滇西剑川始新世富碱岩浆岩锆石U-Pb年代学与Sr-Nd-Hf同位素地球化学及其对岩石成因的制约

滇西剑川富碱岩浆岩位于青藏高原东南缘的三江南段,是金沙江-红河富碱岩浆岩带的重要组成部分。剑川富碱岩浆岩包括花岗岩和正长岩两类岩石,前者主要有花岗斑岩和石英二长斑岩,后者主要是正长斑岩和粗面岩。本文对剑川富碱岩浆岩进行了主微量元素、锆石U-Pb年代学和Sr-Nd-Hf同位素特征研究。锆石U-Pb测年结果显示,剑川花岗岩结晶年龄为35. 1～36. 1Ma,正长岩结晶年龄为35. 7～35. 8Ma,均形成于始新世。花岗斑岩和石英二长斑岩的SiO_2含量为67. 92%～69. 93%,K_2O/Na_2O比值介于0. 86～1. 22,具有高钾钙碱性特征;正长斑岩和粗面岩的SiO_2含量为53. 94%～63. 51%,K_2O/Na_2O比值介于1. 30～2. 68,属于钾玄质岩石系列。两类岩石都富集轻稀土元素(LREE)和大离子亲石元素(LILE),相对亏损高场强元素(HFSE)。其中,花岗斑岩和石英二长斑岩有着较高的Sr、Sr/Y、La/Yb值和低的Y、Yb含量,具有埃达克质岩浆属性。结合Sr-Nd-Hf同位素研究认为,滇西剑川地区花岗岩起源于增厚的镁铁质新生下地壳部分熔融,正长岩是由交代富集的岩石圈地幔熔融产生的基性岩浆演化而来的产物。滇西剑川新生代富碱岩浆活动是对印度与欧亚板块晚碰撞阶段,岩石圈地幔发生对流减薄和软流圈物质上涌过程的响应。

Zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopes of the Eocene alkali-rich magmatic rocks in the Jianchuan area of western Yunnan and their implications for petrogenesis

Located in the southern section of the Sanjiang region on the southeastern margin of the Qinghai-Tibet Plateau, alkali-rich magmatic rocks in the Jianchuan area of western Yunnan are important part of the Jinsha River-Red River alkali-rich magmatism belt. These alkali-rich magmatic rocks include granites and syenites. The former includes granite porphyry and quartz-monzonite porphyry, and the later consists of syenite porphyry and trachyte. In this paper, we report major and trace elemental composition, zircon U-Pb geochronology and Sr-Nd-Hf isotopic composition of these alkali-rich magmatic rocks. The zircon U-Pb dating indicates that the granites and syenites formed at 35.1~36.1Ma and 35.7~35.8Ma, respectively, consistent with the Cenozoic alkali-rich magmatic rocks in western Yunnan. Geochemical results showed that the granite porphyry and quartz-monzonite porphyry belong to high-K calc-alkalic series, with SiO₂ contents of 67.92%~69.93% and K₂O/Na₂O ratios of 0.86~1.22. The syenite porphyry and trachyte belong to shoshonite series, with SiO₂ contents of 53.94%~63.51% and K₂O/Na₂O ratios of 1.20~2.68. They are both enriched in alkali, K₂O, light rare-earth elements (LREEs) and large ion lithophile elements (LILEs), but depleted in high field strength elements (HFSEs) relatively. The granite porphyry and quartz-monzonite porphyry geochemically show adakitic affinity, characterized by high Sr contents and high ratios of Sr/Y, La/Yb, but low Y and Yb contents. These geochemical signatures, along with Sr-Nd-Hf isotopic compositions, reveal that the granites were product of partial melting of thickened mafic lower crust and the syenites were derived from direct partial melting of metasomatized lithospheric mantle with various degree of fractionation. These alkali-rich magmatic rocks in the Jianchuan area formed during the late stage of collision between Indian and Eurasian continents. The Cenozoic alkali-rich magmatism is the response to asthenospheric upwelling, following the thinning of mantle lithosphere in western Yunnan.