17:15 〜 18:30
[SCG44-P06] The trace element composition of sapphire in the xenolith of garnet-bearing andesitic tuff of the Donzurubo Formation, the Nijo Group, Japan.
キーワード：サファイア、コランダム、微量元素、LA-ICP-MS、二上山
The Donzurubo Formation is a member of the Middle Miocene Nijo Group in the Mt. Nijo, Japan. The Donzurubo Formation consists of three sub-formations, the Lower Donzurubo Formation: garnet-biotite rhyolitic welded tuff, the Middle Donzurubo Formation: garnet-biotite andesite (Ishikiriba Volcanics), garnet-biotite andesitic clastic rocks, garnet-hornblend-orthopyroxene andesite (Hata Volcanics), and the Upper Donzurubo Formation: garnet-biotite rhyolite (Medake Volcanics), garnet-biotite rhyolitic clastic rocks. The volcanic rocks in the Nijo Group are a typical Setouchi Volcanic Rocks which are characterized by the occurrence of high-Mg andesite in Miocene. The volcanic rocks in Nijo Group contains abundant garnets. The occurrence of corundum as a xenolith of garnet-bearing andesite were reported in previous studies (e.g. Omori, 1942, Morimoto, 1949). On the other hand, the euhedral sapphires and garnets are found in Takeda river which is downstream from Mt. Nijo.
In this study, we present the occurrence of corundum-bearing xenolith which is found in garnet-biotite andesitic tuff in the Lower Donzurubo Formation. The euhedral pastel blue sapphire grains are also collected from river sands in Takeda river. The mineral assemblage of corundum-bearing xenolith is corundum, orthopyroxene, plagioclase, sillimanite, garnet, spinel, zircon, monazite, and ilmenite. The corundum is replaced by sillimanite, spinel and plagioclase. The sillimanite and spinel are a product of metamorphic reaction as; 5 corundum + garnet = 3 sillimanite + 3 spinel. This reaction takes place at high temperature condition (> 750 oC).
The corundum in xenolith are Fe-poor (1178-1707 μg/g), Ga-poor (43-63 μg/g), Cr-poor (81-159 μg/g), Ti-rich (273-975μg/g), and Mg-rich (114-240 μg/g). Some of sapphires from Takeda river shows similar trace element composition to the corundum in xenolith, but some of them shows the different compositions which may indicate the different origin of sapphires. Trace elements in corundum are often used for origin determination. In the case of blue sapphire, the discrimination diagram of sapphire is difficult to identify their origin (Groat et al., 2019) but trace element compositions of corundum and sapphires in this study clearly shows the metamorphic origin.
The basement rocks of the Nijo Group consist of granitoids and high-temperature metamorphic rocks of the Ryoke belt. The mineral assemblage of xenolith and trace elements of corundum indicate that the origin of xenolith could be lateritic metamorphic rock at high-T condition which might be the Ryoke metamorphic rocks.
Acknowledgement
We appreciate the Hikia Group for obtaining samples.
References
Groat at al. (2019) Gems & Gemology, 55, 512-535.
Morimoto (1949) The Journal of the Geological Society of Japan, 55, 38-43.
Omori (1942) The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists, 27, 27-58.
In this study, we present the occurrence of corundum-bearing xenolith which is found in garnet-biotite andesitic tuff in the Lower Donzurubo Formation. The euhedral pastel blue sapphire grains are also collected from river sands in Takeda river. The mineral assemblage of corundum-bearing xenolith is corundum, orthopyroxene, plagioclase, sillimanite, garnet, spinel, zircon, monazite, and ilmenite. The corundum is replaced by sillimanite, spinel and plagioclase. The sillimanite and spinel are a product of metamorphic reaction as; 5 corundum + garnet = 3 sillimanite + 3 spinel. This reaction takes place at high temperature condition (> 750 oC).
The corundum in xenolith are Fe-poor (1178-1707 μg/g), Ga-poor (43-63 μg/g), Cr-poor (81-159 μg/g), Ti-rich (273-975μg/g), and Mg-rich (114-240 μg/g). Some of sapphires from Takeda river shows similar trace element composition to the corundum in xenolith, but some of them shows the different compositions which may indicate the different origin of sapphires. Trace elements in corundum are often used for origin determination. In the case of blue sapphire, the discrimination diagram of sapphire is difficult to identify their origin (Groat et al., 2019) but trace element compositions of corundum and sapphires in this study clearly shows the metamorphic origin.
The basement rocks of the Nijo Group consist of granitoids and high-temperature metamorphic rocks of the Ryoke belt. The mineral assemblage of xenolith and trace elements of corundum indicate that the origin of xenolith could be lateritic metamorphic rock at high-T condition which might be the Ryoke metamorphic rocks.
Acknowledgement
We appreciate the Hikia Group for obtaining samples.
References
Groat at al. (2019) Gems & Gemology, 55, 512-535.
Morimoto (1949) The Journal of the Geological Society of Japan, 55, 38-43.
Omori (1942) The Journal of the Japanese Association of Mineralogists, Petrologists and Economic Geologists, 27, 27-58.