云南铺台山花岗斑岩的地球化学特征、锆石SHRIMP U-Pb年龄及其地质意义

Geochemical Characteristics and SHRIMP Zircon U-Pb Age of the Putaishan Granite Porphyry and Their Geological Implications

云南省鹤庆县铺台山花岗斑岩是哀牢山-金沙江碱性岩带北衙-六合地区的一个钾质碱性侵入体。本文对该岩体进行了全岩主、微量元素和锆石SHRIMP U-Pb年龄分析。结果显示铺台山花岗斑岩显示出高钾、富碱的特征,同时具有低全岩Mg#值(0.20~0.32),低母岩浆温度和岩浆氧逸度,以及较浅的岩浆起源深度。锆石定年结果为~34 Ma,指示岩体侵位时代为始新世。结合前人对该区域碱性侵入岩Pb同位素研究结果和花岗斑岩锆石核部年龄(218~541 Ma),认为哀牢山-金沙江地区发生强烈剪切-走滑的过程中,哀牢山-金沙江新生代富碱侵入岩带中的氧化性富钾斑岩体可能与古俯冲带下被扰动的碳酸盐化地幔橄榄岩的部分熔融有关;同时印度板块和欧亚板块碰撞造成哀牢山-金沙江地区的强烈伸展和剪切作用诱发地幔上涌,地幔上涌带来的热量使得深部地壳脱水并产生还原性流体,铺台山花岗斑岩可能就是富钾地壳物质在这些还原性流体的参与下发生减压熔融的产物。

The Putaishan granite porphyry is an alkali-rich intrusion in Heqin,Yunnan province.In this study,we conducted whole-rock major and trace element analyses and zircon SHRIMP U-Pb dating of the Putaishan granite porphyry.The Putaishan granite porphyry has high K2O and Na2O+K2O contents,low Mg#(0.20–0.32),low magma temperature,oxygen fugacity and shallow melting depth.Zircon SHRIMP U-Pb dating yielded a crystallization age of ca.34 Ma,indicating that the Putaishan intrusion was emplaced in the Eocene epoch.Combined the obtained ages(218–541 Ma)of zircon cores and previous Pb isotope results on alkali intrusions in this region,we propose that the Cenozoic oxidized alkali-rich intrusions in the Ailaoshan-Jinshajiang belt are possibly resulted from partial melting of carbonated metasomatized mantle peridotite beneath the ancient subduction zone during the divergent strike-slip movements of the Ailaoshan-Jinshajiang faults.Meanwhile,the extension and shearing caused by the collision between the India and the Eura-Asia block in the Ailaoshan-Jinshajiang region triggered the upwelled mantle that supplied heat for dehydration of the deep crust which produced crust-derived reduced fluids,and the decompression melting of the lower crustal materials with the aid of such reductive fluids resulted in the formation of the Putaishan granite porphyry.