江苏赣榆县石桥片麻岩地球化学和锆石U-Pb年龄

通过野外和室内研究结果表明,石桥片麻岩是由沉积岩变质组成。在化学成分上,表现为高硅（SiO₂=75.22%～76.42%）、低铝（Al₂O₃=11.02%～12.29%）、富碱（Na₂O+K₂O=8.00%～8.68%）和贫钙（CaO=0.30%～0.82%）,富集Rb、Ba、Th、U等大离子亲石元素,贫Nb、Ta、Zr、Hf、Ti 等高场强元素,以及轻稀土相对富集（LREE/HREE=6.06～7.14）、铕负异常较明显（δEu=0.18～0.56）等。通过锆石成因类型及定年研究,获得超高压变质作用时间为254 Ma和退变质作用时间为217 Ma,老核（继承锆石）岩浆碎屑锆石是异地多时代（元古代-古生代）,其寄主岩的原岩为沉积岩,时代不会早于古生代。

Geochemical characteristic and zircon U-Pb ages of gneiss in Shiqiao area,Ganyu Country,Jiangsu Province

The gneiss in Shiqiao area are characterized by high SiO₂(SiO₂=75.22%～76.42%), low Al₂O₃(Al₂O₃=11.02%～12.29%), alkali-rich(Na₂O+K₂O=8.00%～8.68%)and low CaO(CaO=0.30%～0.82%). They are further characterized by enrichment in large-ion lithophile elements such as Rb, Ba, Th and U and depletion in high field-strength elements such as Nb, Ta, Zr, Hf and Ti. The concentration of rare earth elements(REE)is in the range 170.51×10^-6～263.49×10^-6. These rocks are enriched in light REE(LREE/HREE=6.06～7.14) and lack of obvious Eu negative anomaly(δEu=0.18～0.56). We yield the zircon U-Pb age of 254 Ma corresponding to the UHP time, and 217 Ma are retrograde time. Above all, a large number of allochthonous inherited zircon are formed in different times, which indicates the original rock of the Shiqiao gneiss should be sedimentary rocks and it may not be earlier than the Paleozoic era.