PETROGRAPHY AGE AND ORIGIN OF THE SCHIEL ALKALINE COMPLEX, NORTHERN TRANSVAAL, SOUTH-AFRICA

U-Pb ion microprobe data for zircon from syenitic phases of the Schiel alkaline complex, South Africa, indicate that it was emplaced at abou t 1.85 Ga and that it contains xenocrystic zircon of about 2.0 Ga and Archaean ages. Zircon with an age of 2.0 Ga may be derived from the ho rnblende granite (fenite) associated with the complex. in contrast, Rb -Sr and uranogenic Pb-Pb whole rock data suggest an age of about 2.1 G a and may reflect mixing of Pb from at least two sources: the lithosph eric source of the magma and the crust into which the magma was intrud ed. Sm-Nd whole-rock isotopic data yield an apparent isochron age of a bout 2.3 Ga, but are interpreted to indicate that the syenitic magma w as derived from a lithospheric source that underwent LREE enrichment b etween about 2.6 and 2.8 Ga ago. This source is chemically similar to that of the 2.05 Ga Phalaborwa complex, suggesting that both complexes were derived from a common, more primitive source but at different ti mes. Emplacement of the Schiel alkaline complex may have been related to rifting leading to deposition of the reeks of the Soutpansberg Grou p.