云南大关新地2井奥陶—志留纪之交钾质斑脱岩岩石地球化学特征分析  

Petrology and geochemistry of the K-bentonites at the Ordovician-Silurian transition in XD2 well,Daguan,Yunnan Province

作　　者：葛祥英[1,2] 牟传龙 余谦[1] 刘伟 门欣[1,3] 何江林[1] 陆俊泽[1] 梁薇 GE Xiangying;MOU Chuanlong;YU Qian;LIU Wei;MEN Xin;HE Jianglin;LU Junze;LIANG Wei(Chengdu Institute of China Geology and Mineral Resources,Chengdu 610081,Sichuan,China;China University of Geosciences(Beijing),Beijing 100083,China;Shandong University of Science and Technology,Qingdao 266590,Shandong,China)

机构地区：[1]中国地质调查局成都地质调查中心,四川成都610081 [2]中国地质大学(北京),北京100083 [3]山东科技大学,山东青岛266590

出　　处：《中国地质》2021年第3期911-924,共14页Geology in China

基　　金：中国地质调查局项目“四川盆地下古生界海相页岩气基础地质调查”(DD20160176);“西南主要成矿带铀矿资源调查”(DD20190122);“四川盆地地质结构与深层油气综合调查”(DD20211210)联合资助。

摘　　要：扬子地台内晚奥陶世末—早志留世初五峰—龙马溪组内沉积了多层钾质斑脱岩,但对于该时期扬子地台西缘钾质斑脱岩的研究报道相对较少。本文旨在通过对云南大关地区新地2井五峰—龙马溪组内沉积的钾质斑脱岩进行矿物学及地球化学分析,进一步确定扬子西缘该时期钾质斑脱岩原始岩浆类型及其所产生的构造环境。矿物学特征表明,钾质斑脱岩主要由黏土矿物和非黏土矿物组成,其中黏土矿物由伊利石和伊蒙混层组成,非黏土矿物以石英、长石、方解石、白云石和黄铁矿等为主。钾质斑脱岩主量元素以高K_(2)O,低TiO_(2)为特征,微量元素特征表现为富集Rb、Ba、Th、U等元素,Ti、P元素相对亏损,Ti/Th值指示了酸性火山灰的性质;ΣREE在(49.86~209.43)×10^(-6);与球粒陨石相比,轻稀土轻微富集、具Eu负异常,无Ce异常;在Nb/Y-Zr/TiO_(2)图解中,数据点主要落在安山岩和粗面英安岩之间,表明钾质斑脱岩源岩浆性质为中酸性岩浆;依据微量元素特征和构造环境判别结果,初步认为原始岩浆可能形成于岛弧环境,其火山灰来源可能与扬子北缘早古生代秦岭洋闭合过程中的板块碰撞有关。Many K-bentonites have been recognized from the Wufeng-Longmaxi Formations(Upper Ordovician-Lower Silurian)in the Yangtze Block,but only a few of them on the western margin of the Yangtze Block are reported.The mineralogical and geochemical studies of K-bentonites in the Wufeng-Longmaxi Formations through Xindi 2 well in Daguan area of Yunnan province were carried out to confirm the original magma type and its tectonic setting.The mineralogical characteristics show that the potassium bentonite is mainly composed of clay minerals and non-clay minerals,in which the clay minerals are composed of illite and illite-montmorillonite mixed beds.The non-clay minerals are mainly quartz,feldspar,calcite,dolomite and pyrite.It is geochemically characterized by high K_(2)O and low TiO_(2),relative enrichment of Rb,Ba,Th and U and depletion of Ti and P elements.The Ti/Th values indicates acidic volcanic ash character.Compared with the chondrite,the total rare earth elements is(49.86-209.43)×10^(-6) with slight rich LREE and negative Eu amomaly,without Ce abnormity.In Nb/Y-Zr/TiO_(2) diagram,the data dots are mainly plotted in the andesite and trachy andesite range,which shows that the volcanic ash is mostly from middle-acid rocks.Various chemical discrimination diagrams and trace elements imply that K-bentonites were possibly derived from an island arc environment,and the volcanic ash was probably related to the subduction and closure of the Qinling Ocean on the northern border of Yangtze Plate in the Early Paleozoic.

关 键 词：新地2井 钾质斑脱岩 奥陶—志留纪之交 地球化学 源岩浆 构造背景 大关地区 云南 

分 类 号：P618.67[天文地球—矿床学]