Granitic rocks in the Hida complex

Abstract

The Hida complex is made up mainly of gneisses and granitic rocks. The latters are classified into three ma-jor groups; older migmatitic granitoids, early Mesozoic granitoids and late Cretaceous to Paleogene granitoids. The first group is characterized by, the close association with gneisses forming migmatite structure, metamorphic texture, CaO-rich bulk chemistry, very low magnetic susceptibility, and high initial Sr isotopic ratios. Their isotopic ages range from 400 to 1200 Ma, but the exact age has not yet been obtained.
The second group is the most widespread granitoids in the complex, and its formation is divided into two stages. The early-stage granitic masses, probably 210 to 220 Ma in ages, are composed mainly of weakly foliated quartz monzodiorite to granodiorite and augen granites showing highly mylonitic nature. They include low microcline with nearly maximurn ordering, and exhibit high magnetic susceptibility on the felsic facies and have scattered initial Sr isotopic ratios. The later-stage masses, mostly 180 Ma in ages, comprise zoned plutons composed mainly of tonalite and granodiorite. They contain intermediate microcline with broad x-ray diffraction patterns, and belong to the I-type and magnetite-series granites.
The third group intrudes into the eastern and western sides of the Hida complex, accompanied with felsic volcanic rocks of nearly same geologic ages. The granitoids contain orthoclase with Δ=0, and their petrographic characteristics suggest rapid cooling of the bodies at shallow depth.
The geologic and genetic features of each granitic mass are summarized here, along the above line of classifications.