The geology, petrology, geochronology and source region character of the layered gabbronoritic Oranjekom Complex in the Kibaran Namaqua mobile belt, South Africa

The Oranjekom Complex, a small mafic to anorthositic layered intrusion situ ated on the eastern margin of the Namaqua Mobile Belt, forms part of a numb er of such intrusions that were intruded early in the evolution of the belt . These have subsequently been deformed and isochemically metamorphosed to amphibole-bearing rocks. Petrographic and geochemical evidence indicates th at the magmas were alumina-rich and that plagioclase was the primary liquid us phase, but in situ differentiation trends are shown mainly by the mafic phases. This implies that plagioclase crystals, although present in the mag ma, remained partially in suspension, whereas the mafic minerals accumulate d. This is supported by both major and trace element character of the rocks . The Rb-Sr and Sm-Nd isotopic systems indicate that the Oranjekom Complex was intruded and metamorphosed at ca. 1100 Ma. This was followed by further metamorphism at ca. 944 Ma. The isotopic character indicates a depleted ma ntle source, that gives an age slightly older but within error of the estim ated time of intrusion. Further data is required to precisely constrain the age of intrusion of these important markers in the evolution of the Namaqu a Orogen. The isotope data indicate that there was limited crustal interact ion indicating a thin (possibly juvenile) crust at that time. (C) 2000 Else vier Science Limited. All rights reserved.