Volcanic evolution of Moloka‘i, Hawai‘i: Implications for the shield to postshield transition in Hawaiian volcanoes

New mapping, geochronology and petrological investigations reveal that the two volcanoes of the island of Moloka‘i, Hawai‘i exhibit striking contrasts in composition and volcanic production in the waning stages of their activity. Postshield lava extrusion on West Moloka‘i produced less than 1km3 of alkalic basalt and hawaiite, most likely in less than 100k.y, beginning ~1.8Ma. West Moloka‘i is unique among Hawaiian volcanoes that reached the postshield stage of evolution in lacking a protracted period of transitional volcanism in the late shield stage. Petrological modeling indicates that volatile-poor (<0.5 wt % H2O) West Moloka‘i postshield magmas evolved in reservoirs in the upper mantle, 20-26km beneath the summit. In contrast, following a protracted transitional period, East Moloka‘i extruded postshield magmas ranging from hawaiite to benmoreite that evolved in magma reservoirs lying within the crust, 11- 17km beneath the summit. Parental magmas to the East Moloka‘i postshield were hydrous and enriched in phosphorous and Sr (2.5 wt % P2O5, 2500 ppm Sr at 6 wt % MgO, 45 wt % SiO2), a magma type known previously only from Kohala volcano. Although the overall volume of East Moloka‘i volcano is less than that of West Moloka‘i, its proportion of postshield extrusives is greater by a factor of 8, and its postshield volumetric production rate (~155km3/m.y.) is greater than that at West Moloka‘i by a factor of 3. The lower volume and deeper magmatic evolution of the West Moloka‘i postshield compared to that of East Moloka‘i is consistent with a thermally controlled relationship between magma supply, as inferred from volcanic production rate, and magma reservoir depth.