湘南金鸡岭辉绿岩脉的岩石成因及动力学意义

Petrogenesis of the Jinjiling diabasic dikes in southern Hunan and its dynamic significance

华南中生代基性岩脉的形成通常与板内伸展活动有关,它们既是探索深部地幔演化的探针,又是研究地球动力学背景的手段。本文选取湘南金鸡岭辉绿岩脉进行了高精度LA- ICP- MS锆石U- Pb同位素定年、全岩主微量元素和Sr- Nd同位素以及矿物地球化学测试分析,旨在探讨它们的岩石成因与岩石圈地幔演化的关系及华南内陆地区的地球动力学背景。研究显示金鸡岭辉绿岩脉的锆石U- Pb年龄为153. 1±1. 0 Ma,为晚侏罗世岩浆活动的产物。该岩石微量元素组成具有类似于洋岛玄武岩(OIB)的特征,表现出轻稀土元素(LREE)、大离子亲石元素(LILE)和高场强元素(HFSE)不同程度的富集,无Nb、Ta负异常,表明岩浆源区与岛弧环境无关,并且地壳物质混染程度较低。87Sr/86 Sr初始比值为0. 7062~0. 7080,εNd( t )值为+1. 76^+3. 75,也类似于OIB地幔源区。研究表明,金鸡岭辉绿岩脉来源于受软流圈熔体侵蚀/交代的岩石圈地幔的部分熔融,相应岩浆在上升侵位过程中发生了分离结晶和轻微的地壳混染作用,是板内拉张环境下的产物,为地幔改造/岩石圈减薄的岩浆岩响应及软流圈-岩石圈之间相互作用的表征。华南内陆地区中生代的岩浆作用可能与古太平洋板块俯冲无关,而是在陆内裂谷背景下软流圈上涌和岩石圈伸展减薄的产物。

The formation of Mesozoic mafic dykes in South China is usually related to intraplate extensional activities. They are not only probes for deep mantle evolution, but also tools for studying geodynamic background. In this paper, high- precision LA- ICP- MS zircon U- Pb isotope dating, whole- rock major and trace element and Sr- Nd isotope and mineral chemistry analyses were conducted for the Jinjiling diabasic dikes, southern Hunan province, to explore the relationship between their petrogenesis and the evolution of the lithospheric mantle and the geodynamic background of the inland area of South China. The results show that the zircon U- Pb age of the Jinjiling diabasic dikes is 153. 1±1. 0 Ma, suggesting that they are the product of late Jurassic magmatism. They are enriched in light rare earth elements (LREEs), large ion lithophile elements (LILEs) and high field strength elements (HFSEs), without Nb and Ta negative anomalies, showing the typical ocean island basalt (OIB)- like geochemical affinity. These characteristics show that the magma source is independent from the island arc environment, and less crustal contamination. The initial 87 Sr/86 Sr ratio of 0. 7062 to 0. 7080 and εNd( t ) of +1. 76 to +3. 75 are also consistent with the characteristic of OIB mantle source. The study shows that the Jinjiling diabasic dikes should be derived from partial melting of the lithosphere mantle which was eroded/metasomatized by the asthenosphere originated melts, and the magma underwent fractional crystallization and slight crustal contamination in the emplacement process. Those magmatic rocks are the response of mantle alteration/lithospheric thinning and the reflection of asthenosphere- lithosphere interaction. The Mesozoic magmatism in the inland area of South China may have no relation with the subduction of the paleo- Pacific plate, but be the product of the asthenosphere upwelling and lithosphere thinning under the background of the intracontinental rift.