岩石鉱物科学
Online ISSN : 1349-7979
Print ISSN : 1345-630X
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44 巻 , 3 号
May
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特集号「日本の花崗岩とテクトニクス」その5
原著論文
  • 亀井 淳志, 内藤 一樹, 高村 早弥香, 加々島 慎一, 奥澤 康一, 関 陽児, 渡部 芳夫
    44 巻 (2015) 3 号 p. 131-154
    公開日: 2015/06/27
    [早期公開] 公開日: 2015/04/28
    ジャーナル 認証あり
      A small stock of leucocratic Grt-two mica granite enriched in high field strength elements (HFSEs) has recently been found in the Kanamaru-Oguni district of the Asahi-Iide mountains in the Ashio Belt of the NE Japan. The granite has a high-K peraluminous composition, and is categorized as an A-type within-plate granitoid, according to several geochemical discriminants based on HFSEs. However, total Zr + Nb + Ce + Y contents are lower (166-192 ppm) and Rb/Ba ratios are higher (19-48) than those typical of A-type granitoids (Zr + Nb + Ce + Y > 350, Rb/Ba < 10). This suggests that this stock is in fact a highly fractionated granite, rather than an A-type intrusive. The mineral assemblage and normative composition of the granite suggests that the stock solidified at shallow depth (~ 3-6 km), under moderately water-rich conditions. Nd isotopic compositions are appropriate for an igneous origin, rather than derivation from the sedimentary rocks. Geochemical modeling shows that the petrogenesis of the granite can be explained by partial melting of mafic igneous crust, with subsequent active fractional crystallization of plagioclase + alkali-feldspar. Many other small stocks composed of high-Si peraluminous granite occur in the Asahi and Iide mountains, and these have been the subject of previous studies. The granitic activity ranges from Late Cretaceous (~ 90 Ma) to Paleogene in age. Although most of these stocks were derived from partial melting of various crustal rocks, some are highly differentiated, and have HFSE concentrations similar to the Kanamaru-Oguni stock studied here.
      HFSE-rich granitoids occur sporadically within the other geological units of southwestern Japan. HFSE-rich granitoids are restricted in the SW Japanese arc because the Ashio Belt in NE Japan also corresponds geologically to the units of SW Japan. The granitoids in the Inner and Outer Zones of SW Japan differ in composition. The HFSE-rich granitoids in the Inner Zone originated from middle to lower crustal materials, and were then strongly differentiated before emplacement. This is similar genesis to the leucogranite in the Kanamaru-Oguni district. In contrast, the solitary HFSE-rich granitoid in the Outer Zone crops out at Cape Ashizuri. This occurrence is the only classic A-type intrusion in Japan, and is considered to have formed by a low degree of partial melting of the upper mantle or mafic lower crust. Although the HFSE-rich granitoids within the SW Japan arc may be similar geochemically to within-plate or ocean-ridge granites, they are in fact volcanic-arc granites produced within the subduction zone by specific activities.
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原著論文
  • 佐藤 景, 石渡 明
    44 巻 (2015) 3 号 p. 155-170
    公開日: 2015/06/27
    [早期公開] 公開日: 2015/04/28
    ジャーナル 認証あり
      Ishikoshi Andesite is the Middle Miocene, amphibole-bearing, two pyroxene andesite lava flow located in the forearc area of the Northeast Japan arc, which covers the area of 8 km EW and 7 km NS and has the thickness of >180 m. In this study it is divided into peripheral lava (PL) and central lava (CL), which are distributed in the peripheral and central parts of the volcanic edifice, respectively. PL bears groundmass pigeonite or shows higher FeO*/MgO trend with fractionation and CL does not bear pigeonite and lower FeO*/MgO trend, therefore PL is tholeiitic and CL is calc-alkaline. PL shows petrological features that are compatible with crystallization differentiation. CL has several features that suggest magma mixing such as non-equilibrium plagioclase phenocrysts, but evidences for magma mixing are not robust. In comparison to similar lava flows, PL and CL seem to form a composite lava flow because clear geological boundary between PL and CL cannot be observed and FeO*/MgO ratio of CL tend to be higher in the marginal portion. We also find that two-pyroxene equilibrium temperature of CL (760-850 ℃)is lower than that of PL (970-990 ℃). CL should have been too viscous at this temperature (>1010 Pa·s in anhydrous state) to form a dike, thus it should have effused through the vent which PL formed to be composite lava flow with PL. Ishikoshi Andesite has columnar joints rather than platy joints. In comparison to other lava flows which have platy joints in Japan, it is possible that less anisotropic crystals, low eruption temperature (high viscosity) and/or large phenocryst size have prevented Ishikoshi Andesite from formation of foliation along which platy joints develop in the lava.
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