Abstract
Pelitic and psammitic metamorphic rock fragments are found in rhyolitic tuff beds of the Lower Oligocene Momijiyama Formation in Yubari-gawa area of central Hokkaido, Japan. The host rhyolitic tuff bed shows typical inverse-to-normal grading, and is interpreted as sediment-gravity-flow deposits. Except for a few cases, the metamorphic rock fragments are considered to be derived as detrital fragments from the metamorphic rocks composing shallow basement beneath felsic volcanics. The metamorphic rock fragments show various metamorphic grades up to the amphibolite facies. Chemical composition of garnets in metamorphic rock fragments and detrital garnets in the tuff bed is characteristically rich in Mn and/or Ca, indicating the greenschist facies to amphibolite facies. The Hidaka metamorphic belt is a nearby low P/T metamorphic belt that is supposed to be a source region of the metamorphic rock fragments. Chemical composition of the garnet grains is similar to that of the Fuyushima metamorphic rocks, a possible low-grade equivalent of Hidaka metamorphic rocks. Timing of the exhumation of the Hidaka metamorphic rocks is generally supposed to be the Late Miocene. The finding of metamorphic rock fragments from the Lower Oligocene tuff beds, however, suggests earlier uplifting of the low-grade Hidaka metamorphic rocks.
