5:15 PM - 6:45 PM
[SMP24-P02] Melt-rock interaction and mantle re-fertilization recorded in symmetrical layers of peridotite-mafic rocks in the Horoman peridotite, Hokkaido, Japan
Keywords:Peridotite, Mafic-rock, re-fertilization, Melt-Rock interaction
The Horoman peridotite is located at the southern end of the Hidaka metamorphic belt in Hokkaido, Japan. Its size is 8*10*3 km. Based on geological and petrological features, the body is divided into an upper part with a thickness of about 1 km and a lower part with a thickness of about 2 km. Many studies have been carried out. The outcrop is composed of layers of peridotite and mafic rocks, and a symmetrical structure is observed around a peridotite layer (Toramaru, 1997, Mem. Geol. Soc. Japan). In this study, mineral and whole-rock chemical compositions are determined for the northern outcrop of Mt. Apoi in the upper part of the Horoman Massif.
The peridotite rocks are divided into plagioclase and non-plagioclase layers classified as plagioclase lherzolite and spinel harzburgite. The internal structure of the plagioclase lherzolite layer consists of a thin, fine-grained layer about 5 mm thick containing plagioclase and an alternating layer several 10 mm thick rich in olivine and pyroxene without plagioclase. In contrast, the grain size of spinel harzburgite is relatively homogeneous and comparable to the average grain size of the plagioclase lherzolite.
The mineral assemblage and chemical composition of the mafic rocks indicate that the mafic rocks are Type I according to the classification of Shiotani and Niida (1997, Mem. Geol. Soc. Jpn.) and Takazawa et al. (1999, J. Petrol.). Whole-rock trace elements in mafic rocks show a pattern similar to that of Mid Oceanic Ridge Basalt (N-MORB), which is poor in light rare-earth elements. The chemical composition of the minerals in the mafic rocks is not homogeneous and compositional gradients due to elemental diffusion are observed within the minerals. Large pyroxenes with grain sizes of about 2 mm are commonly observed at the boundaries between the mafic and peridotite layers. They show a chemical zoning with higher Ti at the rim than in the core. These features are interpreted to record the influx of MORB-like melts into the peridotite forming mafic rock layers and their reaction with the wall rock peridotite. The whole-rock chemical composition of the plagioclase lherzolite is intermediate between that of the spinel harzburgite and the mafic rock. This suggests that the melt flowed into the spinel harzburgite and reacted to form plagioclase lherzolite as a result of fertilization. The negative trend in Mg# and TiO2 content of clinopyroxene in the peridotite suggests that the degree of re-fertilization by melt infiltration varies among peridotite layers. This interpretation is consistent with structural features of the identical rock specimens showing the deformation of partially melted peridotite (to be presented by Hihara et al.).
The thickness of the mafic rock layers is greater than the thin plagioclase-rich layer of the plagioclase lherzolite. We suggest that the thickness is related to the amount of melt that has migrated into the peridotite (Spiegelman et al., 2001, J. Geophys. Res.; Pec et al., 2020, Geochem. Geophys. Geosyst.). If this is the case, the thickness will also correlate with the degree of partial melting. Note that melts with a low degree of partial melting would form a thin layer and melts with a high degree of partial melting would form a thick layer. The degree of partial melting can be expressed as Rb/Th ratio, if Rb has a higher partition coefficient into melts during partial melting than Th, the melts produced from low degrees of partial melting will have a higher value, and melts from higher degrees of partial melting will have a lower value. The value is higher in plagioclase lherzolites and is interpreted as a smaller amount of melt forming a thin layer containing plagioclase. On the other hand, the value is lower in mafic rock layers, suggesting that the melts formed the mafic rock layers with higher degrees of partial melting. This outcrop is thought to have been formed by a reaction between melt and rock, and further study is needed to determine the origin of the symmetrical structure.
We also examine the plagioclase lherzolite from the Horoman River, which is located at east of Mt. Apoi. It consists of a plagioclase-rich layer of about 5 mm thich and plagioclase-poor layer, in which plagioclase is interbedded with other minerals, with a coarse-grained pyroxene layer at the boundary. The mineral composition of the plagioclase-poor part is identical to that of the plagioclase lherzolite in the northern outcrop of Mt. Apoi. On the other hand, the samples collected from an outcrop more than 10 m away has a lower TiO2 content in the clinoperoxene and differs from the northern outcrop of Mt. Apoi. This suggests that there are several types of plagioclase lherzolites in the Horoman peridotite complex.
The peridotite rocks are divided into plagioclase and non-plagioclase layers classified as plagioclase lherzolite and spinel harzburgite. The internal structure of the plagioclase lherzolite layer consists of a thin, fine-grained layer about 5 mm thick containing plagioclase and an alternating layer several 10 mm thick rich in olivine and pyroxene without plagioclase. In contrast, the grain size of spinel harzburgite is relatively homogeneous and comparable to the average grain size of the plagioclase lherzolite.
The mineral assemblage and chemical composition of the mafic rocks indicate that the mafic rocks are Type I according to the classification of Shiotani and Niida (1997, Mem. Geol. Soc. Jpn.) and Takazawa et al. (1999, J. Petrol.). Whole-rock trace elements in mafic rocks show a pattern similar to that of Mid Oceanic Ridge Basalt (N-MORB), which is poor in light rare-earth elements. The chemical composition of the minerals in the mafic rocks is not homogeneous and compositional gradients due to elemental diffusion are observed within the minerals. Large pyroxenes with grain sizes of about 2 mm are commonly observed at the boundaries between the mafic and peridotite layers. They show a chemical zoning with higher Ti at the rim than in the core. These features are interpreted to record the influx of MORB-like melts into the peridotite forming mafic rock layers and their reaction with the wall rock peridotite. The whole-rock chemical composition of the plagioclase lherzolite is intermediate between that of the spinel harzburgite and the mafic rock. This suggests that the melt flowed into the spinel harzburgite and reacted to form plagioclase lherzolite as a result of fertilization. The negative trend in Mg# and TiO2 content of clinopyroxene in the peridotite suggests that the degree of re-fertilization by melt infiltration varies among peridotite layers. This interpretation is consistent with structural features of the identical rock specimens showing the deformation of partially melted peridotite (to be presented by Hihara et al.).
The thickness of the mafic rock layers is greater than the thin plagioclase-rich layer of the plagioclase lherzolite. We suggest that the thickness is related to the amount of melt that has migrated into the peridotite (Spiegelman et al., 2001, J. Geophys. Res.; Pec et al., 2020, Geochem. Geophys. Geosyst.). If this is the case, the thickness will also correlate with the degree of partial melting. Note that melts with a low degree of partial melting would form a thin layer and melts with a high degree of partial melting would form a thick layer. The degree of partial melting can be expressed as Rb/Th ratio, if Rb has a higher partition coefficient into melts during partial melting than Th, the melts produced from low degrees of partial melting will have a higher value, and melts from higher degrees of partial melting will have a lower value. The value is higher in plagioclase lherzolites and is interpreted as a smaller amount of melt forming a thin layer containing plagioclase. On the other hand, the value is lower in mafic rock layers, suggesting that the melts formed the mafic rock layers with higher degrees of partial melting. This outcrop is thought to have been formed by a reaction between melt and rock, and further study is needed to determine the origin of the symmetrical structure.
We also examine the plagioclase lherzolite from the Horoman River, which is located at east of Mt. Apoi. It consists of a plagioclase-rich layer of about 5 mm thich and plagioclase-poor layer, in which plagioclase is interbedded with other minerals, with a coarse-grained pyroxene layer at the boundary. The mineral composition of the plagioclase-poor part is identical to that of the plagioclase lherzolite in the northern outcrop of Mt. Apoi. On the other hand, the samples collected from an outcrop more than 10 m away has a lower TiO2 content in the clinoperoxene and differs from the northern outcrop of Mt. Apoi. This suggests that there are several types of plagioclase lherzolites in the Horoman peridotite complex.