CHAPTER 8: Metamorphic Rocks
1. Metamorphic rocks are those rocks that have undergone changes in mineralogy, texture and/or chemical composition as a result of changes in temperature and pressure. The original rock may have been igneous, sedimentary or another metamorphic rock.
2. The pressure and heat that drive metamorphism are consequences of three forces:
(a) internal heat of the Earth.
(b) weight of the overlying rock.
(c) horizontal or tectonic forces that cause the rocks to deform.
3. Figure 8.1: Pressures and temperatures increase as we go deeper into the earth. Temperatures increase with depth at different rates depending on the location.
4, Figure 8.2: A measure of the rate at which temperatures increase with depth is a measure of the geothermal gradient. In much of the Earth's crust, the geothermal gradient is ~30o/km, but some regions have higher gradients and some lower.
Types of Metamorphism
There are several types of metamorphism.
1. Contact Metamorphism (Figs 8.3, 8.14 & 8.15): usually occurs where high temperatures are restricted to a small area, generally around the margins of an igneous intrusion. Geothermal gradients are high.
2. Hydrothermal Metamorphism (Fig. 8.3): typically occurs along mid-ocean ridge spreading centers where heated seawater percolates through hot, fractured basalt. Chemical reactions between the heated seawater and basalt result in metamorphism of the basalt. Hydrothermal metamorphism can also occur on continents where crustal rocks are metamorphosed by invading, hot fluids associated with igneous intrusions.
3. Burial Metamorphism (Fig. 8.3): occurs when sedimentary rocks that had undergone diagenesis are buried even deeper. Diagenesis grades into burial metamorphism, a relatively mild type of metamorphism resulting from the heat and pressure exerted by overlying sediments and sedimentary rocks. Although partial alteration of the mineralogy and texture may occur, bedding and other sedimentary structures are usually preserved.
4. Regional Metamorphism (Fig. 8.3): When temperatures and pressures increase beyond the range of burial metamorphism, regional metamorphism takes over. Regional metamorphism takes place under high temperature and pressure conditions that may extend over large areas. Regional metamorphism results in intense alteration of the mineralogy and texture of rocks, usually to the point where original sedimentary structures are destroyed. Regional metamorphism is primarily due to tectonic forces associated with the interaction between lithospheric plates. This occurs in areas of active subduction and mountain building.
5. Cataclastic Metamorphism: A high-pressure metamorphism resulting from the crushing and shearing of rock during tectonic movement, mostly along faults. Cataclastic metamorphism is generally localized along fault planes (areas of detachment where rocks slide past one another). Cataclastic Metamorphism produces sheared, highly deformed rocks called mylonites.
Metamorphic Grade and Metamorphic Facies
1. Figures 8.12 & 8.13: The extent of metamorphism can be defined on the basis of metamorphic grade and Metamorphic Facies. The term 'Metamorphic Facies' describes the grouping of rocks of various mineral compositions formed under different temperature and pressure conditions. Metamorphic facies encompass different regions in P-T space and are named on the basis of certain characteristic minerals that form through metamorphism of primarily basalt.
2. Metamorphic rocks formed under the lowest metamorphic temperatures and pressures (< 250o C and <4 kb) can be regarded as very low-grade metamorphic rocks. Basalt metamorphosed under under these very low P-T conditions characteristically forms certain minerals called zeolites (Fig. 8.12), hence the name Zeolite Facies for this range of P-T conditions. Rocks other than basalt metamorphosed under similar conditions, however, may not contain zeolite minerals because they do not have the proper chemical ingredients. These other rocks however, can still be regarded as having formed within the Zeolite Facies of metamorphism.
3. Metamorphic rocks formed within the P-T range of 2-9 kb and 250-450 oC can be regarded as low grade metamorphic rocks. Mafic volcanic rocks metamorphosed under these conditions contain green minerals like chlorite and epidote, hence the name Greenschist Facies. Rocks of other compositions metamorphosed under similar P-T conditions may not contain these minerals but are still said to have been metamorphosed to Greenschist Facies.
4. Rocks metamorphosed within the P-T range of 2-9 kb and 450-700 oC are regarded as medium grade metamorphic rocks. Mafic igneous rocks metamorphosed under medium grade conditions contain abundant amphibole, so this region of P-T space is referred to as the Amphibolite Facies.
5. The highest grade of metamorphosed mafic volcanics are the pyroxene granulites which are course rocks containing pyroxene and Ca-plagioclase. High grade metamorphic rocks of the Granulite Facies form at temperatures >700 oC and pressures ranging from 4-10 kb.
6. Metamorphism under very high pressures and relatively low temperatures, such as occurs along subduction zones, constitutes the Blueschist Facies because basalt and shale metamorphosed under these conditions often contain blue amphiboles called glaucophane.
7. Rocks formed under extremely high pressures (>10 kb) and moderate to high temperatures are called eclogite and are often rich in garnet and pyroxene.
8. The hornfels comprise the series of rocks that result from contact metamorphism under low pressures and a wide range of temperatures.
9. Figure 8.15: Hornfels are metamorphic rocks that have been cooked in place by an adjacent magmatic intrusion. Hornfels are generally products of heated recrystallization of the original rock (typically sedimentary rock), coupled with chemical reactions involving hot fluids from the nearby magmatic intrusion. These hot fluids invade the rocks via fractures and pore spaces and react with the original minerals to produce new minerals. The rim of altered rock around an igneous intrusion is called a contact aureole.
10. Some rocks that have been metamorphosed at higher pressures and temperatures may be re-metamorphosed at lower temperatures and pressures, particularly in the presence of fluids, in a process called retrograde metamorphism. For example, an amphibolite can be partly re-metamorphosed to a greenschist. Serpentine minerals are often products of retrograde metamorphism of ultramafic rocks.
Metamorphic Rocks and Textures
1. Metamorphic rocks are characterized by a certain set of minerals. They include familiar minerals like quartz, feldpar, mica and in some cases pyroxene. New minerals include garnet, staurolite and kyanite which are only found in metamorphic rocks.
2. Metamorphic Rocks can be divided into two groups based on their metamorphic textures, (1) foliated and (2) nonfoliated.
1. Figure 8.4: Deformation of some rocks such as shale and clay-rich sandstone (greywacke) produces textural changes involving formation of flat or wavy parallel planes within the metamorphosed rock. These flat or wavy planes are referred to as foliation.
2. Folation generally cuts the rocks at an angle to the original bedding unless the deformation is such that foliation and bedding are coincident.
3. Fig. 8.5: Foliation is mainly produced in rocks that contain platy minerals like mica and chlorite. These platy minerals generally form when shale and clay-bearing sandstone (greywacke) are metamorphosed. As these platy minerals grow, their planes take on a preferred orientation that is usually perpendicular to the main direction of forces squeezing the rock. Minerals in the original rock that survive metamorphism may rotate during deformation to acquire a preferred orientation parallel with the platy minerals.
4. Minerals like amphiboles with long, elongated crystals also tend to assume a preferred orientation during metamorphism. The elongated crystals of amphibole line up parallel with the foliation plane, and in addition point in a common direction to form lineation.
5. Folated rocks such as slate easily split along planes of foliation, a characteristic referred to as slaty cleavage (Fig. 8.6).
Foliated and Related Rocks
1. Fig. 8.7: Metamorphism of shale and clay-bearing sandstone produces a variety of foliated metamorphic rocks depending on the extent of deformation (metamorphic grade).
2. Fig. 8.8: Foliation due mainly to the orientation of platy minerals form a series of metamorphic rocks termed, in order of increasing metamorphic grade, slate (lowest grade) - phyllite - schist. Schist form within the P-T field of upper low-grade to medium-grade metamorphism. The increase in degree of metamorphism is accompanied by an increase in the size of platy crystals. The platy minerals in slate are too small to be seen. In phyllite, the flakes have grown larger as evidenced by an increase in luster. In schist, the platy minerals are clearly visable to the naked eye.
3. Schist is often named for their most abundant minerals. Thus there are quartz-mica schist, garnet-mica schist (Fig. 8.10), muscovite schist and actinolite schist.
4. Figs 8.7 & 8.8: At high grades of metamorphism, gneiss forms. Gneiss is a high-grade metamorphic rock consisting of light and dark minerals that are segregated into bands, lenses or streaks. In general, the mafic minerals like biotite mica and amphibole are concentrated in the dark bands whereas light minerals like quartz and feldspar are concentrated in light bands. Gneiss is coarse grained and generally exhibits poor foliation due to the increased abundance of non-platy minerals like quartz and feldspar.
5. Amphibolite is largely composed of long, thin crystals of amphiboles aligned in a common direction. Sometimes separate light bands of feldspar are evident. Amphibolite forms through metamorphism of mafic igneous rocks.
6. If metamorphism reaches a temperature > 700 oC, rocks may begin to partially melt. The resulting silica-rich liquid will invade the partially melted rock as a series of veins and stringers to produce a migmatite.
7. Granulite is a high to very high-grade metamorphic rock that displays a granular texture usually consisting of quartz, plagioclase, pyroxene, garnet and an Al-silicate mineral called silliminite. The crystals are generally equant (same diameter lengthwise and widthwise) and seldom exhibit foliation. Granulite forms through metamorphism of shale, greywacke, and many kinds of igneous rocks.
8. Figure 8.11: The metamorphism of shale yields certain index minerals that characterize different metamorphic grades. These minerals can be used to estimate the sequence and degree of metamorphism in the rock record. This is done in the field by mapping the first appearance of an index mineral and constructing isograd lines. Isograds are used to determine the type and extent of metamorphism within a particular region.
Other Nonfoliated Rocks
1. Metamorphism of quartz-rich sandstone produces quartzite (Fig. 8.9), a very hard, nonfoliated metamorphic rock consisting almost entirely of silica.
2. Metamorphism of limestone and dolomite causes recrystallization of calcite to produce marble (Fig. 8.9). Calcite crystals in marble are generally inter-grown and of uniform size.
3. Hornfels are formed through contact metamorphism (Fig. 8.15). Hornfels are essentially cooked in place by the adjacent magmatic intrusion without deformation. The textures therefore reflect simple recrystallization and are usually granular. Any platy minerals in hornfels are randomly oriented.
4. Greenstone forms through low-grade metamorphism of basalt typically involving hot, percolating seawater or other solutions. The mineral chlorite gives greenstone its characteristic green color. Greenstone forms around mid-ocean ridge spreading centers (Fig. 22.25) and on continents where buried mafic igneous rocks react with hot groundwater.