江苏东海片麻状碱性花岗岩地质地球化学及其年代学研究--以驼峰、牛山两地为例

Geological and Geochemical Characteristics of the Gneissic Alkali Granite in Donghai,Jiangsu Province,and Chronology Research: Two Examples from the Tuofeng and Niushan Regions

本文通过对东海县驼峰和牛山片麻状碱性花岗岩的产状、岩石结构构造、碱性铁镁矿物、元素地球化学和锆石成因及SHRIMP定年等综合研究,得出以下结论：①驼峰和牛山片麻状碱性花岗岩,在造岩矿物和岩石化学成分等方面具有与岩浆成因的钠质碱性花岗岩一致的特点：高硅（SiO2=74.25%~77.08%）、富碱（Na2O＋K2O=7.7%~9.31%）、高钠（Na2O〉K2O）、低钙（CaO=0.18%~0.39%）和含霓石等碱性铁镁矿物;②东海县驼峰和牛山片麻状碱性花岗岩片理产状：走向北东、倾向南东、倾角30°~35°。造岩矿物：斜长石聚片双晶常垂直晶体延长方向等,显示在固相条件下结晶的特征,以及继承锆石大部分是不同时代的岩浆碎屑锆石。上述特征均显示其原岩应是沉积岩;③锆石SHRIMP定年结果表明,驼峰和牛山片麻状碱性花岗岩成岩年龄为晚三叠世,其原岩时代不会早于古生代。

Based on the study of altitude of gneissic alkali-granite,rock structure and texture,alkaline ferromagnesian minerals,element geochemistry,zircon genesis and and zircon SHRIMP dating in Donghai,Jiangsu Province,the following conclusions has been achieved.① Tuofeng and Niushan gneissic alkali-granite have the similar characteristics to sodium alkaline granite in rock-forming minerals and rock chemical compositions,characterized by high silicon（SiO2=74.25%~77.08%）,alkaline-rich（Na2O＋K2O=7.7%~9.31%）,high sodium（Na2OK2O）,low calcium（CaO=0.18%~0.39%）,and presence of aegirines.② Tuofeng and Niushan gneissic alkali-granites are characterized by NE-trending schistosity and dip angle of 30°~35°.The rock-forming mineral is predominantly feldspars with vertical-extending polysynthetic twin,exhibiting features of crystallization under a solid phase condition.Most inherited zircons are magmatic and clastic zircons of different times.All above-mentioned characteristics imply that the original rock should be sedimentary rocks.③ The zircons SHRIMP dating results show that the Tuofeng and Niushan gneissic alkali-granites formed at late Triassic.Therefore,its primary rocks may not be ealier than the Paleozoic.