Seafloor Geology of the Basement Serpentinite Body in the Ohmachi Seamount (Izu-Bonin Arc) as Exhumed Parts of a Subduction Zone Within the Philippine Sea - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 109

3.2 Amphibole Schist

Six samples of amphibole schists commonly with relics of high-pressure
metamorphism were collected from the Ohmachi Seamount. They consist of three
lithotypes: epidote (Ep) - albite (Ab) amphibolite (four samples), garnet (Grt) -
zoisite (Zo) amphibolite (one sample), and Grt-Ep amphibolite (one sample:
Fig. 6a ). Rocks of the former two types contain microinclusions of the eclogite
facies minerals such as omphacite as reported by Ueda et al. ( 2004 ). For the Grt-Zo
amphibolite sample, Ueda et al. ( 2004, 2005 ) estimated P-T conditions of the peak
metamorphic stage as 600-700°C, 2.0-2.5 GPa, and of decompression stage as
500-600°C, <1.3 GPa. Grt-Ep amphibolite has glaucophane and albite microin-
clusions in garnet, suggesting early blueschist facies metamorphism. All types of
the amphibole schists show nematoblastic textures of amphiboles and epidote or
zoisite, with evident mineral lineation. The Grt-Zo amphibolite also show compo-
sitional banding of amphibole-rich and zoisite-rich layers.
The collected samples are float stones with diameters of several to ten and
several centimeters, whose surfaces commonly have slicken lines. Their localities
are limited near the northern end of the schistose serpentinite exposures (at dive site
6K#609 and dredge site D06: see the next section), and their amount is probably
quite small relative to that of serpentinites. It is thus assumed that they originated
from faulted blocks or clasts sparsely hosted by the schistose serpentinite.

3.3 Paleogene Volcanic Rocks

Volcanic rocks are the main constituent of the Ohmachi Seamount. They consist
mainly of hornblende andesite and two-pyroxene andesite of calc-alkaline series
(Yuasa and Nohara 1992 ; Yuasa et al. 1999 ). They occur as lava and volcanic or
tuff breccia (Fig. 5f ) accompanied by tuff and volcanic sandstone and conglomerate.
Hypabyssal varieties were also observed in the main parts of the seamount. No
deformation structures have been found in these rocks. Volcanic glass is preserved
in unaltered rocks, whereas altered rocks contain zeolites and clay minerals. From
two-pyroxene andesite cobbles dredged at site D731 (Fig. 4 ), K-Ar ages of
31.9 ± 1.2 Ma and 33.6 ± 1.2 Ma (latest Eocene to Early Oligocene) were reported
by Yuasa et al. ( 1988 ). Another variety of volcanic rocks from site D794 (Fig. 4 )
was reported by Yuasa and Nohara ( 1992 ) and Yuasa et al. ( 1992, 1999 ), mainly
comprising sheared volcanic breccia with slicken sides. Clasts consist of basaltic
andesite to andesite of calc-alkaline series, and contain pseudomorphs after pyroxene
and olivine. Matrix and phenocrysts were extensively recrystallized by prehnite-
pumpellyite facies metamorphic minerals. Severely altered olivine basaltic
andesites were also collected during dive 6K#610 at 3,230 mbsl. Yuasa et al.
( 1999 ) suggested that these metamorphosed volcanic rocks underlie the relatively
unaltered andesites.