Rehydration of eclogites and garnet-replacement processes during exhumation in the amphibolite facies
Published:April 17, 2019
Thais Hyppolito, Aitor Cambeses, Samuel Angiboust, Tom Raimondo, Antonio García-Casco, Caetano Juliani, 2019. "Rehydration of eclogites and garnet-replacement processes during exhumation in the amphibolite facies", Metamorphic Geology: Microscale to Mountain Belts, Silvio Ferrero, Pierre Lanari, Philippe Goncalves, Eugene G. Grosch
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EPMA and LA-ICP-MS trace-element maps have been acquired from amphibolitized eclogites from the Diego de Almagro Metamorphic Complex (Chile). Several garnet growth pulses and garnet resorption stages are revealed by major elements chemical zoning and by heterogeneous Y and rare earth element (REE) behaviour, associated with subduction and exhumation of these rocks. Distribution of REE in prograde garnet is texturally and chemically coupled with the breakdown of REE-bearing minerals while formation of epidote and titanite generations during amphibolitization is recorded by complex textures involving new garnet generation and overprinting phases. The latest overprint stage is characterized by fine-grained intergrowth between garnet and epidote micro-veins, phengite, hornblende, albite and titanite. Garnet cracks have been gradually re-equilibrated during this event witnessing short-scale dissolution–transport–precipitation. Pseudosection modelling shows that local variability in water content during amphibolitization controls garnet stability at the expense of epidote. Overprinting microstructures are explained by the effect of locally-derived aqueous fluids that trigger the ‘unlocking’ of elements from the reacting eclogite-facies paragenesis. These findings highlight the microscopic characteristics of amphibolitization processes documented in exhumed eclogite-facies terranes and shed light on the importance of thorough micro-chemical investigations while undertaking pressure–temperature (PT) estimates on rocks with strong textural disequilibrium.
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Metamorphic Geology: Microscale to Mountain Belts
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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.