This paper details a new discovery of anorthite-bearing diopsidites (= anorthite diopsidites) from the lowermost crust along Wadi Fizh in the northern Oman ophiolite. The anorthite diopsidites occur as networks within layered gabbros 50 m above the gabbro/peridotite boundary, and are mainly composed of high-Mg diopsidic clinopyroxenes and anorthites with various amounts of uvarovite. They are intermediate in mineral chemical characteristics between the crustal gabbros and the diopsidites, and interpreted as an interaction product between hightemperature Ca-rich hydrothermal fluids and peridotites within the mantle section. The anorthite diopsidites were probably formed by interaction between the fluids involved in diopsidization within the mantle section and the layered gabbro. The fluids responsible for the formation of the anorthite diopsidite were also Ca-rich and carried Cr to form uvarovite from the underlying mantle section. High-temperature hydrothermal circulation plays an important role in the transportation of elements, e.g., Ca and Cr, from the upper mantle to the lower crust across the Moho in the ocean floor.