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About: Metamorphism is a(n) research topic. Over the lifetime, 18308 publication(s) have been published within this topic receiving 655859 citation(s).

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Journal ArticleDOI
Abstract: With the aim to link zircon composition with paragenesis and thus metamorphic conditions, zircons from eclogite- and granulite-facies rocks were analysed for trace elements using LA-ICP-MS and SHRIMP ion microprobe. Metamorphic zircons from these different settings display a large variation in trace element composition. In the granulites, zircon overgrowths formed in equilibrium with partial melt and are similar to magmatic zircon in terms of high Y, Hf and P content, steep heavy-enriched REE pattern, positive Ce anomaly and negative Eu anomaly. They are distinguishable from magmatic zircon because of their low Th/U ratio. Independently of whole rock composition, metamorphic zircon domains in eclogite-facies rocks have low Th/U ratio and reduced HREE enrichment and Eu anomaly. In a low grade metamorphic vein, zircon has low Th/U ratio but is extremely enriched in Y, Nb and HREE. Petrological and geochronological data demonstrate that metamorphic zircon overgrowths crystallised at granulite-facies conditions in equilibrium with unzoned garnet. It is thus possible for the first time to calculate trace element distribution coefficients between zircon and garnet. Hf is the elements that most strongly partition into zircon. Y, Nb and REE have distribution coefficients between 90 and 0.9 with minimum values for the MREE. In eclogite-facies rocks, the HREE depletion in metamorphic zircon domains is attributed to concurrent formation of garnet under sub-solidus conditions. In one sample, the zircon/garnet trace elements partitioning indicates that metamorphic zircon formed in equilibrium with the garnet rim, i.e. at the eclogitic peak. The reduced Eu anomaly in the metamorphic zircon is interpreted as indicating absence of feldspars and thus supports zircon formation in eclogite facies. In a metamorphic vein within the eclogite-facies rocks, zircons have larger Eu anomaly with respect to high-pressure zircon. Together with geochronological evidence, the Eu anomaly suggests that these zircons formed during prograde metamorphism, before the break down of feldspars at high pressure. The REE composition of zircon can therefore relate zircon formation to specific metamorphic stages such as eclogite, granulite or greenschist facies. This allows linking zircon U–Pb ages with pressure–temperature conditions, a fundamental step in constraining rates of metamorphic processes.

1,957 citations

01 Jan 1965
Abstract: 1. Definition and Types of Metamorphism.- 2. From Diagenesis to Metamorphism.- 3. Factors of Metamorphism.- General Considerations.- The Composition of the Fluid Phase.- Directed Pressure.- 4. Mineral Parageneses: The Building Blocks of Metamorphic Rocks.- 5. Graphical Representation of Metamorphic Mineral Parageneses.- Composition Plotting.- ACF Diagram.- A'FK Diagram.- How Are ACF and A'FK Diagrams Used?.- AFM Diagrams.- 6. Classification Principles: Metamorphic Facies versus Metamorphic Grade.- 7. The Four Divisions of Metamorphic Grade.- General Considerations.- The Terms Isograd and Isoreaction-Grad.- The Division of Very-Low-Grade Metamorphism.- The Division of Low-Grade Metamorphism.- The Change from Low-Grade to Medium-Grade Metamorphism.- The Change from Medium-Grade to High-Grade Metamorphism.- Granulite-High Grade Regional Hypersthene Zone.- Pressure Divisions of the Metamorphic Grades.- Problems with the Al2SiO5 Species.- 8. General Characteristics of Metamorphic Terrains.- Metamorphic Zones in Contact Aureoles.- Metamorphic Zones in Regional Metamorphism.- Paired Metamorphic Belts.- 9. Metamorphic Reactions in Carbonate Rocks.- General Considerations.- Metamorphism of Siliceous Dolomitic Limestones.- Formation of Wollastonite.- Metamorphism of Carbonates at Very High Temperature and Very Low Pressure.- 10. Metamorphism of Marls.- 11. Metamorphism of Ultramafic Rocks: Systems MgO-SiO2-CO2-H2O and MgO-CaO-SiO2-H2O.- 12. Metamorphism of Mafic Rocks.- Transformations Except Those of Very-Low-Grade Metamorphism at Low Pressures.- Very-Low-Grade Metamorphism at Low Pressures.- Evaluation of Metamorphic Changes at Very-Low Grade.- The Role of CO2 in Very-Low-Grade Metamorphism.- 13. Very-Low-Grade Metamorphism of Graywackes.- 14. Metamorphism of Pelites.- General Statement.- Metamorphism of Pelitic Rocks at Very-Low and Low-Grade.- Metamorphism of Pelitic Rocks at Medium- and High-Grade.- 15. A Key to Determine Metamorphic Grades and Major Isoreaction-Grads or Isograds in Common Rocks.- Very-Low-Grade Metamorphism.- Low-Grade.- Medium- and High-Grade.- Geothermometers and Geobarometers.- Sequences of Isoreaction-Grads or Isograds.- 16. Regional Hypersthene Zone (Granolite High Grade).- Nomenclature and Mineralogical Features of "Granulites".- Metamorphism of Granolites and Related Granoblastites.- Petrogenetic Considerations.- 17. Eclogites.- 18. Anatexis, Formation of Migmatites, and Origin of Granitic Magmas.- Anatexis: General Considerations.- Experimental Anatexis of Rocks Composed of Alkali Feldspar, Plagioclase, and Quartz.- Experimental Anatexis of Rocks Composed of Plagioclase and Quartz but Lacking Alkali Feldspar.- Formation of Migmatites.- Formation of Granitic Magmas by Anatexis.- Appendix: Nomenclature of Common Metamorphic Rocks.- Names of Important Rock Groups.- Prefixes.- Classification.

1,615 citations

Journal ArticleDOI
Abstract: The development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional-scale thermal regimes. These are: the total radiogenic heat supply within the thickened crust, the supply of heat from the mantle, the thermal conductivity of the medium and the length and time scales of erosion of the continental crust. The orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening, during which little temperature change occurs in individual rocks, followed by a lengthier phase of erosion, at the end of which the crust is at its original thickness. The principal features of pressure-temperature-time (PTt) paths followed by rocks in this environment are a period of thermal relaxation, during which the temperature rises towards the higher geotherm that would be supported by the thickened crust, followed by a period of cooling as the rock approaches the cold land surface. The temperature increase that occurs is governed by the degree of thickening of the crust, its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary. For much of the parameter range considered, the heating phase encompasses a considerable portion of the exhumation (decompression) part of the PTt path. In addition to the detailed calculation of PTt paths we present an idealized model of the thickening and exhumation process, which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path. An important feature of these PTt paths is that most of them lie within 50 °C of the maximum temperature attained for one third or more of the total duration of their burial and uplift, and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience. Considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm. A companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes. © 1984 Oxford University Press.

1,509 citations

Journal ArticleDOI
Abstract: An examination of lithological, geochemical, geochronological, structural and metamorphic P–T path data suggests that the basement of the North China Craton can be divided into Eastern and Western Blocks, separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone, called the Trans-North China Orogen. The Eastern Block consists predominantly of Late Archean domiform tonalitic–trondhjemitic–granodioritic (TTG) batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at ∼2.5 Ga, with anticlockwise P–T paths. Some Early to Middle Archean rocks are locally present in the Eastern Block, but their tectonic history is unclear due to reworking by the 2.5 Ga tectonothermal event. The Western Block has a Late Archean assemblage, structural style and metamorphic history similar to that of the Eastern Block, but it differs in the absence of early to middle Archean assemblages and in being overlain by and interleaved with Paleoproterozoic khondalites, which were affected by a ∼1.8 Ga metamorphic event involving clockwise P–T paths. A mantle plume model is proposed for the formation and evolution of Late Archean basement rocks in the Eastern and Western Blocks based on a combination of extensive exposure of TTG gneisses, affinities of mafic rocks to continental tholeiitic basalts, presence of voluminous komatiitic rocks, dominant diaprism-related domiform structures, anticlockwise P–T paths, and a short time span from the primary emplacement of TTG and ultramafic to mafic rocks until the onset of regional metamorphism. Between the two blocks is the Trans-North China Orogen which is bounded by two major fault systems and is composed of Late Archean to Paleoproterozoic TTG gneisses and granitoids, interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra-arc basin environments. These rocks underwent multiple phases of compressional deformation and peak high-pressure metamorphism followed by rapid exhumation during the Late Paleoproterozoic at ∼1.8 Ga as a result of collision between the Eastern and Western Blocks, resulting in the amalgamation of the North China Craton.

1,397 citations

Journal ArticleDOI
Abstract: Protolith zircon in high-grade metagranitoids from Queensland, Australia, partially recrystallized during granulite-grade metamorphism. We describe the zircon in detail using integrated cathodoluminescence, U–Pb isotope, trace element and electron backscatter diffraction pattern (EBSP) analyses. Primary igneous oscillatory zoning is partially modified or obliterated in areas within single crystals, but is well preserved in other areas. A variety of secondary internal structures are observed, with large areas of transgressive recrystallized zircon usually dominant. Associated with these areas are recrystallization margins, interpreted to be recrystallization fronts, that have conformable boundaries with transgressive recrystallized areas, but contrasting cathodoluminescence and trace element chemistry. Trace element analyses of primary and secondary structures provide compelling evidence for closed-system solid-state recrystallization. By this process, trace elements in the protolith zircon are purged during recrystallization and partitioned between the enriched recrystallization front and depleted recrystallized areas. However, recrystallization is not always efficient, often leaving a ‘memory’ of the protolith trace element and isotopic composition. This results in the measurement of ‘mixed’ U–Pb isotope ages. Nonetheless, the age of metamorphism has been determined. A correlation between apparent age and Th/U ratio is indicative of incomplete re-setting by partial recrystallization. Recrystallization is shown to probably not significantly affect Lu–Hf ages. Recrystallization has been determined by textural and trace element analysis and EBSP data not to have proceeded by sub-grain rotation or local dissolution/re-precipitation, but probably by grain-boundary migration and defect diffusion. The formation of metamorphic zircon by solid-state recrystallization is probably common to high-grade terranes worldwide. The recognition of this process of formation is essential for correct interpretation of zircon-derived U–Pb ages and subsequent tectonic models.

1,269 citations

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