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On the contrary, the serpentinite body of the Ohmachi Seamount lacks crustal
rocks which did not suffer high-pressure metamorphism. It consists almost entirely of
serpentinite, with trace amounts of amphibole schists with eclogite and blueschist
relics. No evidence of mud flow has been observed, and the serpentinite body is likely
to occur as a coherent mass except the parts locally fractured by faults and landslides.
Antigorite is ubiquitous, although later low-T serpentines after olivine are also com-
mon. And most notably, the northern part of the body consists of crystalline schist of
antigorite (schistose serpentinite), which has not been found in the forearc serpen-
tinite seamounts. Its folded structure is truncated by the base of the Paleogene. These
contrasting occurrences suggest that the serpentinites of the Ohmachi Seamount do
not have the same mode of exhumation with the forearc serpentinite seamounts, and
might have originated neither from a diapir nor a mud volcano.
Although the extent of the exposed serpentinite body in the Ohmachi Seamount
is quite limited, their observed modes of occurrences are rather similar to some of
the regional high-pressure metamorphic terranes on land such as Sanbagawa Belt in
Japan (Mizukami and Wallis
2005
) or the Zermatt-Saas Ophiolite in the Alps (Li
et al.
2004
). Serpentinites in these terranes are coherent bodies with common occur-
rences of folded antigorite schist or schistose antigorite peridotite with evident
stretching lineation. Accompanied crustal rocks are exclusively high-pressure meta-
morphic. Therefore, it is probable that the serpentinite exposure in the Ohmachi
Seamount is a window of a regional high-pressure metamorphic belt, which formed
in the Philippine Sea Plate, although its extent is obscured by cover sequences.
Although there is no direct observation of contact relationship between the
schistose serpentinite and the amphibole schists, they share the common structural
features such as schistosity, mineral and/or stretching lineation, and folds that
deformed the schistosity. These common features suggest that the amphibole
schists were incorporated into the schistose serpentinite before or during generation
of the schistosity. So far as seen in amphibole schists, the schistosity and the min-
eral lineation were formed during the decompression stage from the eclogite to the
amphibolite facies (Ueda et al.
2004, 2005
). Hence, schistosity of serpentinites
might also have formed during the same decompression stage as the amphibole
schists. Dominantly low-angle foliation and subhorizontal stretching lineation in
the schistose serpentinite suggest that the serpentinite body was subvertically
flattened and laterally extended during its exhumation. The strain might have been
localized in the northern part consisting mainly of schistose serpentinite with
amphibole schists, compared to the overlying southern part mostly of massive
types. This structural relation implies that the massive serpentinites originated from
the hanging-wall of the shear zone which carried up eclogites.
The massive serpentinite occasionally contain edenitic to pargasitic amphiboles
which could be stable in primary peridotite stages (Niida et al.
2003, 2005
) as hydrous
phases, and are intruded by ultramafic cumulate dikes including olivine hornblendite,
which suggest pathways of hydrous magma, before the antigorite-forming metamor-
phism with deformation. Such hydrous natures prefer a wedge mantle origin to
subducted oceanic mantle origins as products of less hydrous igneous activity at the
mid-oceanic ridge. Although they are much less refractory than the typical forearc
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