The Archean Granitoid-Paleoproterozoic Unconformity: Superimposition of Alteration Events in Archean Granitoids, Gabon

The Archean granitoids of the South Gabon, dated from 3.0-to-2.5 Ga, were examined along the Archean-Proterozoic unconformity. The petrography was based on mineralogical identification using electron microscopy, X-ray diffraction, and the electron microprobe. The geochemistry was based on chemical analyses of major and trace elements, and on isotopic dating. Results of this work show that the granitoids have been affected by superimposed chloritization and illitization events that were dated by monazite U-Th-Pd isotopic data. The granitic basement shows three superimposed alteration events: 3.0-2.9 Ga primary magmatic event, 2.6 Ga propylitic, and a late oxic 1.9 Ga diagenetic event.