2:30 PM - 2:45 PM
[SVC31-03] Genesis of vesiculation in dolerite sills of the Shimokawa ophiolite in the northern Hidaka belt, Hokkaido: Genesis of trachyte in the MORB
Keywords:Hokkaito Hidaka belt, dorelite sill, Shimokawa ophiolite, vesiculation
Highly vesiculated dolerite sills are found in three places in the Shimokawa ophiolite. Dolerites which intruded into soft sediments are considerably vesiculated up to 10 % (Miyashita, 1999). Highly vesiculated dolerite sill 1 has different texture and vesiculation texture between the margin part and inside part. There are few researches dealing with texture and vesiculation texture. In this presentation, we clarified texture and vesiculation texture and examined a process of vesiculation in the dolerite sill. We present new finding of Ab-Kfs veins and aggregates. Based of the geological and chemical features of the Ab-Kfs veins and aggregate, we examined the formation process.
The Shimokawa ophiolite is exposed in the northern part of central Hokkaido. 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).
Highly vesiculated dolerite sill 1 about 22 to 24 m thick occurs at the lowest horizon in the highly vesiculated dolerite sills. Dolerite is composed plagioclase, clinopyroxene and opaque minerals. The marginal part of the sill shows hyalo-ophitic textures, inside part show dolerite texture, and the most upper part shows fine-grained dolerite texture. Vesicles are irregular shape, about 1 mm2 size, filled with chlorite and quartz. We measured vesicularities of 22 samples. The marginal part within 50 cm from the contact is not vesiculated. Vesicularities increase as much as 10 vol % upward until about 18 m from the lower contact. But dolerites containing Ab-Kfs veins show high vesicularity. Plagioclase number densities (PND) show a systematic variation. The margin shows a high value (40 number/mm2) and inside shows low values (4-6 number/mm2), implying that the margin is fine-grained and inside is coarse-grained. The PND of basalt containing the Ab-Kfs vein is higher than the surrounding rocks (15 number/mm2), indicating fine-grained than the surrounding basalts. Above facts suggest that the Ab-Kfs veins were formed before the solidification of the sills.
The Ab-Kfs vein and aggregates are found from 8 locations in the highly vesiculated dolerite sill 1. In the marginal part of the sill the Ab-Kfs aggregates appearing intergranular crystals, show irregular shape and <100μm2 size. The Ab-Kfs veins are >2 mm width and thicker than later calcite veins. The border of the Ab-Kfs vein is not straight and waves. It is noted that the Ab-Kfs veins do not cut primary igneous crystals. The Ab-Kfs veins and aggregates are mostly composed of albite and K-feldspar. The bulk composition of the veins estimated from averaged analyses from 44 spot measured by SEM-EDS exhibits a trachytic composition (IUGS igneous rock classification). The bulk compositions are also estimated from color mapping and mineral analyses by EPMA-WDS of the vein and shows again similar trachytic composition. The Ab-Kfs veins have irregular shaped vesicles of 3 mm2 size which are filled with chlorite and quartz. It’s difficult that trachyte melts were produced by fractional crystallization of MORB magma. Trachyte melts are brought from outside the dolerite sill.
The Shimokawa ophiolite generally shows poor vesiculation. Dolerite sill 1 shows high vesiculation except for the chilled margin. PND analyses show a systematic variation. The margin shows a fine-grained and inside shows coarse-grained. Texture made by crystallization after magma intrudes. The chilled margin is not vesiculated, but inside of the sill is vesiculated > 5 %. We found 2 type of Ab-Kfs aggregates one is vein in the inside parts, the other is aggregates in the chilled margin. Composition of the Ab-Kfs vein is trachytic. Here we show trachytic melts is interstitial melt brought from outside the MORB magma. Surrounding dolerites of the Ab-Kfs veins are highly vesiculated. Therefore, the Ab-Kfs veins release volatile components into magma. The factor of the high vesiculation in the dolerite sill 1 is multiple.
[1]Miyashita and Watanabe, 1988. [2]Miyashita, 1999.
The Shimokawa ophiolite is exposed in the northern part of central Hokkaido. 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).
Highly vesiculated dolerite sill 1 about 22 to 24 m thick occurs at the lowest horizon in the highly vesiculated dolerite sills. Dolerite is composed plagioclase, clinopyroxene and opaque minerals. The marginal part of the sill shows hyalo-ophitic textures, inside part show dolerite texture, and the most upper part shows fine-grained dolerite texture. Vesicles are irregular shape, about 1 mm2 size, filled with chlorite and quartz. We measured vesicularities of 22 samples. The marginal part within 50 cm from the contact is not vesiculated. Vesicularities increase as much as 10 vol % upward until about 18 m from the lower contact. But dolerites containing Ab-Kfs veins show high vesicularity. Plagioclase number densities (PND) show a systematic variation. The margin shows a high value (40 number/mm2) and inside shows low values (4-6 number/mm2), implying that the margin is fine-grained and inside is coarse-grained. The PND of basalt containing the Ab-Kfs vein is higher than the surrounding rocks (15 number/mm2), indicating fine-grained than the surrounding basalts. Above facts suggest that the Ab-Kfs veins were formed before the solidification of the sills.
The Ab-Kfs vein and aggregates are found from 8 locations in the highly vesiculated dolerite sill 1. In the marginal part of the sill the Ab-Kfs aggregates appearing intergranular crystals, show irregular shape and <100μm2 size. The Ab-Kfs veins are >2 mm width and thicker than later calcite veins. The border of the Ab-Kfs vein is not straight and waves. It is noted that the Ab-Kfs veins do not cut primary igneous crystals. The Ab-Kfs veins and aggregates are mostly composed of albite and K-feldspar. The bulk composition of the veins estimated from averaged analyses from 44 spot measured by SEM-EDS exhibits a trachytic composition (IUGS igneous rock classification). The bulk compositions are also estimated from color mapping and mineral analyses by EPMA-WDS of the vein and shows again similar trachytic composition. The Ab-Kfs veins have irregular shaped vesicles of 3 mm2 size which are filled with chlorite and quartz. It’s difficult that trachyte melts were produced by fractional crystallization of MORB magma. Trachyte melts are brought from outside the dolerite sill.
The Shimokawa ophiolite generally shows poor vesiculation. Dolerite sill 1 shows high vesiculation except for the chilled margin. PND analyses show a systematic variation. The margin shows a fine-grained and inside shows coarse-grained. Texture made by crystallization after magma intrudes. The chilled margin is not vesiculated, but inside of the sill is vesiculated > 5 %. We found 2 type of Ab-Kfs aggregates one is vein in the inside parts, the other is aggregates in the chilled margin. Composition of the Ab-Kfs vein is trachytic. Here we show trachytic melts is interstitial melt brought from outside the MORB magma. Surrounding dolerites of the Ab-Kfs veins are highly vesiculated. Therefore, the Ab-Kfs veins release volatile components into magma. The factor of the high vesiculation in the dolerite sill 1 is multiple.
[1]Miyashita and Watanabe, 1988. [2]Miyashita, 1999.