云南大平糜棱岩化碱性花岗岩的锆石特征及其地质意义

在哀牢山一金沙江新生代钾质碱性岩浆岩带的南端，云南金平县大平糜棱岩化钠闪石霓石花岗岩中锆石，主要为“变质锆石”和具有老核新壳的“变质复合型锆石”，它们皆具不规则状和圆化的外形，并常高度的富集成堆和显示有一定层位。用ELA-ICP-MS和SHRIMP对锆石定年，结果表明，变质复合型锆石中老核年龄为247．9±6．4Ma，新壳和变质锆石年龄为145．7±3．4Ma。薄片观察发现，在岩体发生糜棱岩化之前还有一次区域变质作用，呈现在糜棱岩化弱的地段岩石发育柱粒变晶结构和花岗变晶结构，而且残留有清楚的层理，由此表明该花岗岩的成因与沉积变质有关。

Characteristics of Zircons for Dating from Daping Mylonitized Alkaline Granite in Yunnan and Their Geologic Implications

The Daping mylonitized riebeckite-aegirine granite is located in the southern part of the Ailaoshan-Jinshajiang Cenozoic alkaline magmatic rock belt. The zircons for dating form this granite are of two types: the metamorphic zircons and the metamorphic composite zircons which have old cores and new shells. Their crystal morphology is irregular and/or rounded. The high-degree concentration and layering of zircons are observed in rock thin sections. The results of dating are as following: the old core U-Th-Pb age is 247 Ma by using ELA-ICP-MS and the new shell U-Pb age is 145 Ma by using SHRIMP in metamorphic composite zircons. At the same time, there was a regional metamorphic process before mylonitization. In the weakly mylonitized segments the rocks have prism-granular blastic texture and granoblastic texture. The rocks have clear layering, which suggests a sedimentary-metamorphic genesis of this granite.