新潟—山形県境の金丸—小国地域に産するHFS 元素に富むザクロ石両雲母花崗岩

抄録

  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.