[S-VC29] 火山噴火のダイナミクスと素過程

[SVC29-P11] The magmatic system beneath maar complex revealed from the geochemistry of ash particles at Lamongan Volcanic Field (LVF), East Java, Indonesia<quillbot-extension-portal></quillbot-extension-portal>

*Andriansyah Gurusinga¹、Tsukasa Ohba¹、Takashi Hoshide¹ (1.Akita University)

キーワード：Lamongan Volcanic Field (LVF), maar, geochemistry, volcanic ash

The Lamongan volcanic field comprises twenty nine maars and sixty one cinder cones located at the surrounding flanks of basaltic Lamongan stratovolcano, East Java, Indonesia. These monogenetic volcanic complex were estimated formed between 13 and 40 ka in the graben section between massifs Tengger-Semeru and Iyang Argapura. Their formation is the result of basaltic magma rising to the surface involved fissure-style eruption in the local extension of strong regional tectonic setting. The petrogenesis study of monogenetic and stratovolcanoes are commonly studied using lava fragments and lapilli size fraction of eruption products. It is leaving a difficult challenges in understand the assigned magmatic processes to the eruptions that produced fine-dominant size fraction. In that case, ash particles of LVF maar eruption products reveal fascinating details about the development of the magmatic system beneath the complex. The brown ash (North and West maars) and grayish-black ash (East maar) have similarity of least evolved basaltic magmas with average composition of 46.43 wt.% SiO₂, 0.57 wt.% K₂O (low-An# plagioclase [48-69]; low-Mg# pyroxene [51-71]; and low-Fo# olivine [42-63]; for brown ash). The black ash (North and West maars) are more evolved basalt to basaltic-andesitic magma (avg. 50.52 wt.% SiO₂, 1.27 wt.% K₂O, low-An# plagioclase [45-65], low-Mg# pyroxene [55-69], and mod-Fo# olivine [61-68]), whereas the orange ash are a batch of magma with a slightly higher silica content (dominant in basaltic-andesitic with 52.61 wt.% SiO₂, 1.5 wt.% K₂O, and low-An# plagioclase [50-63]). However, the most primitive features were dicovered in the North scoria cone deposits (avg. SiO₂ 42.92%, 0.58 wt.% K₂O, high-An# plagioclase [78-93], high-Mg# pyroxene [65-78], and high-Fo# olivine [71-78]; picrobasaltic) and East lava (Ranu Agung; avg. SiO₂ 45.43%, 0.32 wt.% K₂O, high-An# plagioclase [79-95], high-Mg# pyroxene [68-82], and mod-Fo# olivine [62-68]; basaltic). They could be comparable to the initial magma composition that feeds the maar complex of LVF. Therefore, the ash particles implies that at least five batches of magma have contributed to the maar eruptive activities. Our findings show that the heterogeneities of ash particle types consistently reflect variations in geochemical composition and mineral chemistry, which may be related to the diversity and complexity of magmatic system beneath the Maar complex.<quillbot-extension-portal></quillbot-extension-portal>