A detailed analysis on the generation and alteration hysteresis of high temperature skams

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In the high-temperature skarns at Fuka, Okayama Prefecture and Kushiro, Hiroshima Prefecture, spurrite and gehlenite occur characteristically as primary minerals. The skarns were formed by the intrusion into limestone of monzodiorite and quartz-monzonite. The igneous rocks appear to be products of the same magmatic origin. The temperature of the igneous magma affects the type of skarns. The temperatures of the igneous rocks forming the gehlenite-spunite skarns range from 690℃ to 930℃, using the geothermometer. Most of the gehlenite and spurrite crystals were altered to retrograde minerals. In the present study, the alteration process of the gehlenite crystals was clarified by texture and chemical analysis. As a result of detailed analysis, there are five alteration processes in order of formation as follows : (1) Radial alteration process (alteration from gehlenite to calcite, amesite, garnet and vesuvianite). (2) Ti-Fe phases formation process (formation of schoiiomite and perovskite). (3) Leucocratic vein alteration process (alteration to vesuvianite). (4) Melanocratic vein alteration process (alteration to unidentified mineral). (5) Zoning alteration process (alteration to hydrogrossular).