Multiple zircon growth within early Archaean tonalitic gneiss from the Ancient Gneiss Complex, Swaziland

Abstract:

UPb age determinations by ion microprobe reveal multiple episodes of zircon growth and recrystallization within a single sample of tonalitic gneiss from the Ancient Gneiss Complex. The oldest episode at 3644 ± 4 Ma (2σ) produced the dominant type of zircon, characteristically purplish-brown and massive in texture; this probably constitutes unaltered zircon precipitated from the original magma. Recrystallization accompanied (and obscured) by early Pb loss took place within the oldest grains at 3504 ± 6 Ma and 3433 ± 8 Ma. Whole new grains grew at these times also. We interpret the post-3644 Ma growth as due to episodic deformational and metamorphic events that transformed the tonalite pluton into foliated banded gneiss. In addition, many grains are visibly overgrown by two layers of younger zircon of different colour and texture, dated at 2986 ± 20 Ma and 2867 ± 30 Ma. Euhedral, finely-zoned whole grains having the 2986 Ma age are present also, evidently contributed by very thin felsic veins associated with the nearby Lochiel granite. The age of 3644 ± 4 Ma combined with precise zircon UPb dating of volcanics from the Onverwacht Group reported elsewhere demonstrates that at least part of the Ancient Gneiss Complex is older than the Barberton Greenstone Belt. © 1988.

Año de publicación:

1988

Keywords:

Fuente:

scopus