Abstract
Mount Morning is a Cenozoic, alkaline eruptive center in the south-west Ross Sea, Antarctica. New ages on 17 Mount Morning volcanic rocks (combined with 34 existing ages) allows division of Mount Morning volcanism into two phases, erupted between at least 18.7 Ma and 11.4 Ma, and 6.13 and 0.02 Ma. The position of Mount Morning on the active West Antarctic Rift System within the stationary Antarctic plate is a key factor in the eruptive center’s longevity. The earliest, mildly alkaline, Phase I volcanism comprises predominantly trachytic rocks produced by combined assimilation and fractional crystallization processes over 7.3 m.y. Strongly alkaline Phase II volcanism is dominated by a basanite – phonolite lineage, with the youngest (post last glacial maximum) activity dominated by small volume primitive basanite eruptions. The evolution from mildly to strongly alkaline chemistry between phases reflects magma residence time in the crust, the degree of mantle melting, or the degree of magma—country-rock interaction. Phase I magmatism occurred over a comparable area to the present-day, Phase II shield. The 5.2 m.y. volcanic hiatus separating Phase I and II coincides with a cycle of eruption and glacial erosion at the nearby Minna Bluff eruptive center. Mount Morning is the likely source of volcanic detritus in Cape Roberts drill-core (about 24.1 to 18.4 Ma) and in ANDRILL drill-hole 1B (about 13.6 Ma), located 170 km north and 105 km north-east respectively, of Mount Morning. Based upon the timing of eruptions and high heat-flow, Mount Morning should be considered a dormant volcano.