大山倉吉軽石における重鉱物組成および熱磁化特性の層内変化

The vertical variations of heavy mineral assemblage and thermomagnetic property of ferromagnetic minerals were examined on the Daisen Kurayoshi Pumice(DKP). The DKP is a late Pleistocene widespread tephra in Japan. At the eastern foot of Mt.Daisen apart 17 km from eruption center, 3.5 m thick DKP were divided into twelve horizons and specimens were corrected from each horizon. At the investigated outcrop, the DKP is accompanied by thin volcanic ash layer at its base and covered by volcanic soil(loam). In this volcanic soil, the Nh volcanic sand layer and a layer of pumice grains reworked from the DKP are intercalated. The specimens of all these layers were also corrected and examined. The DKP has biotite, hornblende, orthopyroxene, titanomagnetite and hemoilmenite as heavy mineral components. Modal composition is unique in the DKP except its lowest part. From the thin volcanic ash layer to the main part of DKP through the lowest part of DKP, the systematic modal variation is recognizable, the decrease in hornblende and increase in orthopyroxene. The Curie temperatures of titanomagnetite (429±4℃) and hemoilmenite (108±10℃) measured by thermomagnetic balance are also unique in the DKP except its uppermost part. In the uppermost part of DKP, the Curie temperatures of both titanomagnetite and hemoilmenite are systematically varied to higher temperature;446℃ and 147℃respectively. There are no correspondance between veriation of heavy mineral assemblage and that of thermomagnetic property.