江苏东海芝麻房预先导孔(CCSD-PP1)橄榄岩及其反映的上地幔亏损和交代事件

Peridotite from the Pre-pilot Hole (PP1) of the Chinese Continental Scientific Drilling Project and Its Bearing on Depleted and Metasomatic Upper Mantle

芝麻房预先导孔橄榄岩岩石类型有二辉橄榄岩、方辉橄榄岩、单辉橄榄岩和纯橄榄岩,其中前3种多数含有石榴子石。这些岩石在垂向上显示成层性,没有某种方向的韵律变化,它们之间呈渐变过渡关系。对样品进行主量元素、微量元素、稀土元素和Sr、Nd、O同位素分析表明,随着样品MgO含量的增高,Al_2O_3、CaO逐渐降低,呈良好的相关关系,并且大致上从石榴橄榄岩类到不含石榴子石的橄榄岩类主量元素的亏损程度增高,两者一般均比原始地幔主量元素亏损。稀土元素配分型式表现为右倾的轻稀土富集型,在微量元素蛛网图上,也表现为强不相容元素和微量元素到适度不相容元素的总体右倾型,与原始地幔相比,均富集强不相容元素。富集程度的不同与主量元素的亏损无对应关系,说明主量元素和微量元素各自的特点不是同一种事件造成的,而是先前地幔部分熔融作用之后又发生了富含不相容元素流体的交代作用。Sr、Nd同位素具有富集特点,变化范围较大,而且主量元素较亏损的岩石更富集Sr同位索,显示了交代富集的特点。稀土元素(Ce/Yb)_N比值和ε_(Nd)之间的负相关关系和Rb丰度与Sr同位素之间的正相关关系表明,富集Sr、Nd同位素的流体和富集不相容微量元素的流体均来自地幔。

The Zhimafang peridotite from the pre-pilot hole of the CCSDP (Chinese Continental Scientific Drilling Project) is composed of lerzolite, harzbergite, wehrlite and dunite with or without garnet. These rocks show the layering but without rhythmic character. The contact between the layers is transitional. Major, trace (including REE) and isotope analyses have been do on the samples from the hole. A12O3 and CaO decrease with decrease of MgO showing very good correlativity. From garnet peridotite to garnert-free peridotite, these rocks become even more refractory and all of them are more refractory than pyrolite in major composition. REE show enriched LREE. In Spider diagram, the highly incompatible and moderate incompatible elements also show enriched characters. There is no correlation between refractory degree and enrichment of trace elements,which indicates decoupling between major and trace elements were caused by two different deep processes; these two processes might be previous partial melting in the upper mantle and subsequent matsomatism by mantle fluid rich in incompatible elements. Sr and Nd isotope compositions also have the enriched characteristics and variation ranges are large; the more refractory the rocks, the more enriched isotope compositions. Mineral O isotope has the typical mantle compositional character ranging from between +4‰ to +6‰, which indicates the source of metasomatic fluid was in the mantle itself. The inverse correlation between (Ce/Yb)N and εNd and positive correlation between Rb abundance and Sr isotope composition indicate that the sources of fluids enriched in Sr, Nd isotopes and highly incompatible elements were the same.