鄂尔多斯盆地东南部延长组长7段厚层熔结凝灰岩特征及其地质意义  

Characteristics and Geological Significance of Thick Ignimbrite Beds of Yanchang Formation (Chang 7 Section) in Southeastern Ordos Basin

作　　者：许锋 朱增伍 李长春 杨治国 XU Feng;ZHU Zengwu;LI Changchun;YANG Zhiguo(Shaanxi Center of Geological Survey,Xi'an,Shaanxi 710068,China)

机构地区：[1]陕西省地质调查中心,陕西西安710068

出　　处：《现代地质》2019年第2期389-400,共12页Geoscience

基　　金：陕西省公益性地质专项科技研究项目"鄂尔多斯盆地东南部三叠系油页岩富集规律研究和资源潜力评价"(公益20150301)

摘　　要：鄂尔多斯盆地延长组长7优质烃源岩中凝灰岩夹层广泛分布,单层厚度一般为0.1～10 cm,局部可达1～2 m。在盆地南部铜川瑶曲镇新发现了单层厚约14 m的岩屑晶屑熔结角砾凝灰岩层夹透镜状碳酸岩。熔结凝灰岩SiO_2含量在47.09%～66.6%之间,呈中性-偏酸性特征;∑REE为(164.04～245.74)×10^(-6),LREE相对富集,总体特征表现为负Eu异常明显的右倾型曲线;微量元素中大离子亲石元素Sr、P为负异常,高场强元素U、Th为明显的正异常。碳酸岩中SiO_2含量较高,矿物组成简单,可能为壳源碳酸岩; Sr和Ba的含量较高,显示典型的火成碳酸岩特征。根据熔结凝灰岩的野外露头及岩相学特征,结合其分布特征,建立了鄂尔多斯盆地中晚三叠世火山岩岩相模式,认为铜川瑶曲镇的熔结凝灰岩为火山通道相-爆发相。熔结凝灰岩源于火山弧高钾钙碱性岩浆源区,透镜状碳酸岩产生于碰撞造山过程中派生的引张大陆岩石圈断裂带,直接由岩石圈富集、地幔低程度部分熔融作用而产生的碳酸岩浆侵入或喷发所形成,与中晚三叠世秦岭造山带的演化关系密切,是秦岭造山带印支构造活动在鄂尔多斯盆地的沉积耦合效应,该发现为鄂尔多斯盆地中生代构造演化提供了新的岩石学证据。Ignimbrite interbeds are widely distributed in the high-grade hydrocarbon source rocks in the Chang 7 member of the Yanchang Formation in the Ordos basin.Generally,a single layer is 0.1 to 10 cm thick,occasionally reaching 1 to 2 m.Field geological and microscope petrographic studies at Yaoquzhen(Tongchuan city,southern Ordos basin) have identified a ~ 14-meter thick lithic crystal ignimbrite with interbedded lensoidal carbonatite.The SiO2 content is of 47.09% to 66.6%,which show that the rocks are primarily intermediate-felsic.Total REE(∑REE) contents are 164.04×10^-6 to 245.74×10^-6,and the rocks are enriched in LREEs,depleted in HREEs,with right-inclining REE pattern and negative Eu anomalies.LILEs(such as Sr and P)show negative anomalies whereas HFSEs(such as U and Th) show positive anomalies.For the carbonatites,the relatively high SiO2 and simple mineral assemblage suggest a crustal origin,whilst the high Sr and Ba contents show an igneous origin.Based on the geological and petrographic features of the ignimbrite and their distribution characteristics,the Middle-Late Triassic volcanic lithofacies model in the Ordos basin is established and the ignimbrite is likely of volcanic feeder-explosive facies.The ignimbrite was primarily from volcanic arc and had a high-K calc-alkaline magma source.The lensoidal carbonatites may have formed during the extrusion along extensional lithospheric faults resulted from continental collision,with the carbonatites produced directly from lowdegree partial melting of the lithospheric mantle,which was intimately related to the evolution of Qinling orogenic belt that was responsible to mountain-basin coupling.The discovery provides a new petrological basis to understand the tectonic evolution of the Mesozoic Ordos basin.

关 键 词：熔结凝灰岩 碳酸岩 火山通道相-爆发相 火山岩相 铜川瑶曲镇 延长组 鄂尔多斯盆地 

分 类 号：TE121[石油与天然气工程—油气勘探]