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[SMP42-11] Spacial distribution of garnet associated with foliation in the Sanbagawa metamorphic rocks, Kanto Mountains, Japan
Keywords:lithology, Sanbagawa metamorphic belt, temperature
Mineral assemblage was quartz, plagioclase, muscovite, chlorite. Not all the samples had garnet, calcite, titanite, and zoisite. Carbonaceous material was also included in many of the samples. 36 of the 55 samples included garnet in its mineral assemblage. Garnet grains were small, most of them with diameters 50 to 100 micrometers. Most of the garnet grains were euhedral and were found within the relatively mafic layers within the pelitic metamorphic rocks which were mostly composed of muscovite. Chlorite was often associated with garnet.
Occurrence of garnet was not at random, but seemed to form groups with lenticular or tabular distribution. The lens (if it was a lens) was subparallel to the foliation, which is known to be subparallel to the lithologic boundary in this area. Spacial distribution of garnet occurrence seemed to form lenticular or tabular groups subparallel to the lithology.
Chemical analyses of garnet using EPMA showed that the garnet grains were normally zoned, exhibiting euhedral growth of the crystal. Mn (spessartine) content was quite high (<50 Xsps). Some of the grains lacked part of the outermost rim, indicating resorption of garnet grains, but not in large amount. No evidence of large retrograde metamorphism was found. It is explained that the garnet grains started to grow during the last stage of the prograde metamorphism. Those cores were formed and started to grow but the growth did not last long because the grains are all so small compared to the proper garnet grains from the garnet zone.
The sharp euhedral chemical zoning profile indicates that the garnet grains were not kept hot for long time. Spacial distribution of garnet was probably controlled by the lithology parallel layers, namely the bulk rock chemistry.
Hashimoto et al. (1992) Journal of the Geological Society of Japan. 98, 953-965.