ZIRCON AGE MEASUREMENTS OF HIGH METAMORPH IZED ROCKS OF NAPIER COMPLEX AT THE EASTERN ANTARCTIC BY XES-XEN SPECTRA

The Xe(s)-Xe(n) age spectra and spontaneous fission xenon extraction k inetics are studied on the material of 4 zircon samples of Napier comp lex at the Eastern Antarctic. According to the theoretical concept of Xe(s)-Xe(n) age determination method the ideal age spectrum at the the rmal annealing should have a shape of the ascending steps changing to the horizontal plateau corresponding to the true sample age. Three of analysed zircons in the temperature range of 70-1400-degrees-C show th e mentioned plateau corresponding to 70-30% of the total radiogenic xe non. Low temperature gas fractions (less-than-or-equal-to 8% of total xenon) indicate the diminshed age values as a result of spontaneous fi ssion xenon losses from the metamict zircon phases during the geologic al history. At 1420-1800-degrees-C the extracted xenon involves 20-60% of its total content and indicates lower than plateau age values. It is assumed that these ages could be related to new formed mineral phas es in metamorphism or to relaxed metamict zircons in the thermic exper iment. The correlation of age-spectra shape and extrema at xenon extra ction curves is observed. It could be explained by the gas release fro m different phases of zircon grains or from zircon grains of different geochemical history. The interpretation presents some difficulties be cause of large number of grains in examined samples (80-150 mg). The f urther development of the method should be directed on the increase of mass-spectrometry sensitivity in 10-50 times. This would allow the da ting on single zircon grains.