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| 唐古拉山东段莫云地区二叠纪玄武岩地球化学特征及源区性质 | |
| 引用本文： | 段其发,王建雄,白云山,牛志军,何龙清,姚华舟,寇晓虎,张克信.唐古拉山东段莫云地区二叠纪玄武岩地球化学特征及源区性质[J].岩石矿物学杂志,2010,29(2):125-138. |
| 作者姓名： | 段其发 王建雄 白云山 牛志军 何龙清 姚华舟 寇晓虎 张克信 |
| 作者单位： | 1. 中国地质大学,地质过程与矿产资源国家重点实验室,湖北,武汉,430074;宜昌地质矿产研究所,湖北,宜昌,443003 2. 宜昌地质矿产研究所,湖北,宜昌,443003 3. 中国地质大学,地质过程与矿产资源国家重点实验室,湖北,武汉,430074 |
| 基金项目： | 中国地质调查局1:25万直根尕卡幅区域地质调查资助项目，国土资源部百名优秀青年科技人才计划资助项目 |
| 摘 要： | 在青藏高原腹地,二叠纪火山岩分布于唐古拉山东段,古地理上位于北羌塘盆地东缘。对唐古拉山东段莫云地区中二叠世栖霞期尕笛考组火山岩的主量元素、微量元素和Sr-Nd同位素进行了分析。岩相学和地球化学指标显示二叠纪火山岩为玄武岩,该套岩石低SiO2、MgO、K2O和Mg#值(0.39～0.48),富Na2O、TFeO、TiO2、P2O5含量,富集LREE和高场强元素(Nb、Ta、Zr、Hf、P、Ti),(La/Yb)N=14.89～23.23,(Gd/Yb)N=2.30～3.58,Eu异常不明显(δEu=0.76～0.99),具有明显的Th正异常和K、Sr负异常,显示与板内碱性玄武岩相似的地球化学特征;同位素组成以低(87Sr/86Sr)i(0.7033～0.7039)、高εNd(t)(+4.2,t=271Ma)为特点,反映岩浆源区既有亏损地幔源(DMM)又有富集地幔源(EMⅠ)的双重属性。岩浆起源可能与地幔柱诱导的软流圈上涌导致含石榴子石橄榄岩的岩石圈地幔部分熔融有关。莫云玄武岩形成于板内伸展环境(初始裂谷),具有主动裂谷作用性质。 |
| 关 键 词： | 二叠纪 玄武岩 地球化学 地幔源区 唐古拉山东段 |
Geochemistry and mantle source characteristics of the Permian basalts in Moyun area, eastern Tanggula Range |
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| DUAN Qi_f,WANG Jian_xiong,BAI Yun_shan,NIU Zhi_jun,HE Long_qing,YAO Hua_zhou,KOU Xiao_hu and ZHANG Ke_xin.Geochemistry and mantle source characteristics of the Permian basalts in Moyun area, eastern Tanggula Range[J].Acta Petrologica Et Mineralogica,2010,29(2):125-138. | |
| Authors: | DUAN Qi_f WANG Jian_xiong BAI Yun_shan NIU Zhi_jun HE Long_qing YAO Hua_zhou KOU Xiao_hu and ZHANG Ke_xin |
| Institution: | State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;Yichang Institute of Geology and Mineral Resources, Yichang 443003, China;Yichang Institute of Geology and Mineral Resources, Yichang 443003, China;Yichang Institute of Geology and Mineral Resources, Yichang 443003, China;Yichang Institute of Geology and Mineral Resources, Yichang 443003, China;Yichang Institute of Geology and Mineral Resources, Yichang 443003, China;Yichang Institute of Geology and Mineral Resources, Yichang 443003, China;State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China;State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan 430074, China |
| Abstract: | The eastern Tanggula Range in the hinterland of the Qinghai-Tibet plateau constitutes the key to the reconstruction of the eastern palaeo-Tethys tectonic framework. It paleogeographically belongs to the eastern margin of northern Qiangtang basin, lies in the western part of the Sanjiang tectonic belt and is adjacent to the Emeishan Large Igneous Province. Volcanic rocks are widely distributed in the eastern Tanggula Range, with the volcanic activity taking place mainly in Middle Permian. Based on field mapping, the authors divided the volcanic rocks in Moyan area into three basic rock units, namely 2~(nd), 3~(rd) and 4~(th) rock unit, with the 3~(rd) unit occurring in the interlayer form. They are in conformable contact with fine clastic rocks, mudstone and bioclastic limestone of the shallow-sea facies, making up three cycles of sedimentary-volcanic rocks in the Middle Permian Gadikao Formation. A systematical analysis of major and trace element compositions and whole-rock isotopes of the Gadikao Formation volcanic rocks indicates that the volcanic rocks belong to basalt (i.e., Moyun basalts) in the petrologic and geochemical indicators, with the rock types comprising grayish green massive basalt,grayish green massive olivine basalt and thick dark grayish green dolerite. The Moyun basalts have low and insignificantly varying SiO_2, Al2O3, MgO, Ni and Cr content, being 44.24%～49.62% (averagely 45.62%), 12.27%～15.35%, 5.88%～7.52%, 112×10~(-6)～191×10~(-6) and 141×10~(-6)～269×10~(-6), respectively; K_2O content varies remarkably (0.02%～1.27%), Mg~# ratios are low (0.39～0.48), and there are high Na_2O (2.36%～5.26%), TiO_2 (2.12%～5.73%), total FeO (TFeO＞10%) and P_2O_5(0.70%～1.87%), suggesting that the Moyun basalts are similar to Emeishan continental flood basalts with high TiO_2 and low Al_2O_3 content. As for geochemical characteristics, the Moyun basalts are of the transitional basalt series. The chondrite-normalized REE patterns indicate that all the samples have extremely high total content of rare earth elements (∑REE=284.02×10~(-6)～678.88×10~(-6)) and are significantly enriched in light REE (LREE), with (La/Yb)N of 14.89～23.23, (Gd/Yb)N of 2.30～3.58, and weakly Eu negative anomalies whose δEu values are 0.76～0.99. The large ion lithophile elements (LILE) (Ba, Th, Pb) and high field strength elements (HFSE) (Nb, Ta, Zr, Hf, P, Ti) in the basalts have been strongly concentrated. In the primitive mantle-normalized pattern, all the samples show weak negative anomalies of Nb-Ta, Zr-Hf, obvious negative anomalies of K, Sr and prominent positive anomalies of Th, La and Sm. Rb/Sr, Zr/Ba, Ta/Hf, Nb/Zr and Th/Ta ratios are mostly in the range of 0.001～0.012 (averagely 0.006), 0.71～4.65, 0.22～0.56(averagely 0.36), 0.2～0.3 and 3.35～16.93, respectively. The age-corrected initial (~(87)Sr/~(86)Sr)i ratios are low and vary narrowly from 0.703 3 to 0.703 9,~(143)Nd/~(144)Nd ratios are high (0.512 711～0.512 713), and εNd(t) values are +4.2 (t=271 Ma ). These data indicate an insignificant crustal contamination. In the diagrams of (~ (87)Sr/~(86)Sr)i-εNd(t) for basalts, the data are fallen in the depleted mantle (DMM) area. It is thus concluded that the Moyun basalts are geochemically similar to intraplate basalts, characterized by both DMM and enriched mantle (EMⅠ). The basaltic magma might have been derived from lower degree partial melting of a garnet-bearing peridotite lithospheric mantle. The characteristics of the Moyun basalts reveal that their formation was closely related to the mantle-plume activity. The basalts must have been formed in an intraplate spreading zone (incipient rift) and have characteristics of the active rift. The data presented in this paper are of great significance for the study of the mantle evolution of the eastern Tethys in the Permian period. |
| Keywords: | Permian basalts geochemistry mantle source intraplate spreading zone (incipient rift) eastern Tanggula Range |
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