10:45 AM - 12:15 PM
[SMP26-P09] Mantle refertilization recorded in mineral compositions of an outcrop showing a symmetric sequence composed of peridotite and mafic-rock layers of Horoman peridotite, Japan
Keywords:refertilization, peridotite, mafic-rock, symmetry
The Horoman peridotite is located at the southern end of the Hidaka metamorphic belt in Hokkaido, Japan. It has been well studied because it is free of serpentinization and its lithological diversity provides information of the upper mantle processes. One of the characteristics of the Horoman peridotite is two-types of symmetric structures. One symmetric structures is observed on the scale of a few hundred meters and shows a gradual change in the degrees of melt depletion in the wall rock depending on the distance from a melt penetration (Takahashi, 1991, Nature) or reflecting the local thermal structure, hotter harzburgite and cooler plagioclase lherzolite. The other symmetric structure is observed on the scale of a few meters in outcrops and can be formed through repeatedly stretching and folding of two rock layers (Toramaru, 1997, Memoirs of Geological Society of Japan). In this study, mineral compositions are analyzed in rock specimens collected from an outcrop composed of peridotite and mafic-rock layers exposed on the north ridge of Mt. Apoi, where the small-scale symmetric sequence was documented around a peridotite layer (Toramaru, 1997, Memoirs of Geological Society of Japan).
Peridotite and mafic-rocks differ in grain size and mineral assemblages. Peridotite is relatively coarse-grained (~250 µm) and composed mainly of olivine and pyroxene, ranging from lherzolite to harzburgite. Mafic-rocks are finer-grained (~150 µm) and characterized by the presence of plagioclase and hornblende. The mafic-rocks are Type I (Shiotani and Niiida, 1997, Memoirs of Geological Society of Japan; Takazawa et al., 1999, Journal of Petrology), as described below, based on mineral assemblages and their chemical compositions.
The forsterite (Fo) content of olivine and Mg/(Mg+Fe)×100 (Mg#) of pyroxene in the mafic-rock layers has a range from 84 to 88. Both TiO2 and Al2O3 contents of the clinopyroxene rim in the mafic-rock layers are usually higher than peridotite, and the hornblende is Ti-rich pargasite or kaersutite. Based on these characteristics, the mafic layers are Type I mafic-rocks by Shiotani and Niida (1997, Memoirs of Geological Society of Japan) and Takazawa et al. (1999, Journal of Petrology). Among the analyzed specimens, a mafic-rock layer shows the same Fo content of olivine and Mg# of pyroxene as those of peridotite layers. The mafic-rock layer also shows low TiO2 and Al2O3 contents and has no hornblende. The composition of plagioclase is also different from those in the other mafic-rock layers. This rock has clinopyroxene only along boundaries with peridotite layer. This mafic-rock layer looks similar to plagioclase lherzolite.
The Fo content of olivine and Mg# of pyroxene in the peridotite layers are homogeneous within each layer within a range of ±1, but vary from layer to layer, showing a range from 88 to 92, which is higher than those of mafic-rock layers. Those with low values as 88~90 are limited to thin peridotite layer sandwiched between mafic-rock layers. These peridotites are also characterized by high TiO2 and Al2O3 contents of the clinopyroxene rims. These peridotites can be formed through hybridization with "mafic-rock-like materials ".
In addition to the peridotite and mafic-rock layers, finely alternating fine-grained layers have 91~92 Fo content of olivine and 91~92 Mg# of pyroxene. The TiO2 content is higher than that of typical plagioclase lherzolite, and the chemical composition of pyroxene shows have contents intermediate between those in peridotite and mafic-rock layers, suggesting hybridization of peridotite and mafic-rock layers.
Clinopyroxene of mafic-rock layers shows, prominent Ti and Al zoned structures; high Al in the core and high Ti in the rim. The Ti zoning may be formed through reaction with Ti-rich materials. The Al zoning may record phase change from garnet to plagioclase through spinel during its ascent.
The symmetric structure of the outcrop of the north ridge of Mt. Apoi can be formed through the stretching and folding of the strata as proposed by Toramaru (1997, Memoirs of Geological Society of Japan). The chemical refertilization of peridotite with mafic-rock-like materials may be related to the mechanical mixing of peridotite and mafic-rock observed in the outcrop. If this is the case, the alternation layers represent an imperfect hybridization between peridotite and mafic-rock layers.
Peridotite and mafic-rocks differ in grain size and mineral assemblages. Peridotite is relatively coarse-grained (~250 µm) and composed mainly of olivine and pyroxene, ranging from lherzolite to harzburgite. Mafic-rocks are finer-grained (~150 µm) and characterized by the presence of plagioclase and hornblende. The mafic-rocks are Type I (Shiotani and Niiida, 1997, Memoirs of Geological Society of Japan; Takazawa et al., 1999, Journal of Petrology), as described below, based on mineral assemblages and their chemical compositions.
The forsterite (Fo) content of olivine and Mg/(Mg+Fe)×100 (Mg#) of pyroxene in the mafic-rock layers has a range from 84 to 88. Both TiO2 and Al2O3 contents of the clinopyroxene rim in the mafic-rock layers are usually higher than peridotite, and the hornblende is Ti-rich pargasite or kaersutite. Based on these characteristics, the mafic layers are Type I mafic-rocks by Shiotani and Niida (1997, Memoirs of Geological Society of Japan) and Takazawa et al. (1999, Journal of Petrology). Among the analyzed specimens, a mafic-rock layer shows the same Fo content of olivine and Mg# of pyroxene as those of peridotite layers. The mafic-rock layer also shows low TiO2 and Al2O3 contents and has no hornblende. The composition of plagioclase is also different from those in the other mafic-rock layers. This rock has clinopyroxene only along boundaries with peridotite layer. This mafic-rock layer looks similar to plagioclase lherzolite.
The Fo content of olivine and Mg# of pyroxene in the peridotite layers are homogeneous within each layer within a range of ±1, but vary from layer to layer, showing a range from 88 to 92, which is higher than those of mafic-rock layers. Those with low values as 88~90 are limited to thin peridotite layer sandwiched between mafic-rock layers. These peridotites are also characterized by high TiO2 and Al2O3 contents of the clinopyroxene rims. These peridotites can be formed through hybridization with "mafic-rock-like materials ".
In addition to the peridotite and mafic-rock layers, finely alternating fine-grained layers have 91~92 Fo content of olivine and 91~92 Mg# of pyroxene. The TiO2 content is higher than that of typical plagioclase lherzolite, and the chemical composition of pyroxene shows have contents intermediate between those in peridotite and mafic-rock layers, suggesting hybridization of peridotite and mafic-rock layers.
Clinopyroxene of mafic-rock layers shows, prominent Ti and Al zoned structures; high Al in the core and high Ti in the rim. The Ti zoning may be formed through reaction with Ti-rich materials. The Al zoning may record phase change from garnet to plagioclase through spinel during its ascent.
The symmetric structure of the outcrop of the north ridge of Mt. Apoi can be formed through the stretching and folding of the strata as proposed by Toramaru (1997, Memoirs of Geological Society of Japan). The chemical refertilization of peridotite with mafic-rock-like materials may be related to the mechanical mixing of peridotite and mafic-rock observed in the outcrop. If this is the case, the alternation layers represent an imperfect hybridization between peridotite and mafic-rock layers.