11:00 AM - 1:00 PM
[SCG47-P01] Chemical compositions and petrography of segregation vesicles in dolerite sills of the Shimokawa ophiolite in the northern Hidaka belt, Hokkaido
Keywords:segregation vesicle, northern Hidaka belt, MORB
1. Introduction
Authors have investigated genesis of highly vesiculated dolerite sill 1 (referred to as sill 1) of Shimokawa ophiolite in the northern Hidaka belt, Hokkaido by analysis of whole rock chemical composition and vesicle number density and vesicularity. We focused that sill 1 which has a highly vesiculated than other dolerites in the region and has feldspar veins. The veins mainly compose albite and K-feldspar. We believed that the hydrothermal fluid supply from the surrounding sediments to the dolerite (magma) increased the degree of vesiculation and formed feldspar veins.
By the way the segregation vesicle (Smith, 1967) which matrix is filled with a hyalopyritic structure is observed in Shimokawa ophiolite. Jones (1969) proposed that segregation vesicle was formed by residual melt ejection into the early vesicles.
2. Geological overview
Shimokawa Ophiolite is located in the northern part of central Hokkaido. Shimokawa ophiolite is composed of incomplete ophiolite. Serpentinized harzburgite and gabbro occur at the lowest part. The lower part of the main body consists of alternation of dolerite and sediments, the upper part consists of pillow lava, and the middle part shows intermediate features between the upper and lower parts. Dolerites show MORB features (Miyashita and Watanabe, 1988).
This study, the target is the highly vesiculated dolerite sill 3 (referred to as sill 3) where many segregation vesicles are observed. The layer thickness is 10 m. Dolerite consists of plagioclase, clinopyroxene, and opaque minerals. Dolerite is surrounded by black shale which does not has a chilled margin. The surrounding sedimentary rock and dolerite are considered to be in contact with each other by faults.
According to the thin section observation, segregation vesicles are composed of fine-grained matrix of albite and clinopyroxene. In the inner part of the segregation vesicle, there are spherical structure filled with fine-grained green minerals composed of chlorite. The proportion of this green area is varies (Fig. 1).
3. Result
We analysised by EDS scanning microprobe analysis at 2 - 3 points or more by 5 points in segregation vesicles to determine the average composition. We also performed point analysis to identify the mineral that fill segregation vesicles. The minerals that make up the green spots are chlorite and sphene, segregation vesicles are sphene, albite and clinopyroxene. Sphene is thought to be an altered form of ilmenite or glass. Segregation vesicle part has average value of TiO2 = 1.3 to 3.4 wt %, contains almost no K2O. Whole rock chemical composition of host rock dolerite sill 3 are TiO2 = 1.27 to 1.28 wt %. The composition of segregation vesicle is more differentiated from the whole rock chemical composition of dolerite sill 3.
4. Consideration
These results indicate that the residual melt of the differentiated matrix dolerite composition was discharged into the vesicle. Composition of the segregation vesicle is completely different from the trachyte-like composition of the feldspar vein (Kato and Miyashita, 2021). These results support that the origin of the feldspar veins is outside the dolerite magma, not in the residual melt of the differentiated dolerite magma.
References
[1] Smith, 1967 [2] Jones, 1969 [3] Miyashita and Watanabe,1988 [4] Kato and Miyashita, 2021. Japan Geosci. Union Meet. 2010, Abstr., SVC31-03.
Figure. 1
Photomicrographs of dolerite sill 3 (A and B) and sill 2 (C and D). A and C are crossed nicols. B and D are opened nicols.
Authors have investigated genesis of highly vesiculated dolerite sill 1 (referred to as sill 1) of Shimokawa ophiolite in the northern Hidaka belt, Hokkaido by analysis of whole rock chemical composition and vesicle number density and vesicularity. We focused that sill 1 which has a highly vesiculated than other dolerites in the region and has feldspar veins. The veins mainly compose albite and K-feldspar. We believed that the hydrothermal fluid supply from the surrounding sediments to the dolerite (magma) increased the degree of vesiculation and formed feldspar veins.
By the way the segregation vesicle (Smith, 1967) which matrix is filled with a hyalopyritic structure is observed in Shimokawa ophiolite. Jones (1969) proposed that segregation vesicle was formed by residual melt ejection into the early vesicles.
2. Geological overview
Shimokawa Ophiolite is located in the northern part of central Hokkaido. Shimokawa ophiolite is composed of incomplete ophiolite. Serpentinized harzburgite and gabbro occur at the lowest part. The lower part of the main body consists of alternation of dolerite and sediments, the upper part consists of pillow lava, and the middle part shows intermediate features between the upper and lower parts. Dolerites show MORB features (Miyashita and Watanabe, 1988).
This study, the target is the highly vesiculated dolerite sill 3 (referred to as sill 3) where many segregation vesicles are observed. The layer thickness is 10 m. Dolerite consists of plagioclase, clinopyroxene, and opaque minerals. Dolerite is surrounded by black shale which does not has a chilled margin. The surrounding sedimentary rock and dolerite are considered to be in contact with each other by faults.
According to the thin section observation, segregation vesicles are composed of fine-grained matrix of albite and clinopyroxene. In the inner part of the segregation vesicle, there are spherical structure filled with fine-grained green minerals composed of chlorite. The proportion of this green area is varies (Fig. 1).
3. Result
We analysised by EDS scanning microprobe analysis at 2 - 3 points or more by 5 points in segregation vesicles to determine the average composition. We also performed point analysis to identify the mineral that fill segregation vesicles. The minerals that make up the green spots are chlorite and sphene, segregation vesicles are sphene, albite and clinopyroxene. Sphene is thought to be an altered form of ilmenite or glass. Segregation vesicle part has average value of TiO2 = 1.3 to 3.4 wt %, contains almost no K2O. Whole rock chemical composition of host rock dolerite sill 3 are TiO2 = 1.27 to 1.28 wt %. The composition of segregation vesicle is more differentiated from the whole rock chemical composition of dolerite sill 3.
4. Consideration
These results indicate that the residual melt of the differentiated matrix dolerite composition was discharged into the vesicle. Composition of the segregation vesicle is completely different from the trachyte-like composition of the feldspar vein (Kato and Miyashita, 2021). These results support that the origin of the feldspar veins is outside the dolerite magma, not in the residual melt of the differentiated dolerite magma.
References
[1] Smith, 1967 [2] Jones, 1969 [3] Miyashita and Watanabe,1988 [4] Kato and Miyashita, 2021. Japan Geosci. Union Meet. 2010, Abstr., SVC31-03.
Figure. 1
Photomicrographs of dolerite sill 3 (A and B) and sill 2 (C and D). A and C are crossed nicols. B and D are opened nicols.