滇西二叠系玄武岩及其与东古特提斯演化的关系

Geochemistry of the Permian basalt and its relationship with east Palaeo-Tethys evolution in Xiangyun area,Yunnan Province

目的研究云南祥云地区二叠系玄武岩和东古特提斯洋在三江地区演化历史之间的关系。方法运用地球化学及Sr-Nd-Pb同位素特征分析。结果祥云地区二叠系玄武岩属钙碱性系列,TiO2=1.41%~2.05%(平均1.69%),K2O=0.13%~3.04%(平均1.19%),轻稀土中度富集。岩石形成于大陆溢流环境,属于中国南方广泛分布的峨眉山玄武岩中的低钛玄武岩类。该组玄武岩微量及稀土元素地球化学显示出显著的过渡属性,总体表现为板内玄武岩特征,但又具有大洋拉斑玄武岩的部分特征,并在一些特征元素组合方面指示了俯冲物质(或陆壳)的混染。岩石具有富集的同位素地球化学特征,(87Sr/86Sr)i=0.706 350~0.706 635,143Nd/144Nd=0.512 355~0.512 381,εNd(t)=-3.99^-3.3,TDM=1.14~1.18 Ga。结论本区玄武岩可能形成于东古特提斯主洋盆地紧邻的大陆边缘环境,其地幔源区具有大陆富集地幔+大洋亏损地幔的混源特征,并在其源区内或岩浆形成演化过程中受到过俯冲物质(或陆壳)的混染。

Aim To trace the relationship between basalt magmasism and the tectonic evolution of the eastern Paleo-tethys ocean in Sanjiang area. Methods Petrology, major and trace element, Sr-Nd-Pb isotopic research. Resuits The geochemical research indicates that the Permian basalt from Xiangyun area belongs to calc-alkaline se- ries and exhibits TiO2 = 1.41 - 2. 05% （ mean value is 1.69% ）, K2O = 0. 13 - 3.04% （ mean value is 1.19% ）, moderately enrichment in light rare earth elements. As well as evolved Sr-Nd-Pb isotopic composition （（^87Sr/^86Sr） i =0. 706 350 -0. 706 635 ;^143Nd/^144Nd = 0.512 355 - 0. 512 381,εNd （t） = - 3. 99- -3. 3 ,TDM = 1. 14 - 1. 18 Ga） o It erupted within continental plate tectonic setting during Permian period, belonging to low TiO2 continental flood basalt named Emeishan basalt that was extensively distributed in south China. The basalt shows distinctive transitional geochemical characteristics between typical oceanic island basalt and oceanic crust tholeiite. And in some degree, it might be contaminated by subducted oceanic crust and/or continental crust. Conclusion This implies that the basalt in the studied area should be formed adjacent to the active continental margin of the east palaeo-Tethys. It should be originated from a contamination mantle source （ enrichment subcontinental mantle ＋ depleted oceanic mantle）, and contaminated by subducted oceanic crust and/or continental crust during the partial melting and the magma evolutionary process.