Lateral variations of H2O contents in quaternary magmas of Northeastern Japan

Volcanoes of the East Japan volcanic arc are divided into two groups on the basis of their phenocryst assemblages; volcanoes with lavas or pyroclastic rocks containing quartz phenocrysts and no hornblende phenocrysts (type A), and those with rocks containing hornblende phenocrysts and no quartz phenocrysts (type B). Type A volcanoes occur only in the narrow region along the volcanic front, whereas type B volcanoes are distributed in the area closer to the Sea of Japan.Recent experimental studies on calc-alkaline andesite-dacite under H₂O-saturated and -undersaturated conditions indicate that the liquidus temperature (maximum thermal stability limit) of quartz decreases drastically with increasing H₂O content in magma, whereas the liquidus temperatures of hornblende and biotite are relatively constant with variations in the H₂O content and bulk chemical composition of the magma.It is suggested from the lateral variation of mafic phenocryst assemblages [1] and from the above result that the temperature of the parental magmas of these volcanoes increases, and their H₂O contents decrease, towards the volcanic front in the East Japan volcanic arc.Such lateral variations in the H₂O contents of magmas under the East Japan volcanic arc are in agreement with those of other incompatible elements (K, Rb, REE, etc.). If H₂O-undersaturated partial melting of upper mantle peridotite can be represented by the univariant line (olivine, Ca-rich clinopyroxene, orthopyroxene and liquid coexist) in the system H₂OMg₂SiO₄z.sbnd;CaMgSi₂O₆z.sbnd;SiO₂, the decrease of H₂O content in the magma suggests that the melting temperature of the peridotitic mantle may gradually increase, and so the degree of partial melting may increase, towards the volcanic front. The lateral variation of other incompatible elements can also be explained by this model.     

Alkali mapping of the japanese quaternary volcanic rocks

Of ca. 200 Quaternary volcanic centers in the Japanese islands, ca. 120 are characterized by K₂O and Na₂O concentrations of their ejecta normalized to SiO₂ = 60% K₂O values give a geographically well-defined trend of regularly increasing away from the volcanic front. Na₂O values show no geographical trend. Relationships between the SiO₂-normalized K₂O and the depth of the deep seismic zone (K-h relation) are different for different arcs. The Southwest Japan arc shows distinctly higher K₂O and less regular geographic trend than the others probably due to crustal contamination. In the normal segments of the Northeastern Japan belt, K₂O increases ca. 2.5 to 3.0% per 100 km depth of the seismic zone. Groups of volcanic centers near the junction between the Northeast Honshu and Izu-Mariana arcs give characteristically lower K₂O while Rishiri volcano far behind the arc junction between the Kurile and Honshu arcs also gives a low K₂O value.     

Geology of the quaternary volcanic centres of the east Anatolia

Following the collision along the Bitlis–Zagros suture, a north–south convergence between the Arabian Platform and Laurasia has continued uninterrupted until the present. As a result, the continental crust has been shortened, thickened and consequently elevated to form the Turkish–Iranian high plateau. On the high plateau volcanic activity began during the Neogene, intensified during the late Miocene–Pliocene and continued until historical times. Large volcanic centres have been developed during the Quaternary which form significant peaks above the Turkish–Iranian high plateau. Among the Quaternary volcanoes, the major volcanic centres are Ararat, Tendürek, Suphan and Nemrut. Ararat (Ağri Daği) is the largest volcanic center and is a compound stratovolcano, consisting of Greater Ararat and lesser Ararat. The former represents the highest elevation of Anatolia reaching over 5000 m in height. Tendürek is a double-peaked shield volcano, which produced a voluminous amount of basalt lava as extensive pahoehoe, and aa flows. It has an ill-defined semi-caldera. Suphan is an isolated stratovolcano, capped by silicic dome. It represents the second highest topographic elevation in Anatolia, with a height of over 4000 m. A cluster of subsidiary cones and small domes surrounds the volcano. Nemrut is the largest member of a group of volcanoes, which trend north–south. It is a stratovolcano, having a well-defined collapse caldera and a caldera lake. Various volcanic ejecta have been extruded from these volcanic centres over the last 1 to 2 million years. The Quaternary volcanic centres, although temporally and spatially closely associated, display a wide range of lavas from basalt to rhyolite. The volcanoes have diverse compositional trends; Ararat is distinctly subalkaline, Suphan is mildly subalkaline, Nemrut is mildly alkaline and Tendürek is strongly alkaline. The major and trace element compositions together with the isotope ratios indicate that their magmas were generated from a heterogeneous mantle source. Each of the volcanic centres has undergone a partly different magmatic evolution.     

Quantitative distribution of late Cenozoic volcanic materials in Japan

The late Cenozoic orogeny in Japan is briefly reviewed. Amounts of volcanic materials in the three periods of the orogeny are estimated at: early Neogene 150 × 10³ km³ (mafic 40 %, salic 60 %), middle and late Neogene 20 × 10³ km³ (mafic 70 %, salic 30 %), Quaternary 5 × 10³ km³ (mafic 80 %, salic 20 %). The largest volume per unit time is in the early Neogene, and the smallest in the middle and late Neogene. Volume per unit area becomes larger towards the southeastern margin or «front» of the volcanic belt. Thermal energy transported by volcanic materials is compared with the terrestrial heat flow in the belt.     

Content and behavior of fluorine in Japanese quaternary volcanic rocks and petrogenetic application

Fluorine contents in about 160 representative Quaternary volcanic rocks and 15 hornblende and biotite phenocrysts in a calc-alkali series in Japan have been determined by a selective ion-electrode method. Tholeiites have the lowest contents and the narrowest range (58–145 ppm), while alkali basalts have the highest contentws and the widest range (301–666 ppm), high-alumina basalts have intermediate values (188–292 ppm). F contents in basalts clearly increase from east to west across the Japanese Islands, as do alkalies, P₂O₅ REE, U, Th and H₂O.The volcanic rocks studied are divided into two groups on the basis of F: (1) witt, increasing % SiO₂ or advancing fractionation, F contents show either progressive enrichment; or (2) with increasing fractionation, F contents show rather constant values. The former is produced by fractionation of anhydrous phases from basalt to mafic andesite magmas; the tholeiite series of Nasu volcanic zone (outer zone), northeastern, Japan is a typical example. The latter group is derived through separation of amphibole-bearing phases from basaltic magmas at various depths from upper mantle (about 30 km) to upper crust; the alkali series in southwestern Japan and the calc-alkali series of Chokai volcanic zone (inner zone), northeastern Japan, are examples.     

Summary of recent volcanic activity

Summary of recent volcanic activitySmithsonian Institution's, Global Volcanism Network     

Summary of recent volcanic activity

Summary of recent volcanic activitySmithsonian Institution's Global Volcanism Network     

Summary of recent volcanic activity

Summary of recent volcanic activitySimithsonian Institution's Global Volcanism Network     

Summary of recent volcanic activity

Summary of recent volcanic activitySmithsonian Institution's Global Volcanism Network