Abstract
In the Sanbagawa metamorphic belt of central Shikoku, many dunitic and wehrlitic ultramafic bodies occur intercalated with the pelitic schists of the Upper member of the Minawa Formation and of the Ojoin Formation. They are metamorphosed peridotite (P-type) and metamorphosed serpentinite (S-type).
Metamorphic zonal mapping was made on the metamorphosed serpentinites. Mineral assemblages are, in the order of increasing metamorphic grade, antigorite-brucite-diopside (zone I), olivine-antigorite-diopside (zone II), and olivine-antigorite-tremolite (zone III). The distribution of zones is consistent with the thermal structure of the metamorphic terrain.
Relic mineralogy indicates that most of ultramafic bodies of the Sanbagawa metamorphic belt are originally cumulates formed at the spinel and garnet lherzolite facies in the upper mantle. One dunitic metamorphosed serpentinite (Shiraga body), of which chromite indicates Cr-Al substitution (Cr/(Cr+Al)=0.85 to 0.5) without Fe3+ and Ti variations, may have been originally a residue after partial melting of mantle peridotite.
With regard to the origin and thermal history, there are at least 4 types of ultramafic bodies in the Sanbagawa metamorphic belt ; (1) P-type, cumulus garnet peridotite, (2) P-type, cumulus spinel peridotite, (3) S-type, cumulus spinel peridotite, and (4) S-type, residual spinel peridotite. The diversity of origin and thermal history indicates that the Upper member of the Minawa Formation and the Ojoin Formation were probably melange zones into which ultramafic bodies were tectonically emplaced. As metamorphic grade of ultramafic bodies is in harmoneous with that of their surrounding schists, tectonic emplacement of ultramafic bodies ceased before the maximum temperature of the Sanbagawa metamorphism was reached.
