Geologic and petrologic evolution of subvolcanic hydrothermal system: A case on pyroclastic deposits since the 1331 CE eruption at Azuma-Jododaira volcano, central Fukushima, North-Eastern Japan

Abstract

Abstract This study investigated the evolution of the magma and hydrothermal system of the Azuma-Jododaira volcano, focusing on the geology and ash petrology of the tephra layers. Eight tephra layers were identified in the Jododaira-Iwodaira area (bottom to top): L1–1, L1–2, L2, L3, L4, L5, L6, and L7. Layers L1–1 to L1–2 and L3 to L4 are correlated with the 1331 CE Az-OA and 1771 CE JP7 units, respectively, determined by Yamamoto (2005) . Ash component analysis was conducted utilizing petrographic microscopy, X-ray diffraction (XRD), and scanning electron microscope with energy-dispersive spectroscopy (SEM-EDS). The ash grains were classified into four categories: (1) fluidal pyroclasts (andesitic scoria), (2) crystalline pyroclasts (various crystalline-andesitic rocks), (3) hydrothermally altered lithic fragments, and (4) fragmented crystals (plagioclase, orthopyroxene, and augite). Hydrothermal alterations in category 3 were classified into seven types based on the alteration mineralogy: Silica (silica+titanium oxide±pyrite), Pyrophyllite (silica+pyrophyllite±alunite), Kaolin (silica+7 A-kaolin mineral±alunite), Alunite (silica+alunite), Mica-Chlorite (silica+illite+sericite [coarse-grained illite] ± chlorite±biotite), Chlorite (silica+chlorite±epidote), and Mica-K-feldspar (silica+chlorite+biotite+K-feldspar). Layers L1–1 and L1–2 contain hydrothermally altered fragments that underwent all alteration types (Silica, Kaolin, Alunite, Pyrophyllite, Chlorite, Mica-Chlorite, and Mica-K-feldspar). Layers L2–L6 contain crystalline and fluidal pyroclasts with hydrothermally altered (Silica, Kaolin, Alunite, and Pyrophyllite) lithic fragments. Layer L7 contains abundant hydrothermally altered lithic fragments with similar alteration types to those of layers L2–L6. The four-stage evolution of the subvolcanic hydrothermal system was inferred from the observed stratigraphic variations. Stage 1: A zoned alteration column was present before the 1331 CE eruption, which displayed various alteration types (Silica, Kaolin, Alunite, Pyrophyllite, Chlorite, Mica-Chlorite, and Mica-K-feldspar). Stage 2: 1331 CE magma intruded, modifying the zoned alteration column, and produced monotone-acidic alteration types (silica, kaolin, alunite, and pyrophyllite) and incorporated crystalline pyroclasts in layers L2–L6. Stage 3: Successive juvenile magma inputs to the shallower portion maintained the strongly acidic hydrothermal system. Stage 4 (L7 eruption to present): The subvolcanic hydrothermal system remained acidic due to additional crystalline magma input.