河北滦平球状闪长岩岩石地球化学特征与源区性质探讨

Geochemistry and source nature of th e orbicular dioritic rocks in Luanping,Hebei Province

滦平球状闪长岩体具有特殊的地球化学性质,其中最显著的岩石化学特征表现为高度富Al2O3(21.58%～26.80%)而相对贫K2O(0.35%～1.87%)。地球化学方面以异常高的Sr含量和较高的Ba含量、极低的稀土元素总量、明显的Eu正异常为特征而明显不同于SiO2含量类似的其他钙碱性岩类。此外,该球状闪长岩体还表现为贫Nb、Ta、Zr、Hf、Y等高场强元素和Th、U、Rb等强不相容元素,较强的轻重稀土元素分馏程度,岩石地球化学的总体特征可与埃达克岩相对比,但其Eu正异常和Sr正异常更为明显。岩体中球体的Sr同位素初始值为0.70533～0.70537,核部斜长角闪岩捕虏体和壳层的εNd(t)有明显的差别,分别为-12.6和-17.7,Nd同位素地壳保留年龄分别为2.11Ga和2.64Ga,表明它们来自古老地壳的不同层位。滦平球状闪长质岩浆可能是由新太古代亏损型拉斑玄武质斜长角闪岩选择性部分熔融(主要是斜长石)形成的高Al2O3岩浆,它们在上升过程中捕获了古元古代的斜长角闪岩。

Luanping orbicular dioritic pluton has special geochemical characteristics with unusual enrichment in Al 2 O 3 (21.58%～26.80%)and relative poorness of K 2 O(0.35%～1.87%).The pluton has abnormally high concentrations of Sr and high concen trations of Ba,especially low concentrations of rare earth elements(REE)and strong positive Eu anomaly,which are striking different from other calc-alkaline rocks with similar SiO 2 concentrations.The pluton also con tains low concentrations of high fie ld strength elements(HFSE),such as Nb,Ta,Zr,Hf and Y and large ion lithophi le elements(LILE),such as Th,U and Pb.The characteris tics of the pluton can be compared with adakite e xcept stronger positive Eu and Sr anomalies.The initial values of Sr isotope of the pluton are 0.70533～0.70536.T heε Nd (t)values of the core and the shell are -12.6and -17.7,respectively,corresponding to Nd i sotope model ages of 2.11Ga and 2.64Ga,indicating they came from differe nt part of the ancient crust.Therefore,the magma of the pluton is probably h igh-Al 2 O 3 one formed by selected partial melting(mainly by plagioclase)of Neoarchean depleted tholeiitic a mphibolite,and they captured the Paleo-proterozoic amphibolite during their uprising.