Metamorphic history of serpentinite mylonites from the Happo ultramafic complex, central Japan

Research output: Contribution to journalArticle

44 Citations (Scopus)

Abstract

Serpentinite mylonites from the Happo ultramafic complex show evidence of two stages of mylonitization at different temperature conditions. Peridotite mylonites exhibit two types of olivine porphyroclasts and neoblasts - produced at the earlier stage. The olivine neoblasts have a stretching lineation with a fabric suggesting plastic deformation along (0 1 0) [0 0 1]. In addition to the olivine fabric, the stable association of olivine, orthopyroxene and tremolite in the peridotites that survived later serpentinization, and the Si and Na contents of tremolite, suggest that the earlier mylonitization took place at temperatures between 700 and 800 °C. Later mylonitization was associated with high-temperature serpentinization to form serpentinite mylonites. In contrast to a common type of serpentinite in orogenic belts, the serpentinite mylonites are cohesively foliated, rich in olivine and diopside, and poor in antigorite. The diopside has low Al, Cr and Na contents typical of a retrograde origin, and the olivine has a homogeneous composition except in areas subjected to contact metamorphism at a later stage. Modal composition and mineral chemistry suggest that the serpentinite mylonites were formed by a hydration reaction of tremolite and olivine to produce diopside and antigorite under stable conditions of olivine, at temperatures between 400 and 600 °C. Later-stage mylonitization has preferentially been superimposed on the earlier-stage mylonite zone with a common direction of foliation. The difference in temperature between the two mylonitization stages suggests that the shear zone was episodically active during the emplacement of the Happo complex. Conditions of relatively high temperature for serpentinization at a convergent plate boundary and high permeability caused by the early mylonitization favoured the formation of the serpentinite mylonites.

Original languageEnglish
Pages (from-to)711-723
Number of pages13
JournalJournal of Metamorphic Geology
Volume23
Issue number8
DOIs
Publication statusPublished - Oct 2005

Fingerprint

serpentinite
mylonitization
olivine
history
tremolite
serpentinization
diopside
Serpentine Asbestos
antigorite
Temperature
temperature
contact metamorphism
mylonite
plastic deformation
lineation
Chemical analysis
foliation
orthopyroxene
plate boundary
orogenic belt

Keywords

  • Metamorphism
  • Mylonite
  • Peridotite
  • Serpentinite
  • Serpentinization

ASJC Scopus subject areas

  • Geology

Cite this

Metamorphic history of serpentinite mylonites from the Happo ultramafic complex, central Japan. / Nozaka, Toshio.

In: Journal of Metamorphic Geology, Vol. 23, No. 8, 10.2005, p. 711-723.

Research output: Contribution to journalArticle

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abstract = "Serpentinite mylonites from the Happo ultramafic complex show evidence of two stages of mylonitization at different temperature conditions. Peridotite mylonites exhibit two types of olivine porphyroclasts and neoblasts - produced at the earlier stage. The olivine neoblasts have a stretching lineation with a fabric suggesting plastic deformation along (0 1 0) [0 0 1]. In addition to the olivine fabric, the stable association of olivine, orthopyroxene and tremolite in the peridotites that survived later serpentinization, and the Si and Na contents of tremolite, suggest that the earlier mylonitization took place at temperatures between 700 and 800 °C. Later mylonitization was associated with high-temperature serpentinization to form serpentinite mylonites. In contrast to a common type of serpentinite in orogenic belts, the serpentinite mylonites are cohesively foliated, rich in olivine and diopside, and poor in antigorite. The diopside has low Al, Cr and Na contents typical of a retrograde origin, and the olivine has a homogeneous composition except in areas subjected to contact metamorphism at a later stage. Modal composition and mineral chemistry suggest that the serpentinite mylonites were formed by a hydration reaction of tremolite and olivine to produce diopside and antigorite under stable conditions of olivine, at temperatures between 400 and 600 °C. Later-stage mylonitization has preferentially been superimposed on the earlier-stage mylonite zone with a common direction of foliation. The difference in temperature between the two mylonitization stages suggests that the shear zone was episodically active during the emplacement of the Happo complex. Conditions of relatively high temperature for serpentinization at a convergent plate boundary and high permeability caused by the early mylonitization favoured the formation of the serpentinite mylonites.",
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