中部日本土岐花崗岩体の岩相と化学組成の累帯変化

抄録

  The Toki granite, a single pluton, is located in the Cretaceous Sanyo Belt in the Inner Zone of Southwest Japan. 483 samples collected from 19 borehole sites in the Toki granite display spatial variations in mineral assemblage, mode, bulk chemical composition and mineral composition, indicative of a zoned pluton. Based on the mode and the mineral assemblages with or without hornblende, the Toki granite can be divided into three rock facies from the margin to the interior: muscovite-biotite granite (MBG), hornblende-biotite granite (HBG) and biotite granite (BG). The ASI values [molar Al₂O₃/(CaO+Na₂O+K₂O)], Al₂O₃, K₂O and Na₂O change systematically from MBG through HBG to BG, corresponding to a systematic variation from peraluminous in the margin to metaluminous granite in the interior. Fe³⁺/Fe²⁺ ratios gradually increase from MBG through HBG to BG, corresponding to a systematic change from ilmenite-series in the margin to magnetite-series in the interior. The sedimentary rocks of the Mino terrane (Kamiasou unit) were intruded and metamorphosed to hornfels by the Toki granitic magma. Na₂O/K₂O and Fe³⁺/Fe²⁺ ratios become closer to those of the hornfels towards the margin of the body, suggesting the assimilation of the crustal host rock by the Toki granitic magma. However, the same ranges in SiO₂ among three rock facies leave the possibility that each rock facies derived from a different magma. Both petrography and bulk chemistry imply that 1) the chemical variation of the Toki granite is not solely due to fractional crystallization, and 2) assimilation of crustal host rocks or simultaneous intrusion of different magmas has played a significant role during the emplacement process.