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[SMP24-P03] Datolite and associated minerals from micro-diorite block in mantle section of Horokanai Ophiolite, Japan
Keywords:Boron, Serpentinite, Rodingite, Datolite, Mantle wedge, Ophiolite
Some serpentinites from Tunnel construction in Japan have been reported to leach boron concentration exceeding environmental standard (>1mg/L) by leaching tests (Ishigami et al., 2022). Understanding the origin and geological background of the naturally occurring heavy metal elements in serpentinites is required. In this report, we periluminally report the occurrence of boron containing mineral datolite [CaBSiO4(OH)] and associated minerals from micro-diorite, which are closely associated with subduction zone serpentinites.
Datolite has been discovered within a “microdiorite” block in the Takadomari Serpentinite Body in the Kamuikotan zone of the Sorachi-Ezo Belt, Japan. The Takadomari Body is regarded as the mantle section of the Horokanai Ophiolite (Igarashi et al., 1985). Structurally below the Body in contact with fault, there are Kamuikotan metamorphic rocks which include blueschists. We considered that this tectonic setting provides an opportunity to understand of material cycling at the mantle-wedge/slab boundary.
The Takadomari Body consists of massive to foliated serpentinite originating harzburgite and dunite, with numerous blocks of “microdiorite” from gabbro to diorite. Complex serpentinization of high-temperature with meta-olivine and antigorite and low-temperature with lizardite are observed (Takamizawa et al., 2022). Microdiorite is occurred as dike-like or block within massive to foliated serpentinites, from several tens of centimeter to several meters in size, and occasionally exceeding several tens of meters. Typical microdiorite has mineral assemblages of plagioclase, green to brown amphibole, and Fe-Ti oxides with titanite and zircon as accessory minerals. Amphiboles is partially or completely replaced by chlorite. As secondary mineral veins of white color, prehnite or Ca-carbonate are common. The microdiorite is considered to be island arc tholeiite of slightly enriched in Na2O (Kato et al., 1989), and K-Ar age of 125±12 Ma (MITI Japan, 1994). The microdiorite containing datolite was sampled from a horizontal boring core obtained during tunnel construction in 2010 near the Horokanai-Touge. Petrographic description and mineral and mapping analysis using EPMA were conducted.
The sampled microdiorite occurs as light gray dike-like blocks with a width of 1.1 m in foliated serpentinite, and including several white veins (<1.5 cm wide). Both of contact appears dark gray, reminiscent of chilled margins. The igneous rock texture is almost indistinct due to rodingitization, wherein primary minerals altered to chlorite, calcite, andradite, and titanite and notably contains tiny (<10 µm) natural copper grains. This white vein containing datolite consist of a euhedral wollastonite and anhedral calcite in the center and euhedral wollastonite and anhedral datolite around the wall. Datolite is partially replacement of wollastonite, and has a contact to calcite. Additionally, the vein contains aggregates of euhedral Ti-garnet (Titanian andradite) with less than 0.1 mm in size, containing CaO (38-39 wt%), FeO* (1.7-6.1 wt%), and TiO2 (2-11 wt%). Ti-garnet have fibrous core with up to 11.3 wt% of TiO2, indicating replacement of titanate. The thin rim contains up to 6 wt% of FeO*. Wollastonite crosscutting Ti-garnet suggests a crystallization sequence of titanite → Ti-garnet → wollastonite → datolite + calcite, implying the presence of a boron-rich fluid in the Ca-Si metasomatic process of microdiorite within serpentinite body.
This is probably the first report of the coexistence of datolite, calcite, wollastonite, and titanium garnet from a rodingite (microdiorite) within a serpentinite body in the subduction zone settings of Japan Arc. As previous study, datolite has been found in carbonate rocks equivalent to eclogite facies metamorphic rocks of the Sanbagawa Metamorphic Belt, with the estimated P-T condition of 400–650 ℃ and 0.8–1.3 GPa (Yoshida et al., 2021). On the other hand, the occurrence of Ti-garnet, similar compositions to our sample, has also been reported from rodingite associated with serpentinites in the Nagasaki metamorphic rocks and its conditions are estimated to be around ~400℃ and 0.6–0.8 GPa (Schmitt et al., 2019). Our further research will contribute to understanding the geological evolution of the Takadomari serpentinite body and the processes of fluid-rock interaction in shallow mantle wedge setting.
Datolite has been discovered within a “microdiorite” block in the Takadomari Serpentinite Body in the Kamuikotan zone of the Sorachi-Ezo Belt, Japan. The Takadomari Body is regarded as the mantle section of the Horokanai Ophiolite (Igarashi et al., 1985). Structurally below the Body in contact with fault, there are Kamuikotan metamorphic rocks which include blueschists. We considered that this tectonic setting provides an opportunity to understand of material cycling at the mantle-wedge/slab boundary.
The Takadomari Body consists of massive to foliated serpentinite originating harzburgite and dunite, with numerous blocks of “microdiorite” from gabbro to diorite. Complex serpentinization of high-temperature with meta-olivine and antigorite and low-temperature with lizardite are observed (Takamizawa et al., 2022). Microdiorite is occurred as dike-like or block within massive to foliated serpentinites, from several tens of centimeter to several meters in size, and occasionally exceeding several tens of meters. Typical microdiorite has mineral assemblages of plagioclase, green to brown amphibole, and Fe-Ti oxides with titanite and zircon as accessory minerals. Amphiboles is partially or completely replaced by chlorite. As secondary mineral veins of white color, prehnite or Ca-carbonate are common. The microdiorite is considered to be island arc tholeiite of slightly enriched in Na2O (Kato et al., 1989), and K-Ar age of 125±12 Ma (MITI Japan, 1994). The microdiorite containing datolite was sampled from a horizontal boring core obtained during tunnel construction in 2010 near the Horokanai-Touge. Petrographic description and mineral and mapping analysis using EPMA were conducted.
The sampled microdiorite occurs as light gray dike-like blocks with a width of 1.1 m in foliated serpentinite, and including several white veins (<1.5 cm wide). Both of contact appears dark gray, reminiscent of chilled margins. The igneous rock texture is almost indistinct due to rodingitization, wherein primary minerals altered to chlorite, calcite, andradite, and titanite and notably contains tiny (<10 µm) natural copper grains. This white vein containing datolite consist of a euhedral wollastonite and anhedral calcite in the center and euhedral wollastonite and anhedral datolite around the wall. Datolite is partially replacement of wollastonite, and has a contact to calcite. Additionally, the vein contains aggregates of euhedral Ti-garnet (Titanian andradite) with less than 0.1 mm in size, containing CaO (38-39 wt%), FeO* (1.7-6.1 wt%), and TiO2 (2-11 wt%). Ti-garnet have fibrous core with up to 11.3 wt% of TiO2, indicating replacement of titanate. The thin rim contains up to 6 wt% of FeO*. Wollastonite crosscutting Ti-garnet suggests a crystallization sequence of titanite → Ti-garnet → wollastonite → datolite + calcite, implying the presence of a boron-rich fluid in the Ca-Si metasomatic process of microdiorite within serpentinite body.
This is probably the first report of the coexistence of datolite, calcite, wollastonite, and titanium garnet from a rodingite (microdiorite) within a serpentinite body in the subduction zone settings of Japan Arc. As previous study, datolite has been found in carbonate rocks equivalent to eclogite facies metamorphic rocks of the Sanbagawa Metamorphic Belt, with the estimated P-T condition of 400–650 ℃ and 0.8–1.3 GPa (Yoshida et al., 2021). On the other hand, the occurrence of Ti-garnet, similar compositions to our sample, has also been reported from rodingite associated with serpentinites in the Nagasaki metamorphic rocks and its conditions are estimated to be around ~400℃ and 0.6–0.8 GPa (Schmitt et al., 2019). Our further research will contribute to understanding the geological evolution of the Takadomari serpentinite body and the processes of fluid-rock interaction in shallow mantle wedge setting.