Masao Ban

Evidence for a short-lived stratified magma chamber: petrology of the Z-To5 tephra layer (c. 5.8 ka) at Zao volcano, NE Japan

Abstract Volcanic rocks from the Z-To5 tephra layer of Zao volcano, NE Japan, preserve petrological information that reflects the magmatic processes under the volcano. The Z-To5 rocks were formed by the mixing of three magmas that differed in composition and phenocryst assemblage: basalt (1150–1200 °C), with high Mg (Foc. 81) olivine; basaltic andesite (1020–1100 °C), with Mg-rich orthopyroxene (Mg#=c. 78) and clinopyroxene (Mg#=c. 78), lower Mg olivine (Mg#=c. 78), and calcic (Anc. 85) plagioclase; and andesite (900–1000 °C) with Mg-poor orthopyroxene (Mg#=61–66) and clinopyroxene (Mg#=64–68), and An-poor plagioclase. The basaltic magma was formed through fractionation of Foc. 85 olivine from a less differentiated basaltic magma during its fast ascent from the depths. The andesitic magma, which occupied a shallow magma chamber, was heated by underplating of the basaltic magma, resulting in dissolution of some minerals. Subsequently, the basaltic andesite magma was formed by mixing of the basaltic and andesitic magmas in the chamber. Petrological evidence for the rapid growth of phenocrysts in the basaltic andesite magma suggests that the magma residence time was short. The basaltic andesite magma, mixing with a small portion of the andesitic magma, was withdrawn upon eruption. The rates of these processes are inferred to be rapid based on petrological considerations.

Mineralogy and geochemistry of granitoids from Kinnaur region, Himachal Higher Himalaya, India: Implication on the nature of felsic magmatism in the collision tectonics

Felsic magmatism in the southern part of Himachal Higher Himalaya is constituted by Neoproterozoic granite gneiss (GGn), Early Palaeozoic granitoids (EPG) and Tertiary tourmaline-bearing leucogranite (TLg). Magnetic susceptibility values (<3 ×10−3 SI), molar Al2O3/(CaO + Na2O + K2O) (≥1.1), mineral assemblage (bt–ms–pl–kf–qtz ± tur ± ap), and the presence of normative corundum relate these granitoids to peraluminous S-type, ilmenite series (reduced type) granites formed in a syncollisional tectonic setting. Plagioclase from GGn (An10–An31) and EPG (An15–An33) represents oligoclase to andesine and TLg (An2–An15) represents albite to oligoclase, whereas compositional ranges of K-feldspar are more-or-less similar (Or88 to Or95 in GGn, Or86 to Or97 in EPG and Or87 to Or94 in TLg). Biotites in GGn (Mg/Mg + Fet= 0.34–0.45), EPG (Mg/Mg + Fet= 0.27–0.47), and TLg (Mg/Mg + Fet= 0.25–0.30) are ferribiotites enriched in siderophyllite, which stabilised between FMQ and HM buffers and are characterised by dominant 3Fe⇌

Petrologic Study of Explosive Pyroclastic Eruption Stage in Shirataka Volcano, NE Japan: Synchronized Eruption of Multiple Magma Chambers

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Quaternary Volcanism Along the Volcanic Front in Northeast Japan

2Al, 3Mg⇌

New Insights into Andesite Genesis: the Role of Mantle-derived Calc-alkalic and Crust-derived Tholeiitic Melts in Magma Differentiation beneath Zao Volcano, NE Japan

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K – Ar ages of lavas from shield volcanoes in the Michoacan-Guanajuato volcanic field, Mexico

2Al substitutions typical of peraluminous (S-type), reducing felsic melts. Muscovite in GGn (Mg/Mg + Fet=0.58–0.66), EPG (Mg/Mg + Fet=0.31−0.59), and TLg (Mg/Mg + Fet=0.29–0.42) represent celadonite and paragonite solid solutions, and the tourmaline from EPG and TLg belongs to the schorl-elbaite series, which are characteristics of peraluminous, Li-poor, biotite-tourmaline granites. Geochemical features reveal that the GGn and EPG precursor melts were most likely derived from melting of biotite-rich metapelite and metagraywacke sources, whereas TLg melt appears to have formed from biotite-muscovite rich metapelite and metagraywacke sources. Major and trace elements modelling suggest that the GGn, EPG and TLg parental melts have experienced low degrees (∼13, ∼17 and ∼13%, respectively) of kf–pl–bt fractionation, respectively, subsequent to partial melting. The GGn and EPG melts are the results of a pre-Himalayan, syn-collisional Pan-African felsic magmatic event, whereas the TLg is a magmatic product of Himalayan collision tectonics.

Contrasting volcanism in the Michoacan-Guanajuato Volcanic Field, central Mexico: shield volcanoes vs. cinder cones

Some of detailed petrologic studies on rock samples of middle to large sized explosive pyroclastic eruptions recently revealed that the eruptions were caused by simultaneous eruption of multiple distinct magma chambers beneath the volcanoes (e.g., Nakagawa et al. 2003: Shane et al. 2007). It is very important to examine the genetic relationships among the magmas to understand the magma feeding system which caused such explosive eruptions. The explosive pyroclastic eruption stage in Shirataka volcano, NE Japan (Fig. 1) is one of potential candidates for such kind of researches. The aim of this study is to reveal the magma feeding system beneath Shirataka volcano in the explosive pyroclastic eruption stage and examine the genetic relationships among magmas involved in the explosive eruption.

The origin of glasses in hydrous mantle xenoliths from the West Eifel, Germany: incongruent break down of amphibole

Northeast Japan parallels a subduction zone where the Pacific plate converges against the North American plate. The axial part of Northeast Japan is composed of an uplifted mountain range called the Ou Backbone Range, along which a number of Quaternary volcanoes are distributed. The eastern margin of these volcanoes defines part of the Quaternary volcanic front of Northeast Japan (Fig. 1). The chemical composition of the volcanic rocks indicates a strong across variation in the alkali content and other incompatible elements, which are lower along the volcanic front and gradually increase rearward (Nakagawa et al., 1988; Yoshida, 2001). The Sr isotope compositions also indicate across-arc variation; the fore-arc volcanoes have higher 87Sr/86Sr ratios (0.704-0.705) than the rear-arc volcanoes (around 0.703) (Notsu, 1983; Kumura & Yoshida, 2006). Such variations can be ascribed to heterogeneous subcontinental lithosphere and/or additional of components from the subducted slab (e.g., Sakuyama & Nesbitt, 1986; Tatsumi & Eggins, 1995). This trench-parallel chemical zonation in Northeast Japan has been established since ca. 12 Ma (Yoshida, 2001). The late Miocene to Quaternary evolution of the volcanic arc of Northeast Japan has been accompanied by some remarkable features. These include (1) Late Miocene to Pliocene caldera-forming volcanism phase, under a direction of maximum compression oblique to the arc and (2) Quaternary andesite stratovolcano-forming volcanism phase, under orthogonal convergence settings (Acocella et al., 2008). A compressive stress regime under orthogonal convergence is unfavourable to facilitate caldera-forming volcanism requiring the formation of a large magma reservoir at shallow depth (Yoshida, 2001). The predominance of stratovolcanoes is reconciled with compressional tectonic settings in the present-day subduction system. Nevertheless, it remains obscure as to when the andesite stratovolcano-forming volcanism has been established under Quaternary orthogonal convergence in Northeast Japan. In general, characteristics of volcanism such as distribution of volcanoes, type of eruptions, magma discharge rate are closely associated with tectonics surrounding the volcanoes. It is very important to examine the relationship between them for better understanding magmatism in various tectonic settings. In this chapter, the temporal changes in the distribution, type and magma discharge rate of the volcanoes near the volcanic front (i.e., Nasu Volcanic Zone) during the last 2.0 m. y. were clarified based on the age and volume