Volcanic activity in the Akan volcanic field in the southwestern end of the Kurile arc began in early Pleistocene and formed stratovolcanoes. The somma lavas are tholeiite series and range from 51-60 wt% SiO2. The eruption of voluminous calc-alkali rhyolitic magma (68-73 wt% SiO2) with caldera formation took place at middle Pleistocene. The trace-element characteristics of the volcanic rocks from the Akan volcanic field suggest that the felsic magma implies a difficulty for its derivation from basaltic magma such as the somma lava. Modeling the trace element processes in volcanic rocks indicate that the most basic rock of the somma plots near partial melting 20% of the mantle source with the fractional crystallization subsequent to the partial melting. On the other hand, the Akan pyroclastic flow deposits may be produced by variable degree of partial melting of an andesitic crust source with local heterogeneity of the source material. The most plausible heat source for the genesis of the felsic magma may be latent heat from the mantle-derived basaltic magma such as the somma lava.
The granitoid suite of the Homret Mikpid area is categorized as quartz-diorite, granodiorite and adamellite. Quartz-diorite and granodiorite are predominantly metaluminous, whereas adamellite is peraluminous and displays some of the S-type characteristics. While Quartz-diorite and granodiorite are classified respectively as being intrusives in a pre-plate collision and late orogenic tectonic settings, the adamellite is correlated with syn-collision setting. Granodiorite shows the highest values of REE, which could be attributed to the presence of accessory minerals. Quartz-diorite shows irregular chondrite-normalized REE pattern without significant europium anomaly. In contrast, the adamellite is characterized by slightly steep LREE and a flat HREE pattern, with little negative europium anomaly (Eu/Eu* = 0.652-0.826), and LaN/YbN ratios of 3.4-5.3 Uranium and potassium contents show gradual increase from quartz-diorite to adamellite. This is in harmony with the relatively mafic nature of quartz diorite and the crustal source of adamellite. On the other hand, thorium shows a relative enrichment in granodiorite. This enrichment is attributed to the presence of accessory minerals particularly allanite. The Homret Mikpid granitoids provide an example for demonstrating partial melting from different magma sources. The quartz-diorite formed separately as a result of partial melting of the lower crust that formed mainly of amphibolites. At different crustal level, the granodiorite was formed from dacites-tonalites and metagreywackes through the same process. The adamellite could be generated at shallower depth by anatexis of crustal source such as metapelites and metagreywackes.
Morphology and internal textures of negative crystals in igneous quartz were studied by combining microscopic observation and annealing experiments of natural specimens. Judged from its symmetry and the annealing experiments, the equilibrium form of the negative crystals is estimated to be rounded bipyramidal. The rounded bipyramidal shape suggests that only rhombohedral faces are singular under the growth conditions of quartz crystals in felsic magmas. Thermal history of these crystals after the eruption of magmas was estimated by the morphology and the internal texture of the negative crystals.
Cahnite was found in a vein that intruded crystalline limestone in the vicinity of skarns at Fuka, Okayama Prefecture, Japan. This is the first occurrence of cahnite in Japan. It occurs as aggregates of tetrahedral crystals up to 1 mm long on calcite crystals that grow on cavity walls in the vein, and as anhedral crystals in direct contact with johnbaumite in the vein. The other associate minerals are mainly andradite, arsenopyrite and löllingite. Electron microprobe analyses and ICP-MS give the empirical formula Ca2.09B0.95As0.93Si0.06O3.92(OH)40.8 on the basis of O=8. The unit cell parameters in tetragonal system are a=7.101(1) and c=6.192(1)Å. The mineral is optically uniaxial negative with refractive indices ω=1.658(1) and ε=1.657(1). The Vickers microhardness is 413 kg mm−2 and the Moh's hardness number is 4.5. The density is 3.13(2) g cm−3. It is likely that cahnite at Fuka was formed as a secondary mineral by a late-hydrothermal alteration of johnbaumite.