河北省赤城县小张家口超基性岩体主要特征和时代

The Main Characteristics and Age of Xiaozhangjiakou Ultrabasic Rocks in Chicheng County, Hebei Province

小张家口超基性岩体是华北板块北缘超基性岩带的一个典型岩体，岩相分带明显，由透辉岩、闪辉岩、二辉岩和纯橄岩组成，主要矿物组成包括透辉石、贵橄榄石、角闪石等。化学成分上富ＦｅＯ、ＣａＯ而贫ＭｇＯ，ｍ／ｆ平均２．２１，属铁质或富铁质超基性岩。稀土分布形式为轻稀土富集型，具不显著的负铕异常，微量元素以富含不相容元素Ｒｂ、Ｔｉ、Ｓｒ、Ｖ等而贫Ｃｒ、Ｃｏ、Ｎｉ等相容元素为特征。Ｓｍ－Ｎｄ等时年龄１８３７Ｍａ，Ｎｄ（ｔ）为十１０．７２。该岩体由亏损型地幔部分熔融所产生的岩浆经结晶分异作用而形成。

Xiaozhangjiakou ultrabasic rocks is the host rock of gold deposit, mainly consisting of pyroxenide rocks, including pyroxenite, pyroxene-peridotite, hornblende- pyroxenite and diopsidite, etc. Their petrochemistry is charaterized by the richness of FeO and CaO and the poorness of MgO (m/f=2. 21), belonging to the ferruginous ultrabasic rocks. They are of LREE-enriched REE distribution pattern, They are rich in incompatible elements such as Rb, Ti, Sr, V and poor in compatible elements such as Cr, Ni, Co etc. Petrological, geochemical and isotope geochemical evidences indicate that the rocks are the products of differentiation and crystallization of magma which was derived from the partial melting of the depleted mantle. Sm-Nd isochron analysis shows that the age of the ultrabasic intrusive rocks is 1837 Ma, the mid-Proterozoic.