HR: 1340h
AN: V43E-1461    [Abstracts]
TI: Volcanic Rocks Collected With ROV Tiburon From Rodriguez Seamount, Located at the Continental Slope of the California Borderland
AU: * Davis, A S
EM: davisa@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 9644 United States
AU: Clague, D A
EM: clague@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 9644 United States
AU: Paduan, J B
EM: paje@mbari.org
AF: MBARI, 7700 Sandholdt Road, Moss Landing, CA 95039 9644 United States
AB: Volcanic rocks were collected from Rodriguez Seamount at the outer margin off the Continental Borderland with MBARI's ROV Tiburon in October 2003 and April 2004. Six dives recovered lava and volcaniclastic samples from the deep flanks ($\sim$2120 m) to the summit at 630 m. Whole rock compositions of plagioclase-olivine-clionpyroxene bearing lava samples are predominantly alkalic basalt ($<$8% MgO) and hawaiite with minor mugearite (MgO=1.5%). Glass compositions of pillow rims and of volcaniclastic fragments in breccia and bedded sandstone are predominantly hawaiite, mugearite and minor evolved alkalic basalt. The lava samples include one rhyolite and one basaltic andesite with subduction-related chemistry; they are probably erratics. Other clearly identifiable erratics include granite, quartzite, amphibolite, and bored, erosion-sculpted sandstone, resembling typical beach deposits. Most of these erratics are pebble- to small cobble-size and occur in conglomerate and crossbedded sandstone that surround the summit at a break in slope that most likely marks the shoreline when Rodriguez was an island. The lava outcrops on the gently domed platform of the summit are dense, oxidized \`{a}\`{a}-like flows without glassy rinds. Sulfur content of glass, collected from the flanks of the volcano, ranges from 1300 ppm of a glass inclusion in an olivine crystal to $\sim$160 ppm of volcaniclastic grains, indicating extensive degassing. Petrographically and chemically these lavas are virtually identical to those erupted on Miocene seamounts offshore central California (e.g. Davidson, Guide, Pioneer, Gumdrop seamounts, Davis et al, 2002) as well as Northeast Bank on the continental shelf south of Rodriguez and seamounts farther offshore from the Continental Borderland (e.g. Little Joe, San Marcos, San Juan seamounts, Clague et al, unpublished; Davis et al., 1995). Trace element abundances and ratios (e.g. LREE, Zr/Nb, Ta/Nb) also completely overlap with those from the other sites, suggesting similar origins by small percentage of melting of somewhat enriched MORB-like sources. Ar-Ar dating for these samples is in progress but published ages (Davis et al, 2002; Bohrson and Davis, 1994) indicate sporadic eruptions during the late Miocene along the continental margin, extending from the continental shelf out to the seamount province. This volcanism postdates the change from a subduction-related regime by at least 7 million years and does not appear to be related to migration of a triple junction. Instead episodes of localized extension in response to movement along the transform fault systems allowed small degree of decompression melting of mantle rising along zones of weakness.
DE: 8400 VOLCANOLOGY
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 3655 Major element composition
DE: 3670 Minor and trace element composition
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting