HR: 11:50h
AN: V52A-07    [Abstracts]
TI: High-Mg basalts as a Signal of Magma System Replenishment at Lopevi Island, Vanuatu
AU: * Stewart, R B
EM: r.b.stewart@massey.ac.nz
AF: Institute of Natural Resources, Massey University, Palmerston North, New Zealand
AU: Smith, I E
AF: Geology-SGGES, University of Auckland, Auckland, New Zealand
AU: Turner, M B
AF: Institute of Natural Resources, Massey University, Palmerston North, New Zealand
AU: Cronin, S J
AF: Institute of Natural Resources, Massey University, Palmerston North, New Zealand
AB: Lopevi is is a basalt to basaltic andesite island stratovolcano in central Vanuatu and is part of a long-lived, mature Island Arc chain. Central Vanuatu is tectonically influenced by the subduction of the D'Entrecasteaux zone. Primitive rock types that have been identified from the arc include picrites, ankaramites and high MgO basalts. High MgO rocks are generally considered to be a relatively rare component of arc-type magma suites but as detailed sequence sampling of individual volcanoes occurs, they have been identified more often. Here we report on the occurrence of high-Mg basalts in a sequence of lavas erupted in the last 100 years from Lopevi volcano. Activity at Lopevi is characteristically intermittent with eruptive sequences occurring over a c. 6 year period, separated by longer periods of repose. A major eruptive episode in 1939 caused evacuation of the island and the next eruptive episode in the 1960's also led to evacuation. The 1960's cycle of activity ended in 1982. The most recent phase of activity commenced in 1998 with a return to eruption of more siliceous, high alumina basaltic andesite. Geochemical data show that the 1960's lavas were different from those erupted earlier and later. They are olivine basalts with up to 9 wt percent MgO, 70 ppm Ni and 300 ppm Cr; Al2O3 content is about 12 wt percent. The 2003 lavas and pre-1960's lavas, in contrast, are basaltic andesites with c. 4 wt percent MgO, less than 25 ppm Ni, less than 100 ppm Cr and c. 20 wt percent Al2O3. The 1960's Lopevi sequence of eruptions represents an injection of a more primitive, high MgO magma at the end of a 21 year quiescent period after the major eruptions of 1939. Injection of small batches of more primitive magmas over decadal time periods at Lopevi marks the initiation of a new magmatic cycle. The occurrence of high MgO magmas as part of a cycle that includes typically low MgO arc type rocks demonstrates a consanguineous relationship and shows that high MgO arc type rocks are part of a genetically linked suite rather than a distinct magma type. Their comparative scarcity in many subduction related associations is probably a function of tectonic environment rather than of fundamental petrological factors.
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly