西南三江杂岩带景洪南部地区晚泥盆世火山岩的发现及意义

Petrogenesis of Late Devonian volcanic rocks in the Jinghong area,southwestern Sanjiang region and its geological implications

本文对西南三江地区景洪南部新发现的晚泥盆世英安质火山岩进行了系统的锆石U-Pb年龄、Hf-O同位素以及全岩地球化学研究。测年结果显示,该英安质火山岩形成于362.3±3.4Ma。火山岩的SiO_(2)含量为62.87%~66.29%,MgO含量为2.15%~2.49%,Mg^(#)值为44~47,富集Na_(2)O(4.77%~5.51%),高Na_(2)O/K_(2)O比值(2.1~3.3),低Y(12.5×10^(-6)~15.3×10^(-6))和Yb(1.38×10^(-6)~1.70×10^(-6))具有高硅埃达克岩的地球化学特征,表明它们由俯冲的洋壳部分熔融而成。低的锆石ε_(Hf)(t)(+0.87~+3.27),相对较高的锆石δ^(18)O(6.31‰~7.64‰),以及高的全岩Th/Yb(4.86~7.78)和(La/Sm)_(N)(3.62~4.56)比值指示岩浆中混染了大洋沉积物或混入了大洋沉积岩起源的熔体。综合区域岩浆岩和沉积岩资料本文认为南澜沧江带晚泥盆世时经历了板片的俯冲,该俯冲作用很可能代表的是由大中河、大平掌火山岩以及火山块状硫化物矿床(VHMS)指示的弧后洋盆的俯冲消减过程。

In this study,SHRIMP zircon U-Pb age,Hf-O isotopes,and whole-rock geochemical data are reported for the newlydiscovered Late Devonian dacitic volcanic rocks from the Jinghong area,southwestern Sanjiang region.New dating results reveal that the volcanic rocks were formed at 362.3±3.4 Ma.These rocks have moderate SiO2(62.87%~66.29%),MgO(2.15%~2.49%),and Mg^(#)(44~47)values.They are characterized by high Na_(2)O contents(4.77%~5.51%)with Na_(2)O/K_(2)O ratios of 2.1~3.3,and low Y(12.5×10^(-6)~15.3×10^(-6))and Yb(1.38×10^(-6)~1.70×10^(-6))contents.These geochemical results are similar to those of high-SiO_(2) adakites,indicating that they were derived by partial melting of the subducted oceanic slab.The low zirconε_(Hf)(t)values(+0.87~+3.27)and relatively high zirconδ^(18) O(6.31‰~7.64‰)values,and whole-rock Th/Yb(4.86~7.78)and(La/Sm)N(3.62~4.56)suggest that subducted oceanic sediments were assimilated into or sediment-derived melt incorporated into the magma.In combination with regional Paleozoic magmatism and stratigraphy data,it can be concluded that the southern Lancangjiang belt underwent the Late Devonian oceanic slab subduction,and the process is likely to represent the subduction of the back-arc oceanic basin revealed by Dazhonghe and Dapingzhang volcanic rocks and volcanic-hosted massive sulfide(VHMS).