Crystalline limestone in the Hida metamorphic complex, central Japan

Geological characteristics, mineral compositions, texture and mode of occurrences of dolomite

Abstract

The Hida metamorphic rocks intercalate many beds of limestone around 3 to 30% of the total thickness in each area, and this feature is distinct from other major metamorphic belts in Japan. The limestone in the complex is completely crystalline under the amphibolite to granulite facies metamorphism.
It was partly deformed by later mylonitization at the late Permian to Triassic age, and further thermally recrystallized by the intrusion of the early Mesozoic and late Cretaceous to Paleogene granitoids. This complex history is printed on the occurrences, style of folding, mineral compositions and particularly on the texture of crystalline limestone. The limestone in the eastern region is thinned and higly deformed by close to tight folds showing pinch and swelling, characterized by very fine-grained blastomylonitic to ultramylonitic texture, whereas the limestone in the western region is weakly deformed and has the largest scale in total thickness and length of beds having very coarse-grained granoblastic to gneissic and mortar textures. In contrast, the limestone in the central region is similar to the eastern region in the scale of beds and style of folding, but exhibits coarse-to medium-grained typical granoblastic texture without any mylonitic nature, and has higher temperature mineral assemblage such as wollastonite-calcite-quartz.
The limestone is partly dolomitic including dolomite, olivine, clinohumite, serpentine, phlogopite, graphite, clinopyroxene, tremolite and Mg-rich colourless amphiboles, and associated with calc-silicate rocks consisting of quartz, calcite, plagioclase, K-feldspar, wollastonite, grossular, scapolite, epidote, clinopyroxene, hornblende and prehnite. X-ray analyses of powdered samples and microscopic observations on stained specimens reveal that most parts of limestone are free from dolomite except very small thready particles in calcite. The dolomitic parts are found sporadically in the central and western regions of the complex, in which parts, the dolomite compositions are usually 1 to 10 wt.%, but rarely reaching 50 to 90 wt.%. The geological characteristics of limestone in the Hida complex are discussed in comparison with the Paleozoic reef-formed limestone of other areas in Japan.