In situ detrital zircon U-Pb age dating in Lastoursville Sub-Basin carbonate-black shale deposits in Francevillian Basin, Gabon

Abstract

The aim of this master thesis was, first, to test the applicability of the in situ zircon U-Pb dating technique on small zircon grains from carbonate and black-shale rocks and, second, to refine the maximum depositional ages of the Paleoproterozoic sediments in the FB/FC formations of the Francevillian Basin in Gabon. Studied material was sampled in the carbonate-black-shale succession opened in LST12 drill core Lastoursville Sub-Basin of the Francevillian Basin. For detrital zircon dating drill core samples were scanned with a hand-held X-ray fluorescence spectrometer for the highest Zr content, and samples with zircon content over 400 ppm were chosen for further analysis. Three samples had zircon grains suitable for U-Pb dating using laser-ablation inductively coupled plasma mass spectrometry. The zircon grains yielded Pb-Pb ages mostly around 2860 Ma and did not show any systematic variation with respect to the zircon grain size. In one sample from the upper part of the studied succession, there was an indication of a population with Pb-Pb ages at ca 2200 Ma, which defines the maximum age of the sediments. Numerous zircon grains had microscopic galena (PbS) inclusions in them, and such grains were not suitable for U-Pb dating. The galena inclusions are a result of hydrothermal alterations; however, the timing and wider implications of such hydrothermal event(s) on the preservation state of the Francevillian sediments remain debatable.