| 甘蒙北山后红泉地区海相玄武岩地球化学、年代学特征及成因 |
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| 引用本文： | 郭小刚,陈守余,苟瑞,刘鑫,王秦,蒲强林.甘蒙北山后红泉地区海相玄武岩地球化学、年代学特征及成因[J].地球科学,2021,46(11):3945-3964. |
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| 作者姓名： | 郭小刚 陈守余 苟瑞 刘鑫 王秦 蒲强林 |
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| 作者单位： | 1.中国地质大学资源学院, 湖北武汉 430074 |
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| 基金项目： | 甘肃省基础地质调查项目甘国土资勘发2016[33]-6甘肃省基础地质调查项目2017[47]-17甘肃省基础地质调查项目2018[45]-28 |
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| 摘 要： | 甘蒙北山地区发育的海相二叠系地层是中亚造山带南缘最年轻的海相沉积地层之一，为进一步探讨二叠纪古亚洲洋的关闭及后期的碰撞造山作用的时间节点，选取甘蒙北山后红泉地区二叠系海相火山岩-碎屑岩地层的5条代表性剖面进行了岩性-岩相及分布特征研究，并对海相地层中的玄武岩进行了岩相学、元素地球化学以及全岩Sr-Nd同位素等研究，同时选择剖面典型岩石进行了LA-ICP-MS锆石U-Pb定年.获得流纹英安岩与含砂亮泥晶生屑灰岩年龄分别为273±1.7 Ma和275.8±1.4 Ma，结合已发表年龄数据及古生物资料，明确了火山岩的喷发时代介于早二叠世亚丁斯克期-中二叠世沃德期，应厘定为双堡塘组；地球化学研究表明，后红泉地区玄武岩SiO2含量介于47.48%~54.56%，主要为玄武岩和玄武安山岩，属钠质拉斑玄武岩；稀土总量（ΣREE）较高，为74.90×10-6~173.61×10-6，LREE/HREE为2.32~6.55，表现出轻稀土相对重稀土轻微富集，（La/Yb）N=1.30~6.20，在球粒陨石标准化分布曲线整体上呈现LREE轻度富集、配分曲线平缓的特征，相对富集La、Ce、Nd、Zr、Hf等元素，不同程度地亏损Ta、Nb、P、Ti等元素，δEu=0.83~1.20，无明显的Eu异常；（87Sr/86Sr）i为0.702 024~0.706 432，εNd（t）值介于1.99~6.54（除了-0.59和-0.83）.因此，后红泉地区玄武岩岩浆是由接近原始地幔成分的石榴石二辉橄榄岩低程度熔融形成，其演化成岩过程中并未经地壳的强烈混染，而在岩浆源区发生了强烈的混染作用，推测软流圈地幔受到了陆源沉积岩、大陆地壳或蚀变大洋地壳改造，岩石圈拆沉作用可能是其产生的重要机制；说明古亚洲洋在早期已闭合，至早-中二叠世进一步拉张裂解形成了裂谷盆地.
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| 关 键 词： | 北山 二叠系 地球化学特征 年代学特征 构造环境 岩石学 |
| 收稿时间： | 2020-12-29 |
Geochemistry,Chronology and Genesis of Marine Basalts in Houhongquan Area,Beishan, Gansu and Inner Mongolia |
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| Guo Xiaogang,Chen Shouyu,Gou Rui,Liu Xin,Wang Qin,Pu Qianglin.Geochemistry,Chronology and Genesis of Marine Basalts in Houhongquan Area,Beishan, Gansu and Inner Mongolia[J].Earth Science-Journal of China University of Geosciences,2021,46(11):3945-3964. |
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| Authors: | Guo Xiaogang Chen Shouyu Gou Rui Liu Xin Wang Qin Pu Qianglin |
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| Abstract: | The marine Permian strata developed in the Beishan area, Gansu and Inner Mongolia, is one of the youngest marine sedimentary strata in the southern margin of the Central Asian orogenic belt. To further explore the timeline of the Permian paleo-Asian Ocean closure and the later collisional orogeny, five representative sections of the Permian marine volcaniclastic-clastic strata in the Houhongquan, Beishan, Gansu and Inner Mongolia were selected for lithology-petrography and distribution characteristics studies, and petrography, elemental geochemistry and whole-rock Sr-Nd isotope studies were conducted on the basalts in the marine strata. In addition, a typical rock profile was selected for LA-ICP-MS zircon U-Pb dating. The ages of the rhyodacite and the sand-bearing bright micrite bioclastic limestone are 273±1.7 Ma and 275.8±1.4 Ma, respectively. Combined with published age data and paleontological data, it is clear that the eruption age of the volcanic rock is between the Artinskian age of Early Permian to the Ward stage of the Middle Permian. It should be determined as the Shuangbaotang Formation. The SiO2 contents of basalt in Houhongquan area ranges from 47.48% to 54.56%, mainly basalt and basalt andesite, belonging to sodium-tholeiitic basalt. The total content of rare earth elements (ΣREE) is relatively high, ranging 74.90×10-6-173.61×10-6, LREE/HREE is between 2.32-6.55, showing slight enrichment of light rare earth elements relative to heavy rare earth elements, with (La/Yb)N=1.30-6.20. On the whole, chondrite-normalized distribution curve shows slightly enriched LREE with the gentle distribution curve, relatively enriched elements such as La, Ce, Nd, Zr, Hf, and depleted Ta, Nb, P, Ti and other elements to varying degrees, δEu=0.83-1.20, no obvious Eu anomaly. (87Sr/86Sr)i is 0.702 024-0.706 432, the εNd(t)values between 1.99 and 6.54 (except -0.59 and -0.83). Therefore, the basalt magma in the Houhongquan area is formed by low-degree melting of the garnet peridotite, which is close to the original mantle composition. During its evolution and diagenesis, it was not strongly contaminated by the crust, but a strong contaminant occurred in the magma source area. It is speculated that the asthenospheric mantle has been modified by terrigenous sedimentary rocks, continental crust or altered oceanic crust, and lithospheric delamination may be an important mechanism for its generation. It shows that the Paleo-Asian Ocean was closed in the early period, and the rift basin was formed by further extension and rifting in the Early-Middle Permian. |
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