Tectonic evolution of the Paleoproterozoic Francevillian Basin and implications for the Lomagundi Carbon Isotope Excursion

The 2.2–2.06 Ga Lomagundi Carbon Isotope Excursion (LCIE), Earth’s most pronounced and long-lasting carbon isotope anomaly, corresponds to the latter part of the Great Oxidation Episode (GOE), but its timing is poorly constrained, and recent work shows facies-dependent δ¹³C variations in restricted nearshore settings, challenging its interpretation as a global Paleoproterozoic ocean signal. The end of the LCIE has been recognized in the Francevillian Group, an exceptionally well-preserved, unmetamorphosed Paleoproterozoic succession in southeastern Gabon. However, uncertainties in basin evolution and depositional ages have limited interpretations. To address this, we combined detrital zircon U-Pb geochronology, based on two independent methods, with geochemical analyses for the FB to FD formations. The results of the two methods show a consistent signal of a tectonic evolution from extensional (rift to passive margin) to convergent (foreland) settings and indicate that the Francevillian Basin was connected to the global ocean both during the LCIE peak (ca. 2.13 Ga, FB Formation) and its termination (ca. 2.08 Ga, FC–FD transition). Integrating these data with geochronologic constraints from other open-marine carbonate successions characterized by high δ¹³C values demonstrates a synchronous end to the LCIE between ca. 2.13 and 2.08 Ga. This implies that the LCIE reflects global-scale biogeochemical and redox processes rather than local, basin-scale processes. The Francevillian Group record thus provides critical insights into the GOE, early biospheric innovations and ecosystem evolution.