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Unrecognized metamorphic complexities can produce erroneous interpretations when using equilibrium thermodynamics and isotope geochronology. Universally em-ployed methods for determining pressure-temperature conditions during regional meta-morphism are based on the assumption of chemical equilibrium, and geochronology in metamorphic rocks can suffer from cryptic redistribution of isotopes. In this research, the scales of disequilibrium in regionally metamorphosed rocks and the effects of garnet resorption on Lu-Hf garnet ages were examined through numerical simulations of these processes. Concerning scales of disequilibrium, thirteen porphyroblastic datasets, previ-ously measured using X-ray computed tomography, were examined by numerically simulating diffusion-controlled nucleation and growth of garnet while tracking chemical potential gradients to determine reaction affinity Ar (-[delta]rG). Maximum nucleation rates are 10−13̇6-10−9̇8 nuclei cm−3 s−1, interfacial energies are 0.004-0.14 J m−2 assuming shape factors of 0.1-1.0, and Al intergranular diffusion (QD = 140 kJ mol−1) is 10−14̇4-10−11̇1 m2 s−1 at 600 °C. Limitations in determining crystallization kinetics arise from difficulties in constraining rock-specific properties (e.g., porosity and Al solubility). Ar at the time and location of nucleation is 0.4-5.9 kJ mol−1 of 12-oxygen garnet ([delta]T = 4.0-62.0 °C) for the earliest nuclei, and 5.3-29.0 kJ mol−1 ([delta]T = 50-125 °C) for nucleation at maximum Ar. The results demonstrate potential for delayed nucleation and metastability that can generate spurious interpretations. The timing of metamorphic events is also critical for understanding geologic history. In the Makhavinekh Lake Pluton aureole, Labrador, garnet resorption caused redistribution of Lu and loss of Hf from consumed rims, creat-ing spuriously young ages. Garnet-ilmenite Lu-Hf geochronology using bulk separates yields apparent ages that young toward the contact from 1876 ± 21 Ma (4025 m) to 1396 ± 8 Ma (450 m). Toward the contact, garnet crystals are progressively more re-sorbed. Numerical modeling was used to test retention of Lu and loss of Hf during re-sorption as the dominant control on age. More resorption and Lu retention produce younger apparent ages (false ages). Application of the model to the aureole yields model ages from 1850 Ma to 1374 Ma, younging toward the contact. Thus, Lu-Hf geo-chronology applied to resorbed garnets requires careful examination of Lu zoning.
Metamorphic Textures provides definitions, descriptions and illustrations of metamorphic textures, as well as the fundamental processes involved in textural development. This book is composed of 11 chapters and begins with a presentation of the metamorphic processes and the production of metamorphic minerals. The subsequent chapters describe the structural classification of grain boundaries, the metamorphic reactions, mineral transformations, and the crystallization and recrystallization of metamorphic rocks. These topics are followed by the texture examination of thermal metamorphic rocks and minerals and the preferred orientations of these rocks, particularly the dimensional and lattice preferred orientation. Other chapters survey the textures of rocks under dynamic and shock metamorphism. The final chapters describe the textures of regional and polymetamorphism. This book will be of great use to petrologists, physicists, and graduate and undergraduate petrology students.
Geological Society of London Handbook Series Edited by KeithCox Founded in 1807, the Geological Society of London has beenpublishing since 1845 and now distributes its journal to Fellowsthroughout the world. This Handbook is published as part ofa series of authoritative practical guides to field geology. The Field Description of Metamorphic Rocks "This handbook describes how metamorphic rocks and rock masses maybe observed, recorded and mapped in the field. Written at a levelsuitable for undergraduate students of geology, this book (as withits companion volumes in the series) has firmly established itselfas an essential tool for any geologist -- student, professional oramateur -- faced with the task of making a general description ofan area of metamorphic rocks. A clear, systematic frameworktogether with numerous diagrams, illustrations and checklistsenables readers to produce useful and broadly similar descriptions,despite possible differences of background or specialist interest.This well-written and well-produced little text will, I am certain,become standard reading for most geology undergraduates. It willalso interest many geologists who do not regularly work inmetamorphic terrains and will be particularly useful to engineeringgeologists and civil engineers who are often concerned withdescribing the fabrics of metamorphic rocks without being concernedabout their origins." --M.E. Jones, Mineralogical Magazine Contents: * Metamorphic Fieldwork and Mapping * Names and Categories of Metamorphic Rocks and Rock Units * Rock Banding * Minerals * Compositions * Grade * Textures * Fabric Types * Relations to Structures * Undeformed Pods * Augen * Pseudomorphs * Veins * Igneous Contacts * Metasomatism * Reaction Zones * Fault-Zones and Mylonites * Reference Tables and Checklists