Discovery of Paleo-volcanic edifice and determination of its eruptive circles of Emeishan basalt in Zhaojue-Meigu Area, Sichuan Province
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摘要:
四川昭觉—美姑地区峨眉山玄武岩古火山机构的发现及其喷发旋回的确定提要：野外地质调查发现，扬子西缘昭觉—美姑一带存在多处峨眉山玄武岩组古火山机构，为研究其火山喷发旋回与方式提供了良好窗口。古火山机构的岩石类型主要由巨厚玄武岩以及少量的火山碎屑岩和火山碎屑熔岩组成，且均具火山颈爆发相、溢流相和火山洼地相堆积构造特征。系统的地质剖面研究表明，峨眉山玄武岩组存在3次快速喷发亚旋回及25个火山韵律，亚旋回之间不存在火山间歇期。其中，第一亚旋回和第三亚旋回的喷发方式均以爆发相与溢流相为主，而第二亚旋回则以溢流相为主。峨眉山玄武岩组古火山机构的系统厘定和研究，揭示了峨眉山大火成岩省的火山喷发方式和旋回韵律特征，为研究与峨眉山玄武岩组相关的成矿成藏作用提供了重要地层资料。
Abstract:The geological survey revealed that several paleo-volcanic edifices related to Emeishan basalt were identified in the Zhaojue-Meigu area on the western margin of Yangtze Block, which provides an excellent window for studying their eruption cycles and modes. The rock types of paleo-volcanic edifices are mainly composed of thick basalts, small amounts of pyroclastic rocks and volcanic clastic lavas. They have the characteristics of explosive facies, lava overflow facies and volcanic depression facies. Systematic geological profile suggest that there are three rapid eruption cycles (without any interval) and 25 volcanic rhythms during the formation of the Emeishan basalt Formation. Among them, the eruption patterns of the first and third cycles are dominated by the explosive facies and lava overflow facies, while the second cycle is dominated by the overflow phase. The systematic confirmation and study of the paleo-volcanic edifices reveal the volcanic eruption pattern and cycle rhythm characteristics of Emeishan large igneous province, which provides important data for the further study of the Wupo copper deposits in the western margin of Yangtze Block.
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1. 研究目的（Objective）
甘肃省高台县大青山地区地处阿拉善地块龙首山基底杂岩带，位于酒东盆地马营凹陷东段山前沉积盆地北缘（图 1a）。区内主要出露有古元古界—新太古界龙首山岩群、中元古界蓟县系墩子沟群、海西期侵入岩、侏罗系龙凤山组和白垩系庙沟组（图 1b）。
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图 1 甘肃省高台县大青山地区大地构造位置图（a）、区域地质图（b）以及ZK1201岩性柱状图（c），含油气岩心照片（d-g）和油气成藏模式图（h）Figure 1. Tectonic location (a), regional geological map(b), lithology column(c) and drill photos of ZK1201 (d-g), hydrocarbon accumulation pattern (h) in the Daqingshan area of Gaotai County, Gansu Province为实现研究区金属资源和油气资源的综合调查，中国地质调查局发展研究中心联合甘肃省地调院、探矿工程所、吉林大学在前期“甘肃省高台县臭泥墩—西小口子地区三幅1∶5万矿产远景调查”项目基础上，通过开展专题地质填图、矿产综合信息预测、智能找矿预测等工作，部署实施钻孔ZK1201，以期实现找矿突破。
2. 研究方法（Methods）
利用研究区地质调查、磁法、激电测深、化探数据和无人机影像等资料，开展综合信息解译。采用卷积和孪生网络神经网络模型对区内典型金属矿床成矿作用特征标志、油气赋矿层位进行深度学习，提出工程验证建议。钻探验证所采用钻机为汽车钻，整机包括车底盘、动力系统、液压系统、操控系统等。
3. 结果（Results）
在综合研究和智能预测的基础上，布设的ZK1201孔在钻穿早二叠世花岗闪长岩（图 1c）后，钻遇地层，续钻至393.8 m后终孔（图 1c）。此次工作共钻遇中侏罗统龙凤山组地层220 m，共发现14层油层（总厚145 m，单层最大厚度28 m，最小厚度1.4 m）。钻孔含油性由上部砾岩（油斑级以下）向下部砂岩（富含油或饱含油）逐渐增多，其中高角度裂缝普遍见可流动原油（图 1d~g）。经国家地质实验测试中心分析，原油中饱和烃、芳烃含量分别占32.4%和34.6%，为高品质轻质原油。原油中正构烷烃分布完整，主峰碳数、奇偶优势及甾烷和藿烷分布都指示其陆相烃源岩来源。
野外地质调查发现，白垩系庙沟组近水平发育，与下伏侏罗系龙凤山组呈角度不整合接触。庙沟组主要由厚层暗色泥岩组成，并发育薄层暗色粉砂质泥岩，可能为区域烃源岩层。初步判断成熟的烃源岩排出的油气沿角度不整合运移至侏罗系砂砾岩和砂岩储层后，被逆冲推覆花岗岩体封闭，形成构造-岩性油气藏（图 1h）。
研究发现区域内沉积盆地最南缘边界处在祁连山北缘断裂之下，最北缘处在龙首山断裂的下盘，南北跨度约80 km。区域内沉积地层较厚，其中侏罗系龙凤山组厚约2100 m，白垩系庙沟组厚约900 m，说明研究区具有较大的成藏潜力。此次油气藏的发现，预示着大青山地区具有完整的油气成藏系统，显示出良好油气勘探前景。建议进一步加强油气基础地质调查研究工作。
4. 结论（Conclusions）
（1）在大青山地区花岗岩逆冲推覆体之下的中生代沉积地层中发现原油，所发现的高品质轻质原油，具陆相烃源岩来源特征。
（2）研究区具有良好的油气勘探前景，建议进一步加强油气地质调查研究工作。
5. 致谢（Acknowledgement）
感谢甘肃省地质调查院董国强，北京探矿工程研究所渠洪杰、谭春亮以及国家实验测试中心沈斌在野外工作和样品测试过程中的协助。
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图 1 峨眉山玄武岩分布示意图及研究区位置(据Ali et al., 2010; Sun et al., 2010修改)
Figure 1. Schematic map showing the distribution of the Emeishan basalts and the studied area (modified from Ali et al., 2010; Sun et al., 2010)
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图 3 第一喷发亚旋回古火山机构岩石的野外特征
a—瓦侯火山集块岩露头；b—瓦侯火山集块岩中的灰岩集块和石英砂岩集块；c—竹核火山集块岩棱角状灰岩集块；d—竹核火山集块岩熔蚀港湾状的灰岩集块；e—德吉火山集块岩中玄武岩集块；f—黄茅梗火山集块岩中浑圆状的灰岩集块和次棱状的玄武岩集块
Figure 3. Field photographs of ancient volcanic apparatus rocks in the first eruptive cycle
a-Wahou volcanic agglomerate outcrop; b-Limestone and Quartz sandstone aggregates in the Wahou volcanic aggregates; c-Zhouhe nuclear volcanic aggregates and angular limestone aggregates; d-Zhouher volcanic aggregates and melting embayment limestone aggregates; e-Deji basalt aggregates in volcanic aggregates; f-Round limestone aggregates and subangular basalt aggregates in the Huangmaogeng volcanic aggregates
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图 4 第三喷发旋回古火山机构的野外岩石学特征
Figure 4. Field petrological features of the ancient volcanic apparatus formed in the third eruptive cycle
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图 5 峨眉山玄武岩喷发韵律旋回划分
1—砾岩；2—灰岩；3—块状玄武岩；4—杏仁状玄武岩；5—斑状玄武岩；6—杏仁斑状玄武岩；7—多斑状玄武岩；8—玄武质集块岩；9—玄武质含集块角砾岩；10—玄武质火山角砾岩
Figure 5. Division of the eruption rhythm and cycle for the Emeishan basalt
1-Conglomerate; 2-Limestone; 3-Aphyric Basalt; 4-Amygdaloidal basalt; 5-Porphyritic basalt; 6-Amygdaloidal-phyric basalt; 7-Polyporphyric basalt; 8-Basaltic agglomerate; 9-Basaltic agglomerated breccia; 10-Basaltic volcanic breccia
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图 6 峨眉山玄武岩火山机构示意图
1—玄武岩集块；2—灰岩集块；3—灰岩；4—一旋回早期；5—一旋回中期；6—一旋回晚期；7—三旋回
Figure 6. Schematic diagram showing the volcanic apparatus of the Emeishan basalts
1-Basaltic agglomerate; 2-Limestone agglomerate; 3-Limestone; 4-Early Cycle 1;5-Middle Cycle 1;6-Late Cycle 1;7-Cycle 3
表 1 本文厘定的火山机构的地理位置和岩石学信息
Table 1 Information on the position and petrology of the identified volcanic apparatus in this study
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