S. What is the name of this metamorphic rock? 4. What type(s) of rock might have
ID: 115844 • Letter: S
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S. What is the name of this metamorphic rock? 4. What type(s) of rock might have been the protolith for this rock? garnet uscovite Figure A7.3.3 E Analyze the metamorphic rock sample in Fig. A7.3A. 1. Is this rock foliated or nonfoliated (granofelsic)? What features in the photograph did you use to make your interpretation? 2. What is the name of this metamorphic rock? 3. What type(s) of rock might have been the protolith for this rock? plagioclase (white) garnet (red) pyroxene (green) 1 cm Figure A7.3.4 REFLECT & DISCUSS Explain your answer Which one of the rocks in this activity do you think has the highest metamorphic grade? 200Explanation / Answer
D (1) The mineral that defines foliation feature is muscovite. Because these minerals are sheet minerals of the phyllosilicate group, there crystal habit is platy so when undergoes to metamorphic pressure allign themselves perpendicular to the stress direction forming alternate layers commonly known as foliation structure. Foliation is a penetrative structure evenly distributed throughout the rock body.
(2) The overall texture of the rock is foliated. The other typical texture found is the spotted texture or nodular texture. The mineral showing this texture is garnet which are almost ovoid in shape surrounded by foliation planes of muscovite.
(3) The name of the rock is garnet-muscovite schist with muscovite as the dominant mineral.
(4) From sedimentary point of view the protolith rock is shale or mudstone while if look from igneous rocks the protolith it can be from a felsic rock.
E (1) As you can see that the green minerals (pyroxene) and white minerals (plagioclase) are forming alternate layers along a direction so the rock is foliated and the foliated minerals are plagioclase and pyroxene.
(2) The name of the rock is granulite because the rock contain pyroxene, plagioclase and abundant garnet making it a high grade rock suffered upto granulite facies. This assemblage of minerals names it a granulite. The rock is fine grained with abundant garnet.
(3) The protolith can be both sedimentary and igneous origin. It can be from pelitc origin or mafic origin mainly from quartzo-feldspathic rocks composed of OH-free minerals.
F) The rock is obviously granulite having high grade of metamorphism. The minerals defining the granulite are pyroxene, plagioclase and garnet making it metamorphosed upto granulite facies (high temperature and high pressure). Whereas the other rock is a schist having low to medium grade of metamorphism.
C) As the figure 7.5A is missing I cannot make a concrete answer but what i can say that the rock have suffered high strain. These rocks are found in high stress-strain areas like Himalayas. The folded layers that are formed is called the ptygmatic folds where the axial planes are oriented in different direction. The white parallel layers that you say were flat but didnt break due to folding because the minerals were not brittle while the layers were competent layers. The stress acted parallel to the layers making the folded structures.
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