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Lecture Notes 6 Metamorphic Rocks

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Lecture Notes 6 Metamorphic Rocks Lecture notes ‐ Bill Engstrom: Instructor Metamorphic Rocks GLG 101 – Physical Geology Now that we have found out how rocks melt and become igneous rocks, and how weathering occurs and sedimentary rocks are formed, we need to find out what happens where rocks are too cool to melt but where still hot and under high pressures deep within the Earth? In that situation, rocks can still be changed and new rocks can be formed. The process of change in a solid state is called metamorphism. Metamorphism • meta = change morph = form • Physical & chemical changes occur in a solid state • Igneous, sedimentary or other metamorphic (protolith=parent) rocks change to other rock types • There are three (3) main agents of metamorphism ‐ Heat / Pressure / Chemically active fluids Why Metamorphism Occurs: Changes in Temperature, Pressure, and Exposure to Fluids • Change is slow‐may take millions of years to convert minerals to other minerals more stable in the new P‐T (pressure and temperature) environment). Heat drives recrystallization – New mineral structures are more stable in higher T environment Solids readjust to new temperature and pressure conditions. Example: Kyanite (low‐med temp.) – Andalusite (med. Temp.& high pressure) – Sillimanite (med‐high temp.) are all polymorphs (different minerals with the same aluminum silicate composition. Refer to the diagram in the slides. Other polymorphs we have already discussed are diamond & graphite (high and low temp./pressure carbon minerals). Metamorphic Grade & Facies Assemblages of minerals – indicate temperature and pressure These assemblages (groups of minerals) are called Metamorphic Facies (minerals formed in similar T & P and tectonic settings). Refer to the diagram in the slides. • Subduction zone metamorphism – Blueschist facies‐ high pressure/low temperature • Regional metamorphism series‐ from Low T/P to High T/P • Greenstone‐Greenshist‐Amphibolite‐Granulite‐Eclogite facies • Contact metamorphism‐High T/Low P ‐ Hornfels facies (also called greenstone) Note: Because there is a lot of variability in the parent (protolith) rock types, and conditions of mineral stability are also variable, the mineral assemblages are highly complex. Metamorphic Changes: pressure & stresses – two primary types • confining pressure (essentially equal compaction from all sides) – compaction/minerals can recrystallize • differential stress (directed / unequal stress) – grains also reoriented & rocks folded Reorientation of the minerals can occur and a texture called foliation results from differential or directed stress . • Flat minerals like micas will want to orient with the “flat” face perpendicular to directed stress. Foliation will only form if flat or platy minerals are present. However, a weak foliation can form if pebbles are present (e.g. stretched pebble conglomerate) Recrystallization can also occur. Example: Limestone or quartz sandstone These rocks are monominerallic and there are no flat or platy minerals present. Because the minerals are not reoriented, we get very little information about the direction of pressure or temperatures. • Under higher P & T conditions – they simply recrystallize. • Much larger crystals are formed with an interlocking texture • Limestone becomes marble and sandstone becomes quartzite. Types of Metamorphism Local Metamorphism‐ Contact Metamorphism. For example, when an magma is intruded into a limestone (host rock), a baked zone can be formed in the limestone. This zone is called an “aureole”. In rock types that are more “reactive”, the size or thickness of the aureole can be larger. The size varies depending on the type of host rock, the size of the igneous body and the amount of available water. Local Metamorphism‐ Shock. This can occur when a meteor hits the Earth. These are confined to the local area around the impact as high heat and pressure is generated over a very short timeframe. Tektites are small beads of silica rich glass that can be formed when this happens. Local Metamorphism – dynamic. For example, when there is movement along a fault, a fault breccia/gouge (powder) and fault breccias (angular broken bits of rock) can be produced. Fluids can also be present in this environment that alter the rock. Local Metamorphism: Hydrothermal Alteration (Metasomatism) • Contact metamorphism in the presence of fluids • Metasomatism – means that substantial chemical changes take place • Note: Heat alone usually forms new minerals (new crystal structures stable at higher T &P with roughly equivalent chemical compositions) • Ore bodies can be formed in this setting Example: Hydrothermal alteration can occur at Ocean Ridges (hot mineral rich fluids expelled from magma). “Black Smokers” are features where there are metal rich fluids emanate from the sea floor and metal deposits metal deposits precipitate out. Hydrothermal Alteration can occur in veins. Ore deposits are formed as hot mineral rich fluids ascend and minerals are deposited. Regional Metamorphism • Thermal only – burial beyond normal depths (e.g. subsiding basins) where sediment was deposited (Example ‐ Gulf Coast Basin). • Dynamic ‐ pressure in the absence of temp increase, very rare environment • Dynamo‐thermal ‐ both temp & pressure increase – Heat and directed stress ‐Associated with mountain building at Convergent plate boundaries As rocks are metamorphosed, depending on the parent rock type and type of metamorphism, different textures/rocks are formed. Let’s look at the type of parent rocks and the resulting metamorphic rocks. The parent rocks are similar in composition except for the loss of water. However, the minerals are different. Refer to the figure below. In the figure (from your lab manual), the parent rocks are on the left. In the middle are the metamorphic rocks formed through thermal meta only, and on the right are the products of dynamothermal meta, where there is heat and directed stress, and foliation may occur if flat or play minerals are present in the parent rock. Thermal Metamorphism First we’ll look at the rocks formed through thermal metamorphism only (no directed stress). These are non‐foliated. Mudstone, graywacke and shale (parents) can form greenstone/hornfels Sandstones, arkose and chert can form quartzite as grains fuse together/can fracture through qtz grains Limestone and dolostone can form marble – large interlocking calcite crystals Conglomerate and breccia can form a metaconglomerate where the matrix & cement are recrystallized and matrix becomes as durable as clasts, and the rock can fracture across clasts/matrix Felsic and intermediate igneous rocks may not change as they can be stable under thermal metamorphic conditions. Mafic igneous Rocks can form greenstone (non‐foliated) Note: Greenstone ‐ includes minerals like chlorite,epidote,actinolite ‐ green in color Dynamo‐Thermal Metamorphism (heat and directed stress) • Platy or elongate minerals produce foliation under directed pressure • Foliated rocks are classified on the type of foliation • Where there are no elongate (e.g. platy or flat) minerals foliation is not found (e.g. limestone to marble OR sandstone to quartzite) Differential stress typically occurs during mountain building, and foliated meta rocks are usually associated with ancient convergent boundaries. Now let’s take a look at the metamorphic rocks formed when there is an increase in heat , pressure, and with directed stress. Metamorphism of mudstones, shales and greywacke Slate to Phyllite to Schist to Gneiss (pr. Nice), increasing in metamorphic grade as temp./pressure increases. These are all foliated rocks. • Slate – low grade meta rock fine grain (mica)/ with good rock cleavage (slabs) • Phyllite – med. Grade glossy sheen/larger mins.(e.g. mica/chlorite)‐can’t see them • Schist – med./high grade & med‐coarse grained minerals‐e.g. micas, chlorite, graphite,talc with accessory garnet,staurolite,etc. • Gneiss (pr. Nice) – high grade & med‐coarse grain granular elongated crystals and banded (compositional) Mins‐ e.g. Qtz,feldspars,micas,amphiboles w/accessory mins Again, even under directed stress, quartz sandstone, arkose and chert can become quartzite (non‐ foliated) since there are no platy minerals. And, limestone and dolostone can become marble (non‐foliated). The conglomerate can become a stretched pebble metaconglomerate with (“weak” foliation) due to the directed stress. The felsic/intermed. Igneous rocks can become a gneiss (foliated) And, mafic igneous rocks can become schist or gneiss. Migmatite – Another form of metamorphic rock that is a kind of “hybrid” since it is a high grade/partially melted rock. This is not the normal situation. The vast majority of metamorphic rocks are formed through solid state changes. Here are a couple of key points that were covered that you should “take home”. • Regional metamorphic rocks are interpreted as ancient convergent boundaries • Foliation can be used to determine the direction of convergence (by determining the direction of mineral elongation). The “flat” face is perpendicular to the directed stress. Potential human impacts related to folation….. Rock slides can occur in certain situations when the foliation in the rocks are parallel to the hill slopes. 8/2011 .
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