HR: 1330h
AN: V32B-1010 INVITED     [PDF]
TI: The Anatahan Felsic Province in the Mariana Arc System
AU: * Stern, R J
EM: rjstern@utdallas.edu
AF: Geosciences Dept., U. Texas at Dallas, Richardson, TX 75083-0688 United States
AU: Hargrove, U S
EM: ushargrove@student.utdallas.edu
AF: Geosciences Dept., U. Texas at Dallas, Richardson, TX 75083-0688 United States
AB: The May 2003 plinian eruption of Anatahan was a surprise for the residents of the Northern Mariana Mariana Comonwealth (USA). From a petrologic perspective, this eruption of siliceous andesite was also atypical for the Mariana arc, which elsewhere mostly erupts fractionated basalts and basaltic andesites. However, felsic eruptions are not unusual for Anatahan which has previously erupted a wide range of lavas, from basalts through dacites. The reasons and significance of the intermittently felsic nature of Anatahan lavas may reflect either development of a mature magma chamber localized beneath Anatahan or perhaps the presence of an areally-extensive pool of felsic magma in the middle crust, similar to that inferred for the Izu arc to the north. Anatahan lies at the southern end of the Mariana Central Island Province and just north of the Southern Seamount Province, but it has never been clear whether these morphological subdivisions reflect different magma fluxes along the arc or are due to the island volcanoes being older than the seamounts. Synthesis of existing data for Anatahan and data for surrounding seamounts collected during the 2001 Cook 7 expedition and earlier cruises indicates that Anatahan is situated near the middle of an arc segment with an unusually high proportion of felsic lavas compared to typical Mariana Arc magmatic products. The 115 km-long arc segment from E. Diamante seamount East Diamante seamount (15$\deg$55'N) to a seamount NW of Zealandia Bank (17$\deg$N) define the Anatahan Felsic Province (AFP). Seven edifices within this region, including Anatahan, have erupted felsic lavas, often interspersed with more mafic lavas. Volcano size does not seem important in controlling the abundance of felsic lavas within the AFP; the volcanic island of Sarigan does not contain felsic lavas, whereas small parasitic cones NE of Anatahan and SW of E. Diamante erupt lavas with 65% and 72% SiO2, respectively. This region also contains two suspected hydrothermal vents, as identified by a 2003 NOAA cruise (see Embley et al. T05 IBM arc session). AFP lavas define a consistently medium-K, calc-alkaline suite. There are no large, well-defined calderas as have been mapped in the Izu arc segment to the north. The clearest calderas in the AFP are those of Anatahan and E. Diamante volcanoes, both of which have similar, E-W elongated, craters. The abundance of felsic lavas associated with complex bathymetry west of Sarigan and Zealandia Bank may manifest a large caldera complex that has been overprinted by post-collapse volcanism; more field work will be needed to test this hypothesis. The abundance of felsic lavas in the AFP is difficult to explain by the development of magmatic reservoirs that allow fractionation beneath individual volcanoes. The regional nature of felsic magmas in the AFP supports explanations that call for these to be developed in a broad zone within the middle crust of the region, similar to that envisioned for the Izu arc segment. It would be useful to know if the unusually high abundance of felsic lavas in this region is associated with a mid-crustal layer with seismic velocities characteristic of tonalitic plutonic rocks, as has been found for the Izu arc to the north.
DE: 3655 Major element composition
DE: 8404 Ash deposits
DE: 9355 Pacific Ocean
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting