Blueschist blocks at Mochimaru in the Tari-Misaka ultramafic complex: Their petrologic characteristics and significance

Research output: Contribution to journalArticle

4 Citations (Scopus)

Abstract

Blueschist tectonic blocks occur in serpentinites at Mochimaru, Hiroshima Prefecture, Southwest Japan. They contain alkali amphibole coexisting with pumpellyite and chlorite, with or without calcic amphibole. Textural and chemical analyses reveal that the blueschists, together with other mafic schists, have similar metamorphic history. After their capture by serpentinites and before the emplacement of the serpentinites into the present geological position, the tectonic blocks were subjected to high P/T metamorphism around the boundary between the blueschist and pumpellyite-actinolite facies. The amphiboles formed by this metamorphism change from tremolite through glaucophane to ferroglaucophane with increasing FeO/MgO of whole rock compositions. The P-T conditions are estimated to be within 200-350°C and 5-7 kbar: These are higher P/T conditions than those of the regional metamorphism of Southwest Japan. The difference in the P-T conditions implies differences in tectonic situation and timing of metamorphism between the blocks and regional metamorphic rocks. In addition, the high P/T metamorphism of the tectonic blocks probably occurred in more reducing environments than the regional metamorphism. Because the ferric/ferrous iron ratios of the tectonic blocks are within a narrow range, it is stressed that oxygen fugacity was externally buffered during the high P/T metamorphism by the serpentinization process of the host ultramatic rocks. The reducing effect of serpentinization is common throughout the high P/T metamorphic terranes of Southwest Japan.

Original languageEnglish
Pages (from-to)154-167
Number of pages14
JournalIsland Arc
Volume8
Issue number2
DOIs
Publication statusPublished - Jun 1999

Fingerprint

blueschist
metamorphism
P-T conditions
tectonics
amphibole
pumpellyite
serpentinization
regional metamorphism
glaucophane
tremolite
fugacity
metamorphic rock
schist
host rock
chlorite
terrane
emplacement
iron
oxygen
history

Keywords

  • Alkali amphibole
  • Blueschist
  • Buffering
  • Metamorphism
  • Oxygen fugacity
  • Petrology
  • Serpentinite
  • Southwest Japan
  • Tectonic block

ASJC Scopus subject areas

  • Earth and Planetary Sciences (miscellaneous)

Cite this

Blueschist blocks at Mochimaru in the Tari-Misaka ultramafic complex : Their petrologic characteristics and significance. / Nozaka, Toshio.

In: Island Arc, Vol. 8, No. 2, 06.1999, p. 154-167.

Research output: Contribution to journalArticle

@article{c4fced7d84124ab3b4297069291b2b7c,
title = "Blueschist blocks at Mochimaru in the Tari-Misaka ultramafic complex: Their petrologic characteristics and significance",
abstract = "Blueschist tectonic blocks occur in serpentinites at Mochimaru, Hiroshima Prefecture, Southwest Japan. They contain alkali amphibole coexisting with pumpellyite and chlorite, with or without calcic amphibole. Textural and chemical analyses reveal that the blueschists, together with other mafic schists, have similar metamorphic history. After their capture by serpentinites and before the emplacement of the serpentinites into the present geological position, the tectonic blocks were subjected to high P/T metamorphism around the boundary between the blueschist and pumpellyite-actinolite facies. The amphiboles formed by this metamorphism change from tremolite through glaucophane to ferroglaucophane with increasing FeO/MgO of whole rock compositions. The P-T conditions are estimated to be within 200-350°C and 5-7 kbar: These are higher P/T conditions than those of the regional metamorphism of Southwest Japan. The difference in the P-T conditions implies differences in tectonic situation and timing of metamorphism between the blocks and regional metamorphic rocks. In addition, the high P/T metamorphism of the tectonic blocks probably occurred in more reducing environments than the regional metamorphism. Because the ferric/ferrous iron ratios of the tectonic blocks are within a narrow range, it is stressed that oxygen fugacity was externally buffered during the high P/T metamorphism by the serpentinization process of the host ultramatic rocks. The reducing effect of serpentinization is common throughout the high P/T metamorphic terranes of Southwest Japan.",
keywords = "Alkali amphibole, Blueschist, Buffering, Metamorphism, Oxygen fugacity, Petrology, Serpentinite, Southwest Japan, Tectonic block",
author = "Toshio Nozaka",
year = "1999",
month = "6",
doi = "10.1046/j.1440-1738.1999.00228.x",
language = "English",
volume = "8",
pages = "154--167",
journal = "Island Arc",
issn = "1038-4871",
publisher = "Wiley-Blackwell",
number = "2",

}

TY - JOUR

T1 - Blueschist blocks at Mochimaru in the Tari-Misaka ultramafic complex

T2 - Their petrologic characteristics and significance

AU - Nozaka, Toshio

PY - 1999/6

Y1 - 1999/6

N2 - Blueschist tectonic blocks occur in serpentinites at Mochimaru, Hiroshima Prefecture, Southwest Japan. They contain alkali amphibole coexisting with pumpellyite and chlorite, with or without calcic amphibole. Textural and chemical analyses reveal that the blueschists, together with other mafic schists, have similar metamorphic history. After their capture by serpentinites and before the emplacement of the serpentinites into the present geological position, the tectonic blocks were subjected to high P/T metamorphism around the boundary between the blueschist and pumpellyite-actinolite facies. The amphiboles formed by this metamorphism change from tremolite through glaucophane to ferroglaucophane with increasing FeO/MgO of whole rock compositions. The P-T conditions are estimated to be within 200-350°C and 5-7 kbar: These are higher P/T conditions than those of the regional metamorphism of Southwest Japan. The difference in the P-T conditions implies differences in tectonic situation and timing of metamorphism between the blocks and regional metamorphic rocks. In addition, the high P/T metamorphism of the tectonic blocks probably occurred in more reducing environments than the regional metamorphism. Because the ferric/ferrous iron ratios of the tectonic blocks are within a narrow range, it is stressed that oxygen fugacity was externally buffered during the high P/T metamorphism by the serpentinization process of the host ultramatic rocks. The reducing effect of serpentinization is common throughout the high P/T metamorphic terranes of Southwest Japan.

AB - Blueschist tectonic blocks occur in serpentinites at Mochimaru, Hiroshima Prefecture, Southwest Japan. They contain alkali amphibole coexisting with pumpellyite and chlorite, with or without calcic amphibole. Textural and chemical analyses reveal that the blueschists, together with other mafic schists, have similar metamorphic history. After their capture by serpentinites and before the emplacement of the serpentinites into the present geological position, the tectonic blocks were subjected to high P/T metamorphism around the boundary between the blueschist and pumpellyite-actinolite facies. The amphiboles formed by this metamorphism change from tremolite through glaucophane to ferroglaucophane with increasing FeO/MgO of whole rock compositions. The P-T conditions are estimated to be within 200-350°C and 5-7 kbar: These are higher P/T conditions than those of the regional metamorphism of Southwest Japan. The difference in the P-T conditions implies differences in tectonic situation and timing of metamorphism between the blocks and regional metamorphic rocks. In addition, the high P/T metamorphism of the tectonic blocks probably occurred in more reducing environments than the regional metamorphism. Because the ferric/ferrous iron ratios of the tectonic blocks are within a narrow range, it is stressed that oxygen fugacity was externally buffered during the high P/T metamorphism by the serpentinization process of the host ultramatic rocks. The reducing effect of serpentinization is common throughout the high P/T metamorphic terranes of Southwest Japan.

KW - Alkali amphibole

KW - Blueschist

KW - Buffering

KW - Metamorphism

KW - Oxygen fugacity

KW - Petrology

KW - Serpentinite

KW - Southwest Japan

KW - Tectonic block

UR - http://www.scopus.com/inward/record.url?scp=0032754313&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0032754313&partnerID=8YFLogxK

U2 - 10.1046/j.1440-1738.1999.00228.x

DO - 10.1046/j.1440-1738.1999.00228.x

M3 - Article

AN - SCOPUS:0032754313

VL - 8

SP - 154

EP - 167

JO - Island Arc

JF - Island Arc

SN - 1038-4871

IS - 2

ER -