广东罗定龙塘碱性花岗岩锆石SHRIMP定年及地质意义

ZIRCON SHRIMP DATING OF THE LONGTANG ALKALINE GRANITE AT LUODING, GUANGDONG, AND ITS GEOLOGICAL IMPLICATIONS

龙塘碱性花岗岩位于广东云开大山地区,属罗定泗纶混合岩田区,其中的锆石均为具有老核新壳的变质复合型锆石。SHRIMP定年结果,新壳6个测点加权平均年龄为265±1·8Ma,反映该岩体的成岩年龄,相当于中二叠世,属于海西晚期。老核获得最大的U-Pb年龄为1098Ma,相当于中元古代;457Ma和414Ma可能是新壳和老核的混合年龄。根据锆石的成因类型,结合微观所见的岩石结构(反映沉积变质成因的粒状变晶结构和花岗变晶结构等),表明该碱性花岗岩的成因可能是砂泥质沉积物经变质而成。

The Longtang alkaline granite lies in the Luoding migmatites, Yunkaidashan area. All of the zircons from the granite were metamorphic compound ones which composed of old core and young shells. The SHRIMP technique was applied to determining U-Pb age of the Longtang alkaline granite. The results showed that the Longtang alkaline granite was intruded 265 ± 1.8 Ma ago, which should be Guadalupian （Middle Permian） in time and a record of Late Hercynian orogeny. The old core zircon yielded an age of 1098 Ma, which should form in Middle Proterozoic. And the ages of 457 Ma and 414 Ma should be mixture ones by old core and new shells. According to the zircon genetic types, microcosmic structures （ the prism-granular biastic texture and granoblastic texture）, the Longtang alkaline granite should be metamorphic rock by argillo-renaceous sediments.