New high-precision 40Ar/39Ar ages on Oligocene volcanic rocks of northwestern Kenya

• Rhyolites of Mogila and Songot, Turkana, Kenya are of early Oligocene age (∼32 Ma).
• Rhyolites of Lokwanamur and Gatome, Turkana, Kenya are of Oligocene age (26–27 Ma).
• The Lotikipi synformal structure is broader (∼140 km) than previously depicted (∼85 km).
• Basaltic, then rhyolitic volcanism was widespread in northwest Kenya 26–32 Ma ago.

New, high-precision 40Ar/39Ar ages from volcanic rocks in northwestern Kenya are provided for some areas of exposure in this remote area. We report seven 40Ar/39Ar ages generated from single crystal total fusion experiments on alkali feldspar separated from volcanic rocks in the Mogila, Songot, and Lokwanamur Ranges and the Gatome valley. A rhyolite from the lower part of the sequence in the Mogila Range yielded ages of 32.31 ± 0.06 Ma and 32.33 ± 0.07 Ma, and a rhyolite near the top of that sequence yielded 31.67 ± 0.04 Ma. A single sample from the Songot Range yielded an age of 32.49 ± 0.07 Ma, slightly older than the rocks collected from Mogila. In both ranges the early Oligocene rhyolites are underlain by basalts, as is also the case in the Labur Range. Ages of 25.95 ± 0.03 Ma, 25.91 ± 0.04 Ma, and 27.15 ± 0.03 Ma were measured on alkali feldspar from rhyolites from the Lokwanamur Range, and the nearby Gatome valley. All of these rocks are part of an episode of widespread volcanism in northwestern Kenya in the mid-to late Oligocene that is not currently known from the Ethiopian Rift Valley.