The Sekita Mountains and Torikabuto Volcano District lies at the northeastern end of the Central Upheaval Zone located within the North Fossa Magna region. In this area, early Pleistocene volcanic rocks ranging basaltic to dacitic compositions are widely distributed. These volcanic rocks are subdivided into four distinct groups, based on their geological and geochemical characteristics, the Sekita Mountains (SM), the Shikumi River Basin (SRB), the Kenashi (KN) volcano and the Torikabuto (TK) volcano. In this paper we report petrographical, mineralogical, whole rock geochemical, and Sr- and Nd- isotopic characteristics of the SM and SRB volcanic rocks. The SM and SRB volcanic rocks are divided into group 1 and group 2, and these volcanic rocks are composed mainly of basalt, andesite and dacite belonging to the tholeiitic rock series. The ages of SM and SRB rocks range from ∼2 Ma to 1.3 Ma. Our data can concluded as the followings. 1) The SM and SRB volcanic rocks were formed by fractional crystallization processes of tholeiitic magma, involving plagioclase, clinopyroxene, orthopyroxene, and Fe-Ti oxides. 2) The Sr- and Nd-isotopic compositions of the SM and SRB tholeiitic volcanic rocks are nearly identical to that of the TK calc-alkaline volcanic rocks, suggesting a genetic link between the SM, SRB and TK rocks. 3) The tholeiitic primary magmas for the SM and SRB volcanic rocks and the calc-alkaline primary magmas for the TK rocks would be formed by partial melting processes of the mantle source materials having similar Sr- and Nd-isotopic composition. We interpret that the tholeiitic rocks are products of crystal differentiation at middle crustal level (∼5 kbar), while the calc-alkaline rocks are derived by fractional crystallization at shallower level (2–3 kbar).
Four hydrowoodwardite samples, simplified formula [Cu1−xAlx(OH)2](SO4)x/2•mH2O (0.03≦x≦0.38)], collected from the Dogamaru mine, Shimane Prefecture and the Oike mine, Shiga Prefecture were characterized by powder X-ray diffraction, Fourier transform infrared (FT-IR) and electron microprobe analyses. The samples obtained from the Dogamaru and Oike mines occur as green to blue colloidal crusts (0.5 mm to 20 mm in thickness). The samples indicate fine bedded texture (1 μm to 10 μm in thickness) corresponding to the different compositions. Powder X-ray diffraction patterns are close to those of hydrowoodwardite. FT-IR spectra showed absorption bands at around 3450 cm−1 due to O-H stretching vibration, at around 1640 cm−1 due to H-O-H bending vibration, at around 1100 cm−1 due to v3 and v1 SO42− stretching vibrations, and at around 620 cm−1 and 470 cm−1 due to v4 and v2 SO42− bending vibrations, respectively. Weak absorption bands at around 1395 cm−1 due to carbonate group are observed in the Dogamaru samples. Electron microprobe analyses revealed that the main components are Cu, Al, S, and Si; Si is derived from amorphous silica. Small amount of Zn is contained in the Dogamaru samples. The predominant mechanism for changes in chemical composition of the each sample is substitution of Al for Cu with constant Zn ratios. The ranges of Cu, Zn, Al, and S (apfu) in the Dogamaru and Oike samples are as follows: Cu 0.53–0.89 and 0.62–0.96; Zn 0.03–0.17 and 0.00–0.01; Al 0.04–0.38 and 0.03–0.38; and S 0.02–0.16 and 0.02–0.17, respectively. The minimum values of Al are smaller than any of previously reported hydrowoodwardite.
Betafite group mineral, Ti-dominant pyrochlore supergroup mineral, is found from metasomatic skarn distributed into Ryoke metamorphic rocks in Yuge Island of Geiyo Islands in Seto. This mineral generally occurs in alkali granitic pegmatite, but has not been reported in Japan. Betafite group mineral from Yuge Island is associated with closely ilmenite, titanite and anatase in fluorite. The empirical formula is (Ca1.18 Y0.26 U0.21 Th0.11 HREE0.09 Na0.06 LREE0.05)Σ1.96 (Ti1.26 Nb0.59 Ta0.10 W0.05)Σ2.00 (O6.78 F0.14)Σ6.92. This chemical composition is determined to be oxycalciobetafite because of O is dominant Y site and Ca is dominant A site.
Information collected from geological newsmagazines in 2011 is reviewed. First topic is overview of mineral resources such as Archean gold mineral systems, mineral commodity summaries in USA and rare earth element resources of Afghanistan. Second topic is regional geology. This topic includes early Archean rocks of West Greenland and the Geoparks in China and Canada. Third topic is mineralogy such as introduction to sodalite and birefringence spheres for optical mineralogy.