Figure 1. It should be noted however, that contact metamorphic marbles can be coarse grained. The composition of plagioclase feldspar is typically indicated by the general anorthite (% An) or albite (% Ab) fraction and is easily determined by measuring the refractive index or peel angle within the crushed particles by measuring the refraction angle under a thin section. It is an essential or important constituent of many igneous, metamorphic and sedimentary rocks (acidic, neutral and basic, pegmatite igneous rocks, amphibolite schist and gneiss, feldspar sandstone, siltstone and shale (Tables 4.1 and 6.1Table 4.1Table 6.1)). Greenschist and amphibolite facies rocks form in typical regional metamorphism related to a geodynamic setting of continental collision tectonics (Figure 2). For simplicity we assume VIII coordination across the entire solid solution since we have found that assumed coordination number has little effect on calculated partition coefficients. Albite and Anorthite are the end members of the series, with the intermediary minerals Oligoclase, Andesine, Labradorite, and Bytownite. They can be distinguished from garnet pyroxenites because their clinopyroxenes are omphacitic (Na-rich) whereas those in garnet pyroxenites are not. Santo, in Developments in Volcanology, 2005. Although eclogite facies rocks are widespread in some orogenic belts, they typically occur in small isolated outcrops in a matrix of surrounding rocks that reflect conditions of lower grade facies. The plagioclase–spinel transition in complex compositions: pale blue: MORB Pyrolite (MPY), Niida and Green (1999); stippled gray: Hawaiian Pyrolite - HPY, Green and Ringwood (1970); black: Lherzolite in the CaO-MgO-Al_2O_3-SiO_2 system (Lherz) Presnall et al. endstream endobj 103 0 obj <>>>/EncryptMetadata false/Filter/Standard/Length 128/O($~j��+�o�LV�쯸������[��)/P -1324/R 4/StmF/StdCF/StrF/StdCF/U(��SЈ�+��R� )/V 4>> endobj 104 0 obj <> endobj 105 0 obj <> endobj 106 0 obj <>stream Plagioclase has a single large cation site (M) into which most trace elements of interest partition. These rocks have experienced only minor metamorphism (e.g., Takeda and Graham, 1991). Borghini et al. The following table shows their compositio… 102 0 obj <> endobj Plagioclase is rarely present in the peridotite unit, except as an intercumulus phase near the contact with the aphanitic unit. In many orogenic belts worldwide, metamorphic rocks of the greenschist and amphibolite facies predominate over all other metamorphic facies rocks. Hilary Downes, in Reference Module in Earth Systems and Environmental Sciences, 2020. They contain orthopyroxene inverted from low-calcium clinopyroxene (Mg# ∼67–58) and orthopyroxene inverted from pigeonite (Mg# ∼57–45). Hornblende is an aluminous amphibole with a composition that contains predominantly a pargasite component. They are plotted in Figure 85 and coexist with labradorite as an equilibrium assemblage. The greenschist facies represents lower P-T conditions than the amphibolite facies, i.e., a lower metamorphic grade. Polymict eucrites (Figure 23b) are polymict breccias consisting mostly of eucritic material, but they also contain <10 vol.% of a diogenitic component in the form of orthopyroxene. Rocks under the Microscope. First we use, as in the example above, just two characteristic minerals, a mineral pair, occurring in metabasalt to define a series of additional facies. At conditions of very low-grade metamorphism this is typically not the case. White, in Treatise on Geochemistry, 2003. If eclogites are formed by continuous prograde metamorphism, they are always the highest grade rocks. Noncumulate eucrites originally formed as quickly cooled surface lava flows (unequilibrated noncumulate eucrites), but most were subsequently metamorphosed (metamorphosed noncumulate eucrites). ... first plagioclase, then pyroxene. Metamorphic facies scheme, showing the PT fields of the zeolite and prehnite-pumpellyite facies. (1998) and Bindeman and Davis (2000) present SIMS analyses of Drake's (1972) experimental run products. TABLE 2.13. However, the observed changes in mineral chemistry of all coexisting phases within the plagioclase stability field, and the continuous nature of the transition, additionally suggest that, as a result of mass balance condition, the modal proportions of minerals need to change as well. The XFc values indicate how much Fe replaces Mg in the structural formula. ���+c��#Ҝل�_~�����{�}m[�HS����h`�j�>��j�{��S�� �E���J���� ���d�t 9D��9��7�2U�Z.��~��D pA���d�sjW�V)C\���>�\�|E�%Ō^,���c�p����:}*�h���b`��0�|���\������쒂_&S�� Plagioclase is also present in the pegmatoidal gabbroic rocks where it is black in hand specimens (see Fig. On the other hand, it should also be noted that the modal plagioclase found in the experiments probably represents an overestimation with respect to natural occurrences, where low-pressure recrystallization is generally confined to discrete microstructural domains between spinel-facies porphyroclastic minerals (Cannat and Seyler, 1995; Fabries et al., 1998; Hoogerduijn Strating et al., 1993; Montanini et al., 2006; Newman et al., 1999; Obata, 1980; Ozawa and Takahashi, 1995; Rampone et al., 1993, 1995, 2005; Takazawa et al., 1996). This rollover image shows the contrasting visibility of Plagioclase when viewed under plain polarized light versus crossed polars (second image). We use cookies to help provide and enhance our service and tailor content and ads. The two minerals are the major constituents of greenschists. The two minerals rarely occur together in the same rock. There are several plagioclase feldspars in the seriesbetween albite and anorthite. • Explain the optical properties observed under plane-polarized light and crossed polars. In greenschist facies rocks, however, zeolites. Plagioclase contains significant amounts of an anorthite component. It can be clearly observed as a zonal darkening of the grain due to changes in the optical properties of this phenomenon. with less An-component. Omphacite is an calcalkali pyroxene made of jadeite, diopside, and hedenbergite components. Metamorphic mafic rocks that contain orthopyroxene (Opx) and plagioclase belong to the granulite facies. Glaucophane is an alkali-amphibole that is dark or blue in hand specimen and colourless to blue under the microscope.