2:30 PM - 2:45 PM
[SVC34-04] Formation history and magma system of Mt.Issaikyo, Azuma volcano, Fukushima Prefecture
Keywords:Azuma volcano, Formation history, Magma system, Heterogeneous magmas
Reconstructing the formation history and magma system for extinct volcanoes provides us with essential information for evaluating and forecasting the long-term activity of currently active volcanoes. Mt. Issaikyo, located on the eastern part of Azuma volcano group, NE Japan, was active from 520-180 ka based on K-Ar dating by Matsumoto et al. (2018). After the activity of Issaikyo, Azuma-Jyododaira volcano was formed during the Holocene and is still active. In this study, we performed topographic analysis, field survey and petrological investigations to reconstruct the eruptive history and magmatic evolution of Mt. Issaikyo. In addition, we discussed and evaluated the current activity of this volcanic area by comparing these data of Issaikyo and Azuma-Jododaira volcano.
Mt. Issaikyo can be divided into two main edifices based on the topographic peaks: "Issaikyo South (ISS)" and "Issaikyo Middle (ISM)" (Fig.1).
ISS consists of 8 eruptive units: 7 lava flows (SL1 to SL7, bottom to top) and 1 pyroclastic unit (SP), indicating that the dominant eruption style was lava effusion at this stage. Lava flow 3 is likely the unit for which Matsumoto et al. (2018) obtained an age of 470 ka. Magma mingling structures can be commonly observed in ISS products.
ISM consists of 19 units: 11 lava flow units (MAL1, ML1, MAL2, ML2 to ML9; MAL means significantly altered units) and 8 pyroclastic units (MP1 to MP8). Lava flow unit 9 corresponds to the unit dated at 250 ka by Matsumoto et al. (2018). Few magma mingling structures can be recognized in the units above MP1. In addition, the number of lava flow units decreases towards the upper stratigraphy after MP1, suggesting that the activity of ISM transitioned from explosive (before and including MP1) to effusive (after MP1), and then back to explosive at the end of this stage.
We analyzed both the host rocks and the mafic enclaves for ISS samples. The phenocryst assemblage in the host rock consists of Pl+Cpx+Opx+Opq (tr)±Ol±Qtz, showing andesitic composition with SiO2 contents ranging 58.3 ~ 63.9 wt.%. While phenocryst assemblage of mafic enclaves is Pl+Cpx+Opx+Opq(tr) with the composition of basaltic andesite (SiO2 52.8 ~ 58.3 wt.%). In the Cr Harker diagrams, ISS samples show two coherent trends converging at the felsic side (Fig. 3). This suggests magma mixing processes involving one felsic magma and two mafic magmas (high and low Cr, respectively). Few samples plotting between the two linear trends suggest that there was no mixing between the two mafic magmas or between the two mixed magmas.
Although the phenocryst assemblage of ISM samples is the same as the host rocks of ISS, ISM shows andesitic composition with a narrow compositional range. These data suggest that heterogeneous magmas mixed during ISS activity were homogenized during ISM activity of ISM. The time scale for homogenization can be estimated to be less than 200 ky based on the ages of SL3 and ML9. It should be noted that some samples in ISM plot outside the compositional field of ISS, showing lowest Cr trend, which cannot be explained by the homogenization mentioned above. These lowest-Cr magmas could be generated by the fractionation of Ol and Cpx from the low-Cr magmas of ISS.
Compositional fields of all the ISS/IMS samples almost overlap with that of the youngest product of this volcanic area (Azuma-Jododaira volcano). It can be summarized that the three end-member magmas of the initial activity of Issaikyo experienced magma mingling and homogenization over a long period, and the processes including differentiation and reinjection of the mafic end-member magmas are still ongoing under this volcanic area. We need to reveal the genesis of the three end member magmas to fully explain the detailed magma processes and evaluate long-term volcanic activity and magmatism.
References:
Ban et al. (2013), Furukawa et al. (2018), Matsumoto et al. (2018), Yamamoto (2005)
Mt. Issaikyo can be divided into two main edifices based on the topographic peaks: "Issaikyo South (ISS)" and "Issaikyo Middle (ISM)" (Fig.1).
ISS consists of 8 eruptive units: 7 lava flows (SL1 to SL7, bottom to top) and 1 pyroclastic unit (SP), indicating that the dominant eruption style was lava effusion at this stage. Lava flow 3 is likely the unit for which Matsumoto et al. (2018) obtained an age of 470 ka. Magma mingling structures can be commonly observed in ISS products.
ISM consists of 19 units: 11 lava flow units (MAL1, ML1, MAL2, ML2 to ML9; MAL means significantly altered units) and 8 pyroclastic units (MP1 to MP8). Lava flow unit 9 corresponds to the unit dated at 250 ka by Matsumoto et al. (2018). Few magma mingling structures can be recognized in the units above MP1. In addition, the number of lava flow units decreases towards the upper stratigraphy after MP1, suggesting that the activity of ISM transitioned from explosive (before and including MP1) to effusive (after MP1), and then back to explosive at the end of this stage.
We analyzed both the host rocks and the mafic enclaves for ISS samples. The phenocryst assemblage in the host rock consists of Pl+Cpx+Opx+Opq (tr)±Ol±Qtz, showing andesitic composition with SiO2 contents ranging 58.3 ~ 63.9 wt.%. While phenocryst assemblage of mafic enclaves is Pl+Cpx+Opx+Opq(tr) with the composition of basaltic andesite (SiO2 52.8 ~ 58.3 wt.%). In the Cr Harker diagrams, ISS samples show two coherent trends converging at the felsic side (Fig. 3). This suggests magma mixing processes involving one felsic magma and two mafic magmas (high and low Cr, respectively). Few samples plotting between the two linear trends suggest that there was no mixing between the two mafic magmas or between the two mixed magmas.
Although the phenocryst assemblage of ISM samples is the same as the host rocks of ISS, ISM shows andesitic composition with a narrow compositional range. These data suggest that heterogeneous magmas mixed during ISS activity were homogenized during ISM activity of ISM. The time scale for homogenization can be estimated to be less than 200 ky based on the ages of SL3 and ML9. It should be noted that some samples in ISM plot outside the compositional field of ISS, showing lowest Cr trend, which cannot be explained by the homogenization mentioned above. These lowest-Cr magmas could be generated by the fractionation of Ol and Cpx from the low-Cr magmas of ISS.
Compositional fields of all the ISS/IMS samples almost overlap with that of the youngest product of this volcanic area (Azuma-Jododaira volcano). It can be summarized that the three end-member magmas of the initial activity of Issaikyo experienced magma mingling and homogenization over a long period, and the processes including differentiation and reinjection of the mafic end-member magmas are still ongoing under this volcanic area. We need to reveal the genesis of the three end member magmas to fully explain the detailed magma processes and evaluate long-term volcanic activity and magmatism.
References:
Ban et al. (2013), Furukawa et al. (2018), Matsumoto et al. (2018), Yamamoto (2005)