Granulite

Abstract: Geophysical, petrological, and geochemical data provide important clues about the composition of the deep continental crust. On the basis of seismic refraction data, we divide the crust into type sections associated with different tectonic provinces. Each shows a three-layer crust consisting of upper, middle, and lower crust, in which P wave velocities increase progressively with depth. There is large variation in average P wave velocity of the lower crust between different type sections, but in general, lower crustal velocities are high (>6.9 km s−1) and average middle crustal velocities range between 6.3 and 6.7 km s−1. Heat-producing elements decrease with depth in the crust owing to their depletion in felsic rocks caused by granulite facies metamorphism and an increase in the proportion of mafic rocks with depth. Studies of crustal cross sections show that in Archean regions, 50–85% of the heat flowing from the surface of the Earth is generated within the crust. Granulite terrains that experienced isobaric cooling are representative of middle or lower crust and have higher proportions of mafic rocks than do granulite terrains that experienced isothermal decompression. The latter are probably not representative of the deep crust but are merely upper crustal rocks that have been through an orogenic cycle. Granulite xenoliths provide some of the deepest samples of the continental crust and are composed largely of mafic rock types. Ultrasonic velocity measurements for a wide variety of deep crustal rocks provide a link between crustal velocity and lithology. Meta-igneous felsic, intermediate and mafic granulite, and amphibolite facies rocks are distinguishable on the basis of P and S wave velocities, but metamorphosed shales (metapelites) have velocities that overlap the complete velocity range displayed by the meta-igneous lithologies. The high heat production of metapelites, coupled with their generally limited volumetric extent in granulite terrains and xenoliths, suggests they constitute only a small proportion of the lower crust. Using average P wave velocities derived from the crustal type sections, the estimated areal extent of each type of crust, and the average compositions of different types of granulites, we estimate the average lower and middle crust composition. The lower crust is composed of rocks in the granulite facies and is lithologically heterogeneous. Its average composition is mafic, approaching that of a primitive mantle-derived basalt, but it may range to intermediate bulk compositions in some regions. The middle crust is composed of rocks in the amphibolite facies and is intermediate in bulk composition, containing significant K, Th, and U contents. Average continental crust is intermediate in composition and contains a significant proportion of the bulk silicate Earth's incompatible trace element budget (35–55% of Rb, Ba, K, Pb, Th, and U).

Abstract: Seismic techniques provide the highest-resolution measurements of the structure of the crust and have been conducted on a worldwide basis. We summarize the structure of the continental crust based on the results of seismic refraction profiles and infer crustal composition as a function of depth by comparing these results with high-pressure laboratory measurements of seismic velocity for a wide range of rocks that are commonly found in the crust. The thickness and velocity structure of the crust are well correlated with tectonic province, with extended crust showing an average thickness of 30.5 km and orogens an average of 46.3 km. Shields and platforms have an average crustal thickness nearly equal to the global average. We have corrected for the nonuniform geographical distribution of seismic refraction profiles by estimating the global area of each major crustal type. The weighted average crustal thickness based on these values is 41.1 km. This value is 10% to 20% greater than previous estimates which underrepresented shields, platforms, and orogens. The average compressional wave velocity of the crust is 6.45 km/s, and the average velocity of the uppermost mantle (Pn velocity) is 8.09 km/s. We summarize the velocity structure of the crust at 5-km depth intervals, both in the form of histograms and as an average velocity-depth curve, and compare these determinations with new measurements of compressional wave velocities and densities of over 3000 igneous and metamorphic rock cores made to confining pressures of 1 GPa. On the basis of petrographic studies and chemical analyses, the rocks have been classified into 29 groups. Average velocities, densities, and standard deviations are presented for each group at 5-km depth intervals to crustal depths of 50 km along three different geotherms. This allows us to develop a model for the composition of the continental crust. Velocities in the upper continental crust are matched by velocities of a large number of lithologies, including many low-grade metamorphic rocks and relatively silicic gneisses of amphibolite facies grade. In midcrustal regions, velocity gradients appear to originate from an increase in metamorphic grade, as well as a decrease in silica content. Tonalitic gneiss, granitic gneiss, and amphibolite are abundant midcrustal lithologies. Anisotropy due to preferred mineral orientation is likely to be significant in upper and midcrustal regions. The bulk of the lower continental crust is chemically equivalent to gabbro, with velocities in agreement with laboratory measurements of mafic granulite. Garnet becomes increasingly abundant with depth, and mafic garnet granulite is the dominant rock type immediately above the Mohorovicic discontinuity. Average compressional wave velocities of common crustal rock types show excellent correlations with density. The mean crustal density calculated from our model is 2830 kg/m3, and the average SiO2 content is 61.8%.

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.

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.

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.