【作者】 汪云峰；

【导师】 张招崇；

【作者基本信息】 中国地质大学（北京） ， 矿物学、岩石学、矿床学， 2013， 博士

【摘要】 峨眉山大火成岩省(ELIP)是我国目前唯一被国际地学界承认的大火成岩省,已取得了丰硕的成果。但ELIP与地幔柱的关系以及地幔柱轴部位置等方面还存在不同的认识。本文研究了峨眉山大火成岩省丽江县城西北的虎跳峡和丽江县城东南的金安玄武岩岩体的岩石学、矿物学和地球化学特征,旨在为峨眉山地幔柱模型提供重要约束。虎跳峡岩石组合为双峰式火山岩,有少量苦橄岩和安山岩出露,苦橄岩中橄榄石斑晶多数伊丁石化,无扭折现象,指示了橄榄石是在熔体中结晶形成的；斑状玄武岩中出现次钙普通辉石斑晶,Mg#值为84.92%,形成温度为1441。C；虎跳峡熔岩经历了橄榄石和少量辉石的分离结晶作用。金安剖面主要为玄武岩,普通辉石斑晶形成温度为1021～1207℃,形成压力为2.2-9.0Kb。玄武岩经历了橄榄石、辉石和少量斜长石的分离结晶作用；本文推测出虎跳峡熔岩比金安玄武岩岩浆源区更深,形成压力和形成温度更高。峨眉山大火成岩省苦橄岩的分布显示出以丽江为中心,向两侧逐渐减少的趋势。本文系统研究了虎跳峡、金安玄武岩的主量元素、微量元素、稀土元素和Sr-Nd-Pb同位素特征以及锆石的年龄。虎跳峡苦橄岩和玄武岩的Mg#为57.57～75.17wt%, CaO/Al2O3为0.80-1.34；εNd(t)和(87Sr/86Sr)t变化范围小,分别为0.43～0.54(260Ma)和0.70508-0.70526。金安玄武岩的Mg#为30.41-51.89wt%,CaO/Al2O3为0.49～0.79；εNd(t)为(-3.19)-+3.63,(87Sr/86Sr)t为0.70419～0.70552。高的εNd(t)值以及抗蚀变不相容元素的原始地幔标准化图解与洋岛玄武岩相似,并且其重稀土元素特征指示了源区有石榴子石的残余,而且是低部分熔融的产物。同位素比值与抗蚀变不相容元素比值(如Nb/La、(Th/Ta)N、(La/Nb)N、(La/Ta)N、 La/Sm、Nb/U)的相关性表明,岩浆形成过程中有少量的大陆地壳物质或者相对低εNd(t)组份的大陆岩石圈地幔的混染。总体上,苦橄岩和玄武岩的地球化学特征的研究结果支持了峨眉山大火成岩省是地幔柱头部熔融的成因模型。利用PRIMELT2软件计算出ELIP各地区原始岩浆成分。结果显示丽江苦橄岩形成压力在21.2-44.9Kb之间,熔融深度可达148km左右的石榴石稳定相区。地幔潜温在1401～1726℃之间,高出正常地幔温度121-376℃。地幔熔融的P-T分布关系也为峨眉山地幔柱提供了新证据,从峨眉山大火成岩省计算出的各地区原始岩浆熔融温度、熔融压力、形成深度是以丽江为中心,向两侧逐渐降低。结合前人的研究成果,笔者认为峨眉山大火成岩省是地幔柱成因,并且丽江地区可能是地幔柱的中心位置。更多还原

【Abstract】 The Permian Emeishan large igneous province (ELIP) is only internationally admitted large igneous province in China. There are the many achievements about ELIP, but many scholars hold different opinions, such as the classification and the cause of the high-Ti and low-Ti basalts, the relationship between ELIP and the mantle plume, the place of the mantle plume axis position. This paper studied the petrology, mineralogy and geochemistry of Hutiaoxia and jin’an basalt, which are located in the Northwest and southeast of lijiang county, respectively.The rock association characteristics of Hutiaoxia lavas are bimodal volcanic rocks, which are consist of a little picrates and baesites. The olivine and a little of clinopyroxene in the Hutiaoxia’s lavas are products of the melts’ fractional crystallization. Most of the Olive phenocryst in the picrate had been iddingsited without kinking. The Mg#value of low calcium augite is84.92%in the augitophyric basalt, and its initial melt temperature is1441℃. There are mainly composed of basic basalts in the Jin’an. Its Mg#value of the clinopyroxene phenocryst is66.21%-83.21%, and its initial melt temperature and initial melt pressure are1021-1207℃and2.2-9.2Kb, respectively. So, the olivine, clinopyroxene and a little Monoclinic feldspar in the Jinan are products of the melts’ fractional crystallization. The results showed that Hutiaoxia lava has deeper magma source area, higher melt pressure and melt temperature than jinan basalt. The picrite distribution in Lijiang, which is the center of ELIP, reaches to highest point and rimland lower.The paper mainly studies the mineral compositions, major element, trace element, rare earth element, Sr-Nd-Pb isotopic as well as U-Pb ages of the Permian basalts at the Hutiaoxia and Jinan areas. In the Hutiaoxia, the Mg#value of its picrates and basalts is57.57%-75.17%， its CaO/Al2O3ratio is0.80%-1.34%, and its age-corrected Nd-Sr-Pb isotope ratios and εNd(t) value, which cover a rather small range, are0.70508-0.70526and0.43-0.54, respectively. In the Jinan, the Mg#value, CaO/Al2O3ratio, εNd(t) value and (87Sr/86Sr)t ratio of its basalts are30.41%-51.89%,0.49%-0.79%,0.43-0.54and0.70508-0.70526, respectively. The higher-εNd(t) lavas are isotopically similar to those of several modern oceanic hotspots, and have ocean-island-like patterns of alteration-resistant in compatible elements. Heavy rare-earth element characteristics indicate an important role for garnet during melting and that the lavas were formed by rather small amounts of partial melting, Rough correlations of isltope ratios with ratios of alteration-ersistant highly incompatible elements (e.g., Nb/La、(Th/Ta)、(La/Nb)、(La/Ta)、La/Sm and Nb/U) suggest minor amounts of contamination involving continental material or continental lithospheric mantle with a relatively low-εNd component in the source. Overall, our results are consistent with other evidence suggesting a plume-head origin for the Emeishan province.Basalts can be used as melting temperature and pressure probes of the mantle. We calculated primary magmas using PRIMELT2software. The results show that the primary magmas of Emeishan Large igneous could be generated in the pressure range21.2-44.9Kb at an approximate depth of148km from Garnet stable phase sources. And the generated temperatures in range1401-1726℃, which excess normal asthenosphere temperature121-376℃, provide further evidence of Emeishan mantle plume. Furthermore, the distributions of P-T also strengthen the existence of Emeishan mantle plume. The calculated melt temperature, melt pressures and formation depths of the original magma is characteristic of gradual reduction from Lijiang area to all directions of ELIP.Based on the present studies, the paper shows that the axis of the Emeishan mantle plume should be located beneath the Lijiang County Town, Yunnan Province.更多还原