Crumbling Away: 40Ar-39Ar Dating of a Large Miocene Mass Wasting Period on Maio Island (Cape Verde)

Maio Island is situated in the eastern part of Cape Verde archipelago, and comprises early Mesozoic MORB-type pillow lavas and deep-sea sediments, and Miocene-Pliocene igneous rocks. The island is deeply eroded, indicating several phases of intensive erosion and collapse both during and after igneous growth. Maio exposes a central intrusive complex of Miocene age, which is surrounded by Lower Cretaceous pelagic limestones that were uplifted from the surrounding seafloor by igneous and tectonic activity1. The limestones were intruded by dykes and sills during the Miocene (12-8 Ma), with a peak in activity around 11 Ma2. These successions are overlain by lava flows and conglomerates of the Casas Velhas Formation (Fm.) and the Pedro Vaz Fm., which formed between 12-11 Ma, and by plateau lavas of the Malhada Pedra Fm. (9-7 Ma)2. The youngest volcanic units exposed on the island are lava flows of the Monte Penoso Fm., which formed a stratovolcano and have been K-Ar dated at 6.9 ± 0.4 Ma and 6.7 ± 0.4 Ma (2σ )2. At the base of the Monte Penoso Fm., polymict conglomerates up to 100 m thick, which we interpret as landslide deposits, occur at several locations. They contain well rounded clasts of various lithologies. Fresh biotite grains from a xenolith-rich basanite clast within the conglomerate were analysed. Single crystal laser total fusion biotite 40Ar-39Ar ages range from 8.50 ± 0.03 Ma to 11.55 ± 0.34 Ma (2σ, using the decay constants and atmospheric ratio of 3, and age standard TCS2 (27.87 ± 0.04 Ma; 1σ)). The older biotite ages are probably xenocrysts, but the youngest biotites yield a well constrained weighted mean 40Ar-39Ar age of 8.50 ± 0.02 Ma (2σ; n=22). Combining the K-Ar ages of the overlying Monte Penoso Fm.2 and our 40Ar-39Ar single biotite ages for the conglomerate clast, the Miocene period of large scale collapse and erosion on Maio can be confined to a period between 8.50-6.80 Ma, before the final phase of volcanism on the island began with the formation of the Monte Penoso stratovolcano. References: 1 Stillman, C.J. et al. (1982) J. Geol. Soc. 139 (3), 347–361. 2 Mitchell, J.G. et al. (1983) EPSL 64, 61–76. 3 Steiger R.H. & Jäger E. (1977) EPSL 36, 359–362