Characterization of magnetically zoned pluton, Tsushima, Japan

Systematic measurements of magnetic susceptibility were carried out at the outcrops of the Uchiyama granitic pluton, Tsushima Islands. The pluton consists of hornblende-biotite granodiorite and biotite granite. It intrudes concordantly along an anticline axis of the Taishu Group in middle Miocene (16.1±0.5 Ma), and crops out in five areas. The results show that the low magnetic susceptibility facies, (LM-facies, <100×10^-6 emu/g) always occurs in the peripheral part of the pluton, while the high magnetic susceptibility facies (HM-facies, >250×10^-6 emu/g) occupies the core through the medium magnetic susceptibility zone (MM-facies). It is pointed out that the pluton forms a magnetic zoning within a single plutonic body. Chemical compositions of the granitoids show no conspicuous differences in major elements among the LM-, MM-, and HM-facies. Biotites from the LM-, MM-, and HM-facies also indicate uniform compositions with Fe/(Fe+Mg)=0.6. The pluton is principally surrounded by black mudstones of the Lower Formation of the Taishu Group, which is deltaic to shallow sea sediments of the Eocene to early Miocene age, and the carbon contents in them were analyzed. The analyses show that carbon was mostly decomposed and lost in the hornfels zone, while mudstones in the non-hornfels zone usually contain 0.5 to 0.7 % C. From these facts, it is concluded that the magnetically zoned pluton at Tsushima was formed by an external volatile buffer system such as CH₄-CO₂ during the solidifying magma process.