Magmatic epidote and its petrologic significance

Epidote is a major magmatic mineral in tonalite and granodiorite in a belt coextensive with the Mesozoic accreted terranes between northern California and southeastern Alaska. Textural and chemical evidence indicates that epidote crystallized as a relatively late but magmatic mineral that formed through reaction with hornblende in the presence of a melt phase. The observed relations concur with experimental data on crystallization of epidote from synthetic granodiorite at 8 kbar total pressure. Plutonic rocks bearing magmatic epidote must have formed under moderately high pressures, corresponding to lower crustal depths, under fairly oxidizing conditions.