11:45 〜 12:00
[SMP24-10] Origin and pyrometamorphism of gneissose granitoid xenoliths from Mt. Daisen, Tottori Prefecture
キーワード：パイロ変成作用、大山、花崗岩、捕獲岩、トール石
Mt. Daisen, situated in the western part of Tottori Prefecture, southwest Japan, is a Quaternary volcano composed primarily of andesite and dacite. Previous studies have revealed that the basement geology around Mt. Daisen consists of the Hida plutono-metamorphic complex (Triassic and Jurassic granitoids and gneisses) and the San-in batholith (Late Cretaceous to Paleogene I-type granitoids).
In the Motodani valley of Mt. Daisen, granitoids and gneisses are found as xenoliths within dacite gravels. Those xenoliths were initially correlated with the Hida plutono-metamorphic complex (Miura, 1989). However, chemical Th-U-total Pb dating of thorite in a gneissose granitoid xenolith in this study revealed an igneous age of ~75 Ma, suggesting some xenoliths are associated with the San-in granitoids.
These gneissose granitoid xenoliths exhibit tonalitic mineralogical composition with small amounts of corroded quartz grains. The absence of hornblende and the presence of pyroxene aggregates (orthopyroxene ± augite) associated with calcic plagioclase suggest hornblende dehydration melting during pyrometamorphism. Former melt domains exhibit a microcrystalline texture consisting of elongated crystals of tridymite + oligoclase to anorthoclase + sanidine, indicating a melt composition similar to A-type granite. Two-pyroxene and ternary feldspar compositions, as well as the presence of tridymite, suggest that the xenoliths underwent sanidinite-facies pyrometamorphism. Biotite, present in all of the studied xenoliths, shows an elevated fluorine content (~5 wt% F at the rim of biotite grains). Notably, a unique texture associated with pyrometamorphism is very-fine symplectite consisting of chevkinite-(Ce)/perrierite-(Ce) and orthopyroxene, indicating prograde breakdown of a Th-rich mineral, probably allanite-(Ce).
The mineralogical features of pyrometamorphism observed in the granitoid xenoliths are comparable to the low-pressure melting experiments of calc-alkaline granitoids; Patino Douce (1997) demonstrated the generation of A-type granitic melt and orthopyroxene + calcic plagioclase by dehydration melting of a hornblende-bearing tonalite at 0.4 GPa. Additionally, the results of our study may imply the widespread presence of gneissose rocks as deeper members of the San-in granitoids.
References:
Miura (1989) Memoir. Fac. Edu. Shimane Univ. (Nat. Sci.), 23, 25-34.
Patino Douce, A.E. (1997) Geology, 25, 743-746.
In the Motodani valley of Mt. Daisen, granitoids and gneisses are found as xenoliths within dacite gravels. Those xenoliths were initially correlated with the Hida plutono-metamorphic complex (Miura, 1989). However, chemical Th-U-total Pb dating of thorite in a gneissose granitoid xenolith in this study revealed an igneous age of ~75 Ma, suggesting some xenoliths are associated with the San-in granitoids.
These gneissose granitoid xenoliths exhibit tonalitic mineralogical composition with small amounts of corroded quartz grains. The absence of hornblende and the presence of pyroxene aggregates (orthopyroxene ± augite) associated with calcic plagioclase suggest hornblende dehydration melting during pyrometamorphism. Former melt domains exhibit a microcrystalline texture consisting of elongated crystals of tridymite + oligoclase to anorthoclase + sanidine, indicating a melt composition similar to A-type granite. Two-pyroxene and ternary feldspar compositions, as well as the presence of tridymite, suggest that the xenoliths underwent sanidinite-facies pyrometamorphism. Biotite, present in all of the studied xenoliths, shows an elevated fluorine content (~5 wt% F at the rim of biotite grains). Notably, a unique texture associated with pyrometamorphism is very-fine symplectite consisting of chevkinite-(Ce)/perrierite-(Ce) and orthopyroxene, indicating prograde breakdown of a Th-rich mineral, probably allanite-(Ce).
The mineralogical features of pyrometamorphism observed in the granitoid xenoliths are comparable to the low-pressure melting experiments of calc-alkaline granitoids; Patino Douce (1997) demonstrated the generation of A-type granitic melt and orthopyroxene + calcic plagioclase by dehydration melting of a hornblende-bearing tonalite at 0.4 GPa. Additionally, the results of our study may imply the widespread presence of gneissose rocks as deeper members of the San-in granitoids.
References:
Miura (1989) Memoir. Fac. Edu. Shimane Univ. (Nat. Sci.), 23, 25-34.
Patino Douce, A.E. (1997) Geology, 25, 743-746.