Lab Activity - Metamorphic Rocks

PHYSICAL GEOLOGY Metamorphic Rocks The dynamics of plate tectonics change the surface of the Earth; metamorphic rocks record the history of these changes. The textures, mineral assemblages, and compositions of the rocks give insight into what kind of changes have occurred. When a rock is exposed to conditions unlike those in which it had formed the minerals within the rock become unstable. As this occurs the rock will undergo metamorphism (change in form) as the minerals come into equilibrium with the new environment. These changes include growth of new minerals, the alteration of existing minerals, changes in mineral orientation, mineral segregation, and even partial melting. Types of metamorphism Regional metamorphism occurs with the increase of temperature and pressure. This environment occurs in regions of mountain building commonly associated with convergent tectonic boundaries. If there is an increase in temperature with little to no increase in pressure, like a body of magma or a lava flow coming in contact with the surrounding country rock, it is referred to as contact metamorphism. Burial metamorphism occurs as deep sedimentary basins are filled, burying sediments successively deeper under several kilometers of overburden. Relatively low temperatures and pressures aided by fluids act as the agents of alteration. Moderate to high shear stress and low temperature conditions, which occur commonly at transform boundaries and along faults, causes cataclastic metamorphism. Shock metamorphism occurs as an instantaneous high pressure and temperature event due to a meteor impact. Types of alterations Mineral regrowth occurs as some minerals dissolve (not melt) and others grow— this leads to a rock with a texture with larger mineral crystals than that of the protolith (original rock before metamorphism). Mineral alteration may occur due to a change from a less stable to a more stable atomic structure without a change in composition (polymorphs). Mineral alteration may also occur due to a compositional change with the addition of elements due to reaction with infiltrating hydrothermal fluids. Through an increase of heat and directed pressure ( stress ) minerals may reorient themselves perpendicular to the maximum pressure direction. Elongated, platy, sheet-like and prismatic crystals are more prone to this alignment ( foliation ). Elongate, bladed, and needle-like crystals may reorient to define a linear fabric within the rock ( lineation ). If the rock continues to experience an increase in heat and directed pressure the minerals will begin to segregate into bands of darker and lighter minerals as the rock continues to recrystallize. As the pressure and temperature continues to increase; slight melting will occur giving the rock characteristics of both metamorphic and igneous rock ( migmatite ). The light-colored silicate minerals generally have lower melting temperatures; the dark-colored silicate minerals 1

PHYSICAL GEOLOGY Metamorphic Rocks have higher melting temperatures and are generally unaffected by melting except under extreme temperature environments. Metamorphic rock classification schemes are broken down to two major categories: foliated or non-foliated. Non-foliated metamorphic rocks are identified by their composition. Foliated metamorphic rocks are identified not only by their composition but also by the type of foliation. Foliated rocks form in a continuum, grading from lower grade (intensity) to higher grade. Typically, these rocks will start with a sedimentary rock (shale) as their protolith. Shale will first alter to a rock with a slaty cleavage, next develop a schistose foliation and finally show a gneissic foliation. 2

PHYSICAL GEOLOGY Metamorphic Rocks *names are modified by the minerals present ordered from least to most abundant i.e. a garnet, mica, schist would have both garnets and micas with less garnet than mica. Sample # Foliated/ non-foliated Type of Foliation Composition Protolith Rock Name 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 4

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PHYSICAL GEOLOGY Metamorphic Rocks PART 2: Metamorphic Conditions & Environments The presence of certain minerals in metamorphosed rocks can be used as an indicator of the intensity (or grade) of metamorphism – allowing us to determine the peak temperatures and/or pressures that the rock experienced. Together, the assemblage of minerals and the rock texture can therefore allow us to estimate depth of burial, extent of heating, and the overall tectonic environment that caused the rocks to be transformed by metamorphism. Examine the diagram on the following page, which consists of several parts: A) The top image shows the range of pressures and temperatures experienced by rocks as they are metamorphosed in different environments. 1. Use this diagram to determine the peak temperature conditions that a metamorphic rock that achieved intermediate grade experienced: 2. If a rock is buried deeply enough to experience 6 kilobars of pressure, use the diagram to determine its depth in kilometers beneath the Earth’s surface: 3. Which metamorphic agent produces a foliation texture in a metamorphic rock? 4. How would you be able to immediately tell whether a shale parent rock had been subjected to intermediate-grade contact or intermediate-grade mountain belt (regional) metamorphism? (HINT 1: What should the sample look like in either case?) (HINT 2: where do these conditions plot on the diagram?) 7

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PHYSICAL GEOLOGY Metamorphic Rocks Next, examine the figure shown below; this chart strips out all information related to parent rock, and separates portions of the pressure-temperature (P-T) space beneath the Earth’s surface into different metamorphic facies . The idea here is that each metamorphic facies includes all of the different rocks and minerals that form in a particular metamorphic environment, and represent a specific set of pressure and temperature conditions. This means that we can directly compare metamorphic rocks that had different protoliths (and may look quite different!). It’s a powerful tool to create “apples-to-apples” comparisons of different metamorphic rocks one might find in the field. 9. Review your answers to questions 6-8. Now examine the diagram above. Which metamorphic facies do your two intermediate-grade samples most likely record? 10. Which metamorphic facies would be associated with contact metamorphism? 11. Which two metamorphic facies would be associated with subduction-zone metamorphism? 10