forearc (Tararin, 1995; Fryer et al., 2000; Maekawa et al.,
2004).
Relics of HP metamorphism were found in amphibolites collected from the Ohmachi (Omachi) Seamount near
the volcanic front of the Izu-Bonin arc (Ueda et al., 2004).
These rocks are extensively recrystallized during the retrograde stage, resulting in garnet amphibolite and epidotealbite amphibolite mineralogy. Eclogite and blueschist
facies minerals such as omphacite, kyanite, and
glaucophane occur as micro-inclusions in zoisite, epidote,
hornblende, and garnet. The highest grade sample
underwent the peak metamorphism at 600–700
C and
2–2.5 GPa (ca. 60–80 km deep), followed by a retrograde
overprint at 550–600
C and < ~1 GPa (shallower than
approx. 30 km deep). Although the age of these HP rocks
has not been published, pre-Eocene exhumation was
suggested by Ueda et al. (2004) based on the age of cover
volcano-sedimentary sequence. These HP metamorphic
rocks are hosted in part of a serpentinite body consisting
of antigorite schist and massive antigorite serpentinite.
These serpentinites comprise a coherent body without pulverization, and schistose parts exhibit lateral extension in
ductile manners (Hirauchi et al., 2010; Ueda et al.,
2011). These modes of occurrence differ from the forearc
serpentinite seamounts, and Ueda et al. (2011) considered
that the serpentinite body preserves the structure of deep
serpentinite channel in an intraoceanic subduction zone.
Summary
Petrology, structural geology, and numerical modeling of
HPLT rocks commonly depict dynamic flows beneath
forearcs with great vertical and lateral displacements of
regional rock masses, which might be commonly ongoing
in subduction zones. Such deductions have, however, not
been well linked with seafloor geology of forearc areas.
For example, stacks of thrust sheets (nappe piles) and
hanging-wall low-angle normal faults (detachment) are
ones of the typical structures of HPLT terranes exposed
on land. However, such structures have not yet been
detected in forearcs on the seafloors. Serpentinite diapirs
and mud volcanoes that penetrate the Mariana forearc
crusts are in turn not so suitable with dominantly flat-lying
structures of HPLT terranes. Filling these gaps and comprehension of subduction zone dynamics will greatly
owe to further investigations by marine geoscience.
Bibliography
Anczkiewicz, R., Platt, J. P., Thirlwall, M. F., and Wakabayashi, J.,
2004. Franciscan subduction off to a slow start: evidence from
high-precision Lu–Hf garnet ages on high grade-blocks. Earth
and Planetary Science Letters, 225, 147–161.
Ave Lallemant, H. G., and Guth, L. R., 1990. Role of extensional
tectonics in exhumation of eclogites and blueschists in an
oblique subduction setting: northeastern Venezuela. Geology,
18, 950–953.
Baldwin, S., Monteleone, B. D., Webb, L. E., Fitzgerald, P. G.,
Grove, M., and Hill, E. J., 2004. Pliocene eclogite exhumation
at plate tectonic rates in eastern Papua New Guinea. Nature,
431, 263–267.
Carswell, D. A., and Harley, S. L., 1990. Mineral barometry and
thermometry. In Carswell, D. A. (ed.), Eclogite Facies Rocks.
Glasgow/London: Blackie, pp. 83–110.
Chemenda, A. I., Mattauer, M., Malavieille, J., and Bokun, A. N.,
1995. A mechanism for syn-collisional rock exhumation and
associated normal faulting: results from physical modeling.
Earth and Planetary Science Letters, 132, 225–232.
Cloos, M., 1982. Flow melanges: numerical modeling and geologic
constraints on their origin in the Franciscan subduction complex,
California. Geological Society of America Bulletin, 93,
330–345.
Connolly, J. A. D., and Petrini, K., 2002. An automated strategy for
calculation of phase diagram sections and retrieval of rock properties as a function of physical conditions (0.4 Mb). Journal of
Metamorphic Geology, 20, 697–708.
Davis, J. H., and von Blanckenburg, F., 1995. Slab breakoff:
A model of lithosphere detachment and its test in the magmatism
and deformation of collisional orogens. Earth and Planetary Science Letters, 129, 85–102.
Ellis, D. J., and Green, D. H., 1979. An experimental study of the
effect of Ca upon the garnet-clinopyroxene Fe-Mg exchange
equilibria. Contributions to Mineralogy and Petrology, 71,
13–22.
Evans, B. W., 1990. Phase relations of epidote-blueschists. Lithos,
25, 3–23.
Frey, M., de Capitani, C., and Liou, J. G., 1991. A new petrogenetic
grid for low-grade metabasites. Journal of Metamorphic Geology, 9, 497–509.
Fryer, P., Saboda, K. L., Hohnson, L. E., Mackey, M. E., Moore,
G. F., and Stoffers, P., 1990. Conical seamount: SeaMARC II,
Alvin submersible, and seismic reflection studies. In Fryer, P.,
Pearce, J. A., Stokking, L. B., et al. (eds.), Proceedings of the
Ocean Drilling Program Initial Reports, 125. College Station:
Ocean Drilling Program, pp. 69–80.
Fryer, P., Lockwood, J. P., Becker, N., Phipps, S., and Todd, C. S.,
2000. Significance of serpentinite mud volcanism in convergent
margins. In Dilek, Y., Moores, E. M., Elthon, D., and Nicolas,
A. (eds.), Ophiolites and Oceanic Crust: New Insights from
Field Studies and the Ocean Drilling Program. Boulder: Geological Society of America, pp. 35–51.
Gerya, T., and Stöckhert, B., 2006. Two-dimensional numerical
modeling of tectonic and metamorphic histories at active continental margins. International Journals of Earth Science
(Geologische Rundschau), 95, 250–274.
Gerya, T., Stöckhert, B., and Perchuk, A. L., 2002. Exhumation of
high-pressure metamorphic rocks in a subduction channel:
a numerical simulation. Tectonics, 21, 1056, doi:10.1029/
2002TC001406.
Guillot, S., Hatori, K., de Sigoyer, J., Nägler, T., and Auzene, A.-L.,
2001. Evidence of hydration of the mantle wedge and its role in
the exhumation of eclogites. Earth and Planetary Science Letters, 193, 115–127.
Hill, E. J., Baldwin, S. L., and Lister, G. S., 1993. Unroofing of
active metamorphic core complexes in the D’Entrecasteaux
Islands, Papua New Guinea. Geology, 20, 907–910.
Hirauchi, K., Michibayashi, K., Ueda, H., and Katayama, I., 2010.
Spatial variations in antigorite fabric across a serpentinite subduction channel: insights from the Ohmachi Seamount,
Izu-Bonin frontal arc. Earth and Planetary Science Letters,
299, 196–206.
Holland, T. J. B., and Powell, R., 1998. An internally-consistent
thermodynamic dataset for phases of petrological interest. Journal of Metamorphic Geology, 16, 309–343.
Kimura, G., Maruyama, S., Isozaki, Y., and Terabayashi, M.,
1996. Well-preserved underplating structure of the jadeitized
Franciscan complex, Pacheco Pass, California. Geology, 24,
75–78.
HIGH-PRESSURE, LOW-TEMPERATURE METAMORPHISM
315
2004).
Relics of HP metamorphism were found in amphibolites collected from the Ohmachi (Omachi) Seamount near
the volcanic front of the Izu-Bonin arc (Ueda et al., 2004).
These rocks are extensively recrystallized during the retrograde stage, resulting in garnet amphibolite and epidotealbite amphibolite mineralogy. Eclogite and blueschist
facies minerals such as omphacite, kyanite, and
glaucophane occur as micro-inclusions in zoisite, epidote,
hornblende, and garnet. The highest grade sample
underwent the peak metamorphism at 600–700
C and
2–2.5 GPa (ca. 60–80 km deep), followed by a retrograde
overprint at 550–600
C and < ~1 GPa (shallower than
approx. 30 km deep). Although the age of these HP rocks
has not been published, pre-Eocene exhumation was
suggested by Ueda et al. (2004) based on the age of cover
volcano-sedimentary sequence. These HP metamorphic
rocks are hosted in part of a serpentinite body consisting
of antigorite schist and massive antigorite serpentinite.
These serpentinites comprise a coherent body without pulverization, and schistose parts exhibit lateral extension in
ductile manners (Hirauchi et al., 2010; Ueda et al.,
2011). These modes of occurrence differ from the forearc
serpentinite seamounts, and Ueda et al. (2011) considered
that the serpentinite body preserves the structure of deep
serpentinite channel in an intraoceanic subduction zone.
Summary
Petrology, structural geology, and numerical modeling of
HPLT rocks commonly depict dynamic flows beneath
forearcs with great vertical and lateral displacements of
regional rock masses, which might be commonly ongoing
in subduction zones. Such deductions have, however, not
been well linked with seafloor geology of forearc areas.
For example, stacks of thrust sheets (nappe piles) and
hanging-wall low-angle normal faults (detachment) are
ones of the typical structures of HPLT terranes exposed
on land. However, such structures have not yet been
detected in forearcs on the seafloors. Serpentinite diapirs
and mud volcanoes that penetrate the Mariana forearc
crusts are in turn not so suitable with dominantly flat-lying
structures of HPLT terranes. Filling these gaps and comprehension of subduction zone dynamics will greatly
owe to further investigations by marine geoscience.
Bibliography
Anczkiewicz, R., Platt, J. P., Thirlwall, M. F., and Wakabayashi, J.,
2004. Franciscan subduction off to a slow start: evidence from
high-precision Lu–Hf garnet ages on high grade-blocks. Earth
and Planetary Science Letters, 225, 147–161.
Ave Lallemant, H. G., and Guth, L. R., 1990. Role of extensional
tectonics in exhumation of eclogites and blueschists in an
oblique subduction setting: northeastern Venezuela. Geology,
18, 950–953.
Baldwin, S., Monteleone, B. D., Webb, L. E., Fitzgerald, P. G.,
Grove, M., and Hill, E. J., 2004. Pliocene eclogite exhumation
at plate tectonic rates in eastern Papua New Guinea. Nature,
431, 263–267.
Carswell, D. A., and Harley, S. L., 1990. Mineral barometry and
thermometry. In Carswell, D. A. (ed.), Eclogite Facies Rocks.
Glasgow/London: Blackie, pp. 83–110.
Chemenda, A. I., Mattauer, M., Malavieille, J., and Bokun, A. N.,
1995. A mechanism for syn-collisional rock exhumation and
associated normal faulting: results from physical modeling.
Earth and Planetary Science Letters, 132, 225–232.
Cloos, M., 1982. Flow melanges: numerical modeling and geologic
constraints on their origin in the Franciscan subduction complex,
California. Geological Society of America Bulletin, 93,
330–345.
Connolly, J. A. D., and Petrini, K., 2002. An automated strategy for
calculation of phase diagram sections and retrieval of rock properties as a function of physical conditions (0.4 Mb). Journal of
Metamorphic Geology, 20, 697–708.
Davis, J. H., and von Blanckenburg, F., 1995. Slab breakoff:
A model of lithosphere detachment and its test in the magmatism
and deformation of collisional orogens. Earth and Planetary Science Letters, 129, 85–102.
Ellis, D. J., and Green, D. H., 1979. An experimental study of the
effect of Ca upon the garnet-clinopyroxene Fe-Mg exchange
equilibria. Contributions to Mineralogy and Petrology, 71,
13–22.
Evans, B. W., 1990. Phase relations of epidote-blueschists. Lithos,
25, 3–23.
Frey, M., de Capitani, C., and Liou, J. G., 1991. A new petrogenetic
grid for low-grade metabasites. Journal of Metamorphic Geology, 9, 497–509.
Fryer, P., Saboda, K. L., Hohnson, L. E., Mackey, M. E., Moore,
G. F., and Stoffers, P., 1990. Conical seamount: SeaMARC II,
Alvin submersible, and seismic reflection studies. In Fryer, P.,
Pearce, J. A., Stokking, L. B., et al. (eds.), Proceedings of the
Ocean Drilling Program Initial Reports, 125. College Station:
Ocean Drilling Program, pp. 69–80.
Fryer, P., Lockwood, J. P., Becker, N., Phipps, S., and Todd, C. S.,
2000. Significance of serpentinite mud volcanism in convergent
margins. In Dilek, Y., Moores, E. M., Elthon, D., and Nicolas,
A. (eds.), Ophiolites and Oceanic Crust: New Insights from
Field Studies and the Ocean Drilling Program. Boulder: Geological Society of America, pp. 35–51.
Gerya, T., and Stöckhert, B., 2006. Two-dimensional numerical
modeling of tectonic and metamorphic histories at active continental margins. International Journals of Earth Science
(Geologische Rundschau), 95, 250–274.
Gerya, T., Stöckhert, B., and Perchuk, A. L., 2002. Exhumation of
high-pressure metamorphic rocks in a subduction channel:
a numerical simulation. Tectonics, 21, 1056, doi:10.1029/
2002TC001406.
Guillot, S., Hatori, K., de Sigoyer, J., Nägler, T., and Auzene, A.-L.,
2001. Evidence of hydration of the mantle wedge and its role in
the exhumation of eclogites. Earth and Planetary Science Letters, 193, 115–127.
Hill, E. J., Baldwin, S. L., and Lister, G. S., 1993. Unroofing of
active metamorphic core complexes in the D’Entrecasteaux
Islands, Papua New Guinea. Geology, 20, 907–910.
Hirauchi, K., Michibayashi, K., Ueda, H., and Katayama, I., 2010.
Spatial variations in antigorite fabric across a serpentinite subduction channel: insights from the Ohmachi Seamount,
Izu-Bonin frontal arc. Earth and Planetary Science Letters,
299, 196–206.
Holland, T. J. B., and Powell, R., 1998. An internally-consistent
thermodynamic dataset for phases of petrological interest. Journal of Metamorphic Geology, 16, 309–343.
Kimura, G., Maruyama, S., Isozaki, Y., and Terabayashi, M.,
1996. Well-preserved underplating structure of the jadeitized
Franciscan complex, Pacheco Pass, California. Geology, 24,
75–78.
HIGH-PRESSURE, LOW-TEMPERATURE METAMORPHISM
315