Mass-transfer and differential element mobility in metapelites during multistage metamorphism of the Yenisey Ridge, Siberia
Published:April 17, 2019
I. I. Likhanov, 2019. "Mass-transfer and differential element mobility in metapelites during multistage metamorphism of the Yenisey Ridge, Siberia", Metamorphic Geology: Microscale to Mountain Belts, Silvio Ferrero, Pierre Lanari, Philippe Goncalves, Eugene G. Grosch
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Pelitic gneisses and schists from the Garevka Complex (Yenisey Ridge) at the western margin of the Siberian craton contain zoned garnet porphyroblasts that show clear evidence of multistage growth. The three discrete stages define a counter-clockwise pressure–temperature (P–T) path involving initial prograde low pressure heating followed by near-isothermal medium-pressure compression and post-peak retrograde synexhumation decompression and cooling. The combined study of the variations of in-situ major and trace element mineral compositions of coexisting minerals and mineral modes with changing P and T conditions and metamorphic reactions in rocks allowed detailed investigation of metamorphic processes. A significant increase in pressure during prograde collision-related metamorphism correlates with an abrupt increase of Ca in garnets, which in turn is accompanied by a significant decrease in heavy rare earth elements (HREE) and Y contents. Decrease in temperature and pressure during retrograde metamorphism led to an increase in the HREE and Y content in garnets with concomitant decrease of grossular component of garnet. A pronounced systematic negative correlation between HREE including Y content and Ca content in garnet can be attributed to substitution of trivalent rare earth elements (REE) and Y for divalent Ca cations on eightfold sites. The main reasons for the sharp increase in Ca content in garnets during collisional metamorphism are either (1) the redistribution between garnet and plagioclase, which led to less calcium in composition, or (2) consequence of epidote breakdown. A mass balance of major and trace elements between the reactants and products of metamorphic reactions yield very good matches between the measured and reconstructed modal abundances in all cases, indicating that collisional metamorphism was essentially isochemical with respect to most elements, with the exception of HREE. The minimum equilibration volume for the major elements was c. 1 mm3. The total HREE balance requires a rather greater reaction volume involved in the redistribution of HREEs, of the order of 3–8 mm3 which provides evidence for their relatively higher migration mobility during metamorphism in comparison with other REE.
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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.