15:45 〜 16:00
[SCG45-07] 東北日本小木ピクライト質ドレライトシルに産する2種類のかんらん石の起源
キーワード：小木ピクライト質ドレライトシル、ピクライト、かんらん石、多相包有物、元素拡散モデリング
The Miocene Ogi Picritic Dolerite Sill (OPD) is exposed at Sawasaki, Ogi Peninsula, southwest of Sado Island, northeastern Japan. The OPD (20 m thick) has intruded into the tuffaceous mudstone of the Miocene Ogi Basalt Member. Dark greenish to blackish glassy rock is present between the tuffaceous mudstone (wall rock) and the intrusive body. Yokoyama et al. (1992) reported normally and reversely zoned olivine crystals for the first time. They concluded that chemical zoning of olivine had been formed by olivine accumulation and thermal gradient of magma chamber. However, Fujibayashi et al. (2016) argued that boninitic origin of reversely zoned olivine from the compositional similarity of Fo-CaO trend with the Lau Basin olivine. In addition, they discussed mixing with olivine basalt magma after olivine crystallization from boninitic magma. In this way, there are two different hypotheses for the origin of OPD olivine. To validate them, we newly focused the multiphase solid inclusions (MSIs) in the OPD olivine. To investigate the chemical characteristics of MSI could provide significant constraints on the origin of two types of olivine from the OPD. In this presentation, we show the chemical characteristics of two types of olivine and their MSIs and clarify the tectonic implications of them.
The OPD intrusive body is composed of three lithologies. The chilled marginal basalt and the picritic basalt occur within 30 cm and 30 cm to 4 m of the lower contact, respectively. The picritic dolerite is the main lithology of the OPD. The picritic dolerite contains olivine and spinel as phenocryst and plagioclase, clinopyroxene, olivine and Fe-Ti oxide as groundmass crystals. The modal proportions of olivine are 10 vol% in the chilled marginal basalt, 15 vol% in the picritic basalt, and 60–70 vol% in the picritic dolerite. The whole-rock MgO content of OPD increases from the chilled marginal basalt (10 wt% MgO) to the picritic dolerite (28 wt% MgO), suggesting that flowage differentiation (e.g., Simkin, 1967) happened at the time of emplacement.
The OPD olivine can be classified into two types. The type 1 olivine is relatively fine-grained (1–2 mm in size) and shows euhedral to subhedral shapes. The core is rich in Cr-spinel inclusion and MSIs, and also in Fo and Ca contents (Fo88.0–89.8 and 0.21–0.26 wt % CaO, respectively). The type has a compositionally homogeneous core with a normally zoned rim. The type 2 olivine is coarse-grained (2–10 mm in size), and shows euhedral to anhedral shapes. The core is characterized by inclusion-poor, and low-Fo and Ca content (Fo83.5–85.0 and 0.13–0.20 wt% CaO). On the other hand, the mantle is similar to type 1 olivine in the petrographical and chemical characteristics (inclusion-rich, Fo88.1–89.3 and 0.22–0.25 wt% CaO). As seen above, the type 2 olivine exhibits reverse zoning in Fo and CaO contents. The liquidus olivine composition estimated from the whole-rock composition of the chilled marginal basalt is similar to those of the type 1 olivine core and type 2 olivine mantle, suggesting that the type 1 olivine may be autocryst. On the other hand, the type 2 olivine core is disequilibrium with any OPD samples. This suggests that the type 2 olivine core may be antecryst.
The olivine-hosted MSIs can be also classified into two types. The O1-MSIs are included in the type 1 olivine core and composed of plagioclase + clinopyroxene + amphibole with basaltic bulk compositions. The O2c-MSIs are included in the type 2 olivine core and composed of clinopyroxene + orthopyroxene + amphibole. Some of them have high-Mg andesitic compositions. Especially, plagioclase is only found in highly-crystallized O2c-MSIs with SiO2-rich glasses. These indicate that the type 1 and type 2 olivine grains may be crystallized from basaltic and high-Mg andesitic (boninite-like) magmas, respectively.
The diffusion time calculated from Fe-Mg reverse zoning of the type 2 olivine is approximately 140 years and is longer than the solidification time of 20 m thick OPD sill (2–3 years). This implies that after the type 2 olivine mantle overgrew on the type 2 olivine core, type 2 olivine has been retained in a magma chamber for approximately 140 years until intrusion and emplacement into the shallower place.
Recently, peridotites modified by the LREE-rich melt/fluid have been reported from the back-arc basin of the Sea of Japan (Ninomiya et al., 2007; Morishita et al., 2020). The signature of high-Mg andesitic magmatism recorded in the type 2 olivine core and O2c-MSI may indicate that contaminated oceanic lithospheric mantle had existed beneath the Miocene back-arc basin of the Sea of Japan.
The OPD intrusive body is composed of three lithologies. The chilled marginal basalt and the picritic basalt occur within 30 cm and 30 cm to 4 m of the lower contact, respectively. The picritic dolerite is the main lithology of the OPD. The picritic dolerite contains olivine and spinel as phenocryst and plagioclase, clinopyroxene, olivine and Fe-Ti oxide as groundmass crystals. The modal proportions of olivine are 10 vol% in the chilled marginal basalt, 15 vol% in the picritic basalt, and 60–70 vol% in the picritic dolerite. The whole-rock MgO content of OPD increases from the chilled marginal basalt (10 wt% MgO) to the picritic dolerite (28 wt% MgO), suggesting that flowage differentiation (e.g., Simkin, 1967) happened at the time of emplacement.
The OPD olivine can be classified into two types. The type 1 olivine is relatively fine-grained (1–2 mm in size) and shows euhedral to subhedral shapes. The core is rich in Cr-spinel inclusion and MSIs, and also in Fo and Ca contents (Fo88.0–89.8 and 0.21–0.26 wt % CaO, respectively). The type has a compositionally homogeneous core with a normally zoned rim. The type 2 olivine is coarse-grained (2–10 mm in size), and shows euhedral to anhedral shapes. The core is characterized by inclusion-poor, and low-Fo and Ca content (Fo83.5–85.0 and 0.13–0.20 wt% CaO). On the other hand, the mantle is similar to type 1 olivine in the petrographical and chemical characteristics (inclusion-rich, Fo88.1–89.3 and 0.22–0.25 wt% CaO). As seen above, the type 2 olivine exhibits reverse zoning in Fo and CaO contents. The liquidus olivine composition estimated from the whole-rock composition of the chilled marginal basalt is similar to those of the type 1 olivine core and type 2 olivine mantle, suggesting that the type 1 olivine may be autocryst. On the other hand, the type 2 olivine core is disequilibrium with any OPD samples. This suggests that the type 2 olivine core may be antecryst.
The olivine-hosted MSIs can be also classified into two types. The O1-MSIs are included in the type 1 olivine core and composed of plagioclase + clinopyroxene + amphibole with basaltic bulk compositions. The O2c-MSIs are included in the type 2 olivine core and composed of clinopyroxene + orthopyroxene + amphibole. Some of them have high-Mg andesitic compositions. Especially, plagioclase is only found in highly-crystallized O2c-MSIs with SiO2-rich glasses. These indicate that the type 1 and type 2 olivine grains may be crystallized from basaltic and high-Mg andesitic (boninite-like) magmas, respectively.
The diffusion time calculated from Fe-Mg reverse zoning of the type 2 olivine is approximately 140 years and is longer than the solidification time of 20 m thick OPD sill (2–3 years). This implies that after the type 2 olivine mantle overgrew on the type 2 olivine core, type 2 olivine has been retained in a magma chamber for approximately 140 years until intrusion and emplacement into the shallower place.
Recently, peridotites modified by the LREE-rich melt/fluid have been reported from the back-arc basin of the Sea of Japan (Ninomiya et al., 2007; Morishita et al., 2020). The signature of high-Mg andesitic magmatism recorded in the type 2 olivine core and O2c-MSI may indicate that contaminated oceanic lithospheric mantle had existed beneath the Miocene back-arc basin of the Sea of Japan.