Geoscience Reference
In-Depth Information
1
Introduction
Serpentinite bodies accompanied by high-pressure (HP) metamorphic rocks are
common in orogenic belts exposed on land. They are regarded to have originated from
ancient convergent margins in terms of low thermal gradient during the formation of
accompanied metamorphic rocks. Based on highly sheared and mixed-up structures
with metamorphic pressure of upper mantle depths, they are recently regarded as
exhumed parts of subduction channels of mantle depths (Guillot et al.
2001
; Gerya
et al.
2002
; Ganne et al.
2006
; Federico et al.
2007
) as the interface shear zones of
upper and lower plates, where material transports were channelized (Cloos and Shreve
1988
). It is considered that the serpentinites and the HP metamorphic rocks were
formed and exhumed not only in collision settings but also in subduction settings of
oceanic crust as suggested by protoliths of metamorphic rocks (Maruyama et al.
1996
).
They are occasionally associated with hanging-wall ophiolites of supra-subduction
natures, which suggest subduction beneath immature, rather oceanic thin crusts in
intraoceanic settings or continental margin forearcs (e.g. Dickinson et al.
1996
).
Compared to collision and continental margin settings, it is more difficult to
explain the driving forces of exhumation in non-collisional oceanic subduction set-
tings: the buoyancy of the subducted slab or the voluminous subducted sediments
is not expected to lift or squeeze deep seated rocks upward. A possible resolution
to this problem comes from the properties of serpentinite, which is characterized by
much lower densities and shear strength than mantle peridotites (Hermann et al.
2000
; Guillot et al.
2001
), and its volume expansion during hydration of peridotites
(Fryer and Fryer
1987
; Shervais et al.
2004
). The subduction channel structure, in
which serpentinites occur along the interface between peridotites of the hanging
wall and foot wall lithosphere, might be common in subduction zones. Therefore,
the serpentinite bodies with HP metamorphic rocks can be a geological key target
to understand structure and dynamics of deep inside subduction zones, and those
formed in intraoceanic settings might represent an end-member condition where
material input into the channel is minimal. However, serpentinites with HP meta-
morphic rocks occurring in orogenic belts on land are structurally modified by
subsequent tectonics such as continental collision or sediment accretion etc., and it
is somewhat difficult to read primary structures from them.
The Izu-Bonin-Mariana (IBM) arc comprises a typical intraoceanic subduction
system (Fig.
1
), which generated since the Eocene. The earliest-stage basement
rocks of the IBM are ophiolite consisting of serpentinite, gabbro, and dominantly
undifferentiated volcanic rocks including boninites (Bloomer
1983
; Ishii
1985
),
and are referred to modern analogue of supra-subduction zone ophiolites (Stern and
Bloomer
1991
; Bloomer et al.
1995
). Serpentinites are occasionally accompanied
by HP metamorphic rocks (Maekawa et al.
1993
), proving formation and exhuma-
tion of HP metamorphic rocks in normal subduction settings of intraoceanic envi-
ronments. Most of the HP metamorphic rocks occur in forearc seamounts as
millimeter- to decimeter-scale clasts enclosed in serpentine mud flows from mud
volcanoes (Fryer
1992
; Maekawa et al.
1992, 1993
). These occurrences have been
generally regarded as the typical style of exhumation in modern intraoceanic
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