Significance of hornblende in calc-alkaline andesites and basalts

Hornblende occurs in some andesitic and basaltic rocks of calc-alkaline affini1y, where it is commonly associated with olivine. The texture, eruption history, and composition are interpreted to indicate that hornblende commonly forms as a product of reaction between olivine and basaltic and andesitic liquids. The natural hornblendes probably formed within the crust at temperatures between 960o and 1080'C from liquids with less than about 6 weight percent HrO. The formation of hornblende from basaltic liquid within the crust has implications for evolution of continental crust, as well as for the origin of andesite and thermal conditions in subduction zones.