- © The Mineralogical Society Of America
Epidote was first recognized as a magmatic mineral in the alpine Bergell tonalite by Cornelius (1915). Field observations and microscopic textures let Cornelius to conclude “... the only possibility is, that epidote is a primary mineral in our tonalite, crystallizing early from the magma, i.e., before (in part also contemporaneous with) biotite” (translated from German, Cornelius (1915), p. 170). This knowledge disappeared and for the following 70 years, epidote and zoisite were categorized as metamorphic minerals. The petrologic significance of magmatic epidote was then rediscovered when Zen and Hammarstrom (1984) identified epidote as an important magmatic constituent of intermediate calc-alkaline intrusives in plutons of the North American Cordillera. Zen and Hammarstrom (1984) also suggested that epidote indicates a minimum intrusive pressure of about 0.5 to 0.6 GPa. Subsequently, magmatic epidote was described from many granodioritic to tonalitic plutons, but also from monzogranite (e.g., Leterrier 1972), dikes of dacitic composition (Evans and Vance 1987), and orbicular diorite (Owen 1991, 1992). Furthermore, epidote was not only recognized in crystallizing plutons or dikes but also in high pressure migmatites and pegmatites derived from eclogites (Nicollet et al. 1979; Franz and Smelik 1995).
The role of epidote during magmatic crystallization is relatively well understood, and crystallization temperatures and sequences involving epidote in intermediate magmas (granodiorite-tonalite-trondhjemite, TTG) are experimentally determined and confirmed from natural intrusives. In contrast, little attention is directed towards the inverse process, i.e., melting of epidote bearing lithologies. Epidote is omnipresent in eclogite of intermediate temperature (Enami et al. 2004) and denominates three subfacies (i.e., epidote-blueschist, epidote-amphibolite, and epidote-eclogite facies). Indeed the epidote-amphibolite facies intersects the wet granite solidus near 0.5 GPa at 680°C, defining the pressure above which epidote may be present during melting processes. Experiments on natural compositions have confirmed that epidote is stable above …