Ion microprobe ages of zircons from early Archaean granite pebbles and greywacke, barberton greenstone belt, Southern Africa

Abstract:

We report single-grain and grain-domain U-Pb zircon ages for three granitoid clasts from the Moodies conglomerate and for detrital zircons from a Fig Tree greywacke from clastic sediments in the early Archaean Barberton greenstone belt of southern Africa. The crystallization ages obtained for the Moodies clasts vary between 3570 ± 6 Ma and 3518 ± 11 Ma, while the Fig Tree zircons show a surprisingly uniform source terrain age of 3453 ± 9 Ma that may date the stratigraphically underlying Onverwacht Group felsic metavolcanics. The discordancy patterns are ascribed to a combination of variable Pb loss at unspecified times in the past and new zircon growth and/or recrystallization < 3300 Ma ago. The SHRIMP data record details of isotopic variation, even within single zircons, that result in precise definition of upper concordia intercept ages. Our results suggest an age range between ∼ 3460 and ∼ 3570 Ma for the source region of the Barberton belt clastic metasediments, presumably the Ancient Gneiss Complex of Swaziland and felsic volcanics of the Overwacht Group. This source included geochemically evolved granitoids that predate the oldest greenstones by some 100 Ma. © 1988.

Año de publicación:

1988

Keywords:

Fuente:

scopus