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Regional low-grade metamorphism takes place with a small increase in temperature (above °C) at significantly increased directional pressure. The directed.
Table of contents
- Metamorphic Grade
- Classification of Metamorphic Rocks – Physical Geology
- Organic nitrogen chemistry during low-grade metamorphism
- Genesis of jadeite by low-grade metamorphism
The resulting rock, which includes both metamorphosed and igneous material, is known as a migmatite Figure 7. JPG] As already noted, the nature of the parent rock controls the types of metamorphic rocks that can form from it under differing metamorphic conditions.
The kinds of rocks that can be expected to form at different metamorphic grades from various parent rocks are listed in Table 7. Some rocks, such as granite, do not change much at the lower metamorphic grades because their minerals are still stable up to several hundred degrees. Metamorphic rocks that form under either low-pressure conditions or just confining pressure do not become foliated. In most cases, this is because they are not buried deeply, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust.
This is contact metamorphism. Some examples of non-foliated metamorphic rocks are marble , quartzite , and hornfels.
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Marble is metamorphosed limestone. When it forms, the calcite crystals tend to grow larger, and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone was pure calcite, then the marble will likely be white as in Figure 7. Quartzite is metamorphosed sandstone Figure 7. It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica.
Classification of Metamorphic Rocks – Physical Geology
Most sandstone contains some clay minerals and may also include other minerals such as feldspar or fragments of rock, so most quartzite has some impurities with the quartz. On the other hand, any clay present in the original sandstone is likely to be converted to mica during metamorphism, and any such mica is likely to align with the directional pressure.
An example of this is shown in Figure 7. The quartz crystals show no alignment, but the micas are all aligned, indicating that there was directional pressure during regional metamorphism of this rock. Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rock Figure 7. In some cases, hornfels has visible crystals of minerals like biotite or andalusite. If the hornfels formed in a situation without directed pressure, then these minerals would be randomly orientated, not foliated as they would be if formed with directed pressure.
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Organic nitrogen chemistry during low-grade metamorphism
Chapter 7 Metamorphism and Metamorphic Rocks. Exercise 7. The mica crystals are consistently parallel to one another. A very hard rock with a granular appearance and a glassy lustre. There is no evidence of foliation. A fine-grained rock that splits into wavy sheets. The surfaces of the sheets have a sheen to them. A rock that is dominated by aligned crystals of amphibole. Previous: 7. Next: 7. Cruciani et al. P—T pseudosections calculated for the whole-rock composition of a mafic sample M with a typical greenschist-facies assemblage hatched and b felsic sample MB with prograde growth of stilpnomelane.
Isopleths of Si contents in white micas are the black dashed lines. Traces of sodic amphibole are also calculated in both fields, although this mineral was not observed in thin section. In this case the possible range for peak pressure appears large. In a way similar to that illustrated in Figs 6—9 , peak P—T conditions were estimated for seven additional mafic and nine additional felsic samples using maximum Si contents in phengite Table 1. All results strongly overlap and show no consistent differences between samples.
The thermometric approach of McMullin et al. No difference in microfabric grain size, lack of zonation, clustering, degree of recrystallization was identified between samples in which subgreenschist- and greenschist-facies assemblages occur.
Isopleths of water content bound to solids also can be extracted from the calculated P—T pseudosections. Most water was released during breakdown of OH-bearing minerals at very low-grade conditions. This observation has important implications for metamorphic processes at very low grade as discussed in the next section.
This understanding is as important as the attainment of mineral equilibria. Prior to this work, McMullin et al. In a similar way, the systematic variation of X Mg with changing Si content in white mica and chlorite in the samples of this study can also be interpreted to reflect changing P—T conditions during mineral growth within single samples. However, at very low-grade conditions, calculations of mineral equilibria using multivariant reactions e.
Such an approach results in a cluster of points around a P—T path. This application of equilibrium thermodynamics is based on the specific reaction behaviour at very low metamorphic grade, as follows.
Genesis of jadeite by low-grade metamorphism
Reactions at very low grade are governed 1 by grain size and 2 by the availability of water, resulting in different mineral compositions pretending non-equilibrium conditions. However, this circumstance allows us to define locally a series of equilibria during changing P—T conditions Vidal et al. The very small grain size combined with the strong compositional variability of the reaction products as observed, for instance, for white mica Fig. As a consequence nucleation of new grains with a different composition occurs during changing P—T conditions rather than only further growth of the existing grains Fig.
In this way clusters are formed that can recrystallize under low-grade conditions, resulting in larger grains grown at the expense of smaller grains. In addition, a different composition can form at the rim of larger grains compared with the core composition. Sketch showing small phengite crystals grey in a thin section that grew in a matrix light grey of quartz, feldspar and other minerals during a water pulse at very low-grade conditions stage I.
Subsequently at stage II, another water pulse still at very low-grade conditions caused further growth of phengite with a different composition dark grey. Stage III represents metamorphism at low-grade conditions at which small phengite crystals were dissolved and larger phengite grains grew instead, resulting in a more or less concentric zoning pattern. During all three stages I—III deformation did not take place. Nevertheless, this specific kinetic behaviour at very low-grade conditions favouring the crystallization of new grains during changing P—T conditions also results in prograde phases remaining metastably at peak P—T conditions and during retrograde mineral equilibration.
Thus, prograde and retrograde minerals in general cannot be distinguished by their composition in the very low-grade regime owing to a lack of the zoning that is characteristic at higher grade e. Kryza et al. The reason for nucleation rates exceeding growth rates is not yet clear. In rocks of very fine grain size close to contact metamorphic aureoles, Roselle et al. The free energy of nucleation decreases with formation of more stable nuclei. At very low grade, such supersaturation most probably prevails owing to the sluggishness of reactions and fluid transport at low temperature.
Nucleation of new grains is activated by the availability of water, but partly also by deformation. The apparently incomplete and variable consumption of protolith minerals is due to variable access to water. Hence, a variable degree of preservation of primary minerals is a direct indicator of a variable reaction progress. The clustering of reaction products that dominates the metamorphic fabric in all rock types is a prominent indication of water transport, at least at thin-section scale. Nucleation of specific clusters appears to be bound to specific precursor minerals such as plagioclase, where white mica clusters form as a result of the high local concentration of Al, or clusters of epidote form as a result of a high local concentration of Ca.
Water drives the metamorphic reactions and forms abundant OH-bearing minerals such as white mica, amphibole, chlorite, pumpellyite, prehnite, stilpnomelane and epidote.
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