滇西六合正长斑岩和花岗岩包体中锆石U-Pb年代和微量元素地球化学研究

Zircon U-Pb dating and trace element geochemistry in syenite porphyry and granite xenoliths from Liuhe,western Yunnan,China

六合正长斑岩是滇西新生代富碱斑岩的典型代表,以发育各类深浅不一的岩石包体而闻名,其中包括了同期的花岗岩包体;主岩和包体所蕴含的深部地质过程对于富碱岩浆的形成和演化具有重要约束作用。本文以六合正长斑岩和其中花岗岩包体的锆石微量元素地球化学为研究重点,结合主岩主量元素、锆石U-Pb年龄、Ti温度、锆石氧逸度的对比研究,分析和探讨富碱岩浆的形成和演化过程。研究表明:锆石U-Pb年龄所限定的六合富碱岩浆活动时限为(37.89±0.96)^(35.87±0.58) Ma;而花岗岩包体代表的同期富硅酸性岩浆活动时限为(38.21±0.44)^(36.21±0.36) Ma;两者几乎同步活动。锆石微量元素成因判别图显示主岩锆石主要位于钾镁煌斑岩范围,暗示富碱岩浆源区具地幔属性;花岗岩包体锆石大多位于花岗岩范围,两者在花岗岩范围有部分重叠,证实了主岩锆石和花岗岩包体锆石结晶时存在共同的生长环境。Ce^(Ⅳ)/Ce^((Ⅲ))比值估算和Ti温度计算显示,在36.42~36.41Ma范围内富碱岩浆氧逸度和锆石结晶温度发生突变,指示在该时限内富碱岩浆及所裹挟的花岗质岩浆发生了混合作用;氧逸度的振荡降低反映了后期幔源岩浆的补充。综合研究认为,六合地区来自富集地幔的喜山期富碱岩浆底侵熔化上覆深部地壳长英质岩石形成花岗质岩浆,随即以富碱岩浆为主与少量花岗质岩浆不均一混合并同步结晶成岩形成含有花岗岩包体的正长斑岩。

The Liuhe syenite porphyry is a typical representative of the Cenozoic alkali-rich porphyry in western Yunnan.It is famous for various xenoliths developed at different depths,including granite xenoliths of the same period.The deep geological processes represented by these xenoliths have important constraints on the formation and evolution of alkali-rich magma.In this paper,we focused on zircon trace element analyses,and combined with the studies on syenite porphyry main elements,zircon U-Pb dating,Ti thermometer and zircon oxygen fugacity of the Liuhe syenite porphyry and its granite xenoliths to explore the formation and evolution process of alkali-rich magma.The zircon U-Pb dating results suggested that the activity time of Liuhe alkali-rich magma is 37.89±0.96 to 35.87±0.58 Ma,almost synchronous with that of granite inclusions representing the same period of silicon-rich acidic magma at 38.21±0.44 to 36.21±0.36 Ma.The zircon trace element diagram showed that zircons from syenite porphyry mainly overlapped with the potassium-magnesium lamprophyre range,suggesting that alkali-rich magma source has mantle properties;whereas zircons from granite xenoliths overlapped in part with the granite range.The overlapping part confirmed a common growth environment during syenite porphyry and granite xenoliths zircon crystallization.The estimated Ce(^Ⅳ)/Ce(^Ⅲ)ratio and Ti temperature calculations showed that oxygen fugacity of the alkali-rich magma and zircon crystallization temperature changed greatly between 36.42 and36.41 Ma,indicating the effect of mixing alkali-rich and entrapped granitic magma;while decreasing oxygen fugacity oscillation reflects magma supplement from a later mantle source.In summary,we speculate that the Himalayan alkali-rich magma from enriched mantle source in the Liuhe area underplates and melts the overlying felsic rocks to form granitic magma,and a small amount of which mix with alkali-rich magma to form syenite porphyry.