江苏赣榆县石桥浅变质岩地球化学和锆石U-Pb年龄

Geochemical characteristics and zircon U-Pb ages of low-grade metamorphic rocks in Shiqiao region, Ganyu County, Jiangsu Province

对石桥浅变质岩进行了综合研究。结果表明：（1）岩石化学成分富碱（Na2O＋K2O6.39％～10.65％）、高钾、贫钠（Na2O／K2O=0.03-0.05）；（2）微量元素富集Rb、Ba、Th、U等大离子亲石元素和富Pb，贫Sr；（3）稀土元素含量为130.89～232.80gg／g，轻稀土元素富集（LREE／HREE=7.92～14.84），Eu负异常不明显（6Eu=0.73～0.911，模式曲线向右倾斜（La／Yb=8.4～21.3）；（4）锆石定年表明其时代属于寒武纪（554 Ma）；（5）变质复合锆石的老核，其年龄值较多的是在新太古代（2577～2502Ma）至古元古代（2500～1600Ma）。变质复合锆石的新壳，其年龄分别为2483、2370、2211、778和554Ma，表明继承锆石在其源区曾分别经过变质作用。前述特征均不同于大别一苏鲁超高压变质带的副片麻岩，暗示石桥浅变质岩的成因非原地，而是异地（飞来峰）。

The low-grade metamorphic rocks in Shiqiao area are characterized by alkali-rich （Na2O＋K2O= 6.39%-10.65%）, high K20 （Na2O/K2O =0.03-0.05） and low Na20. They are further characterized by enrichment in 1 Rb, Ba, Th, Pb and U and depletion in Sr. The concentration of rare earth elements （REE） is in the range 130.89-232.80 μg/g. These rocks are enriched in light REE （LREE/HREE = 7.92-14.84） and lack of obvious Eu negative anomaly （δEu = 0.73-0.91）. The new shells of metamorphic composite zircon range in age from Proterozoic era （2483 Ma, 2370 Ma, 2211 Ma and 778 Ma） to Paleozoic era （544 Ma）. However, the existence of Paleozoic-age indicates the low-grade metamorphic rock within Shiqiao area belongs to the early Paleozoic （554 Ma）. The U-Pb ages of inherited zircons are concentrated at Mesoarchean, Neoarchean, Paleo-proterozoic. The different age within the news shells of metamorphic composite zircon shows that the inherited zircons were subjected to metamorphism in the source area. As the preceding feature are different from the paragneiss within Dabie-Sulu ultrahigh pressure metamorphic belt, the petrogenesis of the low grade metamorphic rock is a klippe.