The deposition of alumina, silica and magnesia films by electron bombardment evaporation

Abstract

Sm-Nd internal isochron ages involving retrograde garnet determined from three ultrahigh-temperature (UHT) gneisses taken from Tonagh Island, the western part of the Napier Complex, East Antarctica gave 1870±82 Ma, 1897±50 Ma and 1557±35 Ma. These ages are younger than the late Archaean timing of UHT metamorphism in the Napier Complex. The ca. 1900 Ma age is considered to reflect an important tectonothermal event in the Napier Complex including a tholeiite dyke intrusion. On the other hand, the ca. 1600 Ma age represents a thermal modification lacking signs of deformational events, and separates from the ca. 1900 Ma event. The related East Gondwana fragments such as the Rayner Complex in Antarctica and the Eastern Ghats Belt in India record extensive tectonothermal event of ca. 1400-1600 Ma, and rare indications of ca. 1900-2000 Ma. It is stressed that the assembly of East Gondwana including the Napier Complex, the Rayner Complex, and the Eastern Ghats Belt, if it existed, should be before ca. 1600 Ma, and may be traced back to ca. 2000 Ma of the supercontinent “Columbia” era.