Water Rock Interaction (WRI 14) The Archean granitoid-Paleoproterozoic unconformity: superimposition of alteration events in Archean granitoids, Gabon

Abstract 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. © 2012 The Authors. Published by Elsevier B.V. Selection and/or peer-review under responsibility of Organizing and Scientific Committee of WRI 14 – 2013. Keywords : Archean granite, alteration, clay minerals, isotopic dating, Franceville Basin. 1. Introduction The previous studies performed on the sedimentary formation-magmatic basement unconformity dated around 1.8 – 2.9 Ga (Paleoproterozoic to Archean) were mainly focused on the top of the magmatic basement. This unconformity was largely studied in the earth surface: Canada, Scotland, South Africa and Finland. The different authors accord to the fact that the deepest granitic basement was affected by metamorphic-to-hydrothermal alterations, and suggest a diagenitic origin of the sericitic alteration that had affected the top of the granitic basement. Only few authors suggest the superimposition of this diagenetic alteration on an early weathering event.