EXPERIMENT 10

MICROSCOPIC STUDY OF COMMON METAMORPHIC ROCKS

Outline of Experiment______

10.1 Introduction Gneiss Expected Learning Skills 10.5 Non-foliated Metamorphic 10.2 Requirements Rocks 10.3 Basic Concepts Marble 10.4 Foliated Metamorphic Quartzite Rocks 10.6 Laboratory Exercises Slate 10.7 References Phyllite 10.8 Learning Resources Schist

10.1 INTRODUCTION

You have learnt to identify metamorphic rocks in hand specimens in the previous experiment. In this experiment you will learn to identify microscopic characters of metamorphic rocks of foliated metamorphic rocks such as slate,

184

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. phyllite, schist and gneiss and non-foliated metamorphic rocks such as marble and quartzite. Expected Learning Skills______

After performing this experiment, you should be able to:

❖ identify texture and mineral composition of foliated metamorphic rocks such as slate, phyllite, schist and gneiss;

❖ recognise texture and mineral composition of non-foliated metamorphic rocks such as marble and quartzite; and

❖ list the facies and associated protolith of slate, phyllite, schist, gneiss, marble and quartzite.

10.2 REQUIREMENTS

To perform this experiment successfully, following are the requirements: • Polarising microscope with light source • Thin sections of slate, phyllite, schist, gneiss, marble and quartzite • Pen, pencil, eraser, scale, sharpener, coloured pencils and drawing compass • Laboratory file • You are instructed to draw the sketch of the hand specimen observed in the laboratory given by your instructor.

Instructions: You are required to study Block 3 of BGYCT-135 course (Petrology) before performing this experiment. Bring this practical manual along with Block-3 of BGYCT-133 course while attending the Practical Counselling session.

You have read in Block 3 of BGYCT-133 course that the optical properties of minerals are helpful in their identification. These properties of minerals are useful in the identification of rocks. In this experiment, you will have to utilise your knowledge and experience about the optical properties of rock-forming minerals in the identification of rocks in thin section.

You have been instructed to follow the steps mentioned below:

1. In order to describe the petrographic characters of the rock in thin section you will observe under the microscope, draw a circle with the help of

185 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... drawing compass in your laboratory file as done in the previous experiments. 2. You will draw sketches of the petrographic characters of the rock as seen under plane polarised light (abbreviated as PPL) in one half of the circle 3. In the other half of the circle, you must draw the observed petrographic characters of the rock observed the cross nicols or crossed polars (abbreviated as XP). 4. You have read that the rock is an aggregate of minerals. Therefore, to understand optical properties of rock forming minerals is essential which you had learnt in Experiments 7 and 8 of BGYCL-134 course. Such studies done earlier will help you in microscopic studies of rocks. 5. Make use of the coloured pencils to represent colour(s) of the minerals as seen under the microscope in thin sections. After writing about observed petrographic characters of the rocks and draw their sketches both under plane polarised light and crossed nicols, you have to mention name and protolith of the rock in your laboratory file.

Remember!! Descriptions of the petrographic characteristics in the tables and the sketch given in this experiment are generalized. You have to document your own observations and draw the sketch of the hand specimen in the laboratory file.

10.3 BASIC CONCEPTS

You are repeatedly being instructed that your knowledge of recognising minerals acquired from BGYCT-133 and BGYCL-134 courses will be helpful in the experiments related to microscopic study of rocks. It is utmost important that the optical properties of the minerals present in the rock should be observed and identified carefully. You have to document the following microscopic characters as given in Table 10.1 in your laboratory file. We had discussed these characters in the previous experiment, related to the identification of rocks in hand specimens. You are instructed to read this section very carefully to understand different aspects of the microscopic/petrographic character to be identified.

Table 10.1: Microscopic Characters of ……………….

1. Grain Size : 2. Texture : • Foliation : • Lineation :

186

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. • Structure : 3. Mineral Composition : • Essential Minerals : • Accessory Minerals : 4. Metamorphic Facies : 5. Inference/Name : 6. Parent Rock :

Before thin sections examination of foliated and non-foliated rocks, let us discuss their texture, mineral composition, facies and parent rock.

1) Grain Size: Grain size in metamorphic rock is highly variable. It can be broadly described as coarse, medium or fine. Further subdivisions of the grains can be given by standard size scale. You have to measure the grain size with the help of micrometer provided in the eye piece. For this purpose, you have to bring grain below the eye piece in-built with the micrometer. In order to classify the rock in the thin section (as hornblende schist, feldspar bearing gneiss etc.) you have to consider average size of the majority of the grains.

2) Texture: You have read that metamorphic rocks are formed as the result of various types of metamorphic processes the pre-existing igneous and sedimentary rocks which involve changes in the textures, structures and mineralogical compositions. Structures in the metamorphic rocks reflected on large scale or outcrop scale or hand specimen scale or they may be of regional scale also. On the contrary textures are at small scale phenomenon, and vary from microscopic to hand specimen scale. Mutual agreement and arrangement of mineral grains in the rock unit, their shape, size and growth constitute a few important factors responsible for the development of certain texture types.

A) Foliation: In general, foliation is a non-genetic term used for layering in a metamorphic rock. Foliated rocks typically appear as if the minerals are stacked like pages of a book. The term is applied to surfaces which represents relict bedding or to surfaces resulted from the deformation and/or recrystallization of pre-existing minerals. There are several types of foliations: • Compositional layering: The foliation is defined as alternate layers composed of different minerals. You can easily recognise it by differences in the colour of the layers.

187 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... • Schistosity: Schistosity refers to foliation which possesses a "preferred orientation" of platy or flaky minerals and signifies parallel to sub-parallel arrangement of flaky or platy or platy minerals forming bands of distinctive characters/nature. The bands composed of minerals such as garnet and hornblende and layers consists of platy minerals such as mica, chlorite, aluminosilicates or prismatic minerals such as amphibole, tourmaline. Gneissose rocks may or may not possess schistosity. • Gneissosity: It refers to compositional layering in which layers of roughly equidimensional grains (such as quartz, feldspar), alternate with more schistose layers of platy or elongate grains, or with other granoblastic layers. B) Lineation: Lineation refers to any linear structure that occurs repeatedly in the rock. It is a linear fabric element of a primary igneous or sedimentary structure or a secondary feature developed due to deformation. Minerals such as hornblende is longer in one direction and linearity is similar to pencil or a needle. These linear minerals are also aligned within rock and are referred as a lineation. Hornblende, tourmaline, or stretched quartz grains are linear crystals and can be arranged as a foliation, a lineation, or foliation/lineation both.

Metamorphic textures are categorised into four following groups:

i) Palimpsest (Relict) Textures Blasto-Ophitic Texture Blasto-Intergranular Texture Blasto-Porphyritic Texture ii) Typomorphic Textures Porphyroclastic Texture Mylonitic Texture Nodular Texture Granoblastic Texture Crystallization Textures Porphyroblastic Texture Poikiloblastic Texture iii) Reaction Textures Reaction-Rim Texture Kelyphitic Texture Corona Texture iv) Intergrowth Textures

188

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. Symplectite Texture

Let us discuss them in brief. i) Palimpsest Texture: Palimpsest texture or relict refers to the texture inherited from the texture of a protolith preserved even after the metamorphism of the protolith. Low-grade metamorphic rocks show good preservation of relict textures and are formed in such a way that deformation gets limited. Palimpsest textures include; ophitic texture, intergranular texture, porphyritic texture and cumulate texture. • Blasto-Ophitic Texture: Plagioclase laths (Fig.10.1) enclosed within the pyroxene crystal (sometimes olivine). When plagioclase laths are fully surrounded in the matrix, it is said to be ophitic and if partially surrounded, it is called as subophitic texture.

Fig. 10.1: Plagioclase laths enclosed by augite crystal under XP. (Source: www.alexstrekeisen.it)

• Blasto-Intergranular Texture: The relict texture of igneous rock is preserved and viewed in metamorphic rock (under the microscope) shows interstices formed between plagioclase crystals is occupied by ferromagnesium minerals (such as pyroxene, olivine etc.). The interstices are angular and form generally between two large crystals (Fig. 10.2).

189 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…......

Fig. 10.2: Photomicrograph showing coarse-grained intergranular texture in a metamorphic rock bearing textural characteristics of a basalt protolith observed under XP. (Source: www.alexstrekeisen.it)

• Blasto-Porphyritic Texture: Large grains or crystals are set into the fine-grained matrix or glassy groundmass (Fig. 10.3). They are more common in extrusive igneous rocks. Presence of such type of texture in a metamorphic rock is called as a relict microtexture.

Fig. 10.3: Photomicrograph showing plagioclase clustered in a fine-grained groundmass containing biotite, clinopyroxene and plagioclase and shows porphyritic texture. (Source: www.alexstrekeisen.it)

ii) Typomorphic Texture: These textures are developed either by the thermal action or by the effect of dynamic forces. Based on the dynamic effect, following type of textures are observed in metamorphic rocks:

• Porphyroclastic Texture: Due to deformation in the metamorphic rock, the softer minerals get crushed and form groundmass, while the resistant minerals get fragmented and appear as larger grains than the surrounding minerals (Fig.10.4). Such type of appearance of the rock after the deformation is called as porphyroclastic texture and the larger grain is called as the porphyroclast.

190

Experiment 9 Megascopic Study of Common Metamorphic Rocks ……………………………………………………………………………………………………………………….

Fig. 10.4: Porphyroclastic texture with porphyroclast in the centre surrounded by groundmass minerals under XP; Field of View = 2mm. (Source: www.alexstrekeisen.it)

• Mylonitic Texture: Development of foliation planes or oriented mineral grains in the metamorphic rocks due to cataclastic metamorphism is such that the advance in latter renders the platy minerals or quartz (ribbon quartz) to orient in a particular direction (Fig.10.5a). The foliation may be incipient (protomylonitic) or it may be well defined (orthomylonitic; quartz grains get oriented in a ribbon-like fashion) or foliation persisting in the most advanced stage of cataclasis/ultramylonitic (Fig.10.5 b).

Fig. 10.5: a) Mylonite showing S-C fabric on an outcrop scale (Source: www.alexstrekeisen.it); and b) Mylonite in between two ultramylonite bands (XP image; Field of View = 5mm. (Source: Rudolph et al., 2009)

Based on thermal effects alone (and devoid of deformation), following types of typomorphic textures have been recognised in metamorphic rocks:

• Nodular Texture: When growth of an oval shape porpyroblasts is associated with the randomly distributed quartz or some other minerals, the texture so developed is called as nodular texture (Fig. 10.6).

191 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…......

Fig. 10.6: Nodular texture diagrammatic representation in a thermally metamorphosed rock. (Source: Spry, 1969)

• Granoblastic Texture: The granoblastic texture may vary from equidimensional grains with straight grain boundaries and crystal faces well developed as in polygonal granoblastic texture (Fig. 10.7a, b, c) to irregular grain boundaries as in interlobate granoblastic texture. When mineral grains are interlocked, randomly oriented and prismatic or elongated in habit with triple junctions as a common trend then such type of a granoblastic texture is called as decussate granoblastic texture (Fig. 10.7d, e).

Fig. 10.7: a-e) Diagrammatic representations showing possibilities of different granoblastic texture types in a phaneritic non-foliated metamorphic rock. (Source: Spry, 1969)

• Crystallisation Textures: Crystallisation textures also fall in the typomorphic type of metamorphic textures in which coarse-grained 192

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. metamorphic textures used to develop. Some of the crystallisation textures are: a) Porphyroblastic Texture: When coarse-grained minerals occur in a fine-grained groundmass or surrounded by fine-grained mineral grains, such type of a texture is called as porphyroblastic texture and the large mineral grain is called a porphyroblast (Fig.10.8).

Fig. 10.8: Garnet porphyroblast (XP; Field of View = 2mm). (Source: www.alexstrekeisen.it)

b) Poikiloblastic Texture: Such type of texture is marked with the addition of several fine-grained inclusions within the porphyroblast grain. The inclusions within the mineral grains may be helictic or spiral or seemingly lower in orientation (Fig. 10.9).

Fig. 10.9: Poikiloblastic texture showing inclusions with helictic orientation within the porphyroblast (‘R’ = Rotation axis). (Source: Bard, 1980) iii) Reaction Texture: When new mineral is formed either by the replacement of older mineral or by the reaction between the two phases or sequential reactions leading to the formation of new minerals in the form of concentric rings, the micro texture so formed represents a reaction texture. Some of the important reaction textures are:

193 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... • Reaction-Rim Texture: When older mineral phase is replaced by a new mineral phase along the rim, the texture is called reaction-rim texture. The rim so developed is irregular and forms a contact between the two mineral grains, indicating some reactions that have taken place between the two mineral phases. (Fig. 10.10).

Fig. 10.10: Reaction rim of garnet between plagioclase and hornblende. (Source: www.academic.brooklyn.cuny.edu)

• Kelyphitic Texture: In this texture also, replacement takes place but the intergrowth of two or more minerals. The resultant texture is such that the neo-formed mineral species completely encircles the replaced mineral (Fig. 10.11).

Fig. 10.11: Kelyphitic texture showing Garnet-Clinopyroxenite; (a) PPL ; and (b) XP; White arrows represents the plane of unconformity. (Source: Obata, 2007)

• Corona Texture: This type of texture is possible in both prograde and retrograde phases of metamorphism in which one or more minerals form a complete rim around the older phase present in the centre or core of the texture so that the geometry resembles a corona (Fig. 10.12).

194

Experiment 9 Megascopic Study of Common Metamorphic Rocks ……………………………………………………………………………………………………………………….

Fig. 10.12: Photomicrograph showing orthopyroxene (outer rim) – sillimanite (middle layer) corona on sapphirine (blue). (Source: Sandiford, 1985) iv) Intergrowth Texture: This includes symplectite and myrmekite textures. • Symplectite Texture: The texture is represented by worm-like appearance of minerals (Fig.10.13), formed along the boundary of the older mineral grains which have undergone reaction (Fig. 10.14).

Fig. 10.13: XP image showing myrmekite in plagioclase (Field of View = 2mm). (Source: www.alexstrekeisen.it)

Fig. 10.14: Symplectite of Orthopyroxene – Garnet – Cordierite (XP). (Source: Prakash et al., 2017)

195 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... 3) Mineral Composition: To observe, describe and document mineralogical compositions, you have to use the terms such as essential and accessory. You have read about these terms earlier also. Abundantly present mineral(s) in thin section suggests that they are essential mineral(s). Essential minerals are those minerals whose presence are necessary and considered to be essential for naming or nomenclature of the rock. Mineral(s) present in subordinate or lesser amount signifies that they are accessory minerals. Presence of accessory minerals is not necessary and not used in naming of a particular rock. Therefore, mineral(s) present abundantly are essential minerals and those present in subordinate amounts are accessory minerals. Some of the common metamorphic minerals are andalusite, kyanite, sillimanite, staurolite, garnet, chlorite, biotite, muscovite, glaucophane, talc, serpentine, actinolite and epidote.

You have read about essential and accessory minerals, while identifying igneous rocks megascopically and microscopically in Experiments 1 to 6. Similarly, you have to identify essential and accessory minerals in metamorphic rocks. You have to briefly mention the optical properties of the mineral(s) identified.

While discussing the mineralogical composition of the rock, please note down the diagnostic optical properties of each mineral identified by you. It is not necessary that you will find all the accessory minerals in a thin section provided to you in the study centre.

4) Metamorphic Facies: Metamorphic facies represent a set of mineral assemblages for metamorphic rocks formed under similar pressure s and temperature conditions. Two-dimensional pressure vs. temperature plot/graph represents a set of conditions under which a typical assemblage of metamorphic minerals that exists (Fig. 10.15). On the basis of mineral assemblage, you have to infer corresponding metamorphic facies. As you have already studied about metamorphic facies in Unit 2 of BGYCT-135 course.

196

Experiment 9 Megascopic Study of Common Metamorphic Rocks ……………………………………………………………………………………………………………………….

Fig. 10.15: Metamorphic facies in pressure-temperature space. (Source: Redrawn from https://en.wikipedia.org/wiki/Metamorphic_facies)

5) Inference/Name: On the basis of textural and mineralogical compositions you have to infer name of the rock.

6) Parent Rock: You have to mention about the parent rock or protolith i.e. the lithological composition of the rock that existed prior to the metamorphism as given in Table 10.2.

Table: 10.2: Protolith of Metamorphic Rocks. Name of Rocks Parent Rocks Grain Sizes

Slate Shale, mudstone, siltstone Very fine

Phyllite Slate Fine Schist Phyllite Medium to coarse Foliated Gneiss Schist, granite, basalt, dolerite Medium to coarse

Medium to coarse

- Marble Limestone Non

oliated Medium to coarse Quartzite Quartz sandstone F

Now you will identify microscopic characters of foliated and non-foliated metamorphic rocks such as slate, phyllite, schist, gneiss, quartzite and marble.

197 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... 10.4 FOLIATED METAMORPHIC ROCKS

As discussed in Experiment 9, you know that foliated rocks show development of conspicuous parallelism in the mineralogical and structural constitution. These parallel features include slaty cleavage, schistosity and gneissose structures. The foliated metamorphic rocks in accordance with the grade are; slate, phyllite, schist and gneiss types. Let us discuss them one by one in the following sections.

10.4.1 Slate

Slate is a fine-grained foliated metamorphic rock formed by low-grade regional metamorphism of shale or mudstone. In thin sections, slate is texturally fine- grained and new growth of mica minerals. The oriented direction of platy minerals is perpendicular to the growth-line of the micaceous minerals in which rock has undergone compression which is perpendicular to the growth line of mica minerals. Due to very fine-grained nature and less development of mica minerals, individual grains of mica cannot be marked. Interlocking crystals are seen which are parallel to each other. The microscopic characters of the slate are given in Table 10.3. With the help of photomicrographs (Figs. 10.16 and 10.17) and sketches (Fig. 10.18), identify slate in thin section.

Table 10.3: Microscopic Characters of the Slate. 1. Grain Size : Very fine grained, less than 0.1mm. 2. Texture : Texturally very fine-grained. • Foliation : Well foliated; parallel foliation (layering) of fine-grained platy minerals (chlorite) in a direction perpendicular to the direction of maximum stress. Crenulation cleavage (in granite rich slate). • Lineation : Quartz grains show s alignment parallel to c axis. • Structure : Slaty cleavage. 3. Mineral Composition: • Essential Minerals : Biotite (colourless, 1 set of cleavages, 2nd order brown to red interference colour, parallel extinction, flaky and lamellar form). Chlorite (light green, 1 set of cleavages, 1st and 2nd order interference colours, Parallel extinction, occurs as grain and also as cement). Muscovite (colourless, 1 set of cleavages, 2nd order yellow and red interference colours, parallel extinction, flaky and 198

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. lamellar form). • Accessory Minerals : Apatite, graphite, magnetite, tourmaline, or zircon as well as feldspar. 4. Metamorphic Facies : Greenschist. 5. Inference/Name : SLATE 6. Parent Rock : Mudstone/ siltstone / shale (pelitic).

Fig. 10.16: Clay size crystals in slate under PPL and XP.

Fig. 10.17: Clay size crystals in slate: sketch of the field of view under the microscope under PPL and XP.

199 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…......

Fig. 10.18: Indiscernible mica crystals in slate under PPL and XP.

10.4.2 Phyllite

Phyllite is a foliated rock, formed by metamorphism of slate. Phyllite is a low- grade metamorphic rock. In thin section phyllite seems to be very similar to slate, but could be differentiated from the slate with respect to mica grains. Mica flakes in phyllite are large enough. When viewed under the crossed polar conditions the mica grains are oriented and reflect pink or shades of pink interference colours. Other minerals could be identified based on their individual optical properties. The microscopic characters of phyllite are given in Table 10.4. With the help of photomicrographs (Figs. 10.19 and 10.20) and sketches (Fig. 10.21), identify phyllite in thin section.

Fig. 10.19: Parallelly oriented mica flakes in phyllite under PPL and XP.

200

Experiment 9 Megascopic Study of Common Metamorphic Rocks ……………………………………………………………………………………………………………………….

Fig. 10.20: Discernible mica crystals in phyllite under PPL and XP.

Table 10.4: Microscopic Characters of Phyllite. 1. Grain Size : Fine to medium grained (0.1-1.00 mm particle size). 2. Texture : Mylonitic texture. • Foliation : Foliated (mm scale) commonly wrinkled or wavy; schistosity well-developed. • Lineation : Platy minerals aligned perpendicular to c axis and giving rise to lineation. 3. Mineral Composition: • Essential Minerals : Muscovite (colourless, 1 set of cleavages, 2nd order yellow and red interference colours, parallel extinction, flaky and lamellar form). Biotite (colourless, 1 set of cleavages, 2nd order red interference colour, parallel extinction, flaky and lamellar form). Quartz (colourless, no cleavage, 1st order grey, yellow interference colours, undulose or wavy extinction and twinning, occurs as clast and also cement). Plagioclase (colourless, 2 sets of cleavages intersecting at an angle of 90o, low relief, 1st order white, grey, yellow interference colour, oblique extinction, polysynthetic twinning, shows zoning). • Accessory Minerals : Sericitic mica, graphite, chlorite. 4. Metamorphic Facies : Low Grade; Greenschist. 5. Inference/Name : PHYLLITE 6. Parent Rock : Shale or mudstone.

201 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…......

Fig. 10.21: Parallel orientation of mica crystals in phyllite: sketch of the field of view under the microscope under the PPL and XP.

10.4.3 Schist

In general, schist is a strongly foliated medium grained rock, formed on the continental side of a convergent plate boundary. It is formed by the metamorphism of rocks of higher grade than the slate. In thin section, schist is identified by its coarse texture (coarser than Phyllite) with light and dark bands of minerals. Light coloured minerals being the felsic minerals and darker minerals are of mafic minerals. Unlike in gneiss, the banding in schist is continuous. The micaceous minerals are well oriented and identified easily. Under the microscope, schistosity is clearly visible due to the parallel orientation of mica flakes. Under the XP, the mica appears bright shades of pink or red. Presence of other minerals such as kyanite, hornblende, garnet etc. is identified by making their individual analyses. Schist is wrinkled due to deformational activities that would have taken place in the history of the rock. The microscopic characters of schist are given in Table 10.5. With the help of photomicrographs (Figs. 10.22 and 10.23) and sketches (Fig. 10.24) identify schist in thin section.

202

Experiment 9 Megascopic Study of Common Metamorphic Rocks ……………………………………………………………………………………………………………………….

Fig. 10.22: Garnet mica schist under the PPL and XP.

Table 10.5: Microscopic Characters of Schist. 1. Grain Size : Fine to medium grained. 2. Texture : Coarse texture and of light and dark minerals; Intergranular texture. • Foliation : Well-developed foliation (scale: mm to cm scale) due to the preferred orientation of sheet silicates (mainly biotite and muscovite). Quartz and feldspar grains however show no preferred orientation. • Lineation : Parallel orientation of mica flakes. • Structure : Schistose structure. 3. Mineral Composition: • Essential Minerals : Muscovite (colourless, 1 set of cleavages, 2nd order yellow and red interference, parallel extinction, flaky and lamellar form). Quartz (colourless, no cleavage, 1st order white, grey, yellow interference colours, undulose or wavy, occurs as clasts and also as cement). Plagioclase (colourless, 2 sets of cleavages intersecting at an angle of 90o , low relief, 1st order white, grey, yellow interference colours, oblique/inclined extinction, polysynthetic twinning, shows zoning). Porphyroblasts of garnet are common. • Accessory Minerals : Quartz, feldspars, kyanite, chlorite, garnet, staurolite and sillimanite. 4. Metamorphic Facies : Intermediate/medium metamorphic grade between phyllite and gneiss; Amphibolite facies. 5. Inference/Name : SCHIST 6. Parent Rock : Shale and mudstone, basalt and dolerite in case of amphibolite schist.

203 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…......

Fig. 10.23: Hornblende schist under the PPL and XP.

Fig. 10.24: Garnet mica schist: sketch of the field of view under the microscope under PPL and XP.

10.4.4 Gneiss

Gneiss is a high-grade, foliated metamorphic rock. The rock is medium to coarse grained. In thin section, gneiss is identified by the much coarser texture (coarser than schist) and bandings of light and dark mafic minerals, respectively. Unlike in schist, the bands in gneiss are discontinuous and wane off after some indefinite scale. On analysis, it has been found that schistosity is poorly developed as there is only little mica in the rock. Less mica in the rock makes it more compact and less breakable unlike slate, phyllite and schist. Minerals such as feldspar, pyroxene, amphiboles, mica and quartz are identified based on the individual mineral properties. The microscopic characters of slate are given in Table 10.6. With the help of photomicrographs (Figs. 10.25, 10.26 and 10.27) and sketches (Fig. 10.28) identify gneiss in thin section.

204

Experiment 9 Megascopic Study of Common Metamorphic Rocks ……………………………………………………………………………………………………………………….

Fig. 10.25: Feldspar bearing gneiss under the PPL and XP.

Table 10.6: Microscopic Characters of Gneiss. 1. Grain Size : Medium to coarse grained. 2. Texture : Mylonitic, porphyroblastic texture. • Foliation : Foliated, banded appearance and is made up of granular mineral grains. • Lineation : Quartz, feldspars, mica and amphibole grains alternate as light and dark coloured bands giving the rock a gneissic banding. 3. Mineral Composition: • Essential Minerals : Light band of felsic minerals such as feldspar and quartz. Quartz (colourless, no cleavage, 1st order white, grey, yellow interference colours, undulose wavy, occurs as clast and also as cement). Plagioclase (lath shaped, colourless, low relief, 2 sets of cleavages intersecting at an angle of 90o, polysynthetic twinning, 1st order grey, yellow interference colours). Dark bands of mafic minerals such as biotite, pyroxene, and amphibole. Biotite (colourless, 1 set of cleavages, 2nd order red interference colour, parallel extinction, flaky and lamellar form). Garnet (pink, isotropic, high relief). Pyroxene (colourless/pale green/pale brown, 2 sets of cleavages intersecting at an angle of 90o, moderate relief, second order interference colours. Amphibole (green, 2 sets of cleavages intersecting at an angle of 120o, pleochroic, moderate interference colours, inclined extinction, may alter to chlorite (pale green).

205 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... • Accessory Minerals : Hornblende, garnet. 4. Metamorphic Facies : High grade metamorphic rock. 5. Inference/Name : GNEISS 6. Parent Rock : Shale, mudstone, or felsic igneous rocks.

Fig. 10.26: Gneiss showing bright bands of biotite under PPL and XP.

Fig. 10.27: Gneiss: sketch of the field of view under the microscope under the PPL and XP.

10.5 NON-FOLIATED ROCKS

As discussed in Experiment 9 we know that the non-foliated rocks are those metamorphic rocks by partial or complete absence of foliation or parallelism of mineralogical constituents. Typical examples of non-foliated rocks are marbles, quartzites etc. Let us discuss microscopic characters of marble and quartzite in the following section.

10.5.1 MARBLE

In general, marble is a non-foliated, produced at high temperature and pressure by regional or mostly contact metamorphism of limestone/dolomite or 206

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. calcareous rocks. In thin sections, marble shows crystalline and sugary texture with interlocking calcite crystals. Calcite crystals could be identified by the three sets of cleavages, rhombic or rhombohedral grains and lamellar twinning. If dolomite is present, then it is identified by lamellar twinning, perfect rhombohedral cleavage and coarse to fine grained aggregates of dolomite crystals. The microscopic characters of marble are given in Table 10.7. With the help of photomicrographs (Figs. 10.28 and 10.29) and sketches (Fig. 10.30) to identify marble in thin section.

Fig. 10.28: Crystalline sugary texture in marble under the PPL and XP.

Table 10.7: Microscopic Characters of Marble. 1. Grain Size : Medium to coarse grained. 2. Texture : Granular, sugary texture. • Foliation : Granular. • Lineation : Granular. • Structure : Granulose. 3. Mineral Composition: • Essential Minerals : Calcite (colourless, 3 sets of cleavages, 3rd order bright interference colours, symmetrical extinction/ twinkling, occurs as grain and cement) or dolomite (colourless, perfect rhombohedral cleavage, lamellar twinning, inclined extinction). • Accessory Minerals : Clay minerals, mica, quartz, pyrite, iron oxides, and graphite. 4. Metamorphic Facies : Variable grade regional or contact metamorphism; Granulite facies. 5. Inference/Name : MARBLE 6. Parent Rock : Limestone or dolostone.

207 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…......

Fig. 10.29: Heteroblastic, mortar texture showing calcite crystals sutured and embayed margins under the PPL and XP.

Fig. 10.30: Sugary texture in marble: sketch of the field of view under the PPL and XP.

10.5.2 QUARTZITE

Quartzite is non-foliated metamorphosed sandstone generally form along the convergent plate boundaries and along the orogens. Quartzite is a common metamorphic rock. The protolith is sandstone which occurs a commonly. In thin sections, quartzite shows crystalline and sugary texture like marble with interlocking quartz crystals. Quartz crystals are identified as colourless grains under the plane polarized light and show 1st order grey interference colours. Quartz is identified by the absence of cleavages, anhedral to subhedral form/habit and undulatory extinction. Other minerals if present must be studied individually based on their respective optical properties. The microscopic characters of quartzite are given in Table 10.8. With the help of photomicrographs (Figs. 10.30 and 10.31) and sketches (Fig. 10.32) identify quartzite in thin section.

208

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. Table 10.8: Microscopic Characters of Quartzite. 1. Grain Size : Medium grained, equigranular, interlocking grains of quartz. 2. Texture : Granular, sugary texture (Fig.10.32). • Foliation Granular. • Lineation : None. • Structure : Granulose. 3. Mineral Composition: • Essential Minerals : Abundant quartz grains (colourless, very low relief, cleavage absent, undulose extinction, 1st order grey, yellow interference colours). Mono and polycrystalline quartz. Other mineral impurities like hematite may be present. • Accessory Minerals : Micas, feldspars, garnets and some amphiboles. 4. Metamorphic Facies : Variable grade, regional or contact metamorphism metamorphism; granulite facies. 5. Inference/Name : QUARTZITE 6. Parent Rock : Pure sandstone, quartz arenite.

Fig. 10.30: Sugary texture, fused quartz grains under PPL and XP.

Fig. 10.31: Sutured grain boundaries between quartz grains under PPL and XP. 209 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…......

Fig. 10.32: Sugary texture, fused quartz grains: sketch of the field of view under PPL and XP.

10.6 LABORATORY EXERCISES

Study microscopic characters of the rock samples such as slate, phyllite, schist, gneiss, marble, quartzite given to you by your counselor. Follow the instructions as given below: 1. Get a polarising microscope and rock thin sections from your counsellor. 2. Make all necessary adjustments of the microscope and be sure that light the source is working properly and illuminating the field view of the microscope. 3. Please do not disturb centring of the microscope. 4. Place thin section of a rock on the rotatable stage of the microscope. 5. Study the optical properties of minerals in the rock under plane polarised light (PPL) and between crossed nicols/crossed polars (XP) as you have done for igneous and sedimentary rocks. 6. If you face any problem during the examination of rock thin sections in the microscope, consult your counsellor. 7. Draw neat sketches of the rock (as observed) in your laboratory file under plane polarised light in one half and between crossed Nicols in the other half of the circle. 8. It is important for you to note that the photomicrographs and sketches given in the text are for your understanding. 9. You are required to make observations of the rock thin sections given to you both under plane polarised light and cross nicols. 10. Please do not copy the same sketches given in this experiment in the laboratory file. 210

Experiment 9 Megascopic Study of Common Metamorphic Rocks ………………………………………………………………………………………………………………………. 11. Handle the polarising microscope carefully and do not damage it. 12. If number of students is more and microscopes are les then you are advised to work in groups. Instructions: Make careful observations of the microscopic study of the rock in thin sections given to you. As shown in Table 10.1, document the observations made during the study and draw neat sketches as instructed in Section 10.2 under plane polarised light and between cross Nicols. Give answers of the following exercises in your laboratory file and submit it to your Academic Counsellor.

Exercise 1: Identify gneiss under plane polarised light and cross nicols. Write its microscopic characters and draw a neat sketch.

Exercise 2: Identify phyllite under plane polarised light and cross nicols. Write its microscopic characters and draw a neat sketch.

Exercise 3: Identify schist under plane polarised light and cross nicols. Write its microscopic characters and draw a neat sketch.

Exercise 4: Recognise slate under plane polarised light and cross nicols. Write its microscopic characters and draw a neat sketch.

Exercise 5: Identify marble under plane polarised light and cross nicols. Write its microscopic characters and draw a neat sketch.

Exercise 6: Recognise quartzite under plane polarised light and cross nicols. Write its microscopic characters and draw a neat sketch.

10.7 REFERENCES

• Alan Spry (1969) Metamorphic Textures, 1st Edition, Elsevier, 358p. • Alexander, Pramod, O. (2009) Minerals, Crystals, Rocks and Ores. New India Publishing Agency, 675p. • Bard, J.P (1980) Microtextures of Igneous and Metamorphic Rocks (Petrology and Structural Geology), Springer, 264p. • Michael Sandiford (1985) The metamorphic evolution of granulites at Fyfe Hills; Implications for Archaean crustal thickness in Enderby Land, Antarctica; 3-2, pp. 155 – 178. • Obata, M. (2007) Petrography revived: the science of rock texture, (in Japanese with English abstracts and figure captions), Japanese Magazine of Mineral. Petrol. Sci., 36, pp168-181. • Prakash et al. (2017) Geochronology and phase equilibria modelling of ultra-high temperature sapphirine + quartz-bearing granulite at Usilampatti, 211 BGYCL-136 Petrology: Laboratory ……………………………………………………………………………………………….…...... ….…...... Madurai Block, Southern India: Geochronology and Phase Equilibria Modelling of UHT Granulites, Geol. J. • Rudolph A.J. Trouw, Cees W. Passchier, Dirk J. Wiersma (2009) Atlas of Mylonites and related microstructures, Protomylonite, Mylonite and Ultramylonite, pp 101-112. https://link.springer.com/chapter/10.1007/978-3- 642-03608-8_7 • Tyrell, G. W. (1973) The principles of Petrology. John Wiley & Sons, 349p. • https://en.wikipedia.org/wiki/Metamorphic_facies • https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/schist.html • http://geomaps.wr.usgs.gov/parks/noca/nocaft4.html • http://www.impact-structures.com/impact-rocks-impactites/the-impact- breccia-page/monomictic-breccia-monomictic-impact-breccia-monomictic- movement-breccia/ • https://www.tulane.edu/~sanelson/eens1110/metamorphic.htm • www.alexstrekeisen.it • www.academic.brooklyn.cuny.edu (Websites accessed between 10th December 2019 and 9th May 2020)

10.8 LEARNING RESOURCES

• Metamorphic rock in thin section Link: https://www.youtube.com/watch?v=n5kyADbNhdc • Study of schist in thin section under microscope Link: https://www.youtube.com/watch?v=F9MZKRlyDJM • Petrology- A Quartzite cross polarised Link: https://www.youtube.com/watch?v=0k-fYy8gQ8E • Study of gneiss in thin section under microscope Link: https://www.youtube.com/watch?v=qmeL9Cz_Jeg • Deformed metamorphic quartz thin section under XPL Link: https://www.youtube.com/watch?v=-OpsQoII520

212