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 107

brecciated variations also locally occur (Fig. 5d ). Microscopic textures vary
from pseudomorphic mesh textures by low-T serpentines to completely recrys-
tallized interpenetrating textures mainly by antigorite ± olivine (Fig. 6e-f ).
Bastite is common. A few samples preserve relic orthopyroxene, clinopyroxene,
Cr-spinel, and pargasite as the primary phases. Most of olivine crystals may have
originated from an igneous phase, but were extensively modified to be cleavable
with antigorite films along partings, as suggestive of crystallographic reequili-
bration along with antigorite-forming metamorphism (Uda 1984 ; Akikawa and
Tokonami 1987 ). Minor secondary growth of olivine with inclusions of serpen-
tine and magnetite also occurred. Low-T serpentines occasionally crosscut anti-
gorite, so that the mesh textures generated after the main metamorphism, where
antigorite + olivine assemblage was stable. Degree of serpentinization (antigori-
tization) in the main metamorphic stage (before generation of low-T serpentines
in later stages) are highly variable from nearly zero to 100%, but greater than
90% in the majority .
In addition to olivine and serpentine minerals, they commonly contain chlorite
and magnetite, occasionally with diopside and tremolite, as metamorphic pro-
ducts. Typical metamorphic mineral assemblages are Ol + Atg + Di + Chl + Mag and
Ol + Atg + Tr ± Di + Chl + Mag (mineral abbreviations after Kretz 1983 ), repre-
senting upper greenschist to lower amphibolite-grade temperatures (Evans 1977 ;
O'Hanley 1996 ).
Textures and relic and/or metamorphic mineral assemblages suggest that the
massive serpentinite originated from residual peridotite (lherzolite and harzburgite)
and lesser amounts of dunite. Relic spinels show lesser degrees of partial melting
than typical arc peridotite, but edenitic to pargasitic amphibole as a high-temperature
hydrous phase is common (Niida et al. 2001, 2003, 2005 ). These less refractory but
hydrous features were interpreted to reflect one of the three candidates of their
origins as: (a) relics of sub-continental lithosphere on whose margin the Philippine
Fig. 6 Microscopic and mesoscopic photographs of amphibole schists and serpentinites.
( a ) Micro-folds in garnet-epidote amphibolite (D06-1; unpolarized light) in the YZ section normal
both to foliation and to lineation. ( b ) Schistose serpentinite (6K#609R007; cross-polarized light) in
a view of XZ section normal to foliation and parallel to lineation. Relics of early massive textures
(ML: microlithon) are surrounded by highly schistose neoblasts of antigorite. ( c ) Microfolds
with lineation-parallel axes in the YZ section of schistose serpentinite (6K#609R009; cross-
polarized light). ( d ) Scanned thin section of schistose serpentinite (6K#609R008; cross-polarized
light) in XZ section. Note the highly elongated magnetite porphyroblasts and lineation-normal
crack-seal veins of fibrous antigorite. ( e ) Massive serpentinite (6K#1067R013; cross-polarized
light) consisting mainly of cleavable olivine and antigorite. ( f ) Massive serpentinite (6K#1065R006;
cross-polarized light) with an inter-penetrating texture of antigorite. ( g ) A fold structure in
schistose serpentinite (6K#1066R026; oblique view of the slab of XZ section). Fold axes crosscut
lineation and crenulation (microfold axes). Slicken sides correspond to foliation-subparallel slip
surfaces with low-temperature serpentine. ( h ) Hinge part of a fold (crosscutting lineation) with
fractures filled by low-T serpentine in schistose serpentinite (6K#1066R020; open-polarized
light). Mineral abbreviations: Atg antigorite, Cb carbonate, Di diopside, LTS low-temperature
serpentine, Mt magnetite, Ol olivine, and Tr tremolite