冀北独石口古元古代变辉长闪长岩的成因及其地质意义

Petrogenesis and geological significance of the Paleoproterozoic Dushikou metagabbro-diorite in northern Hebei Province

为了对华北克拉通古元古代岩浆-变质-构造过程有更清晰的认识,本文对冀北赤城北独石口地区红旗营子杂岩中新发现的古元古代变辉长闪长岩进行了全岩主、微量元素,LA-ICP-MS锆石U-Pb年龄、微量元素和Hf同位素测试。独石口变辉长闪长岩具有典型拉斑系列岩浆岩的特征,SiO2含量变化范围为51.27%~54.39%,Na2O+K2O含量变化范围为4.65%~6.00%,Al2O3含量变化范围为14.74%~15.51%,MgO含量变化范围为2.19%~4.65%,Mg^#为28~47。岩石富集轻稀土元素和大离子亲石元素,亏损Nb、Ta等高场强元素,具有受到交代的岩石圈地幔部分熔融形成的岩浆作用的特征,代表壳-幔相互作用产物。独石口变辉长闪长岩中锆石呈自形-半自形晶,具有较高的Th/U比值,为岩浆成因,对2个变辉长闪长岩样品LA-ICP-MS锆石U-Pb定年结果分别为1808±18Ma和1825±18Ma,表明该岩体形成时代稍晚于古元古代区域变质作用峰期。锆石εHf(t)值变化范围为-5.4~+0.1,单阶段Hf模式年龄tDM1介于2236~2418Ma之间,说明除了太古宙岩石圈物质贡献,也存在新生地幔或者地壳物质的贡献。综合研究表明,独石口变辉长闪长岩可能代表古元古代造山带去根作用造成的区域抬升(伸展)的岩浆作用产物。

A combined analysis of whole rock major and trace elements,LA-ICP-MS zircon U-Pb ages,trace element and Hf isotope of the Paleoproterozoic Dushikou metagabbro-diorite,which is newly discovered in the Hongqiyingzi complex,to the north of Chicheng,Hebei Province,are performed to better understand the Paleoproterozoic magmatism-metamorphism-tectonics of the North China Craton.This pluton has the features of typical tholeiitic magmatic rocks,with SiO2 contents of 51.27%~54.39%,Na2O+K2O of 4.65%~6.00%,Al2O3 of 14.74%~15.51%,MgO of 2.19%~4.65%and Mg^#ranging from 28 to 47.The rocks are rich in light rare earth elements and large ion lithophile elements,and depleted in high field strength elements such as Nb,Ta,etc.They are characterized by magmatism formed by partial melting of metasomatic lithospheric mantle,representing the products of crust-mantle interaction.Zircons in the metagabbro-diorite show high Th/U ratios,suggesting a magmatic origin.LA-ICP-MS zircon U-Pb dating results of two metagabbro-diorite samples are 1808±18Ma and 1825±18Ma,respectively,indicating that the pluton emplacement postponed the peak stage of regional metamorphism at Paleoproterozoic.TheεHf(t)values of dated zircons range from-5.4 to+0.1,with tDM1 from 2236Ma to 2418Ma,indicating that a mixture of juvenile mantle,Archean lithosphere mantle and some crustal materials in its magma generation.Consequently,the Dushikou metagabbro-diorite may represent the magmatic product of regional uplift(extension)probably induced by the loss of the root of the Paleoproterozoic orogeny.