Introduction from Morijiri and Nakagawa(2005)
Serpentinite blocks are often found intruding along tectonic lines all over Japan. They are locally observed as matrix of me'lange.
Serpentinite blocks are known to have strong magnetization (Makino et al., 1992).
However, magnetic studies on serpentinite blocks are rarely done because they are difficult to implement in routine paleomagnetic investigations.
Many serpentinite sites show scattered directions of characteristic remanent magnetization (ChRM) in each serpentinite block, even if the sites are located in the same geologic unit.
Normally, samples from the same geologic unit have similar magnetizations.
The geological term “me'lange” (French: mixture) is a nongenetic term, meaning a feature of a (1) block in matrix fabric, (2) ductile matrix, and (3) mappable body.
The term has been widely accepted after the Penrose Conference on Me'langes in 1978 (Silver and Beutner, 1980).
Me'langes are also defined as mappable bodies of deformed rocks characterized by the inclusion of tectonically mixed fragments or blocks,
which may range from 1 to 10 km long, in a pervasively sheared, fine-grained and commonly pelitic matrix (Hsu", 1968).
However, me'lange does not necessarily mean the presence of “exotic” lithologic component (Cowan, 1974).
Ductile me'lange matrix is generally serpentinite or argillite (pelitic rocks).
Some of the argillite-matrix me'langes have been genetically interpreted in terms of the deformation pattern in an accretion prism (Cowan, 1985, Barber et al., 1986, Taira et al., 1988 and Kimura and Mukai, 1991).
However, in the case of serpentinite matrix, detailed observation of the genetic relationship between their outcropping mechanism and the me'lange fabric is never done.
The ideal serpentinization process of olivine (Fo: around 90) is equation(1)(Eckstrand,1975)
30*(Mg0.9Fe0.1)2SiO4 + 41*H2O = 15*Mg3Si2O5(OH)4 +2*FeOFe2O3 + 9*(Mgx,Fe1-x)(OH)2 + 2H2
However, there are some variations of the volume of magnetite produced during the serpentinization process originating from the same peridotites (Toft et al., 1990).
Generally, it is difficult to determine these variations in paleomagnetic studies.
This study determines the features of the magnetic properties of serpentinite. The evidence of the small-scale me'lange fabric of serpentinite matrix is also presented.
The results of this study provide a better understanding of the nature of serpentinization processes occurring in subduction-related accretion prism.
Serpentinite blocks are often found intruding along tectonic lines all over Japan. They are locally observed as matrix of me'lange.
Serpentinite blocks are known to have strong magnetization (Makino et al., 1992).
However, magnetic studies on serpentinite blocks are rarely done because they are difficult to implement in routine paleomagnetic investigations.
Many serpentinite sites show scattered directions of characteristic remanent magnetization (ChRM) in each serpentinite block, even if the sites are located in the same geologic unit.
Normally, samples from the same geologic unit have similar magnetizations.
The geological term “me'lange” (French: mixture) is a nongenetic term, meaning a feature of a (1) block in matrix fabric, (2) ductile matrix, and (3) mappable body.
The term has been widely accepted after the Penrose Conference on Me'langes in 1978 (Silver and Beutner, 1980).
Me'langes are also defined as mappable bodies of deformed rocks characterized by the inclusion of tectonically mixed fragments or blocks,
which may range from 1 to 10 km long, in a pervasively sheared, fine-grained and commonly pelitic matrix (Hsu", 1968).
However, me'lange does not necessarily mean the presence of “exotic” lithologic component (Cowan, 1974).
Ductile me'lange matrix is generally serpentinite or argillite (pelitic rocks).
Some of the argillite-matrix me'langes have been genetically interpreted in terms of the deformation pattern in an accretion prism (Cowan, 1985, Barber et al., 1986, Taira et al., 1988 and Kimura and Mukai, 1991).
However, in the case of serpentinite matrix, detailed observation of the genetic relationship between their outcropping mechanism and the me'lange fabric is never done.
The ideal serpentinization process of olivine (Fo: around 90) is equation(1)(Eckstrand,1975)
30*(Mg0.9Fe0.1)2SiO4 + 41*H2O = 15*Mg3Si2O5(OH)4 +2*FeOFe2O3 + 9*(Mgx,Fe1-x)(OH)2 + 2H2
However, there are some variations of the volume of magnetite produced during the serpentinization process originating from the same peridotites (Toft et al., 1990).
Generally, it is difficult to determine these variations in paleomagnetic studies.
This study determines the features of the magnetic properties of serpentinite. The evidence of the small-scale me'lange fabric of serpentinite matrix is also presented.
The results of this study provide a better understanding of the nature of serpentinization processes occurring in subduction-related accretion prism.
文献一覧
和田信彦・高橋功二・渡辺 順・蟹江康光(1992) 5万分の1地質図幅「三石」及び説明書 (地域地質研究報告). 北海道立地質研究所 (旧北海道立地下資源調査所)，73p.
和田信彦・高橋功二・渡辺 順・蟹江康光(1992) 5万分の1地質図幅「三石」及び説明書 (地域地質研究報告). 北海道立地質研究所 (旧北海道立地下資源調査所)，73p.
- 磁化を担っている鉱物は蛇紋岩化作用で形成されたマグネタイトである
- 蛇紋岩になってから熱変成を受けた形跡がない
- 地質構成としては一体と考えられる蛇紋岩自身の産状がblock in matrixのメランジェである
- 蛇紋岩化作用（Fo90)の一般式(Eckstrand,1975)
30*(Mg0.9Fe0.1)2SiO4 + 41*H2O = 15*Mg3Si2O5(OH)4 +2*FeOFe2O3 + 9*(Mgx,Fe1-x)(OH)2 + 2H2
- 帯磁率→マグネタイトの体積比にほぼ比例
- マグネタイトの体積比→原岩が同じであれば、ほぼ蛇紋岩化度に比例するはず
１．露頭写真
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軍艦山南露頭の拡大版．蛇紋岩体としてはメランジのマトリックスだが，露頭オーダーでみると蛇紋岩自体も block in matrix の構造になっている．ppt |
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南よりみた三石蛇紋岩体．クリーム色に見える路頭が新第三系堆積岩，青白く見えるのが蛇紋岩．蛇紋岩体が新第三系の分布域の中で地塁状に盛り上がった南東端．岩体は北西－南東方向へ細長くのびている．ppt |
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蛇紋岩体中に取り込まれた角閃岩ブロック．蛇紋岩が比較的柔らかいため河川等の差別浸食により角閃岩ブロックのみが突出したノッカー地形を形成する．三石蛇紋岩体には大小様々な変成岩ブロックが取り込まれている．（スケールは汽車）ppt |
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軍艦山南の路頭写真．スケールは露頭に散らばったもの．ppt |
2．地質図類
5万分の1地質図幅「三石」→地質図Naviへ
3．岩石磁気データ
整理中→機関アーカイブへ