The zircon Xe_s–Xe_n spectrum dating of primary events in high-grade metamorphic terrains—the example of Archaean Napier Complex (East Antarctica)

Abstract

The Xe–Xe spectrum dating results are presented for zircons from early Archaean Napier Complex (East Antarctica). Experimental procedure includes measurements of Xe isotopes ratios during stepwise heating (900 to 1800°C, step interval 100°) of samples after preliminary irradiation in a thermal column of a nuclear reactor (fluence 3 × 10¹⁶ n/cm²). The most reliable Xe-Xe zircons ages of ca. 3.3–3.9 Ga (gas increments in the range of 1100–1500°C) are similar to the known dates of the crust formation in the region. In turn, the closure temperature (T_c) estimates for the studied zircons mostly agree with the maximum conditions of the “primary” (M₁) granulite metamorphism in Napier Complex and exceed those for later processes. The correlation of the derived Xe-ages with M₁ demonstrates that the oldest observed regional metamorphism shortly postdated the crust formation. Hence, previously considered tremendous gap (0.5 b.y. or more) between the two events seems doubtful.