Alternating layers of pelitic and basic bands with occasional semi-pelitic band, millimeter to centimeter in width, occur between the epidote–amphibolite (metagabbro) and the pelitic schist in the epidote–amphibolite facies region of the Sanbagawa metamorphic belt, central Shikoku. The whole-rock major, trace, and rare earth element compositions of the semi-pelitic band are intermediate between those of the basic and pelitic bands. The peak metamorphic conditions were estimated at 1.0–1.2 GPa/600–630 °C for the mineral assemblage of the pelitic and semi-pelitic bands. The evolution of the CO₂-rich fluid [X(CO₂) = CO₂/(CO₂ + H₂O)] at the lithologic boundary between the epidote–amphibolite and the pelitic schist, during the Sanbagawa prograde metamorphism, is discussed in the context of a titanite, rutile, calcite, dolomite, and quartz assemblage. The X(CO₂) of the semi-pelitic band and basic and the pelitic bands increased during prograde metamorphism from the stability field of titanite to those of rutile + dolomite + amphibole + quartz and rutile + calcite + amphibole + quartz, respectively. The X(CO₂) values of the metamorphic fluid at the peak metamorphic stage estimated by the matrix assemblages were higher in the order of the pelitic schist and epidote–amphibolite (less than 0.12–0.23), basic and pelitic bands (0.23–0.38), and semi-pelitic band (0.38–0.57), suggesting variations in the fluid compositions on a millimeter to centimeter scale. The CO₂-rich fluid in the alternating layers, especially in the semi-pelitic band, was probably generated by a redox reaction between Fe³⁺-bearing silicate phases, such as amphibole and epidote in the basic band and carbonaceous material in the pelitic band. This reaction is thought to have been triggered by the chemical and/or mechanical mixing of these two bands during prograde metamorphism, resulting in the formation of the semi-pelitic band.

在四国中部三叶川变质带的绿帘石-角闪岩相区域的绿帘石-角闪岩（变辉长岩）和泥质片岩之间出现泥质带和基性带的交替层，偶尔有半泥质带，宽度为毫米到厘米。半泥质岩带的全岩主、微量和稀土元素组成介于基性和泥质岩带之间。泥质岩和半泥质岩带矿物组合的峰值变质条件估计为 1.0-1.2 GPa/600-630 °C。富CO ₂流体的演化[ X (CO ₂ ) = CO ₂ /(CO ₂  + H ₂O)] 在绿帘石-角闪岩和泥质片岩之间的岩性边界处，在三叶川前行变质作用期间，在钛铁矿、金红石、方解石、白云石和石英组合的背景下进行了讨论。半泥质岩带和碱性带和泥质岩带的X (CO ₂ )在从钛铁矿稳定场到金红石+白云石+角闪石+石英和金红石+方解石+角闪石+石英稳定场的变质过程中分别增加。的X（CO ₂) 由基质组合估计的变质峰值阶段的变质流体值较高，依次为泥质片岩和绿帘石-角闪岩（小于 0.12-0.23）、基性和泥质带（0.23-0.38）和半-泥质带（0.38-0.57），表明流体成分在毫米到厘米范围内的变化。交替层中的富含CO _{2 的}流体，尤其是半泥质带中，可能是由 Fe ³⁺之间的氧化还原反应产生的。-含硅酸盐相，例如碱性带中的闪石和绿帘石以及泥质带中的碳质材料。这种反应被认为是在顺行变质过程中这两个带的化学和/或机械混合引发的，导致半泥质带的形成。