秦祁昆结合部晚三叠世OIB型玄武岩岩石学、地球化学及SHRIMP锆石U-Pb年代学研究  

Petrology,geochemistry and SHRIMP U-Pb dating of zircons from Late Triassic OIB-basalt in the conjunction of the Qinling-Qilian-Kunlun orogens

作　　者：闫臻[1] 郭现轻[1] 付长垒[1] 王涛[1] 王宗起[2] 李继亮[3] 

机构地区：[1]中国地质科学院地质研究所,北京100037 [2]中国地质科学院矿产资源研究所,北京100037 [3]中国科学院地质与地球物理研究所,北京100029

出　　处：《地学前缘》2012年第5期164-176,共13页Earth Science Frontiers

基　　金：国家自然科学基金项目(40772137);中国地质调查局地质调查项目(1212010911033;1212011120159)

摘　　要：秦祁昆结合部夏河县八角城作海枕状玄武岩,不整合于中二叠统大关山组生物灰岩之上。枕状构造保存完好,气孔、杏仁构造发育,具斑状结构。斑晶主要为橄榄石,其次为斜长石和辉石;基质由板条状斜长石微晶、粒状辉石、橄榄石及钛铁矿等矿物组成。橄榄石斑晶发生伊丁石化、滑石化和绿泥石化,部分橄榄石完全被伊丁石化。TiO2质量分数为1.99%～2.24%,Mg#值为58～64,∑REE为(106～119)×10-6,LaN/YbN为5.6～7.1,富集Nb、Ta、Zr,以及Rb、Ba、Th、U、K等元素,高Nb/Y低Zr/Ti,并且呈现出与夏威夷、土耳其等地区OIB型碱性玄武岩相一致的稀土、微量元素配分模式曲线。橄榄石斑晶Fo值为82.73～84.57,w(CaO)均大于0.1%;斜长石斑晶An为64.72%～70.39%,基质中斜长石An为58.88%～61.53%,且斜长石斑晶较基质中斜长石呈现出低Si、Fe、Na、K、Ti和高Al、Ca特征。这些事实表明,形成该套岩石组合的原始岩浆曾经经历了橄榄石和斜长石等矿物的结晶分异作用。SHRIMP锆石U-Pb测年结果表明,八角城作海玄武岩形成时代为晚三叠世Norian期((220.5±4.2)Ma)。结合区域地质资料,在秦祁昆结合部古特提斯洋于晚三叠世俯冲消减结束并进入陆-陆碰撞造山作用阶段,该时期陆-陆碰撞作用引发的岩石圈拆沉作用导致岩石圈的减薄,进而形成八角城作海OIB型枕状玄武岩。The pillow lava,which crops out in Zuohai of Xiahe County at the conjunction of Qinling-Qilian-Kunlun orogens,overlies mid-Permian limestones of the Daguanshan Group disconformably.These rocks are porphyritic with abundant olivine,minor plagioclase and clino-pyroxene phenocrysts.The matrix is composed of tabular plagioclase microlites,granular pyroxene,olivine,and minor ilmenite.The olivine phenocrysts have altered into iddingsite,talcum and chlorine,and some of them have totally converted into iddingsite.Their geochemical characteristics are very similar to the OIB-type alkali basalts from Hawaii and NW Turkey,with high TiO2 content（1.99%-2.24%） and Mg# values（58-64）.The Fo values of olivine phenocrysts range from 82.73 to 84.57,with CaO contents higher than 0.1%.The plagioclase phenocrysts（An=64.72-70.39） have lower contents of Si,Fe,Na,K and Ti and higher Al and Ca contents than those plagioclase（An=58.88-61.53） in groundmass.These suggest that the magma has undergone crystallization differentiation of olivine and plagioclase.SHRIMP zircon U-Pb dating yields a mean age of（220.5±4.2） Ma,indicating that these pillow lava should form in the Norian.Regional data also indicate that the Paleo-Tethys Ocean died out and continent-continent collision took place in the late Triassic within the conjunction of the Qinling-Qilian-Kunlun orogens.Then,the delamination of lithosphere probably gave rise to a thinner lithosphere that resulted in volcanism with OIB geochemical affinities.

关 键 词：OIB型玄武岩 晚三叠世 构造背景 秦祁昆结合部 

分 类 号：P588.1[天文地球—岩石学]