抄録
About 320 Tertiary granitoids from the Outer Zone of southwest Japan and the Hidaka Belt of central Hokkaido are statistically compared on major element chemistry. These granitoids occur along the tectonic zones facing fore-arc basin, and have similarities in geological age and opaque mineralogy (il-series). The main conclusions are as follows:
(1) The granitoids from the Hidaka Belt are similar to those from the Outer Zone of southwest Japan, being rich in K2O and FeO but poor in CaO and Fe2O3 compared with the average of Japanese granitoids (Aramaki et al., 1972).
(2) A considerable difference, however, is noted in Na2O content between granitoids of both regions. The granitoids of the Hidaka Belt have higher Na20 content than those of the Outer Zone of southwest Japan. The sodic character of granitoids in the Hidaka Belt may be ascribed to the sodic nature of the source material which is the sedimentary and metamorphic rocks of the Hidaka supergroup that contained originally abundant volcanic materials. The granitoids of the Outer Zone of southwest Japan may have interacted with both the gneissic basement and the Shimanto supergroup having low Na2O and high K2O contents.
(3) TiO2 content of granitoids from both zones are higher than the average of Japanese granitoids.
(4) Al2O3 content of the granitoids of the two zones is similar to that of the Japanese average, but their excess Al2O3 calculated as normative corundum (corundum index) is higher than the average.
(5) In the Hidaka Belt, occurrence of S-type granitoid besides I-type one is suggested from the major element chemistry, which is consistent with recent finding of cordieritebearing granitic rocks from this belt.
