Listvenite formation by the carbonation of pelitic schist and serpentinite in the Median Tectonic Line shear zone, Urayama River, Ehime Prefecture, Japan

Abstract

Listvenites and carbonated serpentinites are found in the shear zone of the Median Tectonic Line at the Urayama River, Shikokuchuo City, Ehime Prefecture, Japan. The listvenites are classified into schistose and massive types, and they mainly consist of quartz, magnesite, and dolomite, with minor amounts of Cr-bearing clay minerals. Furthermore, the listvenites contain chromite and small amounts of sulfide minerals, such as millerite and the gersdorffite-cobaltite series. The occurrence of schistose listvenites in the pelitic schist side through the shear zone is a characteristic feature of the listvenites from the Urayama River. The carbonated serpentinites consist of antigorite, magnesite, and quartz, exhibiting a texture in which the lath-shaped antigorite is partially replaced by magnesite. The protoliths of the massive and schistose listvenites are considered to be serpentinite and pelitic schist, respectively, based on their texture, Mg# of magnesite, presence of chromite, and whole-rock chemical compositions. The massive listvenites at the Urayama River are formed from serpentinites that reacted with fluids containing Ca, Sr, K, Na, and CO₂. The fluids that reacted and were released from serpentinites, containing Ca and CO₂, enriched in Mg, Cr, Ni, and Co, likely interacted with the pelitic schist through the shear zone, producing the schistose listvenite.