2014 年 48 巻 1 号 p. 29-49
Trace element and Sr–Nd–Pb isotopic compositions of basic schists in the Cretaceous Sanbagawa metamorphic belt, a typical regional metamorphic belt in the circum-Pacific orogeny, have been investigated for constraining hydration processes based on the observations of thin-sections and outcrops. The basic schists have undergone significant hydration from 0.8 GPa, 550°C to 0.3 GPa, 400°C during decompression towards the surface at the final stage of metamorphism. High-field-strength and rare-earth element compositions of the basic schists, as well as the Sr–Nd–Pb isotopic ratios, are different among three mineral zones with different peak P–T metamorphic conditions; the basic schists in the low-grade chlorite zone shows N-MORB-like compositions whereas those in the higher-grades, garnet and oligoclase–biotite zones, show more enriched compositions (E-MORB-like). On the other hand, there is a common feature to all the metamorphic zones; the enrichment degree of some group of elements (e.g., large-ion lithophile elements) relative to high-field-strength and heavy-rare-earth elements is proportional to loss on ignition that approximately measures the bulk rock H2O content. This correlation suggests that Li, B, K, Cr, Ni, Rb, Sr, Cs and Ba have been added to the basic schists during hydration. The addition of these elements amounts to as much as 60–80% of the bulk abundance, indicating that significant amounts of elements were transported via pervasive fluid flow, which overprinted the variation in the bulk rock compositions of the protolith. The estimated compositions of hydration fluid are dense in large-ion lithophile elements, lead and light-rare-earth elements (10–100 times denser than primitive mantle) and are similar to those of the slab-derived fluids that induce arc volcanism. These elements (Cs, Rb, Ba, K, La, Ce and Pb) are thought to have been preferentially partitioned into the fluid when it was generated at depth. Such high concentrations indicate a high temperature origin of the hydration fluid, and are consistent with a model of hot slab subduction during exhumation of the Sanbagawa belt.
