Abstract: The early Cretaceous basalts in the Siziwangqi area, Inner Mongolia, are characterized by shoshonite. These rocks are enriched in large ion lithophile elements (LILE),light rare earth elements (LREE), and depleted in high field strength elements (HFSE). The chemical characteristics are similar to the oceanic island basalt (OIB). Trace element ratios such as Rb/Nb, K/Nb, Zr/Nb, Th/Nb and Th/La are consistent with the EM1-OIB geochemical signatures. Their ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios are in the ranges of 17.18 to 17.81, 15.40 to 15.50 and 37.54 to 38.05, respectively, showing the EM1 geochemical signatures. The progressive enrichment of lead concentration with almost unchanging Pb isotopic ratios, preclude the possibility that the magmas were contaminated by crustal materials. The shoshonitic parental magma was derived from the small degree decompression melting of asthenosphere mantle. The Siziwangqi shoshonite is the product of lithospheric thinning of the North China craton.

Key words: Siziwangqi shoshonite, Pb isotope, lithospheric thinning, decompression melting

摘要： 内蒙古四子王旗地区出露的早白垩世玄武岩属于钾玄岩。岩石富集大离子亲石元素（LILE）、轻稀土元素（LREE），亏损高场强元素（HFSE），其地球化学特征总体与洋岛玄武岩(OIB)相似。w(Rb)/w(Nb)、w(K)/w(Nb)、w(Zr)/w(Nb)、w(Th)/w(Nb)和w(Th)/w(La)比值主要落在EM1-OIB范围内。w(²⁰⁶Pb)/w(²⁰⁴Pb)、w(²⁰⁷Pb)/w(²⁰⁴Pb)和w(²⁰⁸Pb)/w(²⁰⁴Pb)分别变化在17.18～17.81, 15.40～15.50和37.54～38.05之间，岩石的Pb同位素组成接近EM1型地幔端元。随着Pb含量(质量分数)的增加，Pb同位素比值几乎没有变化，表明四子王旗钾玄岩没有经历明显的地壳物质混染。钾玄质母岩浆源于软流圈地幔的减压低度部分熔融。四子王旗钾玄岩是华北克拉通岩石圈减薄事件的产物。

关键词: 四子王旗钾玄岩, Pb同位素, 岩石圈减薄, 减压部分熔融