冀东孤山子基性-超基性岩体锆石U-Pb年代学、岩石地球化学特征

Zircon U-Pb geochronology and geochemistry of Gushanzi basic-ultrabasic rocks in eastern Hebei,China

对孤山子基性-超基性岩体的岩相学、锆石U-Pb年代学和全岩地球化学等方面研究,确定了该岩体的形成时代、岩石成因及其形成的构造环境。该杂岩体主要由辉石岩和角闪辉石岩组成,元素地球化学上,贫SiO_2(34.28%~49.22%)、K_2O>Na_2O、富MgO(4.54%~13.26%)特征,属碱性系列岩石。岩石稀土元素球粒陨石标准化配分模式为轻稀土富集型,(La/Yb)_N=8.14~36.19,Eu异常不明显,稀土元素原始地幔标准化分布模式相似,为上地幔富集地幔熔融。岩石富集大离子亲石元素(如Rb、Ba、K)和LREE(如La和Ce),明显亏损高场强元素(Nb、Ta、Ti)和HREE。样品Nb/U、Ce/Pb和La/Sm比值显示岩体经历了有限的地壳混染。锆石的LA-ICP-MS U-Pb年代学研究表明,岩石锆石U-Pb年龄分为3类:一类锆石U-Pb年龄为(1 392±10)Ma,应为孤山子超基性侵入体侵位过程中捕获的碎屑锆石;第二类锆石U-Pb年龄为(274.5±2.1)Ma,为岩浆侵位时形成的锆石;第三类锆石U-Pb年龄为(53.02±1.1)Ma,为热液锆石。研究认为,岩体的岩浆源区主要为亏损地幔,可能受晚古生代古亚洲洋俯冲洋壳的脱出流体迭加作用和地壳物质的混染。结合区域构造演化,认为岩体形成于古亚洲洋闭合后挤压向伸展转换的构造背景。

Based on the study of petrography, zircon U-Pb chronology and geochemical data, the authors determined the formation age, petrogenesis and tectonic setting for the basic-ultrabasic complex from Gushanzi. The complex is mainly composed of pyroxenite and hornblende pyroxenite. The geochemical analysis results show that the complex is enriched in MgO （4.54%-13.25%）, low in SiO2 （34.28%-49.22%）, and K2O〉Na2O, which belong to alkaline series. Chondrite-normalized rare-earth element pattern shows that the rocks are enriched in light rare-earth element, the ratio of （La/Yb）N=8.14-36.19, with light Eu anomalies. These rocks are characterized by enrichment in LILE （Rb, Ba,K）and LREE（La, Ce）, relatively depletion in HFSE（Nb, Ta, Ti）and HREE. The ratios of Nb/U, Ce/Pb and La/Sm indicate that the complex experienced limited crustal contamination. LA-ICP-MS U-Pb dating results of zircons from Gushanzi complex can be separated into three groups. The first group is with U-Pb age of（1 392±10）Ma, which is detrital zircons captured during the emplacement of ultrabasic intrusion. The second group is with U-Pb age of （274.5±2.1）Ma, representing the formation age of ultrabasic rocks. The third group is with U-Pb age of （53.02±1.1）Ma, which are typical hydrothermal zircons. These rocks are derived from partial melting of depleted mantle wedge modified by subduction-related fluids from the subduction of Paleo-Asian Ocean crust during Paleozoic and crustal contamination. Combined with the research of regional tectonic evolution, the authors conclude that the complex was formed after the closure of the Paleo-Asian Ocean in a tectonic setting of transition from compression to extension.