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Metamorphic Rocks
Published in Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough, Earth Materials, 2019
Dexter Perkins, Kevin R. Henke, Adam C. Simon, Lance D. Yarbrough
As discussed in Chapter 2, most metamorphic rocks form when heat, pressure, or fluids cause changes in preexisting rocks. The preexisting, or parent, rocks are called protoliths. Protoliths can be igneous, sedimentary (as at Willsboro), or metamorphic rock of all sorts. The changes that occur during metamorphism may involve changes in rock texture, in the minerals present, and in overall rock composition. So, metamorphic rocks, and the processes that create them, are key parts of the rock cycle that also includes igneous and sedimentary processes. Thus, metamorphic rocks record Earth history. And metamorphic petrologists try to interpret the rocks to interpret the history.
Metamorphic rocks
Published in W.S. MacKenzie, A.E. Adams, K.H. Brodie, Rocks and Minerals in Thin Section, 2017
W.S. MacKenzie, A.E. Adams, K.H. Brodie
Metamorphic rocks can be named according to their texture or to their mineralogy, although is some cases it may be more appropriate to use their protolith in the name. Metamorphism affects sedimentary and igneous rocks as well as previously metamorphosed rocks. The effects of metamorphism, which is often accompanied by deformation, may not obliterate previous features of the protolith, especially at low metamorphic grades. Original compositional banding is often preserved even at high metamorphic grades. Hence in some cases it is more appropriate to use the prefix meta- followed by the protolith, e.g., metagabbro, meta-arkose.
Improving geological logging of drill holes using geochemical data and data analytics for mineral exploration in the Gawler Ranges, South Australia
Published in Australian Journal of Earth Sciences, 2021
E. J. Hill, A. Fabris, Y. Uvarova, C. Tiddy
Metamorphic and metasomatic rocks present several challenges for geochemical discrimination. Metamorphism occurs in a closed, or near-closed chemical system, and therefore metamorphic rocks have similar chemistry to their protolith (Putnis & Austrheim, 2010). For instance, an amphibolite facies pelite may have the same chemical composition as a mudstone. In contrast, metasomatism results in a change in rock composition and texture owing to interaction with hydrothermal or other fluids. The resultant metasomatic effect on the rock is therefore influenced by the chemistry of the altering fluids and can result in dramatic changes to its chemical composition (Putnis & Austrheim, 2010). Drill hole MSDP11 intersected several metasomatised intervals, logged as skarn, between 320 and 450 m. The effect on the chosen compositional variables ranges from dramatic to more subtle; however, all intervals are delineated in the multivariate mosaic plots (Figure 17). The main skarn zone between ∼320 and 390 m is identified in the multivariate mosaic plot at a scale of <90, whereas narrow intervals logged as skarn below 408 m show a weaker chemical contrast to surrounding lithologies and are identified at a scale of <10. The variation in chemical composition of the skarn highlighted in the multivariate plots demonstrates an advantage of integrating geochemistry with logging. Broad geological classifications such as the use of the term skarn can more easily be refined to provide additional sub-classification. For example, intervals of skarn in drill hole MSDP11 could clearly be sub-divided using tessellation of geochemical data to provide improved lithological classification (Figure 17).
Two belts of HTLP sub-regional metamorphism in the New England Orogen, eastern Australia: occurrence and characteristics exemplified by the Wongwibinda Metamorphic Complex
Published in Australian Journal of Earth Sciences, 2020
K. Jessop, N. R. Daczko, S. Piazolo
Variably foliated granitoids intrude amphibolite facies metasedimentary and metavolcanic rocks. Protoliths include volcaniclastic sandstone to conglomerate, pelite, and occasionally vesicular mafic lava. Metasedimentary rocks are now biotite schists and paragneiss that is compositionally layered and grades into migmatite. Muscovite, as well as biotite, is found in the more pelitic schists, and cordierite porphyroblasts have been noted locally. Fine-grained mafic rocks are characterised by green hornblende and interstitial plagioclase. The pelitic mineral assemblage is here used to infer HTLP metamorphism with peak conditions of ∼3.5 kb and ∼675 °C.