HR: 1340h
AN: T53A-1401    [Abstracts]
TI: Decompression Basaltic Volcanism in the Lzu Arc Extensional Zone
AU: * Tollstrup, D L
EM: darrent@es.ucsc.edu
AF: UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AU: Gill, J B
EM: jgill@pmc.ucsc.edu
AF: UC Santa Cruz, 1156 High St., Santa Cruz, CA 95064 United States
AB: Current understanding of the sources of Izu arc volcanism is largely based upon studies of the Quaternary volcanoes of the volcanic front and the 12-3 Ma backarc western seamount chains. Th/La ratios increase, and Nd and Hf isotope ratios decrease across the arc from the volcanic front to the backarc, suggesting more subducted sediment in the backarc source. Evidence of slab-derived fluid decreases across the arc. These contrasts are both temporal and spatial. The greatest amount of sediment implied for the Izu backarc is less than the least for the Mariana volcanic front if the initial mantle were the same. The Izu volcanic front source has the least sediment of all NW Pacific arcs. Basaltic volcanism within the ~100 km wide extensional zone between the volcanic front and western seamount chains has been widespread along the full length of the arc since 3 Ma. Its mantle source is less affected by subducted sediment than in the rear-arc seamounts, and is less affected by slab-derived fluids than the volcanic front. This is particularly true of the active (<1 Ma) rifts a mere 25 km west of the volcanic front. Such rifts are characterized by lower Pb and Sr isotope, Th/La, and Hf/Sm ratios, and higher Nd and Hf isotope ratios, than in either the volcanic front or backarc seamount chains. Along-strike and across-strike chemical variations within the extensional zone are minor and apparently unrelated to extrapolations of the seamount chains. Decompression melting minimally affected by subduction seems to have dominated uniformly within the zone since the inception of rifting at ~3 Ma.
DE: 1021 Composition of the oceanic crust
DE: 1025 Composition of the mantle
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Tectonophysics [T]
MN: Fall Meeting 2005