攀西地区二叠纪赋存铌钽矿的正长岩脉的成因探讨  

Petrogenisis of Permian Nb-Ta mineralized syenitic dikes in the Panxi district,SW China

作　　者：王汾连[1,2] 赵太平[3] 王焰[3] 

机构地区：[1]国土资源部海底矿产资源重点实验室、广州海洋地质调查局,广州510075 [2]中山大学海洋学院,广州510006 [3]中国科学院广州地球化学研究所矿物学与成矿学重点实验室,广州510640

出　　处：《岩石学报》2015年第6期1797-1805,共9页Acta Petrologica Sinica

基　　金：国家基础研究发展计划(973项目)(2011CB808903);中国科学院知识创新工程重要方向项目(KZCX2-YW-Q04-06-03)联合资助

摘　　要：攀西地区广泛发育正长岩脉,其中有些岩脉富含铌钽等稀有金属元素。这些岩脉多侵入于二叠纪辉长岩体中,并与该区玄武岩及长英质岩体共生。锆石U-Pb年代学表明这些正长岩脉与长英质岩体及辉长岩体形成时代相一致(258~256Ma),但是否有成因联系并不清楚。本文对攀西地区炉库和白草两个Nb-Ta矿区中的富矿正长岩脉、贫矿正长岩脉及相关正长岩体在进行了详细的岩石学、矿物学及主微量地球化学的基础上,又开展了全岩Sr-Nd同位素研究。研究表明,岩石的εNd(t)非常均一(富矿正长岩脉εNd(t)=-0.2^+0.2,贫矿正长岩脉εNd(t)=-0.3^+0.7,正长岩体εNd(t)=-0.3^+0.4);两矿区岩石初始86Sr/87Sr值同位素则表现出宽泛的范围,正长岩体和贫矿正长岩脉的(86Sr/87Sr)i分别变化于0.7032~0.7090和0.7044~0.7064,富矿正长岩脉的(86Sr/87Sr)i变化于0.7049~0.7091,较大的初始86Sr/87Sr比值可能由于岩石遭受一定程度的蚀变所致。本文两矿区的富矿正长岩脉及贫矿正长岩脉和正长岩体的Sr-Nd同位素特征,与攀西地区红格镁铁质/超镁铁质层状侵入体的一致(εNd(t)=-2.7^+1.0,(86Sr/87Sr)i=0.7058~0.7064),表明其与峨眉山玄武岩及镁铁质侵入体来自于相同的地幔源区。结合作者及其他研究者对本区正长岩脉和相关正长岩体的主微量地球化学研究,我们认为本区炉库和白草矿区的富矿正长岩脉及相关正长岩体可能是峨眉山地幔柱岩浆活动造成的底侵在下地壳底部的辉长质岩石低程度(5%~10%)部分熔融形成的产物,且较少受到地壳物质混染。Syenitic dikes are widespread in the Panxi district and some of them host Nb and Ta deposits. The mineralized dikes intruded into gabbro intrusions and are spatially and temporally associated with basalts and felsic plutons（ granite and syenite plutons）in the Panxi district,which are part of the - 260 Ma Emeishan large igneous province. These dikes are recently dated to be 258 -256Ma by LA-ICP-MS zircon U-Pb dating technique,contemporaneous with the gabbro and felsic plutons. The Sr-Nd isotopes of the mineralized syenitic dikes,barren syenitic dikes and syenitic plutons of Luku and Baicao Nb-Ta deposits in the Panxi district show similar and uniform initial εNd（ t） values（- 0. 2 to ＋ 0. 2 for mineralized syenitic dikes,- 0. 3 to ＋ 0. 7 for barren syenitic dikes and- 0. 3 to ＋ 0. 4 for syenitic plutons） but they have highly variable（87Sr /86Sr）iratios（ 0. 7049 to 0. 7091 for the mineralized syenitic dikes and 0. 7044 - 0. 7064 for barren syenitic dikes and 0. 7032 to 0. 7090 for barren syenitic plutons）,comparable to the contemporaneous Hongge mafic / ultramafic layered intrusion（ εNd（ t） =- 2. 7 to ＋ 1. 0 and（86Sr /87Sr）i= 0. 7058 to 0. 7064） in the same region,suggesting that these lithologies may have evolved from a common basaltic magmas. According to the whole rock geochemistry and Sr-Nd isotopic characters,We consider that the syenitic plutons and dikes may be the products of low degree（ 5% -10%） partial remelting of underplated gabbro cumulates.

关 键 词：SR-ND同位素 正长岩脉 攀西地区 峨眉大火成岩省 

分 类 号：P588.122[天文地球—岩石学]