Félix Gervais, 2019. "Three modes of isograd formation in the northern Monashee Complex of the Canadian Cordillera", Metamorphic Geology: Microscale to Mountain Belts, Silvio Ferrero, Pierre Lanari, Philippe Goncalves, Eugene G. Grosch
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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.
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Metamorphic Geology: Microscale to Mountain Belts
CONTAINS OPEN ACCESS
In Earth evolution, mountain belts are the loci of crustal growth, reworking and recycling. These crustal-scale processes are unravelled through microscale investigations of textures and mineral assemblages of metamorphic rocks. Multiple episodes of metamorphism, re-equilibration and deformation, however, generally produce a complex and tightly interwoven pattern of microstructures and assemblages. Over the last two decades, the combination of advanced computing and technological capabilities with new concepts has provided a vast array of novel petrological tools and high-resolution/high-sensitivity techniques for microanalysis and imaging. Such novel approaches are proving fundamental to untangling the enigma represented by metamorphism with an unprecedented level of detail and confidence. As a result, the first decade and a half of this century has already seen the tumultuous development of new research avenues in metamorphic petrology. This book aims to provide a timely overview of the state of the art of this field, of newly developed petrological techniques, future advancements and significant new case studies.