阿拉善地块新生代玄武岩和粗玄岩地球化学特征及其地质意义  

Geochemical characteristics of Cenozoic basalts and doleritesin the Alxa block and its geological implications

作　　者：田荣松 解国爱[2] 林灿楠 TIAN Rongsong;XIE Guoai;LIN Cannan(Resource and Environmental Engineering College,Guizhou University,Guiyang,Guizhou 550025,China;School of Earth Sciences and Engineering,Nanjing University,Nanjing,Jiangsu 210093,China)

机构地区：[1]贵州大学资源与环境工程学院,贵州贵阳550025 [2]南京大学地球科学与工程学院,江苏南京210093

出　　处：《地质学报》2022年第7期2365-2379,共15页Acta Geologica Sinica

基　　金：国家自然科学基金项目(编号42072218);国家重点研发计划项目(编号2018YFC0603703)联合资助的成果。

摘　　要：新生代时期受太平洋板片俯冲影响,华北克拉通中东部广泛发育具有洋岛玄武岩特征的板内岩浆活动。然而华北克拉通西段目前并无新生代火山岩年代学及地球化学相关的研究。本文首次报道华北克拉通西部阿拉善巴隆乌拉新生代岩浆活动。来自巴隆乌拉玄武岩和粗玄岩锆石U-Pb定年限定这些岩石形成于始新世时期(37.2~36.5 Ma)。新生代玄武岩和粗玄岩SiO_(2)(50.38%~51.65%)和Al_(2)O_(3)(16.9%~17.5%)含量,富集轻稀土元素和大离子亲石元素,无Eu负异常和Nb、Ta亏损,显示洋岛玄武岩特征。玄武岩和粗玄岩样品^(87)Sr/^(86)Sr值为0.7054~0.7057、全岩ε_(Nd)(t)值及锆石ε_(Hf)(t)值分别为-7.2~-6.7和-3.2~-1.1之间,显示富集的同位素组成。地球化学研究揭示这些新生代岩石在上升过程中经历了有限的地壳混染作用。与华北克拉通东部新生代玄武岩相比,阿拉善新生代玄武岩具有更高的大离子亲石元素和更加富集的同位素组成。阿拉善地块北部地区的新生代玄武岩和粗玄岩起源于部分熔融曾被俯冲熔体交代富集的石榴子石二辉橄榄岩,新生代由于岩石圈伸展和软流圈地幔上涌导致交代富集的地幔源区发生部分熔融,在阿拉善地区形成具有洋岛玄武岩地球化学特征的火山岩。Cenozoic continental basalts with oceanic island basalt(OIB)-like geochemical compositions in the middle and eastern North China craton(NCC) are associated with the subduction of the Pacific slab.However,the geochronological and geochemical data of Cenozoic volcanic rocks in the western part of the North China craton are rarely reported.This paper reports for the first time the Cenozoic magmatism in Balongwula,northern Alxa block,western NCC.The basalts and dolerites in the Balongwula are constrained to the Eocene(37.2~36.5 Ma) by zircon U-Pb dating.These basalts and dolerites show high SiO_(2)(50.38%~51.65%) and Al_(2)O_(3)(16.9%~17.5%) contents,are characterized by enrichment in light rare earthelements(LREEs) and large ion lithophile elements(LILEs),without negative Eu anomalies and Nb,Ta depletion,indicating an affinity with OIB.These rocks also display enriched isotopic compositions with initial ^(87)Sr/^(86)Sr ratios(0.7054~0.7057),ε_(Nd)(t)(-7.2 to-6.7) and ε_(Hf)(t)(-3.2 to-1.1).Geochemical analyses reveal that crustal contamination in these Cenozoic rocks has been negligible.Compared with the Cenozoic basalts in the eastern NCC,the Cenozoic basalts in the northern Alxa block have more enriched isotopic compositions and higher LILEs contents.The Cenozoic basalts and dolerites in the northern margin of the Alxa block were derived from the partial melting of garnet lherzolite that was metasomatized by subducted-related melts.During the Cenozoic,the extension of the lithosphere drove the upwelling of the asthenospheric mantle,leading to partial melting of the metasomatized enriched mantle,forming volcanic rocks with OIB-like geochemical features in the Alxa block.

关 键 词：阿拉善地块 新生代 洋岛玄武岩 古亚洲洋板片 

分 类 号：P588.145[天文地球—岩石学]