A PETROLOGICAL STUDY ON YOUNGER GRANITIC ROCKS AND REALATED METAMORPHIC ROCKS IN THE ABUKUMA MOUNTAINS.

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THE CRETACEOUS HANAWA GRANITIC PLUTON AND NEIGHBORING HIGH-GRADE METAMORPHIC ROCKS WERE STUDIED IN DETAIL, AND A GEOLOGIC MAP IS MADE. THE HANAWA PLUTON IS CONSTITUTED BY. IN ORDER OF INTRUSION FROM OLD TO YOUNG, FINE-GRAINED QUARTZ DIORITE AND TONALITE, FOLIATED TONALITE AND GRANODIORITE, AND MAIN GRANODIORTIC ROCKS. THE GRANODIORITIC ROCKS CAN BE DIVIDED INTO FOUR TYPES; THAT IS PORPHYRITIC POTASH FELDSPAR- AND QUARTZ-BEARING, LAGRE SPHENE-BEARING, MYLONITIC, AND FINE-GRAINED EQUIGRANULA TYPES. SEVENTY-ONE MODAL ANALYSES INDICATE・THAT IN GENERAL THE YOUNGER ROCKS ARE, THE RICHIER IN POTASHI FEDLSPAR AND QUARTZ AND THE POORER IN MAFIC MINERALS. CHEMICAL VARIATION OF 13 GRANODIORITIC ROCKS ARE FAIRLY LIMITED (SiO_2 67-73%), AND ARE SIMILAR TO THOSE OF AVERAGE JAPANESE GRANITIC ROCKS. GM/(MG+FE) RATIO OF BIOTITE AND AN CONTENT OF PLAGIOCLASE EXHIBIT AND OVERALL TREND OF DECREASE TOWARD A SOUTH WESTERN SIDE IN THE GRANODIORITIC PLUTON. MINERAL ASSEMBLAGE AND INDEX MINERAL INDICATE THAT A METAMORPHIC GRADE GENERALLY DECREASES TOWARD THE HANAWA PLUTON, FROM THE GRANULITE FACIES TO THE AMPHIBOLITE FACIES. THIS FACT MAY SUGGEST THAT THE INTRUSION OF THE HANAWA PLUTON WAS NOT IMPORTANT IN FORIMG THE HIGH-GRADE METAMORPHIC ROCKS. TWO TYPES OF METAMORPHISM ARE INFERRED, THAT IS A HIGH TEMPEARTURE AND MODERATE PRESSURE TYPE, AND A MODERATE TEMPERATURE AND LOW PRESSURE TYPE.