河北省宽城县孤山子超基性岩体锆石U-Pb年龄及Hf同位素特征

U-Pb geochronology and Lu-Hf isotopes of zircons from Gushanzi ultrabasic rocks of Kuancheng County in Hebei Province

华北克拉通北缘在早石炭世晚期—中二叠世出露大量中酸性侵入岩体,但基性-超基性岩体出露较少,因而缺乏对该时期地幔性质的认识。笔者对位于内蒙古地轴以南百余千米的孤山子超基性岩体开展锆石U-Pb定年、微量元素、Lu-Hf同位素组成测试分析。结果显示,孤山子超基性岩体LA-ICP-MS锆石U-Pb加权平均年龄为296.9±2.6 Ma,为早二叠世的岩浆活动产物。锆石(样号:GSZ-B3)微量元素含量低,具有强Ce正异常和弱Eu负异常特征以及较低的εHf(t)=-1.9^-7.5(n=17),表明其源区主要为富集的岩石圈地幔。锆石tDM、tCDM远大于其U-Pb年龄,表明岩石圈地幔形成过程中有再造古老地壳加入。对比内蒙古地轴上的同时代波罗诺岩体含角闪辉长岩(297±1 Ma)的锆石εHf(t)(-17.0^-14.1),孤山子超基性岩体的锆石εHf(t)明显偏大,表明孤山子超基性岩体的源区富集程度相对较小。此外,孤山子超基性岩体还有2个锆石测点εHf(t)分别为3.2、4.0,指示母岩浆中有少量亏损幔源物质加入,这暗示了华北克拉通北缘局部地区,在晚古生代已有岩石圈拉张减薄的趋势。

Lots of late Early Carboniferous to Middle Permian acid-intermediate intrusive bodies are outcropped on the northern margin of the North China Craton. Because of rare exposure of the contemporaneous mantle-de- rived basic-ultrabasic rocks, it is very difficult to obtain some information concerning the mantle compositions and processes in this area. For the better understanding of the nature of the source rock, the authors chose the representative intrusions, i.e., Gushanzi ultrabasic rocks, which are about one hundred kilometers away from the southern Inner Mongolian Axis, for zircon U-Pb dating as well as trace element and Lu-Hf isotope analyz- ing. The LA-ICP-MS zircon U-Pb age of Gushanzi ultrabasic intrusion is 296.9 ± 2.6 Ma, which indicates an Early Permian magma activity. The analytical results show the characteristics of low content of trace elements,strong positive Ce anomalies, slight negative Eu anomalies and relatively low εHf（ t ） values of - 1.9～ - 7.5 （ n = 17） in the sample Gushanzi-B3, indicating the source of enriched lithosphere mantle. Both tDM and tCDM are higher than the U-Pb age, implying that the source rock was contaminated by ancient crust during its forma- tion. Compared with zircon εHf（ t ） （ - 17.0 - 14.1） values of Boluonuo hornblende-bearing gabbro in the In- ner Mongolian Axis whose rock-forming age is 297 ± 1 Ma, zircon εHf（ t ） values of Gushanzi ultrabasic rocks are obviously higher, implying a less enriched origin of Gushanzi ultrabasic rocks. Additionally, zircon εHf （ t ） of the other two testing plots of Gushanzi uhrabasic rocks are 3.2 and 4.0, respectively, which implies that a small amount of depleted mantle-derived substance was added to the primitive magma. The authors thus infer the exis-tence of a local tensional and thinning setting of northern margin of North China Craton during Late Paleozoic.