GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
Subject Geology
Paper No and Title Metamorphic and Igneous Petrology Module No and Title Petrology and Mineralogy of Igneous Rocks
Module Tag IV
Principal Investigator Co-Principal Investigator Co-Principal Investigator Prof. Talat Ahmad
Vice-Chancellor Jamia Millia Islamia Delhi
Prof. Devesh K Sinha Department of Geology University of Delhi Delhi
Prof. P. P. Chakraborty Department of Geology University of Delhi Delhi
Paper Coordinator Content Writer Reviewer Prof. Pulak Sengupta
Department of Geological Sciences,
Jadavpur University Kolkata
Dr. Meraj Alam Department of Geology Indira Gandhi National Tribal University,
Amarkantak (M.P.)
Prof. Pulak Sengupta Department of Geological Sciences,
Jadavpur University Kolkata
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
Table of Content
1. Introduction 2. Volcanic settings
2.1 Petrography 2.2 Origin
3. Igneous rocks of the continental lithosphere 3.1 Granitic Rocks
3.2 Pegmatites
3.3 Continental Rhyolite 3.4 Continental Flood Basalts 3.5 Peralkaline Rhyolites 4. Plutonic rocks
4.1 Description of different plutonic igneous rocks 4.1.1 Granite
4.1.2 Gabbro 4.1.3 Granodiorite 4.1.4 Peridotite 5. Volcanic rocks
5.1 Description of different volcanic igneous rocks 5.1.1 Basalt
5.1.2 Dacite 5.1.3 Andesite 5.1.4 Rhyolite
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
1. Introduction
The ocean basins cover the largest (2/3rd) area of the Earth's surface. Because of plate tectonics, however, most oceanic lithosphere eventually is subducted. Thus the only existing oceanic lithosphere is younger than about Jurassic in age and occurs at locations farthest from the oceanic spreading centers or the mid oceanic ridges where the youngest rocks are recorded. Except in areas where magmatism is intense enough to build volcanic structures above sea level, most of the oceanic magmatism is difficult to access. An ophiolite is a sequence of rocks that appears to represent a section through oceanic crust. Ophiolites occur in areas where obduction (the opposite of subduction) has pushed a section of oceanic lithosphere onto continental crust. During this process, most ophiolite sequences have been highly deformed and hydrothermally altered. Nevertheless, it is often possible to look through the deformation and alteration and learn something about the structure of oceanic lithosphere.
Figure 1: An idealized ophiolite sequence
An idealized ophiolite sequence shows an upper layer consisting of deep sea sediments (limestones, cherts, and shales), overlying a layer of pillow basalts (Figure 1). Pillow basalts have a structure consisting of overlapping pillow-shaped pods of basalt. Such pillow structure is typical of lavas erupted under water. The
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
pillow basalts overlie a layer consisting of numerous dikes, some of which were feeder dikes /sheeted dykes for the overlying basalts. Beneath the sheeted dike complex are gabbros that likely represent the magma chambers for the basalts. The upper gabbros are massive while the lower gabbros show layering that might have resulted from crystal settling.
At the base of the layered gabbros there is a sharp increase in the density of the rocks, and the composition changes to ultramafic rocks. This sharp change in density is correlated with what would be expected at the base of the crust, and is thus referred to as the petrologic moho. At the top of the ultramafic sequence the rock type is harzburgite (Ol + Opx), a rock type expected to be the residual left from partially melting peridotite. The base of the ultramafic layer is composed of peridotite.
Because most ophiolites have been hydrothermally altered, most of the mafic rocks have been altered to serpentinite. Note that ophiolite means "snake rock".
2. Volcanic Settings
Volcanism occurs at three different settings on the ocean floor.
1) Oceanic Ridges- these are the oceanic spreading centers where a relatively small range of chemical compositions of basalts are erupted to form the basaltic layer of the oceanic crust. This chemical type of basalt is referred to as Mid Ocean Ridge Basalts (MORBs). In some areas, particularly Iceland, where there has been a large outpouring of basalts on the oceanic ridge, basalts called Enriched Mid Ocean Ridge Basalts (EMORBs) have been erupted.
2) Oceanic Islands- these are islands in the ocean basins that generally occur away from plate boundaries, and are often associated with hot spots. A wide variety of rocks occur in these islands, not all are basaltic, but appear to be related to the basaltic magmas. In general, these rocks are referred to as Oceanic Island Basalts (OIBs).
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
3) Large Igneous Provinces (LIPs)- these are massive outpourings of mostly basaltic lavas that have built large submarine plateaus. Most are mid- Cretaceous in age. They are not well studied, but most have compositions similar to OIBs, and some may have once had oceanic islands on top, but most of these have been removed by erosion.
4) Island Arc- these are subduction related bodies mostly elongated running parallel to the subduction zones, which may intra oceanic or ocean-continent subduction zones. They exhibit large varieties of igneous rocks ranging from basalt – andesite – dacite – rhyolite and their plutonic equivalents.
Collectively they are call arc rocks.
The convergent plate margins are the most intense areas of active magmatism above sea level at the present time. Most of world's violent volcanic activity occurs along these zones. In addition, much magmatism also has resulted (and probably is resulting at present) in significant additions to the crust in the form of plutonic igneous rocks.
Figure 2: The "Pacific Ring of Fire"
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
The "Pacific Ring of Fire" is often discussed in relation to both earthquakes and volcanoes. It surrounds the Pacific Ocean basin and extends into the Indian Ocean and Caribbean Sea. Active subduction is taking place, along these convergent plate boundaries, as evidenced by the zone of earthquakes, called a Benioff Zone, that begins near the oceanic trenches and extends to deeper levels in the direction of plate motion. Earthquake focal depths reach a maximum of about 700 km in some areas. Volcanism occurs on the upper plate about 100 to 200 km above the Benioff Zone. For this reason, volcanism in these areas is often referring to as subduction related volcanism.
Two situations occur:
1) In areas where oceanic lithosphere is subducted beneath oceanic lithosphere the volcanism is expressed on the surface as chains of islands referred to as island arcs. These include the Caribbean Arc, the Aleutian Arc, the Kurile Kamchatka Arc, Japan, the Philippines, the South Sandwich Arc, The Indonesian Arc, the Marianas, Fiji, and Solomon Islands.
2) In areas where oceanic lithosphere is subducted beneath continental lithosphere volcanism occurs as chains of volcanoes near the continental margin, referred to as a continental margin arc. These include the Andes Mountains, Central American Volcanic Belt, Mexican Volcanic Belt, the Cascades, the part of the Aleutian arc on Continental crust, and the North Island of New Zealand.
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
2.1 Petrography:
Probably the most distinguishing feature of subduction-related volcanic rocks is their usually porphyritic nature, usually showing glomeroporphyritic clusters of phenocrysts. Basalts commonly contain phenocrysts of olivine, augite, and plagioclase. Andesites and dacites commonly have phenocrysts of plagioclase, augite, and hypersthene, and some contain hornblende. The most characteristic feature of the andesites and dacites is the predominance of fairly calcic plagioclase phenocrysts that show complex oscillatory zoning. Rhyolites occur as both obsidians and as porphyritic lavas and pyroclastics. Phenocrysts present in rhyolites include plagioclase, sanidine, quartz, orthopyroxene, hornblende, and biotite.
2.2 Origin:
Subduction carries oceanic crust and sediment to depth. As the pressure and temperature rise, the MORB crust and sediments undergo metamorphism that releases hydrous fluids.
These hydrous fluids carry with them high concentrations of LILE and REE, but leave behind the relatively insoluble HFSE. They also carry the isotopic signature of the basaltic crust sediment mixture that released the fluids, and thus have higher 87Sr/86Sr ratios and lower 143Nd/144Nd ratios, reflecting
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
the isotopic composition of the subducted material. If the sediments are young, they may contribute 10Be to these fluids.
The fluids act to metasomatize the overlying mantle wedge, enriching it in LILE, REE, B, 87Sr/86Sr, and possibly 10Be and lowering the 143Nd/144Nd ratio of this mantle.
Adding H2O to the mantle wedge lowers the solidus temperature allowing for partial melting of this metasomatized mantle and generating hydrous basaltic magmas.
This hydrous basaltic magma become saturated with water at crustal depths and differentiate by crystal fractionation, possibly accompanied by contamination of crustal material, to generate the andesites, dacites, and rhyolites of the calc-alkaline suite.
3. Igneous Rocks of the Continental Lithosphere
3.1 Granitic Rocks:Here we discuss a group of plutonic igneous rocks usually referred to as
"granitic rocks", "granitoids", or loosely as granites. Included are true granites, but our discussion will include all medium to coarse-grained rocks that are mostly felsic with a few mafic minerals. Note that true granites have between 10% and 65% of their feldspars as plagioclase, and between 20% and 60%
quartz. All rocks will likely contain mafic minerals such as biotite, hornblende, and perhaps pyroxenes, along with opaque oxide minerals.
3.2 Pegmatites:
Pegmatites are very coarse grained felsic rocks that occur as dikes or pod like segregations both within granitic plutons and intruded into the surrounding country rock. They appear to form during the late stages of crystallization which leaves H2O-rich fluids that readily dissolve high concentrations of alkalies and silica. Thus, most pegmatites are similar to granites and contain the minerals alkali feldspar and quartz.
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
3.3 Continental Rhyolites:
Rhyolites are much more common and voluminous on the continents than in the ocean basins. They range from small domes and lava flows to much larger centers that have erupted volumes measured in 100s of km3. Most of the preserved volume is represented as pyroclastic flow deposits, often termed "ash flow tuffs" or "ignimbrites. Large quantities of these deposits were erupted during the middle Tertiary in the western United States, northern Mexico, throughout Central America, and on the western slopes of the Andes mountains.
The composition of these deposits is usually metaluminous although peralkaline varieties are known. None are peraluminous in composition. Although the recent examples occur near continental margins. Most seem to be associated with episodes of continental extension, such as in Basin and Range Province of the Western U.S. and Mexico.
3.3 Continental Flood Basalts:
Like the large submarine plateaus discussed in our lecture on the ocean basins, large volumes of basaltic magma have erupted on the continents at various times in Earth history. The most recent of these outpourings, but by no means the largest, is the Columbia River basalts erupted in Oregon and Washington states in the mid-Miocene.
3.4 Peralkaline Rhyolites:
Peralkaline rhyolites are common in continental rift settings, although they also occur in oceanic island settings, and our discussion here includes such settings.
In nearly all cases, peralkaline rhyolites are associated with mildly alkaline silica-saturated basalts, hawaiites, mugearites, and trachytes.
4. Plutonic rocks
Plutonic rocks are igneous rocks that solidified from a melt at great depth. It’s made of tightly packed mineral grains of medium size or larger which means that it has phaneritic texture. In addition, the grains are of roughly equal size meaning that it has granular or equigranular texture. Examples of few plutonic rocks are Granite,
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
Gabbro, Granodiorite, Peridotite, Monzonite, Monzodiorite, Nepheline syenite, Norite, Syenite, Tonalite.
4.1 Description of different plutonic igneous rocks
4.1.1 Granite: Granite is a coarse grained, light coloured igneous rock composed mainly of quartz and feldspar with minor amounts of mica and amphibole minerals.
Figure 3: Photomicrograph of Granite
Colour: colourless grains, some grey or dark grains
Mineralogy: essentially quartz, alkali feldspar and plagioclase in variable amounts, usually with hornblende and or biotite, muscovite may occur.
Classification: Acid plutonic igneous rock, Occurrence: intrusive, most commonly occurring in batholiths.
Texture: coarse to very coarse grained, usually granular may be porphyritic with well-shaped phenocrysts (large crystals of feldspar), sometimes foliated, coarse intergrowth of quartz and feldspar can form a graphic texture.
Structure: commonly contain xenoliths, may contain cavities into which well- formed crystals project, may be associated with much late stage mineralization.
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
4.1.2 Gabbro: Gabbro is a dark, medium to coarse grained intrusive igneous rock composed of calcium plagioclase, pyroxene and minor olivine, but no quartz. It is the intrusive equivalent of basalt.
Figure 4: Photomicrograph of Gabbro
Colour: dark grey, greenish, brownish Mineralogy: essential plagioclase and pyroxene, other minerals include olivine, hornblende, spinel, ilmenite, magnetite, apatite.
Classification: basic plutonic igneous rock. Occurrence: deeper continental crust.
Texture: coarse grained, granitic texture, crystals intergrown, may have lath like crystals of feldspar aligned parallel to layering.
Structure: commonly layered, may show alternating light (feldspar rich) and dark (pyroxene and/or olivine) layers.
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
4.1.3 Granodiorite: Granodiorite is a phaneritic textured intrusive igneous rock similar to granite but containing more plagioclase feldspar than orthoclase feldspar.
Figure 5: Photomicrograph of Granodiorite
Colour: black, grey, orange, pink, white Mineralogy: quartz, plagioclase feldspar and minor amount of muscovite. Biotite and amphibole often in the form of hornblende are more abundant in granodiorite than in granite. Pyroxene may present.
Classification: felsic plutonic igneous rock.
Occurrence: Granodiorite is an intrusive igneous rock which is very hard, crystalline and is visibly homogeneous in texture and forms by melting of continental rocks.
Texture: granular, phaneritic
Structure: layered structure occurs in many plutons sometimes they occur together with non -layered granodiorites.
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
4.1.4 Peridotite: Peridotite is a very dense, coarse grained, olivine rich, ultramafic intrusive rock. It is noted for its low silica content and contains very little or no feldspar. It is a common component of oceanic lithosphere and derived from upper mantle.
Figure 6: Photomicrograph of Peridotite
Colour: generally dark greenish grey Mineralogy: generally, olivine with lesser pyroxene, always contains some metallic minerals, e.g. chromite, magnetite.
Classification: plutonic igneous rock
Occurrence: It is found on land as part of oceanic crust sequences called ophiolites which have been thrust in or on to a continental mass or as localized intrusion. It has two primary modes of origin, as mantle rocks formed during the accretion and differentiation of the earth or as cumulate rocks formed by precipitation of olivine + pyroxene from basaltic or ultramafic magmas.
Texture: Phaneritic (coarse grained)
Structure: Layered peridotites may form the base layer of gabbroic complexes.
Some peridotites rich in amphibole have a concentric layered structure and form parts of pluton.
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
5. Volcanic rocks
Volcanic rocks are also known as extrusive igneous rocks because it formed when lava cools and solidifies on earth surface. Volcanic rocks are usually fine grained or aphanitic to glass in texture. They often contain clasts of the rocks and phenocrysts.
Few examples are Basalt, Andesite, Dacite, Rhyolite, Phonolite, Kimberlite, Komatiite, Trachyte, Tholeiite, Nephelinite.
5.1 Description of different volcanic igneous rocks
5.1.1 Basalt: Basalt is one of the most common finely crystalline basic igneous rocks, which is sometimes glassy and are essentially composed of plagioclase feldspar and pyroxene.
Figure 7: Photomicrograph of Basalt
Colour: black, very dark grey, greenish or reddish if altered
Mineralogy: essential plagioclase and pyroxene in variable amounts, with spinel, olivine, ilmenite, magnetite or apatite.
Classification: basic igneous rock
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
Occurrence: widespread, seafloor crust, oceanic islands, continental volcanoes and flood plateau lavas.
Texture: usually large crystals (phenocryst) set in finely crystalline matrix.
May have vesicles (air pockets).
Structure: commonly vesicular or amygdaloidal, filled with zeolites, carbonates or silica. May form columnar joints on cooling, surface forms of lava may be rough or clinkery or have a ropy appearance.
5.1.2 Dacite: Dacite is a felsic extrusive rock, intermediate in composition between andesite and rhyolite. It is the volcanic equivalent of granodiorite.
Figure 8: Photomicrograph of Dacite
Colour: variable, but generally bluish grey or pale grey
Mineralogy: groundmass generally of plagioclase with amphibole, biotite, pyroxene, quartz and glass
Classification: volcanic igneous rock
Occurrence: usually forms as an intrusive rock such as sill or dyke
Texture: generally, have an aphanitic to porphyritic texture. However, it contains quartz as rounded, corroded crystals or grains or as a constituent of the ground mass.
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
Structure: Dacite is primarily associated with andesite and trachyte and forms lava flows, dikes and sometimes massive intrusions in the centres of old volcanoes.
5.1.3 Andesite: Andesite is an extrusive rock intermediate in composition between rhyolite and basalt. It is the volcanic equivalent of diorite
Figure 9: Photomicrograph of Andesite
Colour: variable but typically bluish grey or grey
Mineralogy: groundmass generally of pyroxene and plagioclase, possibly with minor amounts of amphibole and glass, phenocrysts of plagioclase and often pyroxene, occasionally olivine or amphibole.
Classification: volcanic igneous rock Occurrence: typically formed at convergent plate margins but may occur in other tectonic settings.
Texture: aphanitic to porphyritic texture
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
5.1.4 Rhyolite: It is an igneous volcanic rock of felsic composition. It is the extrusive equivalent of granite.
Figure 10: Photomicrograph of Rhyolite Colour: variable but light colured
Mineralogy: groundmass generally of quartz and plagioclase with lesser amount of orthoclase, biotite, amphibole, pyroxene and glass. Phenocryst of plagioclase and quartz often with amphibole and or biotite.
Classification: volcanic igneous rock
Occurrence: due to high silica content rhyolite melts are highly polymerized and form highly viscous lava. They also occur as breccias or in volcanic plugs and dikes
Texture: Aphanitic, glassy, porphyritic. Frequently shows altering layers that differ slightly in granularity or colour, flow banding is sometimes evident.
Structure: vesicles or amygdales may be present, may contain spherulites that are spherical body, comprising radial aggregates of needles usually of quartz and feldspar.
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
Frequently Asked Questions-
Q1. Define the following: (a) ophiolite sequence, (b) ocean ridge, (c) oceanic island (d) Large Igneous Province?
Q2. What conditions favour generation of silica undersaturated magmas from melting of the mantle?
Q3. Summarize the current theory on the origin of the calc-alkaline suite?
Q4. Compare and contrast I-type, S-type, and A-type granitic rocks?
Q5. Compare and contrast Catazonal, Mesozonal, and Epizonal plutons?
Q6. What are pegmatites and how do they form?
Q7. What rocks suites and rock types are found in continental rift valleys?
Multiple Choice Questions-
1. Andesite is most often associated with what type of plate boundary a. divergent plate boundaries
b. transform plate boundaries c. convergent plate boundaries d. reversible plate boundaries Ans: a
2. You discover a rock with minerals large enough to be seen containing amphibole, biotite and plagioclase. This is:
a. diorite b. gabbro c. granite d. hornblende Ans: a
3. Basalt and gabbro
a. have the same mineral
b. formed from magma with the same temperature c. formed from magma with the same silica content d. all of these
Ans: d
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
4. Igneous rocks that form entirely beneath earth surface are said to be a. volcanic
b. extrusive c. plutonic d. platonic Ans: c
5. Volcanic rocks are also a. Intrusive b. Plutonic c. Extrusive d. Explosive Ans: c
6. Andesite was named for a rock type commonly found:
a. in the Rocky Mountains b. in Hawaii
c. in the Andes Mountains d. in Mayberry, North Carolina Ans: c
7. A rock with mineral crystals too small to be seen and low temperature minerals is:
a. gabbro b. rhyolite c. andesite d. granite Ans: b
8. Magma extruded at high temperature tends to be:
a. very fluid b. very viscous c. very explosive d. less viscous Ans: b
9. Which of the following pairs of intrusive and extrusive rocks have the same mineral composition:
a. granite and andesite b. diorite and basalt c. gabbro and basalt d. gabbro and rhyolite Ans: c
GEOLOGY
Paper: Metamorphic and Igneous Petrology Module: Petrology and Mineralogy of Igneous Rocks
10. Which of the following best describes a basalt
a. A dark-coloured, fine-grained igneous rock rich in silica b. A dark-coloured, fine-grained igneous rock poor in silica c. A dark-coloured, coarse-grained igneous rock rich in silica d. A dark-coloured, coarse-grained igneous rock poor in silica Ans: b
11. Which of the following rock crystallizes near earth surface a. basalt
b. gabbro c. diorite d. granite Ans: a
Suggested Readings:
1. Ehlers, Ernest G., & Blatt, H. (1999). Petrology: Igneous, Sedimentary, and Metamorphic, 1st Edn. CBS Publications. ISBN: 0716737434, 978- 0716737438.
2. McBirney, Alexander R. (2006). Igneous Petrology, 3rd Edn. Jones and Bartlett Publishers, Inc. ISBN: 0763734489, 978-0763734480.
3. Philpotts, Anthony R., & Ague, Jay J. (2009). Principles of igneous and metamorphic petrology, 2nd Edn. Cambridge University Press. ISBN:
0521880068, 978-0521880060.
4. Winter, John D. (2001). An Introduction to Igneous and Metamorphic Petrology, 1st Edn. Prentice-Hall Inc., New Jersey. ISBN: 0132403420, 978- 0132403429.
