11:00 AM - 11:15 AM
[R6-04] Magma genesis of the Iwafune diorites, Ibaraki Prefecture
[Presentation award entry]
Keywords:Diorite, Magma genesis, Iwafune pluton, Tsukuba Mountains
It is important to reveal the magma genesis of plutonic rocks in Southwest Japan during Late Cretaceous to Paleogene to understand crustal growth in Japan because large amounts of plutonic rocks were formed in this age. However, the magma genesis of diorite in eastern Southwest Japan has not been reported. Because of its large surface exposure area, the Iwafune diorite represents diorite in this region. Therefore, we reveal the primitive melt genesis of the Iwafune diorite to understand magma genesis in this region.
The Iwafune pluton consists of diorite, orthopyroxene (opx) diorite, and granite. The opx diorites are lower SiO2 contents and higher enstatite contents of pyroxene than the diorite. The compositional variation of the Iwafune diorite can be explained by the fractional crystallization of clinopyroxene and plagioclase from the opx diorite (Yamazaki et al., 2023). Therefore, the opx diorite composition is the closest to the primitive melt composition of the Iwafune diorite.
In order to consider the genesis of the primitive melt of the Iwafune diorite, the Sr isotopic ratios of the upper mantle were estimated to be SrI of the gabbro in the Tsukuba Mountains (Arakawa and Takahashi, 1989; Shibata and Ishihara, 1979). The Sr isotopic ratios of the lower crust were estimated to be the SrI of the granites in this area (Arakawa and Takahashi, 1989; Shibata and Ishihara, 1979). The SrI of the Iwafune diorite (Shibata and Takagi, 1989) is different from the Sr isotopic ratios of the upper mantle, but it is equal to the Sr isotopic ratios of the lower crust. These results suggest that the primitive melt of the Iwafune diorite was formed by partial melting of the lower crust.
We examined whether partial melting of the lower crust would form the primitive melt composition of the Iwafune diorite. Because the HREE contents of the opx diorites are not depleted, and the temperature of the lower crust is 500-700℃ (Coinde, 2005), we estimate that the lower crust was not saturated with water at about 0.8 GPa when the Iwafune diorite formed. The composition of the melt, which is formed by the partial melting of basalt under these physical conditions (Rapp and Watson, 1995), is nearly equal to the composition of the opx diorite.
Therefore, we conclude that the primitive melt of the Iwafune diorite was most likely formed by partial melting of the lower crust.
The Iwafune pluton consists of diorite, orthopyroxene (opx) diorite, and granite. The opx diorites are lower SiO2 contents and higher enstatite contents of pyroxene than the diorite. The compositional variation of the Iwafune diorite can be explained by the fractional crystallization of clinopyroxene and plagioclase from the opx diorite (Yamazaki et al., 2023). Therefore, the opx diorite composition is the closest to the primitive melt composition of the Iwafune diorite.
In order to consider the genesis of the primitive melt of the Iwafune diorite, the Sr isotopic ratios of the upper mantle were estimated to be SrI of the gabbro in the Tsukuba Mountains (Arakawa and Takahashi, 1989; Shibata and Ishihara, 1979). The Sr isotopic ratios of the lower crust were estimated to be the SrI of the granites in this area (Arakawa and Takahashi, 1989; Shibata and Ishihara, 1979). The SrI of the Iwafune diorite (Shibata and Takagi, 1989) is different from the Sr isotopic ratios of the upper mantle, but it is equal to the Sr isotopic ratios of the lower crust. These results suggest that the primitive melt of the Iwafune diorite was formed by partial melting of the lower crust.
We examined whether partial melting of the lower crust would form the primitive melt composition of the Iwafune diorite. Because the HREE contents of the opx diorites are not depleted, and the temperature of the lower crust is 500-700℃ (Coinde, 2005), we estimate that the lower crust was not saturated with water at about 0.8 GPa when the Iwafune diorite formed. The composition of the melt, which is formed by the partial melting of basalt under these physical conditions (Rapp and Watson, 1995), is nearly equal to the composition of the opx diorite.
Therefore, we conclude that the primitive melt of the Iwafune diorite was most likely formed by partial melting of the lower crust.