| 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 |
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| 引用本文： | 许锋,朱增伍,李长春,杨治国.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[J].现代地质,2019,33(2):389-400. |
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| 作者姓名： | 许锋 朱增伍 李长春 杨治国 |
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| 作者单位： | 陕西省地质调查中心,陕西 西安 710068 |
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| 基金项目： | 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) |
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| 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。
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| 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 |
| 收稿时间： | 2018-06-15 |
| 修稿时间： | 2018-11-07 |
Characteristics and Geological Significance of Thick Ignimbrite Beds of Yanchang Formation (Chang 7 Section) in Southeastern Ordos Basin |
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| XU Feng,ZHU Zengwu,LI Changchun,YANG Zhiguo.Characteristics and Geological Significance of Thick Ignimbrite Beds of Yanchang Formation (Chang 7 Section) in Southeastern Ordos Basin[J].Geoscience——Journal of Graduate School,China University of Geosciences,2019,33(2):389-400. |
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| Authors: | XU Feng ZHU Zengwu LI Changchun YANG Zhiguo |
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| Institution: | Shaanxi Center of Geological Survey, Xi’an, Shaanxi 710068,China |
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| Abstract: | 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 low-degree 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. |
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| Keywords: | ignimbrite carbonatite volcanic feeder-explosive facies volcanic lithofacies Yaoquzhen Tongchuan city Yanchang Formation Ordos basin |
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