HR: 1340h
AN: V43E-1457    [Abstracts]
TI: Preliminary geology of the Tanaga Island volcanic cluster, western Aleutians (Alaska)
AU: Coombs, M L
EM: mcoombs@usgs.gov
AF: Alaska Volcano Obervatory, U.S. Geological Survey 4200 University Drive, Anchorage, AK 99508 United States
AU: * Browne, B L
EM: ftblb@uaf.edu
AF: University of Alaska Fairbanks, Geophysical Institute 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AU: Larsen, J F
EM: faust@gi.alaska.edu
AF: University of Alaska Fairbanks, Geophysical Institute 903 Koyukuk Drive, Fairbanks, AK 99775 United States
AB: During 2003, the northwestern portion of Tanaga Island (178$\deg$W) was mapped in detail for the first time during fieldwork by Alaska Volcano Observatory geologists in conjunction with the installation of a volcano monitoring seismic network. The northern half of the island is approximately 20 km across and comprises four discrete volcanic centers, from west to east: Sajaka (area = 22 km$^{2}$), Tanaga (20 km$^{2}$), East Tanaga (15 km$^{2}$), and Takawangha (54 km$^{2}$). The three western centers are steep-sided cones of Holocene age, and have grown in the scar formed by a catastrophic Pleistocene sector collapse directed to the northwest. To the east, a $>$300 m-thick sequence of volcanic and volcaniclastic rocks (Pre-Tanaga unit) underlies Takawangha, which has been active throughout the Pleistocene and Holocene. Holocene eruptive products from all four centers are predominantly lava flows, with minor explosive activity recorded in tephra sections. Additionally, Sajaka experienced a relatively young (mid-Holocene?) sector collapse of its west flank, accompanied by eruption of laterally-directed pyroclastic flows. A morphologically young cone of scoria and thin basalt flows has grown in the collapse scar. Whole-rock geochemical data on 130 samples of lava and scoria from the four centers and the Pre-Tanaga unit are basalts through low-SiO$_{2}$ andesites. All but ten lavas have molar Mg\# between 0.35 and 0.5 and the remainder are between 0.5 and 0.61; no primitive lavas were discovered on Tanaga Island. Lava flows of Holocene age from Tanaga and East Tanaga follow medium-K trends, all lavas from Takawangha are high-K, and Sajaka and Pre-Tanaga lavas fall along both trends. High-K lavas are enriched in other LILE (Rb, Ba, Pb) as well, and fall near or above the high end of published Aleutian lavas for these elements. The lavas exhibit petrographic as well as compositional diversity: mafic phases in Tanaga lavas are olivine, two pyroxenes, and amphibole, at East Tanaga lavas contain two pyroxenes, amphibole, and minor olivine, and Sajaka, Takawangha, and Pre-Tanaga lavas are predominantly olivine- and clinopyroxene-bearing. Thus the character of the sources as well as crustal storage conditions for magmas within the Tanaga Island cluster likely vary, despite close spacing in space and time.
DE: 8499 General or miscellaneous
DE: 3640 Igneous petrology
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting