吉林省辽源晚中生代碱性玄武岩成因及岩石圈地幔性质

Origin of Late Mesozoic Alkaline Basalts and Nature of Lithospheric Mantle in Liaoyuan Area,Jilin Province

报道了吉林辽源晚中生代玄武岩的全岩K-Ar定年结果和岩石地球化学资料,以及其中幔源捕虏体的矿物化学分析结果,探讨了晚中生代碱性玄武岩的成因及岩石圈地幔的性质.全岩K-Ar定年结果显示,辽源玄武岩形成于晚白垩世(91.7±1.7Ma、82.5±1.9Ma、99.3±1.6Ma、82.2±2.4Ma).该玄武岩的SiO2含量介于45.1%～45.9%之间,全碱(Na2O+K2O)含量介于4.50%～6.72%之间,Mg#[100×Mg/(Mg+Fe2+)]值介于60～63之间,属于碱性玄武岩系列.该玄武岩相对富集轻稀土元素(LREEs)、亏损重稀土元素(HREEs),(La/Yb)N值介于12.0～14.0之间,无Eu异常,具有洋岛玄武岩(OIB)型微量元素配分型式.辽源玄武岩中的幔源捕虏体以尖晶石二辉橄榄岩为主,捕虏体中橄榄石的Mg#值较低(87.1～91.2),结合捕虏体中其他矿物的主量和微量元素地球化学特征,认为辽源晚白垩世岩石圈地幔以新增生的岩石圈地幔为主.捕虏体中单斜辉石的LREEs具有从富集到亏损的变化特征,暗示岩石圈地幔经历了部分熔融和交代作用的改造,地幔交代类型以硅酸盐熔体交代为主,同时存在碳酸盐熔体交代作用.辽源晚白垩世玄武岩来源于软流圈地幔的部分熔融,结合区域研究成果,认为辽源晚白垩世玄武岩形成于古太平洋板块俯冲背景下的弧后伸展环境.

This paper presents whole-rock K-Ar dating results and geochemical data of the Irate Mesozoic basalts in the Liaoyuan area, Jilin Province, and geochemical data of minerals from the entrained mantle xenoliths, with the aim of constraining the origin of the basahs and the nature of the Late Mesozoic lithosphere mantle. The whole-rock K-Ar dating results indicate that Liaoyuan basahs formed in the Late Cretaceous （91.7±1.7 Ma, 82.5±1.9 Ma, 99.3±1.6 Ma, 82. 2±2. 4 Ma）. They have SiO2= 45. 1%--45.9%, Na2O±K2O= 4. 50%-6.72%, and Mg# [100）〈Mg/（Mg＋F32＋ ）]=60--63, suggesting that they belong to the alkaline series. Liaoyuan basahs are enriched in light rare earth elements （LREEs）, and depleted in heavy rare earth elements （HREEs） [（La/Yb）N = 12. 0--14. 0], and have no Eu anomalies. Their trace element patterns are similar to that of OIB. The entrained peridotite xenoliths consist mainly of spinel lherzolites in which olivines have relatively low Mg# （87. 1-91.2）. The major and trace element data of the minerals from the xenoliths indicate that the Late Cretaceous litho sphere mantle beneath the Liaoyuan area be composed mainly of the juvenile one. The incompatible elements of clinopyroxenes from the peridotite xenoliths vary from the enrichment to the depletion, suggesting that the lithosphere mantle has experienced partial melting and fluid/melt metasomatism, and that the metasomatizing agents are mainly silicate melt and minor carbonate melt. The Late Cretaceous basalts in the Liaoyuan area were derived from partial melting of asthenospheric mantle. Combined with the studies on the regional geology, we conclude that the basalts formed under the Back-arc extensional setting related to the subduction of the Paleo-Pacific plate.