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An in-depth exploration of metamorphic rocks, their formation, and the roles of temperature, pressure, and fluids in their transformation. It covers various aspects such as metamorphic grades, minerals, and the impact of tectonic processes. The document also discusses the importance of fluids in metamorphic reactions and the formation of new minerals.
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Metamorphic rocks are:
Metamorphic rocks are the oldest rocks on Earth.
Metamorphism describes the mineralogical, chemical, and textural changes to preexisting rocks that occur without substantial melting.
Metamorphism occurs in Earth’s crust and mantle at conditions differing from those under which the rock originally formed.
Conditions of metamorphism range from those lithifying sediment into sedimentary rocks to conditions of temperature and pressure just before rock melts to make magma (see Figure 6.2 on page 136).
Metamorphic petrologists describe the intensity of temperature and pressure during metamorphism as ranging from:
The original composition of the rock is essential in determining the reactions that actually occur during metamorphism. However, significant changes in rock composition may occur if large amounts of chemical active fluids are involved in the metamorphism.
Intrusion of magma into rocks can also increase the temperature (see Figure 6.5 on page 140).
When magma intrudes into rocks, it raises the temperature in the surrounding rock. The transfer of heat from magma may result in metamorphism of the surrounding rocks.
High-temperature metamorphism causes the break down of minerals that contain water or gas molecules, and results in the release of these components. These reactions are called:
KAl 3 Si 3 O 10 (OH) 2 + SiO 2 → Al 2 SiO 5 + KAlSi 3 O 8 + H 2 O
muscovite + quartz → sillimanite + potassium fsp + water
Dehydration reactions may increase the rate of metamorphic reactions that require the presence of water.
The presence of water can also decrease the melting temperature of silicate minerals and enhance the formation of magma.
Dehydration reactions during metamorphism of subducted oceanic crust lead to magma generation at convergent plate boundaries.
CaCO 3 → CaO + CO (^) 2(g)
Calcite → Lime + Carbon dioxide
The most common degassing reactions release carbon dioxide from carbonate minerals.
Rock texture may change during metamorphism without changing chemical composition or minerals.
One example of recrystallization occurs when quartz sandstone undergoes metamorphism (see Figure 6.9 on page 143).
Once metamorphosed, the individual, round quartz grains within the sedimentary rock have a texture of straight- sided crystals that form three-sided junctions.
Strain during metamorphism results in changes in the texture of a rock.
Crystals are rearranged into planes, called foliation, depending on the type and orientation of the stress. Foliation planes are recognized by:
Foliation forms by mechanical rotation of preexisting minerals or dissolution and new mineral growth along preferred orientations.
All types of foliation are planes in the rock that are perpendicular to the greatest normal stress or parallel to shear stress.
Chemical reactions between solids do not occur, or take place very slowly, in the absence of water (see Figure 6. on page 146).
The results of laboratory experiments have demonstrated repeatedly that metamorphic reactions occur more quickly in the presence of water, and often at lower temperature, than by placing the dry ingredients together.
This is true even if the new minerals do not contain water. In fact, even dehydration reactions occur more readily when the minerals are immersed in water.
Fluids may either be part of the original parent rock or be introduced into the metamorphic environment.
The presence of these fluids facilitates metamorphic reactions at these less extreme temperature and pressure conditions.
This fluid not only delivers water, but also heat and ions from the magma.
In summary,
In summary, once metamorphic rocks have formed, the minerals present are not converted back to the original minerals in the parent rock. There is not sufficient temperature, pressure, fluid, or time to facilitate the necessary reverse reactions.
Many minerals are stable over limited ranges in temperature and pressure. Changes in one or both of these variables cause a mineral to react and form a new mineral or minerals (see Figures 6.15 through 6.18 on pages 149- 150).
The results of numerous laboratory studies performed with different minerals and under controlled conditions of temperature and pressure document the conditions where metamorphic minerals (and other minerals) are stable.
Some minerals, referred to as index minerals, are used to estimate the temperature and pressure conditions of metamorphism (see Figure 6.19 on page 151).
Index mineral reveal metamorphic grade, either low, medium, or high.
Only minerals with limited ranges of stability are useful as key index minerals. For example, minerals such as quartz and feldspar, are stable over a large range of temperature and pressure conditions and do not indicate the grade of metamorphism.
The variation in mineral content and texture of metamorphic rocks reveal the temperature and pressure of metamorphism and the composition of parent rocks and reactive fluids.
Composition and texture, therefore, are used as criteria for classifying metamorphic rocks (see Figure 6.20 on pageS 153-154). The primary textural attributes in metamorphic rocks are the presence or absence of foliation and mineral grain size.
Rocks with Foliation
Gneiss forms under conditions of high grade metamorphism and is identified by its characteristic foliation of parallel compositional layers of light-colored (quartz, feldspar) and dark-colored (biotite, amphibole, pyroxene, garnet) minerals. At the highest metamorphic temperatures gneisses lack mica or amphibole because these water-bearing minerals are broke down by dehydration reactions.
Rocks with Foliation
Migmatite forms if the high-grade temperature, pressure, and fluid conditions are appropriate for melting to begin (see Figure 6.22 on page 156).
Migmatite resembles gneiss except that the light-colored bands have the igneous crystallizations texture of granite, and the dark layers show metamorphic crystal growth and recrystallization.
Rocks without Foliation
If the metamorphic rock is nonfoliated or only weakly foliated, then the mineral content is more important for classification purposes (see Figure 6.20 on page 154).
Marble is the rock composed primarily of metamorphically recrystallized calcite.
Quartzite consists of metamorphically recrystallized quartz.
These nonfoliated rocks are distinct from limestone and sandstone because they have metamorphic recrystallization textures, such as large grain sizes meeting along straight edges, or both.
Rocks without Foliation
Amphibolite is a metamorphic rock composed primarily of amphibole minerals with plagioclase feldspar.
Serpentinite is a metamorphic rock composed almost entirely of serpentine.
Rocks without Foliation
Other nonfoliated or weakly foliated metamorphic rocks containing many minerals include greenstone and eclogite formed by metamorphism of mafic igneous rocks.
Greenstone is a metamorphic rock that contains abundant green minerals. The Fe-Mg mica, chlorite, is generally the primary constituent, although green amphibole, feldspar and quartz are typically present as well.
Eclogite is a very high grade metamorphic rock that lacks water bearing minerals. It is dominated by garnet and pyroxene, sometimes with minor quartz. Eclogite is denser than peridotite.
Metamorphosed Sedimentary Rocks
Quartz sandstone and limestone typically metamorphose to nonfoliated rocks composed of a single mineral, because a single, nonplaty mineral dominates the rock (see Figure 6.23 on page 157).
Arkose or lithic sandstone containing significant nonquartz grains may form micas at the expense of feldspar and other minerals during low-grade metamorphism in the presence of water. The resulting rocks may be weakly foliated mica-rich quartzite or even schist if the amount of quartz in the sandstone is minimal.
Metamorphosed Igneous Rocks
Low- to medium-grade metamorphism of mafic and intermediate igneous rocks in the presence of water generates dark-colored metamorphic rocks with abundant Fe- and Mg-bearing mica, like chlorite or biotite (see Figure 6.23).
If foliated these rocks are chlorite or biotite schists, and if not foliated, they are greenstones.
Metamorphosed Igneous Rocks
At higher metamorphic grade, hornblende becomes increasingly stable and forms amphibolites.
At the highest grade of metamorphism, the rock is gneiss or eclogite.
