江苏省东海县片麻状碱性花岗岩的成因及年代学研究

江苏省东海地区位于苏鲁造山带的西南缘，区内广泛出露片麻状碱性花岗岩.本文对东海地区磨山、虎山和房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定年等综合研究结果表明：该类岩石具有高硅、富碱、富铁贫镁和含碱性铁镁矿物等特征，说明其为碱性花岗岩；岩体还保留有沉积岩特有的层理（走向北东、倾向南东等一致的产状），长石等造岩矿物显示在固相条件下结晶的特征，以及继承锫石大部分是不同时代的岩浆或变质锆石碎屑等，均证实其原岩是沉积岩.SHRIMP定年对变质锆石的边部获得两组206Pb-238U加权平均年龄，分别为224.7±9.4 Ma和209.4±2.5～212±3.5Ma，反映了东海片麻状碱性花岗岩成岩时间属于印支期，相当于晚三叠世，变质锆石的核部206Pb-238U年龄变化于866～248 Ma，反映原岩时代不会早于晚古生代.

Chronology and meta-sedimentary genesis for gneissic alkaline granites in Donghai county,Jiangsu province

Donghai region is tectonically located in the southwestern margin of the Su-Lu Orogenic Belt,where the gneissic alkaline granites were outcropped.This paper made an integrated study of petrology,mineralogy and geochemistry for the Moshan,Hushan and Fangshan gneissic alkaline granites in Donghai.It was shown that these rocks should belong to A-type granite.These plutons were characterized by sedimentary layers,the orthogonal intersection of the elongate direction and polysynthetic twin in the plagioclase,the thick arrangement of zircons in the thin section and the clastic old cores of the metamorphic zircons in the cathodeluminescence images.The allochthonous and polygenetic characteristics of the old cores of the metamorphic zircons suggested the protolith of the gneissic alkaline granite in Donghai being sedimentary rocks,rather than magmatite.The ²⁰⁶Pb-²³⁸U mean ages of 224.7±9.4 Ma and 212±3.5 Ma (or 209.4±2.5 Ma) for the rims of metamorphic zircons indicated that the metamorphic time of the granite should be the Indosinian epoch,correspond to Late Trassic,which were also the diagenetic time of these granites.The ²⁰⁶Pb-²³⁸U ages for the old cores ranged from 866 Ma to 248 Ma,which indicated that the formation time of the protolith of the these granites should not earlier than Late Paleozoic.