峨眉地幔柱轴部的榴辉岩-地幔岩源区:主元素、痕量元素及Sr、Nd、Pb同位素证据  

The Axial Zone Consisting of Pyrolite and Eclogite in the Emei Mantle Plume: Major, Trace Element and Sr-Nd-Pb Isotope Evidence

作　　者：侯增谦[1] 卢记仁[1] 林盛中[1] 

机构地区：[1]中国地质科学院矿产资源研究所,北京100037

出　　处：《地质学报》2005年第2期200-219,共20页Acta Geologica Sinica

基　　金：国家重点基础研究规划 973项目"印度-亚洲大陆主碰撞带成矿作用"(编号 2 0 0 2 CB412 610 )资助的成果

摘　　要：峨眉山大陆溢流玄武岩(ECFB)的西南部以丽江、大理和攀枝花三角区为中心的苦橄岩分布区,面积约5×10 4 km2 ,为峨眉地幔柱的轴部区。Sr、Nd、Pb同位素和痕量元素研究表明,大部分火山岩样品落在洋岛火山岩成分范围内,并存在类似FOZO、HIMU和EM - 的三个端元。这说明它们是在地幔柱轴部,由地幔岩和榴辉岩(古玄武质洋壳)组成的源区产生的岩浆形成的。岩浆源区再循环玄武质洋壳的存在可能是该区超大型钒钛磁铁矿床形成的根本原因。少部分分布在洋岛火山岩成分范围之外的样品,一部分属于地幔柱岩浆与地壳混染产物,另一部分低Ti岩石可能与岩石圈反应有关。地幔端元的地球化学特征如下:FOZO端元以白林山苦橄玄武岩(YB-0 1)为代表,低87Sr/86 Sr(0 .70 36 ) ,高1 43Nd/1 44 Nd(0 .5 12 7) ,中等2 0 6 Pb/2 0 7Pb(18.5 6 93) ;Nb/U =36 .6 7,Th/Nb =0 .0 82 ,L a/Nb=0 .91,Zr/Nb=6 .2 3。HIMU端元以丽江苦橄岩(JL - 2 9)为代表,高2 0 6 Pb/2 0 4 Pb(2 0 .6 4 12 )和2 0 7Pb/2 0 4 Pb(15 .74 89) ,低87Sr/86 Sr(0 .70 4 8)。EM - 端元包括两部分:1以二滩苦橄岩-玄武岩(R- 1、3、5、8)为代表,高87Sr/86 Sr(0 .70 73) ,低1 43Nd/1 44 Nd(0 .5 12 3) ,低2 0 6 Pb/2 0 4 Pb(17.996 8)和2 0 8Pb/2 0 4 Pb(37.94 5 0 )The picrite field, located in the southwest of Emeishan continental flood basalts (ECFB) and with the center in Lijiang—Dali—Panzhihua triangular area, is about 5×104 km2 and regarded as the axial area of the Emei mantle plume. The studies of Sr, Nd and Pb isotopes as well as trace elements indicate that most samples of volcanic rocks are located in the field of oceanic island basalt, and FOZO-like, HIMU-like and EMⅠ-Ⅱ-like end-members exist. The evidences indicate that the magmas are formed by melting of pyrolite and eclogite (ancient basaltic oceanic crust) in the axial zone of mantle plume. The existence of recycled basaltic oceanic crust in the axial zone of mantle plume may be the key of the formation of giant V-Ti-magnetite deposits. The rest samples are located outside the field of ocenic island basalt. Some of them belong to the contamination product between mantle-plume magmas and crust, and some low-Ti samples may be related to the reaction with lithosphere.Three mantle end-members, FOZO, HIMU and EMⅠ-Ⅱ, are identified by studying of Sr, Nd and Pb isotopes as well as trace elements. The end-member, FOZO represented by Bailinshan picritic basalt (YB-01) is characterized by low 87Sr/ 86Sr (0.7036), high 143Nd/ 144Nd (0.5127), moderate 206Pb/ 207Pb (18.5693), Nb/U (36.67), Th/Nb (0.082), La/Nb (0.91) and Zr/Nb (6.23). The end-member, HIMU represented by Lijiang picrite (JL-29) is characterized by high 206Pb/ 204Pb (20.6412) and 207Pb/ 204Pb (15.7489), and low 87Sr/ 86Sr (0.7048). The end member, EMⅠ-Ⅱ comprises two groups: (1) Ertan picrite-basalts (R-1, 3, 5 and 8) characterized by high 87Sr/ 86Sr (0.7073) and low 143Nd/ 144Nd (0.5123), 206Pb/ 204Pb (17.9968) and 208Pb/ 204Pb (37.9450), and Nb/U (27), Th/Nb (1.8), La/Nb (1.25), Sm/Yb (5.4～6.3), Zr/Y (11～13) and Nb/Y (1.7～2.1). (2) Binchuan picrite-basalts (BH-8, 10 and 11) with isotope composition similar to those of EMⅠare characterized by 87Sr/ 86Sr (0.7052), 143Nd/

关 键 词：峨眉山 地幔 榴辉岩 主元素 痕量元素 Sr Nd Pb 同位素 玄武岩 

分 类 号：P588.125[天文地球—岩石学]