THE ZIRCON XE-S-XE-N SPECTRUM DATING OF PRIMARY EVENTS IN HIGH-GRADE METAMORPHIC TERRAINS - THE EXAMPLE OF ARCHEAN NAPIER COMPLEX (EAST ANTARCTICA)

The Xe-Xe spectrum dating results are presented for zircons from early Archaean Napier Complex (East Antarctica). Experimental procedure inc ludes measurements of Xe isotopes ratios during stepwise heating (900 to 1800 degrees C, step interval 100 degrees) of samples after prelimi nary irradiation in a thermal column of a nuclear reactor (fluence 3 x 10(16) n/cm(2)). The most reliable Xe-Xe zircons ages of ca. 3.3-3.9 Ga (gas increments in the range of 1100-1500 degrees C) are similar to the known dates of the crust formation in the region. In turn, the cl osure temperature (T-c) estimates for the studied zircons mostly agree with the maximum conditions of the ''primary'' (M(1)) granulite metam orphism in Napier Complex and exceed those for later processes. The co rrelation of the derived Xe-ages with R Il demonstrates that the oldes t observed regional metamorphism shortly postdated the crust formation . Hence, previously considered tremendous gap (0.5 b.y. or more) betwe en the two events seems doubtful.