中国河北省張家口地区万全県漢淖坎玄武岩の地球化学的研究

Geological, petrographical and geochemical characteristics of the Baidrag granite pluton from the Tsagaan Tsahir Uul area can be summarized as following; The granitic complex is composed of two types of granites, distinguished by its biotite content. The modal abundance of biotite varies gradually between the two rock types. Major and trace element geochemical characteristics of the granitic rocks are similar, but they differ by SiO_2 content. FeO, CaO, Sr, and Ba contents of the biotite granite are slightly higher than those of the two mica granite. Both granites are medium to high potassium calc alkaline, and have peraluminous chemical features. Studied mineralogical composition and major and trace element geochemistry show that the granitic rock of Tsagaan Tsahir Uul area has volcanic arc granite (VAG) characteristics.論文Articl

Characteristics of ash particles from the maar complex of Lamongan Volcanic Field (LVF), East Java, Indonesia: How textural features and magma composition control ash morphology

The Lamongan Volcanic Field (LVF), East Java, Indonesia, has experienced numerous maar eruptions, producing varied properties and morphologies of ash particles. This study conducted textural, morphometric, and geochemical analyses of the juvenile particles to elucidate the factors governing their heterogeneous characteristics. Two distinct types of juvenile ash were identified: A (black and brown ash) and B (orange-brown ash), reflecting different fragmentation processes. The blocky to slightly elongate shapes of juvenile A across heterogenous basaltic compositions (resulting in variable textures, rheological properties, and/or cooling histories) highlight the phreatomagmatic process as the primary control of their shape. In contrast, the irregular-fluidal shapes of juvenile B particles indicate magmatic fragmentation of basaltic andesite magma. This study reveals that variable magma properties yield diverse ash components, yet fragmentation dynamics govern pyroclast shapes in the LVF maar complex. Our integrated approach emphasizes the importance of considering multiple variables when interpreting heterogeneous volcanic ash deposits

Temporal variations of petrological characteristics of Tangkil and Rajabasa volcanic rocks, Indonesia

Tangkil and Rajabasa Volcanoes are neighbouring subduction-zone volcanoes located on the southeast tip of Sumatra Island (Sunda Strait, Indonesia). Stratigraphic correlation of lavas in Tangkil-Rajabasa volcanic area was established from field observations, morphological analysis, and K-Ar dating analysis. Detailed petrography and geochemical data of two and eleven lava units from Tangkil and Rajabasa, respectively, were then integrated with the stratigraphy to show the temporal variations. Early stage (> 4.3 Ma) effusives of Tangkil Volcano are dacitic to rhyolitic (67-71 wt. % SiO2; Tklf), whereas the later (c. 4.3 Ma) rocks are basalt to basaltic andesite (c. 52 wt. % SiO2; Tklm). Tangkil shows bimodal magmatism, of which the felsic endmember is > 71 wt. % SiO2 and < 0.1 wt. % MgO. Lavas of Rajabasa Volcano are comparatively younger (c. 0.3 to 0.1 Ma) with compositions ranging from basalt to andesite (51-62 wt. % SiO2; Rbs). Chemical variations of Rajabasa accounts for the interactions of at least three endmembers: Mg-rich medium-K basalt magma, low-Mg medium-K basalt magma, and high-K andesitic magma. During the long evolution of Rbs magma system, the temporal chemistry shows rising-falling variation in SiO2 and MgO indicating the three magmas were active. The felsic endmember magma of Rajabasa is fixed in composition (at ~62 wt. % SiO2; ~2.2 wt. % MgO). The rocks from the last Tklf and Rbs indicate open system processes by containing plagioclase and pyroxene phenocrysts that show resorption of evolved core and overgrowth of less evolved mantle. The multiple zones of dissolution-overgrowth in plagioclase crystals and the fluctuating trend in temporal whole-rock variation suggest that the changes of magmatic condition in temperature, H2O, or chemical composition were repetitive

Hydrothermal Aluminum-Phosphate-Sulfates in Ash from the 2014 Hydrothermal Eruption at Ontake Volcano, Central Honshu, Japan

Aluminum-phosphate-sulfates (APS) of the alunite supergroup occur in igneous rocks within zones of advanced argillic and silicic alteration in porphyry and epithermal ore environments. In this study we report on the presence of woodhouseite-rich APS in ash from the 27 September 2014 hydrothermal eruption of Ontake volcano. Scanning electron microscope coupled with energy dispersive X-ray spectrometer (SEM-EDS) and field emission (FE)-SEM-EDS observations show two types of occurrence of woodhouseite: (a) as cores within chemically zoned alunite-APS crystals (Zoned-alunite-woodhouseite-APS), and (b) as a coherent single-phase mineral in micro-veinlets intergrown with similar micro-veinlets of silica minerals (Micro-wormy-vein woodhouseite-APS). The genetic environment of APS minerals at Ontake volcano is that of a highly acidic hydrothermal system existing beneath the volcano summit, formed by condensation in magmatic steam and/or ground waters of sulfur-rich magmatic volatiles exsolved from the magma chamber beneath Mt. Ontake. Under these conditions, an advanced argillic alteration assemblage forms, which is composed of silica, pyrophyllite, alunite and kaolinite/dickite, plus APS, among other minerals. The discovery of woodhouseite in the volcanic ash of the Ontake 2014 hydrothermal eruption represents the first reported presence of APS within an active volcano. Other volcanoes in Japan and elsewhere with similar phreatic eruptions ejecting altered ash fragments will likely contain APS minerals derived from magmatic-hydrothermal systems within the subvolcanic environment. The presence of APS minerals within the advanced argillic zone below the summit vent of Ontake volcano, together with the prior documentation of phyllic and potassically altered ash fragments, provides evidence for the existence within an active volcano in Japan of an alteration column comparable to that of porphyry copper systems globally

Geologic Background of the Huge Landslide at Aratozawa, Kurihara, Miyagi Prefecture

Geology around the Aratozawa huge landslide triggered by the Iwate-Miyagi Inland Earthquake was surveyed several days after the landslide. Lacustrine sedimentary rocks are overlain by a thick pumice tuff which exposes at the lower scarp. The thick pumice tuff is overlain by a welded tuff at the scarp. A review on previous studies on the area revealed inconsistency among interpretations for their sedimentation ages, ranging from Miocene to Quaternary. Similar past huge landslides, which are identified from topographies, around nearby volcanoes are attributed to the similarity of geology and geomorphology

韓国,鬱陵島火山岩の主成分及び微量元素組成

Geochemical investigation has been carried out for the volcanic rocks from Ulreung island, Korea. The volcanic rock suite is characterized by highly alkalic nature, and some of them contain more than 6wt.% K_2O and Na_2O. The igneous activity may not be related to Pacific plate subduction, or the Japan Sea spreading. It will be hardly possible to explain how to concentrate the abundant incompatible elements in the mantle source in a limited short period beneath Ulreung island. Most of the chemical features of the volcanic rocks are consistent with simple fractional crystallization. It should be pointed out, however, that significant compositional gap was recognized only in K_2O content, but not in the other LILEs. The difference between K_2O and the other LILEs will be hardly explained if fractional crystallization differentiation plays a major role. The volcanic rocks from the island are either highly enriched in K_2O or not so enriched in K_2O. No intermediately K_2O-enriched sample was found. This is contrasted with gradual increase in Na_2O content. This chemical features will require complicated mechanisms.論文Articl