High-pressure and low-pressure mineral assemblages at the same outcrop near Yokokawa, Abukuma metamorphic terrane, Japan

Evidence for unique metamorphic evolution

Abstract

At the same outcrop near Yokokawa, Abukuma metamorphic terrane, occur high-grade lateritic and calc-silicate rocks. The lateritic rocks constitute a discrete block enclosed within calcite marble, whereas the calc-silicate rock is a thin layer intercalated between calcite marble and pelitic-psammitic gneiss.
Textures and compositions of minerals in the lateritic rocks suggest high-temperature loading and subsequent unloading. The characteristic mineral assemblages before loading, at the peak of loading and after unloading are hercynite+sillimanite, corundum+almandine and hercynite+sillimanite, respectively. The hercynite+sillimanite assemblage before loading is well preserved as inclusions in corundum and almandine porphyroblasts, and is texturally distinct from the similar mineral assemblage formed after unloading.
On the other hand, the calc-silicate rock shows no evidence for such a P-T history, and has mineral assemblages stable at high temperatures and low pressures, such as anorthite+wollastonite. In addition, decomposition of CO₃-scapolite to anorthite and calcite suggests recrystallization upon cooling, which may have taken place coevally with the emplacement of the nearby Cretaceous plutons. The occurrence of wollastonite with calcite and quartz only in the major part of the calc-silicate rock suggests that X_co2 of the fluid phase in the major part was lower than that in the marginal part (next to pelitic-psammitic gneiss) over a distance of a few mm during metamorphism.