New 40Ar/39Ar dates and eruptive dynamics of tuff and lava flows in the early stages of the East African Rift (Early Miocene, Napak Palaeovolcano, Uganda)

The Napak Early Miocene palaeovolcano (Uganda, East African Rift-EAR) is listed in the World Heritage Geopark Sites List from UNESCO since 2022. It is an important site for the fossil record in the EAR, because iit was active during the radiation of the hominoid apes. We present the first 40 Ar/ 39 Ar dates obtained from three nepheline lava-bearing samples from the Napak site. These dates range between 20.23 ± 0.07 and 20.20 ± 0.06 Ma (Early Miocene, Early Burdigalian), and are consistent with the known biostratigraphic data. In addition, we performed a bed-by-bed description of the stratigraphy of Napak and a petrographic and major element analyses on clinopyroxenes from tuff and lava flows using Electron microprobe analysis (EMPA). The results show that a mixing process occurred between at least two alkali-rich liquids (i.e nephelinite, phonolite) that accumulated in two distinct magmatic chambers at depths of about 23–30 km and 3–10 km, beneath the volcanic edifice. The eruption style evolved through time, showing a progressive decrease in energy of volcanic eruptions and explosions. However, surprisingly, this evolution was not associated with changes in magma composition through time. We propose alternatively that increased water infiltration through faults or other natural conduits may have controlled the water/magma ratio in the subterranean volcanic environment prior to the eruptions, and ultimately drove the style of eruption (more or less explosive) at the surface. Finally, Napak palaeovolcano, probably shared similar complex magmatic history with other carbonatite-alkaline volcanoes in Kenya and Tanzania ; including a plumbing system with multiple reservoirs associated with recharge/mixing processes and similar carbonatite-alkaline compositions.