河北高寺台含铬超基性岩杂岩体成岩成矿时代及岩石成因

Geochronology and petrogenesis of the Gaositai Cr-bearing ultramafic complex,Hebei Province,China

河北高寺台含铬超基性岩杂岩体产于华北克拉通北缘,具有环状分带的特征,核部为纯橄岩,向外依次为辉橄岩、辉石岩、角闪石岩,各岩相之间为过渡接触关系,在纯橄岩相中赋存有铬铁矿床。本文对该杂岩体的锆石年龄、Hf同位素和主量、微量元素进行了测试和研究。结果表明,随着基性程度的降低,各岩相岩石在主量元素特征上呈现连续的变化特征,Al2O3、TiO2和K2O逐渐升高,MgO含量逐渐降低;微量元素特征上配分曲线形态相似,具有富集Ba、K等LILEs,亏损Nb、Zr等HFSEs,稀土元素具一定程度的分异,无明显Eu异常的特征,说明各岩相为同源岩浆结晶分异作用的产物。通过对3件铬铁矿石和1件辉石岩的LA-ICP-MS锆石U-Pb定年,确定高寺台含铬超基性岩杂岩体的成岩成矿时代为213±1Ma,为晚三叠世,形成于华北板块和西伯利亚板块碰撞后的伸展阶段;年龄值介于214～211Ma之间的锆石的176Hf/177Hf比值介于0.282589～0.282693之间,εHf(t)介于-2.0～+1.6之间,说明高寺台含铬超基性岩杂岩体的岩浆源区组成为亏损地幔物质和富集地幔物质的混合,在岩浆上升过程中有大陆地壳物质的混染,其侵位时代晚三叠世可能代表了华北克拉通北缘岩石圈减薄的开始。高寺台杂岩体年龄测试样品中广泛存在继承锆石,其年龄值介于2800～244Ma之间,分布范围宽,但相对集中于2.6～2.5Ga、400～360Ma、300～280Ma、244Ma共4个年龄段,很可能是岩浆上升过程中的捕获锆石,记录了区域内在新太古代、中泥盆世、早二叠世、晚二叠世-中三叠世存在的岩浆活动。

Located in the northern margin of the North China Craton （NCC）, the concentric zoning Gaositai ultramafic complex show features of chromite-bearing dunite in the core, surrounded subsequently by wehrlite and clinopyroxenite, and minor hornblendite in the rim. Chemically, the Gaositai ultramafic complex display systematic decrease of Mg#, negatively sloped REE patterns with no Eu anomalies and high LILEs with low HFSEs such as Nb and Zr from core to rim, implying a fractional crystallization trend during magma evolution. LA-ICP-MS zircon U-Pb ages of three chromite samples and one clinopyroxenite sample show that emplacement of the Gaositai ultramafic complex was 213±1Ma, which occurred in Late Triassic, implying the geodynamic setting of lithospheric extension after final collision of the Mongolian arc terranes in the NCC. The εHf（t） values of the zircons aged from 214Ma to 211Ma for the clinopyroxenite range form -2.0 to ＋1.6, implying that the parental magma was derived from a mixed source composed of depleted mantle source and enriched lithospheric mantle source, with significant crustal contamination during its ascending. Inherited zircons were widespread in the Gaositai ultramafic complex, with ages from 2800Ma to 244Ma. Four groups were detected： 2.6～2.5Ga, 400～360Ma, 300～280Ma and 244Ma, recording Neoarchaean, Middle Devonian, Early Permain and Late Permain-Middle Triassic magmatism in the northern margin of the NCC.