岩石鉱物科学
Online ISSN : 1349-7979
Print ISSN : 1345-630X
ISSN-L : 1345-630X
35 巻, 2 号
March
選択された号の論文の4件中1~4を表示しています
Original Articles
  • 山本 裕朗, 石川 賢一
    2006 年 35 巻 2 号 p. 53-69
    発行日: 2006年
    公開日: 2006/04/10
    ジャーナル フリー
    The Banji-iwa volcanic rocks (BVR), spread across the central mountainous backbone of the Northeast Honshu arc, form a volcanic complex comprising four composite volcanoes and more than fourteen monogenetic centres. Volcanic activity of the BVR began in the late Pliocene and can be classified into three eruptive stages: Stage I, II and III. Stage I is characterized by the formation of subaqueous andesitic to dacitic lava domes, lava flows, volcaniclastic rocks and feeder dikes. The volcanic products of Stage II largely consist of subaqueous andesitic and dacitic lava flows, volcaniclastic rocks, and feeder dikes. In Stage III, four subaerial composite volcanoes (Senohara-yama, Yamagata Kamuro-dake, Sendai Kamuro-dake and Daito-dake), one dacitic lava dome (Takakura-yama) and three andesitic lava domes (Sanpoukura-san, Koazuma-dake and Minamiomoshiro-yama) were formed.
         The BVR consists of two-pyroxene andesite and dacite with small amounts of olivine-bearing two-pyroxene andesite. The lavas in the eruptives of Stage I volcanoes include the hypersthenic rock series; in contrast, the hypersthenic rock series and the pigeonitic rock series are found to coexist in the eruptives of Stage II and III volcanoes. Lavas of the BVR are distinctively poor in K2O and Rb as compared to those of the Quaternary Sekiryo volcanic rocks.
  • 高島 勲, 村上 英樹, ディク グエン ホン, スチプタ エディ, 毛利 陽司, 柴田 能辰
    2006 年 35 巻 2 号 p. 70-77
    発行日: 2006年
    公開日: 2006/04/10
    ジャーナル フリー
    Thermoluminescence (TL) dating of volcanic rocks and pumice flows around Onikobe-Naruko area was carried out by checking suitability of the sample for this technique. Samples for TL dating were divided into three ranks, A, B and C, on the basis of physical/petrographical characters and geological origin. Rank A sample is hard and compact rock which is chemically stable. Rank B is soft pumice flow or equivalent sample which is chemically unstable for long geologic time. Rank C is sediment deposited in water. For precise and reliable TL dating, only rank A sample must be used.
         TL ages obtained from samples of rank A are 30.4 ka for Naruko dome lava, 187 ka for Takahinatayama lava, 169 ka and 170 ka for Shimoyamazato Tuff Formation, and 296 ka and 268 ka for Ikezuki Tuff Formation. Mitsuzawa Formation at western part of Naruko, pyroclastic flows at Yubama and Byobuiwa are identified as Ikezuki Tuff Formation due to their TL ages of 258 ka, 231 ka to 345 ka and 238 ka, respectively. TL ages of two sites are not well coincide to Ikezuki Tuff Formation. TL ages of the samples from Yubama Onsen and Gyojadaki are 169 ka and 362 ka, which correspond to the age of other eruption products rather than Ikezuki Tuff Formation.
         TL age data for 83 samples of rank B and C are checked by comparison with those of rank A and existence age data. It could be reconciled with stratigraphic evidence and corresponding ages of rank A. Rank B and C data are selectively apply for preliminary stage or special purposes that the rough ages are still useful for research program.
  • 安井 光大, 山元 正継
    2006 年 35 巻 2 号 p. 78-96
    発行日: 2006年
    公開日: 2006/04/10
    ジャーナル フリー
    Lavas and pyroclastic rocks belonging to the Joboji Andesite Member of Itsukaichi Formation (Miocene) and to the Inaniwadake Formation (Pliocene) distribute in the Inaniwadake volcanic district, northern part of the volcanic front of NE Japan arc. The former are basaltic andesite to andesite and the latter basalt. All of the volcanic rocks are classified into extremely low-K tholeiitic series. Pigeonites occur as phenocrysts, microphenocrysts, groundmass minerals and reaction rims around orthopyroxene phenocrysts.
         These tholeiites can be divided into two sub series (the Joboji and the Inaniwadake tholeiites). The Inaniwadake tholeiites are more typically tholeiitic than the Joboji and the Quaternary tholeiites on the volcanic front. Magnetite phenocrysts are rarely found in the Inaniwadake tholeiite but are common in the Joboji and the Quaternary tholeiites. The Inaniwadake magmas are characterized by lower oxygen fugacity and H2O content and by higher temperature (high-temperature type tholeiite) than the Joboji and the Quaterenary magmas (low-temperature type tholeiite) based on the whole-rock and mineral chemistry. The high-temperature type tholeiites of Pliocene age are not restricted to study area but also occur in Kurohanayama and Sasamoriyama, southern part of the volcanic front. Temporal decrease in temperature of magmas is consistent with an increase in volume of coexisting calc-alkaline magmas from Pliocene to Quaternary.
  • 松本 啓作, 平島 崇男
    2006 年 35 巻 2 号 p. 97-108
    発行日: 2006年
    公開日: 2006/04/10
    ジャーナル フリー
    We propose a new modal analysis method using a scanning electron probe microanalyzer with an energy dispersive X-ray analytical system. The procedure is based on the multipoint analysis software of EDAX® Inc. The analysis points are selected as equal interval grids in the secondary electron image (SEI), which covers 3.2 × 2.45 mm2 area as the maximum. The mineral identification of each X-ray spectrum can be done by the combination of the assemblage and content of mineral forming elements. This method can identify most of rock-forming minerals easily, what is an advantage comparing with the conventional modal analysis method under the optical microscope. To obtain the reliable modal data, we can adjust grid intervals depending on the grain size of rock forming minerals and the numbers of the beam acquisition areas depending on the degree of heterogeneity of samples. This technique is available for the rock sample with average grain size between 0.2 and 2.0 mm by the restriction of the minimum enlargement size of the SEI. The new method gives the concordant modal data to those obtained by the conventional point-counting method by several beam acquisition areas for homogeneous samples and by a few tens beam acquisition areas for heterogeneous samples.
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