要：系统地调查与研究弧后盆地岩浆作用能为探究汇聚板块边缘的壳幔相互作用、板块俯冲动力学和弧后盆地形成-演化机制等提供重要信息。本文以最近在青海玉树北部地区发现的二叠纪镁铁质岩类为研究对象,开展了岩相学、锆石U-Pb年代学、岩石地球化学与同位素地质学等研究,以期为深入认识大陆弧后盆地形成机制及有关镁铁质岩浆的起源提供关键线索。锆石LA-ICP-MS U-Pb年代学测定结果表明,玉树北部辉长岩的结晶年龄为260±1 Ma。玄武岩样品具有相对低的TiO2和Nb/Y值以及相对高的Ce/Nb值,其相对于原始地幔要显著富集轻稀土和Th等元素,亏损Nb、Ta、Ti和P等元素,可与世界上典型弧后盆地玄武岩(例如Okinawa Trough BABBs)成分特征类比。而辉长岩样品具有相对高的TiO2和Nb/Y值以及相对低的Ce/Nb值,其相对于原始地幔要显著富集轻稀土和Th等元素,轻微亏损Nb、Ta和Ti,总体上具有与OIB类似的成分特征。玄武岩样品具有变化较大的ISr(0.706~0.709)和εNd(t)值(-1.8~+1.3),而辉长岩样品则具有相对均一的ISr(0.709)和εNd(t)值(-1.8~-1.5)。综合本次研究成果可以确定,玄武岩应该起源于一个受俯冲组分改造的岩石圈地幔,而辉长岩应该来自于一个与地幔柱相关的源区。结合区域最新研究资料可以进一步判断研究区镁铁质岩应该形成于大陆弧后盆地扩张的背景中,并且该弧后盆地的产生很可能与洋壳深俯冲导致的板块回撤、峨眉山地幔柱活动密切相关。The systematic investigations and studies of the back-arc basin magmatism can provide important information for exploring the crust-mantle interaction at the converging plate margin,plate subduction dynamics,and the formation-evolution mechanism of back-arc basins.This paper presents a comprehensive study of petrography,zircon U-Pb chronology,rock geochemistry and isotope geology for the Permian mafic rocks recently discovered in the northern Yushu area,and the data could offer key clues to understand the formation mechanism of continental back-arc basins and the origin of mafic magmas.The zircon LA-ICP-MS U-Pb dating results show that the crystallization age of the gabbro is 260±1 Ma.The basalt samples have relatively low TiO2 content and Nb/Y ratio and relatively high Ce/Nb ratio.Compared with the primitive mantle,those samples are significantly enriched in elements such as light rare earth elements and Th,and depleted of Nb,Ta,Ti,and P,etc.,which are similar to the composition characteristics of typical back-arc basin basalts(such as the Okinawa Trough BABBs)in the world.The gabbro samples have relatively high TiO2 content and Nb/Y ratio and relatively low Ce/Nb ratio.Those samples are significantly enriched in elements such as light rare earth elements and Th,and slightly depleted in Nb,Ta,and Ti,which could be comparable to those of OIB.The basalt samples have variable ISr(0.706~0.709)andεNd(t)(-1.8~+1.3),while the gabbros samples have relatively uniform ISr(0.709)andεNd(t)(-1.8~-1.5).Based on the results of this study,it can be concluded that the basalts should be derived from a lithospheric mantle modified by subducted components,while the gabbros might be originated from a mantle plume-related source.Combining with new regional studies,we propose that the mafic rocks in the study area should be formed in a continental back-arc basin environment,and the formation of the back-arc basin could be attributed to the slab-rollback caused by the deep oceanic subduction and the activity of the Emeishan mantle p

关 键 词：镁铁质岩 大陆弧后盆地 二叠纪 北羌塘 青藏高原 

分 类 号：P588.12[天文地球—岩石学] P597+.3[天文地球—地质学]