湘东北应家山古近纪玄武岩地球化学特征及其成因意义  

Geochemistry of Paleogene basalts from Yingjiashan in northeastern Hunan Provi nce and their significance for petrogenesis

作　　者：彭头平[1] 王岳军[1] 彭冰霞[1] 韦牡[2] 

机构地区：[1]中国科学院广州地球化学研究所同位素年代学和地球化学重点实验室 [2]福建省第八地质大队,福建龙岩364012

出　　处：《地球化学》2006年第4期395-404,共10页Geochimica

基　　金：国家自然科学基金(40473019;40421303;40334039)

摘　　要：湘东北应家山玄武岩的K-Ar年龄为(62.1±0.9)Ma,喷发于古近纪,属亚碱性玄武岩。低MgO(5.65%～7.08%),Mg#(0.52～0.58),Nb-Ta明显亏损,无Eu负异常(Eu/Eu=0.93～1.11),(La/Yb)N=6.2～7.0,Zr/Nb=8.56～9.53,Ce/Y=1.57～1.72,表明其岩浆来源于较浅部的尖晶石二辉橄榄岩地幔大比例部分熔融。样品明显亏损Nb-Ta,富集LREE和LILE,初始87Sr/86Sr=0.708266～0.708464,εNd(63Ma)=-1.51～-2.02,暗示其源区为受到过早期俯冲流体交代的富集岩石圈地幔。结合区域上粤北、赣西北、闽西和琼南晚中生代基性岩脉的研究资料,表明华南内部存在一个相似的受到过早期流体交代的EMⅡ型富集岩石圈地幔。这一研究为了解华南内部中新生代以来陆下岩石圈地幔的属性以及构造演化提供了重要的信息。The K-Ar age [ （62.1 ± 0. 9） Ma] for basalts from Yingjiashan in northeastern Hunan Province indicates that the basaltic magma erupted in the Paleogene. These basalts belong to the sub-alkaline series in composition. They are characterized by slightly low MgO （5.65% -7.08% ）, Mg^# （0. 52 - 0. 58）, markedly Nb-Ta depletion and no Eu （Eu/Eu^＊ = 0. 93 - 1.11 ） anomaly. Their moderate LREE/HREE fractionation [ （La/Yb） N = 6. 2 - 7.0], relatively high Zr/Nb （8.56 - 9.53） and Ce/Y （1.57 - 1.72） ratios suggest that these rocks can be the products of partial melting of spinel-lherzolite mantle. Combined with their significant Nb-Ta depletion, Th, LREE, LILE and Sr isotopic composition, and relatively low εNd（t） values, it implies that the origin source of basaltic magma may have been metasomated by the fluid from the subducted slab and enriched in the past. Their geochemical features are similar to those of the basic dykes derived from the EM Ⅱ-type enriched lithospheric mantle in northern Guangdong Province and northwestern Jiangxi Province. The generation of the Cretaceous-Paleogene basaltic rocks is the result of interaction of EM Ⅱ-type enriched lithospheric mantle and asthenosphere in the tectonic setting of muhi-lihtospheric extension and thinning. This study will provide important information for well understanding and probing into the nature and tectonic evolution of sub-continental lithospheric mantle since the Mesozoic-Cenozoic.

关 键 词：古近纪 亚碱性玄武岩 部分熔融 伸展-减薄 应家山 湘东北 

分 类 号：P595[天文地球—地球化学] P581[天文地球—地质学]