大别山苏家河地区榴辉岩和大理岩的地球化学研究

Geochemical Studies of Eclogites and Marbles from Sujiahe in the Western Dabie Mountains

对大别山西段苏家河地区熊店和杨冲低温榴辉岩及胡家湾榴闪岩进行了详细的氢、氧和锶、钕同位素以及稀土元素和矿物化学分析。熊店榴辉岩和胡家湾榴闪岩的全岩δ^(18)O值较高(8.8‰～11.2‰),而杨冲榴辉岩的全岩δ^(18)O价值较低(4.3‰),它们的全岩δD值均为-55‰～-75‰。结合锶、钕同位素和稀土元素组成特征,推测苏家河榴辉岩(榴闪岩)的原岩可能为岛弧玄武岩,变质前曾与海水在不同温度下进行过氢、氧同位素交换。苏家河榴辉岩围岩大理岩的δ^(13)C值较低(-2.6‰～0.1‰),与大别山东段大理岩δ^(13)C(0.5‰～5.7‰)较高明显不同,反映其原岩灰岩沉积环境存在差异。熊店大理岩的δ^(18)O值低达2.0‰～2.2‰,说明岩石在成岩—变质作用过程中曾与低δ^(18)O流体(可能是大气降水)发生过相互作用。

The isotope compositions of hydrogen, oxygen, strontium and neodymium and the compositions of rare earth elements (REE) were measured on mineral seperates and/or whole-rocks from Xiongdian and Yangchong eclogites and Hujiawan garnet amphibolite in the Sujiahe district, the western Dabie Mountains. δ18O values of whole-rocks for the Xiongdian eclogites and Hujiawan garnet amphibolite are as high as +8. 8‰ to +11. 2‰, those for the Yangchong eclogite are as low as +4. 3‰.The whole-rock δD values range from - 55‰ to - 75‰. These results indicate that the eclogite precursors exchanged oxygen and hydrogen isotopes with ancient seawater under both high and low temperatures. The Sr and Nd isotope compositions as well as REE distribution patterns show that the Sujiahe eclogites resemble island arc basalts and thus were probably originated from subducted ancient oceanic crust between the Yangtze and Sino-Korean plates. Marbles occur as the country rocks of the eclogites in Sujiahe and have lower δ13C values ( - 2. 6‰-+ 0. 1‰) than those from the eastern Dabie Mountains ( + 0. 5‰-+5. 7‰). This indicates adiffernce in sedimentary environment between these marble protoliths (i. e. limestones), maybe due to a difference in their diagenetic ages. δ18O values of marbles from Xiongdian are as low as + 2. 0‰-+2. 2‰, suggesting a water-rock interaction with a low δ18O fluid (probably ancient meteoric water) during diagenetic and/or prograde metamorphism.