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Megascopic Study of Common Ultramafic Igneous Rocks

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Megascopic Study of Common Ultramafic Igneous Rocks EXPERIMENT 4 MEGASCOPIC STUDY OF COMMON ULTRAMAFIC IGNEOUS ROCKS Outline of Experiment_____________________________ 4.1 Introduction 4.5 Megascopic Study of Dunite Expected Learning Skills 4.6 Laboratory Exercises 4.2 Requirements 4.7 References 4.3 Basic Concepts 4.8 Learning Resources 4.4 Megascopic Study of Peridotite 4.1 INTRODUCTION You have learnt to identify felsic rocks such as granite and rhyolite in the Experiment 1, and intermediate and mafic rocks in Experiment 2 and Experiment 3 respectively. In this experiment you will learn to identify megascopic characters of ultramafic rocks. You will learn about the megascopic identification of peridotite and dunite. You have learnt about ultramafic rocks while discussing the basic concepts of four subgroups of igneous rock viz. felsic, intermediate, mafic and ultramafic. Expected Learning Skills__________________________ 51 Petrology: Laboratory ………BGYCL…-136…………………………………………………………………… ……………….….............….….... After performing this experiment, you should be able to: ❖ identify megascopic characters, of peridotite and dunite; ❖ recognise mineral compositions and textures in peridotite and dunite; and ❖ discuss uses and Indian occurrence of peridotite and dunite. 4.2 REQUIREMENTS You will be required with the following items to perform this experiment successfully: • Hand specimens of peridotite and dunite • Hand lens • Laboratory file, pen/ pencil and eraser Note: • Do not use pen/pencil/marker pen to mark the hand specimen of the rock. • Please do not attempt to break the specimen in the laboratory. Many of the specimens cannot be readily replaced. • You are advised to identify the hand specimen provided to you at your study centre with the help of megascopic characters provided in the table and photographs of the hand specimen. Instructions: You are required to study Units 4, 6 and 7 of BGYCT-133 course (Crystallography, Mineralogy and Economic Geology) and Unit 7 of BGYCT-135 course (Petrology) before performing this experiment. Bring this practical manual along with Block 1 in Volume 1 of BGYCT-135 course while attending the Practical Counselling session. You have read in Unit 4 Minerals of BGYCT-133 course that the physical properties of minerals are helpful in the identification. Each mineral displays a few physical properties that can be recognised megascopically. These minerals show some of the important physical properties that are useful in identification of rocks. 4.3 BASIC CONCEPTS Before identification of ultramafic rocks, let us recall basic concepts of the ultramafic subgroup of rocks that we have read in Unit 3 Classification of Igneous Rocks of BGYCT-135 course. Ultramafic igneous rocks contain <45% SiO2, >18% MgO, high FeO and low K2O. It generally contains 90% mafic 52 Megascopic Study of Common Ultramafic Igneous Rocks …………………………Experiment 4 ……………………………………………………………………….............….…...... minerals. Ultramafic rocks are not very much abundant on the Earth’s surface. Commonly, they are intrusive in nature. Ultramafic igneous rocks are melanocratic (dark coloured) and contain >90% ferromagnesian minerals (magnesium and iron bearing mafic minerals) with almost no plagioclase. The common ultramafic rocks are peridotite, pyroxenite and dunite. Dunite and pyroxenite are monomineralic rocks (i.e. made up more than 90% of one mineral only). In the following section, you will learn to identify the megascopic characters of the peridotite and dunite. You have read in Unit 7 of the BGYCT-135 course the megascopic characters which are used in the identification of peridotite and dunite in the hand specimens. You will use following characters (Table 4.1) in the identification of rocks in the hand specimens. Table 4.1: Megascopic Characters of ………………. 1. Colour Index : 2. Mineral Composition: • Essential : • Accessory : 3. Texture: • Crystallinity : • Granularity : • Shape of grains : • Mutual relationship : • Other textures : 4. Diagnostic Characters : 5. Inference/Name : 6. Mode of Occurrence : 7. Important Uses : 8. Indian Occurrences : You have already learnt about above mentioned points in Experiment 1. We have discussed in detail about the above tabulated points i.e. texture, mineralogical composition, mode of occurrence etc. You are advised to follow these instructions. 4.4 MEGASCOPIC STUDY OF PERIDOTITE Generally, peridotite refers to coarse-grained plutonic ultramafic rocks which are dense, dark green to black having high specific gravity. Peridotite is 53 Petrology: Laboratory ………BGYCL…-136…………………………………………………………………… ……………….….............….….... ultramafic, as the rock contains less than 45% silica. It contains high amount of magnesium (> 40%) and reflecting high proportions of magnesium-rich olivine (forsterite), pyroxenes (both clino- and ortho- pyroxenes) and hornblende. The nomenclature of peridotite depends upon varying proportions of olivine and pyroxene. Accessory minerals are garnet, spinel, plagioclase, ilmenite, chromite and magnetite. Peridotite is a dense, coarse-grained igneous rock, consisting mostly of olivine and pyroxene. Peridotite is the dominant rock of the upper part of the Earth's mantle. They are commonly associated with the layered igneous complexes. You have to identify megascopic characters of the peridotite. You are advised to identify hand specimens of gabbro provided to you at your study centre with the help of megascopic characters given in the Table 4.2 and photographs of the hand specimens given in Figure 4.1. Fig. 4.1: Hand specimen of peridotite. Table 4.2: Megascopic Characters of Peridotite. Melanocratic (>67% dark coloured minerals). 1. Colour Index : High proportion of ferromagnesian minerals. Bright green with a few black specks. But it may exhibit a wide range of colours from blackish green, yellow, brown to even red. 2. Mineral Composition: • Essential : Olivine >40%, orthopyroxene and clinopyroxene. Olive green olivine, black orthopyroxenes grass-green clinopyroxenes grains that are visible with unaided eyes. • Accessory : Garnet, spinel, plagioclase, ilmenite, chromite and magnetite. 54 Megascopic Study of Common Ultramafic Igneous Rocks …………………………Experiment 4 ……………………………………………………………………….............….…...... 3. Texture: • Crystallinity : Holocrystalline. • Granularity : Coarse to medium-grained Phaneritic texture. • Shape of grains : Subhedral, hypidiomorphic. • Mutual relationship : Equigranular. • Other textures : Mostly occurs in massive form, but it is also present in layered form. It may show cumulate texture, formed by settling of olivine crystals in the bottom of the magma chamber. It also occurs as nodules with irregular equigranular texture. 4. Diagnostic : Holocrystalline, coarse-grained, phaneritic, Characters coarse-grained, granular. 5. Inference/Name : PERIDOTITE 6. Mode of Occurrence : Plutonic/intrusive. Associated with layered igneous complexes and ophiolite belts. 7. Important Uses : Economically important because they often contain chromite - the only ore of chromium; they can be source rocks for diamonds. 8. Indian Occurrences : Associated with ophiolite belts of Ladakh Himalaya, Naga hills and Andaman Nicobar. Fig. 4.2: Sketch of peridote. 4.5 MEGASCOPIC STUDY OF DUNITE You have learnt that ultramafic rocks contain more than 40% olivine are classifies as peridotites. Peridotite that contains more than 90% olivine is called as dunite (named in 1864 after Dun Mountain in New Zealand). Dunite 55 Petrology: Laboratory ………BGYCL…-136…………………………………………………………………… ……………….….............….….... is an ultramafic plutonic rock that is composed exclusively of olivine. It is a monomineralic rock with >90% olivine. Dunite is mostly composed of bright green olivine. Fresh rock is green as well. However, olivine readily alters and loses its bright green colour quickly. This rock usually contains chromite as pods, lenses and layers. Dunite occurs in layered gabbroic complexes. It probably forms from the accumulation of dense, early crystallising olivine grains that sink to the bottom of the low silica magma. Dunite Intrusions form sills or dikes. Olivine in dunite alters into serpentine. You are advised to identify dunite in the hand specimen provided to you at your study centre with the help of megascopic characters given in the Table 4.3 and photographs of the hand specimens given in Figure 4.3. Fig. 4.3: Hand specimen of dunite. Table 4.3: Megascopic Characters of Dunite. 1. Colour Index : Melanocratic (contains >67% mafic minerals). Olive green colour. 2. Mineral Composition: • Essential : More than 90% olivine, monomineralic rock. • Accessory : Pyroxene, chromite, magnetite, pyrope. 3. Texture: • Crystallinity : Holocrystalline. • Granularity : Coarse grained (Fig.4.3) Phaneritic texture. • Shape of grains : Subhedral, hypidiomorphic. • Mutual relationship : Commonly equigranular. 4. Diagnostic : Typical olive green colour, holocrystalline and 56 Megascopic Study of Common Ultramafic Igneous Rocks …………………………Experiment 4 ……………………………………………………………………….............….…...... Characters hypidiomorphic granular texture. 5. Inference/Name : DUNITE 6. Mode of Occurrence : Plutonic/intrusive. Occurs as sills and dykes and associated with layered gabbroic complexes. 7. Important Uses : Used in construction industry, as dimension stone, cement manufacture, construction aggregate. 8. Indian Occurrences : Associated with ophiolite belts of Ladakh Himalaya, Naga hills and Andaman Nicobar. Fig. 4.4: Sketch of dunite. 4.6 LABORATORY EXERCISES Study the megascopic characters of common mafic rocks such as peridotite and
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