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[SCG51-04] Geochronological constraint on hydrothermal alteration process deduced from titanite U-Pb dating on the Tono Plutonic Complex, Kitakami Mountains
Keywords:Titanite, U-Pb age, Hydrothermal alteration, the Tono Plutonic Complex
Introduction
Alteration processes of granitic rocks at deep underground are important geological phenomena closely related to mass transfer through rock-fluids interactions (Nishimoto et al., 2008) and the formation of microcracks as a pathway of ground water (Yuguchi et al., 2021). Although K-Ar dating for illite produced by plagioclase alteration (Yuguchi et al., 2019) has been reported as a dating method to constrain the timescale of such alteration events, the methods is still limited. Hydrothermal alteration in granitic rocks produces titanite (sphene) associated with biotite chloritization (Eglrton and Banfield, 1985). Determining the age of the titanite crystallization is potentially able to constrain on timescale of hydrothermal alteration event. To constrain the timing of alteration events in the plutonic rocks, we conduct U-Pb dating on secondary titanite in plutonic rocks and compare the results with other radiometric dating results from previous studies to interpret the age values.
Occurrence and U-Pb dating of titanite
The analyzed sample was collected from central facies (defined by Mikoshiba and Kanisawa, 2008) of the Tono Plutonic Complex (TPC), Kitakami Mountains, northeast Japan. Zircon U-Pb age of 117±2 Ma (Tsuchiya et al., 2015) and biotite K-Ar age of 122.6±2.7 Ma (Yuguchi et al., 2020) have been reported from central facies of TPC, respectively.
Titanite occurs at grain boundary between biotite (and/or alkali feldspar) and magnetite (Fig.1a), implying secondary precipitation around primary minerals during sub-solidus stage. Some titanite crystals occur as a patchy shape along cleavage of the chlorited biotite (Fig.1a). This occurrence is similar to the titanite accompanied biotite chloritization in Toki granite, central Japan (Yuguchi et al., 2015). Because the titanite crystals were 5 to 30 mm width in thin-sections (Fig.1a) and hence it was difficult to secure a sufficient analytical area by laser ablation–inductively coupled plasma–mass spectrometry, we conducted U-Pb analysis for separated titanite. The U-Pb isotopic analysis yielded an age value of 118.7±1.5 Ma (Fig.1b).
Interpretation on titanite U-Pb age
Some of titanite crystals in TPC are found at grain boundaries between primary biotite and other minerals or along cleavages of the chlorited biotite (Fig.1a), suggesting that these were secondary formed during hydrothermal alteration associated with biotite chloritization. Since biotite chloritization generally occurs at about 200-300℃ (Yoneda and Maeda, 2008), the formation of titanite may also have occurred under such sub-solidus temperature conditions. If titanite occurred by the alteration of biotite, the age value of titanite would reflect the period of hydrothermal activity around 200-300℃.
Zircon U-Pb age of 117±2 Ma (Tsuchiya et al., 2015) and biotite K-Ar age of 122.6±2.7 Ma (Yuguchi et al., 2020) have been reported from central facies of TPC, respectively. These ages suggest that the TPC cooled within a few million years from closure temperature of the zircon U-Pb systematics to that of biotite K-Ar systematics (400-350℃; Grove and Harrison, 1996). The U-Pb age of titanite obtained in this study overlaps with these reported ages to a range of uncertainties. These indicate that following solidification and alteration processes had occurred in a geologically short period at ca. 120 Ma: (1) solidification of the TPC, (2) its cooling to 400–350°C, and then (3) hydrothermal alteration crystallizing titanite grains (biotite chloritization). Combining radiometric dating at different closure temperatures, such as zircon U-Pb dating, biotite K-Ar dating, and (secondary) titanite U-Pb dating, may make it possible to determine time gaps in alteration and metamorphism in granitoids. Therefore, titanite U-Pb dating will enable us to see new view to geological phenomena difficult to understand only by zircon U-Pb dating.
References
Eggleton and Banfield, 1985, American Mineral., 70, 902-910.
Grove and Harrison, 1996, American Mineral., 81, 940-951.
Mikoshiba and Kanisawa, 2008, Earth Science (Chikyu Kagaku), 62, 183-201.
Nishimoto et al., 2008, J. Japan Society of Engineering Geol., 49, 94-104.
Tsuchiya et al., 2015, Japanese Magazine of Mineralogical and Petrological Sci., 44, 69-90.
Yoneda and Maeda, 2008, J. MMIJ, 124, 694-699.
Yuguchi et al., 2015, American Mineral., 100, 1134-1152.
Yuguchi et al., 2019, J. Asian Earth Sci., 169, 47-66.
Yuguchi et al., 2020, Lithos, 372-373, 105682.
Yuguchi et al., 2021, American Mineral., 106, 1128-1142.
Alteration processes of granitic rocks at deep underground are important geological phenomena closely related to mass transfer through rock-fluids interactions (Nishimoto et al., 2008) and the formation of microcracks as a pathway of ground water (Yuguchi et al., 2021). Although K-Ar dating for illite produced by plagioclase alteration (Yuguchi et al., 2019) has been reported as a dating method to constrain the timescale of such alteration events, the methods is still limited. Hydrothermal alteration in granitic rocks produces titanite (sphene) associated with biotite chloritization (Eglrton and Banfield, 1985). Determining the age of the titanite crystallization is potentially able to constrain on timescale of hydrothermal alteration event. To constrain the timing of alteration events in the plutonic rocks, we conduct U-Pb dating on secondary titanite in plutonic rocks and compare the results with other radiometric dating results from previous studies to interpret the age values.
Occurrence and U-Pb dating of titanite
The analyzed sample was collected from central facies (defined by Mikoshiba and Kanisawa, 2008) of the Tono Plutonic Complex (TPC), Kitakami Mountains, northeast Japan. Zircon U-Pb age of 117±2 Ma (Tsuchiya et al., 2015) and biotite K-Ar age of 122.6±2.7 Ma (Yuguchi et al., 2020) have been reported from central facies of TPC, respectively.
Titanite occurs at grain boundary between biotite (and/or alkali feldspar) and magnetite (Fig.1a), implying secondary precipitation around primary minerals during sub-solidus stage. Some titanite crystals occur as a patchy shape along cleavage of the chlorited biotite (Fig.1a). This occurrence is similar to the titanite accompanied biotite chloritization in Toki granite, central Japan (Yuguchi et al., 2015). Because the titanite crystals were 5 to 30 mm width in thin-sections (Fig.1a) and hence it was difficult to secure a sufficient analytical area by laser ablation–inductively coupled plasma–mass spectrometry, we conducted U-Pb analysis for separated titanite. The U-Pb isotopic analysis yielded an age value of 118.7±1.5 Ma (Fig.1b).
Interpretation on titanite U-Pb age
Some of titanite crystals in TPC are found at grain boundaries between primary biotite and other minerals or along cleavages of the chlorited biotite (Fig.1a), suggesting that these were secondary formed during hydrothermal alteration associated with biotite chloritization. Since biotite chloritization generally occurs at about 200-300℃ (Yoneda and Maeda, 2008), the formation of titanite may also have occurred under such sub-solidus temperature conditions. If titanite occurred by the alteration of biotite, the age value of titanite would reflect the period of hydrothermal activity around 200-300℃.
Zircon U-Pb age of 117±2 Ma (Tsuchiya et al., 2015) and biotite K-Ar age of 122.6±2.7 Ma (Yuguchi et al., 2020) have been reported from central facies of TPC, respectively. These ages suggest that the TPC cooled within a few million years from closure temperature of the zircon U-Pb systematics to that of biotite K-Ar systematics (400-350℃; Grove and Harrison, 1996). The U-Pb age of titanite obtained in this study overlaps with these reported ages to a range of uncertainties. These indicate that following solidification and alteration processes had occurred in a geologically short period at ca. 120 Ma: (1) solidification of the TPC, (2) its cooling to 400–350°C, and then (3) hydrothermal alteration crystallizing titanite grains (biotite chloritization). Combining radiometric dating at different closure temperatures, such as zircon U-Pb dating, biotite K-Ar dating, and (secondary) titanite U-Pb dating, may make it possible to determine time gaps in alteration and metamorphism in granitoids. Therefore, titanite U-Pb dating will enable us to see new view to geological phenomena difficult to understand only by zircon U-Pb dating.
References
Eggleton and Banfield, 1985, American Mineral., 70, 902-910.
Grove and Harrison, 1996, American Mineral., 81, 940-951.
Mikoshiba and Kanisawa, 2008, Earth Science (Chikyu Kagaku), 62, 183-201.
Nishimoto et al., 2008, J. Japan Society of Engineering Geol., 49, 94-104.
Tsuchiya et al., 2015, Japanese Magazine of Mineralogical and Petrological Sci., 44, 69-90.
Yoneda and Maeda, 2008, J. MMIJ, 124, 694-699.
Yuguchi et al., 2015, American Mineral., 100, 1134-1152.
Yuguchi et al., 2019, J. Asian Earth Sci., 169, 47-66.
Yuguchi et al., 2020, Lithos, 372-373, 105682.
Yuguchi et al., 2021, American Mineral., 106, 1128-1142.