Sm-Nd garnet ages of retrograde garnet bearing granulites from Tonagh Island in the Napier Complex, East Antarctica: A preliminary study

摘要

Proterozoic Sm-Nd garnet ages were dated from a garnet-bearing quartzo-feldspathic gneiss (Grt-felsic gneiss) and an iron rich garnet-pyroxene gneiss (Fe-rich Grt-Py gneiss) in the northern part of Tonagh Island, the Napier Complex. The Grtfelsic gneiss occurs as a member of layered gneiss. The Fe-rich Grt-Py gneiss appears as a block in the felsic gneiss. Since the layered gneiss contains a critical mineral assemblage of ultrahigh-temperature (UHT) metamorphism such as sapphirine+quartz, the Grt-felsic and the Fe-rich Grt-Py gneisses have undergone UHT metamorphism whose age is believed to be Late Archaean. The inclination line connecting with garnet and whole rock in the Grt-felsic gneiss yields an age of 1897 Ma in the Sm-Nd system. The internal isochron using garnet, pyroxenes and whole rock for the Fe-rich Grt-Py gneiss gives an age of 1557±35 Ma. Both Sm-Nd internal isochron ages correspond to Early Proterozoic. Garnet-quartz symplectite and quartz occur between orthopyroxene and plagioclase in the Grt-felsic gneiss. This texture indicates the following reaction: orthopyroxene+plagioclase=garnet+quartz. The garnet in the Fe-rich Grt-Py gneiss occurs as interstitial mineral with small grained clinopyroxene and quartz. Taking reaction textures into account, the garnet forming reactions in the Grt-felsic and the Fe-rich Grt-Py gneisses would occur during retrograde events after UHT peak metamorphism. The ages of c. 1900 and c. 1550 Ma would therefore reflect tectonothermal events in the Napier Complex.