Three modes of isograd formation in the northern Monashee Complex of the Canadian Cordillera

This contribution re-examines textural relationships in metapelitic schist from five metamorphic zones separated by four isograds along the west flank of the Frenchman Cap dome in the southeastern Canadian Cordillera. There is a muscovite-out isograd formed by muscovite-dehydration melting in the kyanite-field. Two others, including one coinciding with the muscovite-out isograd, mark the appearance of sillimanite, which was produced along with biotite by garnet breakdown in the presence of melt. Two others mark the disappearance of kyanite: one formed during retrogression, the other coinciding with a shear zone (the Monashee décollement). Only the muscovite-out isograd, therefore, corresponds with the classical definition of an intersection of the topography with a prograde, isogradic-surface. This contribution exemplifies the need to use a descriptive definition of isograd as the trace of a surface across which a specific change in metamorphic mineralogy takes place. It is further proposed to specify their origin whenever possible with terms such as prograde-, retrograde- and structural-isograds. This revised terminology for isograds will improve our understanding of metamorphic terrains by helping to localize cryptic shear zones, gain a better understanding of the retrograde path and put firmer constraints on viable tectonic models.