10:15 AM - 10:30 AM
[SMP36-12] Generation of mixed calc-alkaline andesite: A case study at Rishiri volcano, Northern Hokkaido
Keywords:Calc-alkaline andesite, Magma mixing, Rishiri volcano
Calc-alkaline andesite is a typical rock series in many island-arcs (eg. Miyashiro, 1974) and the composition is similar to the average composition of the continental crust (e.g. Rudnick and Gao, 2003). The petrographical features and the isotope ratios suggest that calc-alkaline andesite magmas are produced by external magma mixing between mantle-derived mafic magma and crust-derived felsic magma (Tatsumi, 2003). However, calc-alkaline andesite commonly coexists with basalt and/or tholeiitic-series andesite in individual volcanoes. In general, basalt and tholeiitic andesite do not show evidence of magma mixing and they evolved by crystal differentiation or AFC process. Why do calc-alkaline andesite magmas coexist with non-mixed basalt and tholeiitic-series andesite magmas in island-arcs? What is the dominant factor to produce calc-alkaline magmas? In order to clarify this problem, we studied calc-alkaline andesite from Rishiri Volcano, northern Hokkaido. We estimated the features of mafic end-member magma for the andesite, and compared them with those of basalts and tholeiitic andesites from the volcano.
Rishiri volcano has produced alkali basalts and tholeiitic-series andesites, as well as calc-alkaline andesite (Ishizuka and Nakagawa, 1999). The calc-alkaline andesite can be divided into A-type (olivine-bearing two pyroxene andesite) and D-type (two pyroxene dacite). Whole-rock SiO2 content of the A-type and D-type ranges from 58.2 wt.% to 62.5 wt.% and 63.9 wt.% to 65.3 wt.%, respectively. Olivine phenocrysts are anhedral and they have reaction rim of orthopyroxene. The Mg-number of the olivine cores show wide range (71-89). Pyroxenes phenocrysts in A-type are reversely or normally zoned with wide core compositions. In contrast, pyroxenes phenocrysts in D-type only show normal zonation with narrow core composition. An-content of the plagioclase cores in A-type (45-87) is wider than in D-type (49-59). With increasing the SiO2 contents, the 87Sr/86Sr and 206Pb/204Pb ratios tend to increase. The 143Nd/144Nd ratios of A-type is higher than those of D-type.
The petrological and geochemical features suggest that the A-type magma was produced by magma mixing between mantle-derived mafic magma and crust-derived felsic magma. The basaltic end-member for the A-type magma was estimated to be primitive alkali basalt with H2O=~5.4 wt.%. The chemical composition is not so different from those for Kutsugata-Tanetomi lava (Kuritani, 1999a; 1999b; Kuritani et al., 2005) and Nozuka lava (Kuritani and Nakagawa, 2016). On the other hand, H2O concentration is significantly higher than those of Kutsugata lava (~4 wt.%) and Nozuka lava (2.1-2.2 wt.%).
The model calculation suggests that water saturation of the high-H2O (5.4 wt.%) primitive melt would have been attained when it crystallized ~10-20 wt.%, whereas water saturation of the low-H2O (2.1 wt.%) primitive melt was attained when it crystallized up to ~60-70 wt.%. This observation indicates that the hydrous primitive end-member magma for the calc-alkaline andesite released much more vapors compared with the low-H2O alkali basalt and tholeiitic andesite magmas when they crystallized due to cooling. The vapor phases would have been transported to the surrounding crust, and caused its partial melting because of the depression of the solidus temperatures. Then, the calc-alkaline andesite magmas were produced by mixing of the primitive basalt magmas with the crustal melt. For these considerations, we suggest that H2O concentration of primary magma can control if calc-alkaline andesite magma is produced or not.