Relation Between Metamorphic Zoning and Graphitizing Degree in Sambagawa Metamorphic Belt

根据泥质和沙质片岩的矿物组合，将关东山三巴川变质地划分为3个带。等级由低到高依次为1区、2区、3区。地带性边界非常复杂，往往难以精确追踪。泥质和泥质岩石的石墨化程度是变质温度的函数，同一岩石的石榴石-绿泥石对的石墨化程度与Mg-Fe分配系数呈良好的正相关关系。因此，建立了分区分类的基础如下。1区；绿泥石，g.d. <27，第2区；绿泥石+石榴石，g.d.28 -35，和3区；绿泥石+石榴石+黑云母和绿泥石+黑云母，g.d. bbb2010。2区少量薄透镜状岩体在1区夹层，1区和3区岩体在2区夹层，2区岩体在3区夹层。该区域的带状结构被解释为许多透镜体的堆叠，每个透镜体都有特定的组合和g.d.。此外，在与第3区相邻的第2区透镜体中，g.d.不会向其边界增加，而是减少。因此，该边界不是变质结晶的热等梯度。与四国三巴川中部地区获得的岩石学资料相比，发现了显著的差异。1)两地区泥质片岩中石榴石的呈现温度大致相同（g.d约28）；2)另一方面，关东山区黑云母形成温度（g.d约36）明显低于四国（g.d约50）；3)关东地区石榴石带的温度宽度比四国窄。结果表明，前者的变质压力也比后者低。

The Sambagawa metamorphic terrain of Kanto Mountains is classified into three zones on the basis of mineral assemblages of pelitic and psammitic schists. The zones are, from lower to higher grade, Zone 1, Zone 2 and Zone 3. The zonal boundaries are highly complicated and often difficult to trace precisely. The graphitizing degree measured on pelitic and psammitic rocks is a function of metamorphic temperature, and a good positive correlation is observed between the degree and the Mg-Fe partition coefficient in garnet-chlorite pair of the same rock. Therefore, the basis of zonal classification has been established as follows. Zone 1 ; chlorite, G. D. <27, Zone 2 ; chlorite+garnet, G. D. 28-35, and Zone 3 ; chlorite+garnet+biotite and chlorite+biotite, G. D. >36. A few thin lenticular masses of Zone 2 are interlayered in the Zone 1 area, those of Zone 1 and Zone 3 rocks in Zone 2, and those of Zone 2 in Zone 3. The zonal structure of the area is interpreted to be stacks of a number of lenses, each of which has a particular assemblage and G. D. Furthermore, in a Zone 2 lens contiguous to Zone 3, G. D. does not increases towards their boundary but decreases. Therefore, that boundary is not a thermal isograd of metamorphic crystallization.As compared with the petrological data obtained in the central Shikoku Sambagawa terrain, remarkable differences have been found. 1) The appearance temperature of garnet in pelitic schists are roughly the same in both areas (G. D. about 28), 2) on the other hand, the temperature of biotite formation is distinctly lower (G. D. about 36) in Kanto Mountains than in Shikoku (G. D. about 50), and 3) the temperature width of the garnet zone is narrower in Kanto than in Shikoku. It is suggested that the metamorphic pressure also is lower in the former than in the latter areas.