2015 年 110 巻 5 号 p. 197-213
Garnet with a complex compositional zoning was found from the northern edge of the Western Iratsu body in the Sanbagawa metamorphic belt of the Besshi district, southwest Japan. The studied garnet shows incipient Mn–reverse (increase) zoning from the center part and subsequent Mn–bell shape type (decrease) zoning towards the outer part, which is almost identical to the ‘Mn–caldera shaped zoning’ described by Xu et al. (1994) and Banno and Nakamura (2004). In order to display the chemical characteristics sterically, three–dimensional X–ray chemical mapping was performed for one very–coarse grain of garnet. The result clearly displays that there is a high–Mn layer with faceted euhedral shape of the garnet at the intermediate part, and that its composition continuously changes from the inside to the outside. Inclusion arrays in garnet getting across the high–Mn layer also suggest the continuous growth of the garnet. In the same sample, garnets without Mn–caldera shaped zoning are ubiquitously observed, which are relatively small in size up to 5 mm in diameter. Inclusion mineral assemblage in garnet and Raman barometry suggest the peak P–T conditions of the studied sample not having reached the eclogite facies, but being the epidote–amphibolite facies. The formation process of the ‘Mn–caldera shaped zoning’ can be best explained by the disequilibrium crystal growth under oversaturation of garnet in MnO–(MgO + FeO) binary system. Such oversaturation can be triggered by rapid heating. The finding of the remnant of rapid heating event would provide an important clue to discuss the exhumation history of the Sanbagawa metamorphic belt.
