11:15 AM - 11:30 AM
[SCG48-03] Magmatic evolution and magma genesis of the Iwafune diorite, Ibaraki Prefecture
Keywords:whole-rock geochemical data, diorite, Iwafune area, Tsukuba Mountains, Magmatic evolution
The Iwafune diorite (Shirosato Town, Ibaraki Prefecture; zircon U–Pb ages of 64.5 ± 1.1 Ma and 64.4 ± 1.0 Ma; Yamazaki et al., 2021) intrudes sedimentary rocks of the Yamizo Group. Because it is located between Early Cretaceous diorites and gabbros in the Yamizo Mountains (109–102 Ma; Ejima et al., 2018, 2019) and Late Cretaceous–Paleogene granites and gabbros in the Tsukuba Mountains (66–61 Ma; Koike and Tsutsumi, 2017, 2018; Wang et al., 2021), the magmagenesis and evolution of the Iwafune diorite are important to understanding the transition between these periods and location of regional magmatism. However, whole–rock geochemical data have never been reported for the Iwafune diorite.
Here, we analyzed the whole–rock geochemical data of the Iwafune diorite by X–ray fluorescence and laser ablation inductively coupled plasma mass spectrometry, as well as those of diorites and gabbros from the Tsukuba Mountains. Based on the obtained major element compositions, we modeled the evolution of the Iwafune diorite using MELTS for Excel (Gualda and Ghiorso, 2015). We assumed the composition of the initial melt to be that of the most mafic diorite. Our model results indicate that the composition of the most felsic diorite can be reproduced by the fractionation of 18.2% plagioclase and 22.8% clinopyroxene from the initial melt. We also used a Rayleigh fractionation model to investigate the evolution of the trace element compositions based on the proportions of fractionated minerals and available mineral–melt partition coefficients (Rollinson and Pease, 2021). Except for heavy rare earth elements (REEs), the calculated melt compositions are again similar to the most felsic diorite. These results indicate that the range of Iwafune diorite compositions was produced by the fractional crystallization of plagioclase and clinopyroxene from the initial melt.
The major (MgO, CaO, Na2O, K2O, and P2O5) and trace element trends (Sr, Rb, Ba, Nb, Ta, Zr, Hf, Y, U, Th, [Eu]N/[Eu]N*, and [La]N/[Yb]N, where the subscript N denotes values normalized to chondritic values; Boynton, 1984) of the Iwafune diorite are similar to those of Tsukuba diorites and gabbros (Wang et al., 2021). However, the Sr, Nb, Y, and [Eu]N/[Eu]N* trends of the Iwafune diorite differ from those of Yamizo diorites and gabbros in the Yamizo Mountains (Takahashi et al., 2005; Koizumi et al., 2006; Ejima et al., 2019). The ΣREE values and slopes of the REE patterns of the Iwafune diorite (ΣREE = 88.4–136 ppm, [La]N/[Yb]N = 5.10–5.94, respectively) are similar to those of Tsukuba diorites and gabbros (ΣREE = 120–137 ppm, [La]N/[Yb]N = 5.94–6.10). Whole–rock geochemical data and zircon U–Pb ages suggest that the Iwafune diorite is genetically related to the diorites and gabbros of the Tsukuba Mountains. The results suggest that the source materials of the plutonic rocks from the Iwafune pluton are the same as the plutonic rocks of the Tsukuba Mountains (Wang et al., 2021). Therefore, the Iwafune diorite was derived from partial melting of the mantle peridotite.
In summary, the initial melt of the Iwafune diorite derived from the partial melting of mantle peridotite and it intruded the sediment rocks of the Yamizo Groupe in 65.6–63.4 Ma. The diorite was formed by fractional crystallization of plagioclase and clinopyroxene from the initial melt.
Here, we analyzed the whole–rock geochemical data of the Iwafune diorite by X–ray fluorescence and laser ablation inductively coupled plasma mass spectrometry, as well as those of diorites and gabbros from the Tsukuba Mountains. Based on the obtained major element compositions, we modeled the evolution of the Iwafune diorite using MELTS for Excel (Gualda and Ghiorso, 2015). We assumed the composition of the initial melt to be that of the most mafic diorite. Our model results indicate that the composition of the most felsic diorite can be reproduced by the fractionation of 18.2% plagioclase and 22.8% clinopyroxene from the initial melt. We also used a Rayleigh fractionation model to investigate the evolution of the trace element compositions based on the proportions of fractionated minerals and available mineral–melt partition coefficients (Rollinson and Pease, 2021). Except for heavy rare earth elements (REEs), the calculated melt compositions are again similar to the most felsic diorite. These results indicate that the range of Iwafune diorite compositions was produced by the fractional crystallization of plagioclase and clinopyroxene from the initial melt.
The major (MgO, CaO, Na2O, K2O, and P2O5) and trace element trends (Sr, Rb, Ba, Nb, Ta, Zr, Hf, Y, U, Th, [Eu]N/[Eu]N*, and [La]N/[Yb]N, where the subscript N denotes values normalized to chondritic values; Boynton, 1984) of the Iwafune diorite are similar to those of Tsukuba diorites and gabbros (Wang et al., 2021). However, the Sr, Nb, Y, and [Eu]N/[Eu]N* trends of the Iwafune diorite differ from those of Yamizo diorites and gabbros in the Yamizo Mountains (Takahashi et al., 2005; Koizumi et al., 2006; Ejima et al., 2019). The ΣREE values and slopes of the REE patterns of the Iwafune diorite (ΣREE = 88.4–136 ppm, [La]N/[Yb]N = 5.10–5.94, respectively) are similar to those of Tsukuba diorites and gabbros (ΣREE = 120–137 ppm, [La]N/[Yb]N = 5.94–6.10). Whole–rock geochemical data and zircon U–Pb ages suggest that the Iwafune diorite is genetically related to the diorites and gabbros of the Tsukuba Mountains. The results suggest that the source materials of the plutonic rocks from the Iwafune pluton are the same as the plutonic rocks of the Tsukuba Mountains (Wang et al., 2021). Therefore, the Iwafune diorite was derived from partial melting of the mantle peridotite.
In summary, the initial melt of the Iwafune diorite derived from the partial melting of mantle peridotite and it intruded the sediment rocks of the Yamizo Groupe in 65.6–63.4 Ma. The diorite was formed by fractional crystallization of plagioclase and clinopyroxene from the initial melt.