A pumpellyite-occurring zone can be distinguished from a biotite-occurring zone in the Paleozoic to Mesozoic terrain of the Kuzakai-Morioka area, northern Kitakami Mountains. The former is presumably a remnant of earlier low-temperature regional metamorphism, while the latter a recrystallized product by the later contact metamorphism due to Cretaceous granitic intrusions.
In the pumpellyite-occurring zone the characteristic mineral assemblage is epidote+pumpel-lyite+chlorite+actinolite in greenstones and phengitic muscovite+chlorite in pelitic rocks. Prehnite occurs only as a vein-forming mineral in greenstones, but is found rarely in the matrices of calcarious pelitic and psammitic rocks.
This area had been metamorphosed in the pumpellyite-actinolite facies grade before the Cretaceous intrusions. However, actinolite is confined to occur in relatively Mg-rich greenstones. In the epidote+pumpellyite+chlorite+actinolite assemblage, the Fe/(Fe+Al) ratios of epidote and pumpellyite and the Mg/(Mg+Fe) ratios of chlorite and actinolite tend to be higher in the east than in the west of this area, suggesting a decrease of prevailing temperature eastward during the metamorphism.
The marked gap in the celadonite content of muscovite between the pumpellyite- and biotite-occurring zones, and the sporadic occurrence of zoned epidote showing an abrupt decrease in Fe/ (Al+Fe) at its core-rim boundary in a part of the pumpellyite-occurring zone support the idea that the low-grade regional metamorphism took place prior to the contact metamorphism.
抄録全体を表示