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 108

Sea Plate initiated, (b) peridotites beneath a slow-spreading back-arc basin, and
(c) island-arc upper mantle with secondary enrichment in hydrous basaltic melt
components (Niida et al. 2003, 2005 ).
The ultramafic dikes in massive serpentinites consist of clinopyroxenite-wehrlite
and olivine-hornblendite, and show cumulate textures in less deformed and less
recrystallized specimens. These ultramafic dikes (except for Ol-poor clinopyroxen-
ite) commonly contain antigorite, and olivine-rich varieties show the same meta-
morphic mineral assemblages as the host massive serpentinites. They occasionally
show weak metamorphic mineral foliation, which suggest their intrusion prior to
the main metamorphic and deformation event.

3.1.2 Schistose Serpentinite

The schistose serpentinite is characterized by lepidoblastic textures consisting
solely of antigorite, thus comprising antigorite schist (Fig. 6b-d ). The rocks also
commonly contain minor amounts of magnetite and chlorite, and are occasion-
ally accompanied by carbonate, diopside, and olivine. Acicular crystals of diop-
side also partly contribute to the schistosity. Modal abundance of olivine and its
pseudomorphs are small (approximately less than 10%), and the majority of the
schistose serpentinite does not contain olivine. The lowest-variance metamor-
phic mineral assemblages is Atg + Ol + Di + Chl + Mag, representing upper
greenschist- to lower amphibolite-grade temperatures (Evans 1977 ; O'Hanley
1996 ) consistent with retrograde (decompression) conditions of amphibole
schists as described later.
Magnetite occurs as ellipsoidal porphyroblasts and/or pseudomorphs after
spinel (Fig. 6d ), whose long axes define a stretching lineation. Development of
schistosity is variable and transitional from the massive serpentinite. Hirauchi
et al. ( 2010 ) showed that alignment of crystallographic axes of antigorite in mas-
sive, transitional, and schistose serpentinites progressively increases in ascending
order (c-axis parallel to the schistosity, and b-axis to the lineation). The transition
from massive to schistose serpentinites is also occasionally visible on cut sur-
faces of hand specimens, where parts of massive serpentinites are stretched along
their margins or shear zones and grade into schistose serpentinite. Here we
describe as schistose serpentinite when lepidoblastic antigorite exceeds 50 modal
%. Schistosity is occasionally recognizable on some outcrop surfaces, whereas it
is not evident and apparently massive on some outcrops. In some cases the folia-
tion surfaces are accompanied by slicken lines, whose orientations are regardless
of stretching lineations by magnetite porphyroblasts. Apparent foliation on rock
surfaces probably resulted from overprint of later slip and/or weathering parallel
to the schistosity. Fold structures are not uncommon (Fig. 6c, g, h ). Lineation-
orthogonal veins of fibrous to acicular antigorite crystals (picrolite), whose long
axes are parallel to stretching lineations, are rarely seen in schistose serpentinite
(Fig. 6d ).