Characterization of Magnetically Zoned Pluton, Tsushima, Japan

Abstract

: 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. 10. 5 Ma), and crops out in five areas. The results show that the low magnetic susceptibility facies (LM–facies, <100A‐10‐6 emu/g) always occurs in the peripheral part of the pluton, while the high magnetic susceptibility facies (HM–facies, > 250 A 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.

Variation of Cl and SO3 Contents of Microphenocrystic Apatite in Intermediate to Silicic Igneous Rocks of Cenozoic Japanese Island Arcs: Implications for Porphyry Cu Metallogenesis in the Western Pacific Island Arcs

“…Among them, granitoids in Tsushima that are associated with vein-type Ag-Zn-Pb deposits are the representative examples in the inner zone of the Southwest Japan arc. The Tsushima granitoids exhibit magnetite-series character in their central portions whereas they are ilmenite-series (occasionally having accessory pyrrhotite) along the contact with the enclosing carbonaceous sedimentary formations (Shimada et al, 2000;Ishihara and Imai, 2000). The SO 3 contents of apatite in Tsushima granitic intrusions are quoted from Ishihara and Imai (2000).…”

Thermochronology for the Granitic Pluton Related to Lead‐Zinc Mineralization in Tsushima, Japan

“…These values vary with chemical compositions, grain sizes and cooling rates (Dodson, 1973). Harrison et al (1985) have demonstrated that biotite has a strong compositional effect, with increasing Fe/Mg ratio corresponding to an increase in diffusivity of argon, but biotite grains from the pluton indicate uniform compositions with Fe/(Fe+Mg) ≈ 0.6 (Shimada et al, 2000). Although variation of Fe/(Fe+Mg) ratios can be widely seen within the amphibole samples (Fig.…”

“…The decrease in magnetic susceptibility of the Uchiyama pluton is considered to have occurred due to reduction by external volatiles from pelitic wall rocks of the Taishu Group (Shimada et al, 2000;Ishihara and H. IKEMI, N. SHIMADA and H. CHIBA 234 RESOURCE GEOLOGY : Imai, 2000). It means that the granitic body, which indicates low magnetic susceptibility, may have reacted well with the volatiles due to its slow cooling rate.…”

“…The pluton belongs essentially to the magnetite-series (Ishihara, 1981), but magnetic susceptibility shows systematical decrease from core to rim in the granitic body due to reduction by external volatiles from the pelitic wall rocks during emplacement (Shimada et al, 2000;Ishihara and Imai, 2000). Chemistry of the pluton shows I-type after Chappell and White (1974), and indicates that the pluton should form a continuous body (Shimada et al, 2000). Terashima and Ishihara (1980) documented that Cl contents of 700 to 1400 ppm in the pluton were higher than those of common granitoids in Japan.…”

Major and Trace Element Chemical Compositional Signatures of Some Granitic Rocks Related to Metal Mineralization in Japan

“…Intrusions in the Taishu [4], Obira (granite porphyry) [5] and Chichibu mining districts were designated as granitic rocks related to Pb-Zn mineralization. We studied samples from the Ohkawame [6] and Daito-Yamasa [7] mining districts as granitic rocks related to Mo mineralization.…”