Clinopyroxene (generally augite of variable composition) and orthopyroxene characterize the highest grades of metamorphism. With more metamorphism, garnet and hornblende develop as the lower-grade minerals disappear, and rocks evolve to become amphibolites. And, commonly, relict (left over from the protolith) plagioclase and augite persist at low- and medium-grades. At slightly higher grades, chlorite, epidote, and the green amphibole actinolite form as rocks become greenschists. Often, these first metamorphic minerals fill vugs or fractures, and the overall rock may still have the appearance of the protolith. Table 13.1 Common Metamorphic Minerals in Mafic Rocksġ3.1.1 Variations as Metamorphic Grade IncreasesĪt the lowest grades, zeolite minerals, prehnite, and pumpellyite form during diagenesis and the onset of metamorphism of mafic rocks. So, the sizes of the red boxes and their order in Figure 13.1 are approximate. One complication with metabasites is that many minerals in mafic rocks have quite variable compositions – and thus variable stability ranges. Table 13.1 lists the compositions of the minerals included in Figure 13.1. This is not true for metasedimentary rocks because, in those rocks, the protolith minerals formed at low temperature. Consequently, during low-grade metamorphism, new minerals develop as high-temperature minerals reequilibrate to lower-temperature conditions. Most metabasites form by metamorphism of high-temperature mafic igneous rocks, commonly basalt. Higher-grade minerals, including garnet, diopside, and orthopyroxene, are not. Lower-grade minerals, including zeolites, prehnite, pumpellyite, chlorite, and epidote are all hydrous. Thus, the minerals or mineral assemblage in a rock tell us the general metamorphic grade. Many of the minerals in Figure 13.1 are only stable over discreet temperature ranges. And, ending with hornblende, garnet, biotite, diopside, or orthopyroxene at high grade. A spectrum of minerals is possible, beginning with zeolites, prehnite, and pumpellyite, at low grade. Serpentinites and other ultramafic rocks may be obducted and become part of alpine peridotites and continental lithosphere.ġ3.1 The Minerals in Metamorphosed Mafic Rocks 13.1 Key minerals in metamorphosed mafic rocksįigure 13.1 shows some common minerals found in metabasites (metamorphosed mafic rocks) at different metamorphic grades. Ultramafic rocks that originate as part of the oceanic lithosphere, may become serpentinites.Different minerals and mineral assemblages characterize different facies and facies series. Facies series reflect the settings in which metamorphism occurred. A facies series is a sequence of different metamorphic zones that increase in grade across a metamorphic terrane.We differentiate facies based on the metamorphic minerals present, on rock color and fabric, and on grain size.A metamorphic facies is a general range of P-T conditions that produces distinctive looking rocks.Many of the minerals in metabasites have variable and complex compositions and, consequently, metamorphic reaction are continuous and do not plot as lines on phase diagrams.Low-grade metamorphism of mafic rocks commonly involves high-temperature minerals reequilibrating to lower-temperature conditions.Most metabasites form by metamorphism of high-temperature igneous protoliths such as basalt.The minerals present in metabasites (metamorphosed mafic rocks) vary with metamorphic grade.
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