単一の花崗岩体内で認められるI-type/S-type系列と磁鉄鉱/チタン鉄鉱系列の累帯構造; 中部日本の土岐花崗岩体を例にして

S-type/I-type and ilmenite-series/magnetite-series zoning within a granitic body; Case study of the Toki granitic body, Central Japan

湯口 貴史; 鶴田 忠彦  ; 西山 忠男*

Yuguchi, Takashi; Tsuruta, Tadahiko; Nishiyama, Tadao*

The Toki granite, a single pluton, is located within the Cretaceous Sanyo belt in the Inner Zone of Southwest Japan. Samples collected from 19 borehole sites in TKG reveal the spatial variation of mineral assemblage, mode, bulk composition and mineral composition, representing a zoned pluton. Based on the petrography, TKG is divided into three rock facies from the margin to the interior: the muscovite-biotite granite (MBG), the hornblende-biotite granite (HBG) and the biotite granite (BG). ASI(mol. AlO/(CaO+NaO+KO)), AlO, KO and NaO change systematically from MBG through HBG to BG, representing a systematic variation from S-type in the margin to I-type in the interior. Fe/Fe become gradually higher from MBG through HBG to BG, representing a systematic change from the ilmenite series in the margin to magnetite series in the interior. The sedimentary rocks of the Mino terrane was intruded and metamorphosed to hornfels by the Toki granitic magma. ASI, NaO/KO and Fe/Fe of TKG become closer to those of the hornfels towards the margin of the body, indicating the assimilation of the crustal host rock by the Toki granitic magma. Both petrography and bulk chemistry suggest that (1) the chemical variation is not solely due to a fractional crystallization, and (2) assimilation has played a significant role in its margin.