A PETROLOGICAL AND RB-SR ISOTOPIC STUDY OF AN EARLY ARCHEAN FERTILE GRANITE PEGMATITE SYSTEM - THE SINCENI-PLUTON IN SWAZILAND

The Sinceni Pluton in the eastern Kaapvaal craton of Swaziland is one of the world's earliest examples of a fertile granite with associated rare-metal pegmatites. The Rb-Sr isochron age of the pluton is 2990 +/- 43 Ma, Sr-87/Sr-86 (i) = 0.711 +/- 7 (95% confidence limit errors). Cogenetic, rare-metal LCT-type pegmatites yield K-feldspar and muscovite Rb-Sr ages of 3000 +/- 100 Ma. The Sinceni Pluton is asymetrically zoned in chemical and mineralogical composition, and the Sinceni pegmatite field occurs at the most differentiated southern end of the pluton. Geochemical modelling shows that the internal differentiation of the pluton is consistent with 25-35% fractional crystallization, and that over 90-99% fractional crystallization would produce the observed Rb, Sr, Ba and Cs contents of the pegmatites. The geochemically primitive pre-3000 Ma Archean crust in Swaziland is unlikely to constitute a direct source of the Sinceni Pluton according to the Sr isotopic and age data. The origin of the pluton can best be explained as a late differentiate from a large volume of granitic magma which formed penecontemporaneous batholiths.