滇西“三江”地区临沧花岗岩基中三叠世碱长花岗岩的发现及其意义

在野外调研的基础上，对滇西“三江”地区临沧花岗岩基中勐库碱长花岗岩体的年代学、岩石学和地球化学开展了较系统的研究。结果显示，勐库岩体的锆石LA-ICP-MS U-Pb年龄为236.2±3.7 Ma，表明岩体应形成于中三叠世，而非前人认为的新生代。全岩主、微量元素，Sr同位素和锆石微量元素特征显示岩体属于S型花岗岩系列，来自于与临沧花岗岩基相类似的地壳沉积物源区的部分熔融，并经历了高程度的结晶分异作用。以勐库岩体为代表的碱长花岗岩与临沧花岗岩基形成时代一致，表明碱长花岗岩应为临沧花岗岩基的重要组成部分。除已报道的黑云母二长花岗岩类和黑云母花岗闪长岩类外，临沧花岗岩基还应包括较广泛分布的碱长花岗岩类。本文同时揭示，由于临沧花岗岩基中以勐库岩体为代表的碱长花岗岩并非早前认为的新生代岩体，“三江”南段特别是临沧地块前人划分的新生代岩体分布范围和规模可能需要重估。

The discovery of Middle Triassic alkali feldspar granite from the giant Lincang batholith in southern Three River region and its geological implications

The Lincang batholith located in southern Three River region is the largest felsic intrusion in southwestern China. The Mengku pluton is one of the major alkali feldspar granite intrusions within the Lincang batholith. Based on field investigations, the authors studied the geochronology, petrology and geochemistry of the Mengku pluton. The results show that the pluton was emplaced in the Middle Triassic (236.2±3.7 Ma), much older than the previously conceived Early Cenozoic. This means that the distribution and abundance of magmatism of the Cenozoic intrusion province in the southern Three River region should be reevaluated. After a careful examination of whole-rock major and trace elements, Sr isotope and zircon trace elements, the authors believe that the Mengku pluton was of S-type affinity and originated from high extent of fractional crystallization of melts generated by metasedimentary crustal anatexis. The Mengku pluton is part of the Lincang batholith. The alkali feldspar granite is another important rock type that has never been reported in the Lincang batholith, which was previously considered to be composed of biotite monzogranite and granodiorite.