Chapter 8 - Metamorphic Rocks

• Metamorphism

  • occurs deep within Earth
  • metamorphic rocks are produced from preexisiting sedimentary and igneous rock.
  • every metamorphic rock has a parent rock - the rock from which it was formed.
  • "to change form", a process that transforms the mineralogy, texture, and sometimes chemical composition of parent rock.
  • the mineralogy also changes becuase the rock is subjected to a new condition (elevated temperature and pressure)
  • also alter's a rock's texture, producing larger crystals, layered or banded appearance.
• Metamorphism

  • e.g. clay minerals are stable only at the Earth's surface. when buried to a depth, they are transformed into chlorite and/or muscovite mica. under more extreme conditions, chlorite become biotite mica.
  • Metamorphic grade: degree to which a parent rock changes during metamorphism.
  • Varies from low grade (less than 200°C) to high grade (more than 600°C)
• Metamorphism

  • e.g. In low grade environments, shale becomes more compact metamorphic rock slate. The clay minerals in shale are changed into tiny chlorite and muscovite mica flakes.
  • during metamorphism, the rock remains solid. if melting occurs, the rock has entered igneous activity.
• What drives metamorphism?

  • agents include heat, pressure, differential stress and chemically active fluids.
  • Heat

  > most important factor since it provides the energy needed for recrystallization of existing minerals.

  > recrystallization - formation of new/enlarged mineral grains at the expanse of original grains.

  > mineralogy may not change by grains tend to become larger.

  > e.g. quartz sandstone metamorphosed to form quartzite.
• What drives metamorphism?

  1. Heat

  • Source of Heat: 2 primary ways

  1. may be more deeply buried; Earth's temperature rise with depth
  2. magma body may be emplaced nearby; heating surrounding rocks.

  • Geothermal gradient - rate of increase in temperature with depth.
  • In the upper crust, this increase in temperature averages about 25°C per km.
  • Many silicates, especially those found in crystalline igneous rocks such as quartz and feldspars, remain stable at these temperature. Thus, their metamorphism occur at higher temperature.
• What drives metamorphism?

  • Several conditions in which heat drives metamorphism.

  > rocks may be carried to great depths & heated include convergent plate boundaries.

  > rocks may also be deeply buried in large basins where gradual subsidence results in thick accumulation of sediments.

  > continental collisions for mountain building causes uplifting while others are thrust downward, where metamorphism happen.

  > heat may also be transported from mantle into the shallowest layers. when magma intrudes shallow depths, the magma cools and releases heat, which bakes & transforms the surrounding rock.
• What drives metamorphism?

  2. Confining pressure

  > pressure increases with depth because thickness of overlying rock increases.

  > buried rocks are subjected to confining pressure in which the forces are applied equally in all directions.

  > this causes spaces between mineral grains to close, producing more compact rocks with greater densities. if the pressure becomes extreme, it can produce a new, denser mineral.

  > phase change - transformation of 1 mineral to another

  > confining pressure does not fold or fracture rocks.
• What drives metamorphism?

  3. Differential Stress

  • occurs when convergent plate boundaries where slabs of lithosphere collide.
  • forces that deform rocks in different directions.
  • squeezes a rock mass as if it were placed in a vise is called compressional stress.
  • along convergent plate boundaries, the greatest differential stress is directed horizontally in the direction of plate motion.
  • in these settings, the crust is shortened (horizontally) and thickened (vertically), resulting in mountainous topography.
• What drives metamorphism?

  3. Differential Stress

  • in high temperatures and pressures, rocks are ductile which allow their mineral grains to flatten.
  • e.g. Metaconglomerate - "stretched pebble" conglomerate
  • The parent rock, conglomerate, consisted of nearly spherical pebbles that have been flattened into elongated structures by differential stress.
  • On the other hand, rocks that are ductile deform by flowing rather than breaking. Deeply buried rocks develop folds when deformed.
  • In near surface environments, rocks are brittle and tend to fracture.
• What drives metamorphism?

  4. Chemically Active Fluids

  • enhance metamorphism by dissolving and transporting ions, facilitating recrystallization.
  • hot fluids transport mineral matter over considerable distances.
  • e.g. Wollastonite (CaSiO3), when a silica rich hydrothermal solution invades limestone, calcite reacts with silica to generate wollastonite and CO2 is driven off.
• Metamorphic Textures

  • Foliation
  • any planar arrangement of mineral grains within a rock.
  • may occur in some sedimentary and igneous but a fundamental characteristic of metamorphosed rocks that have been deformed by folding.
  • driven by compressional stress that shorten rock units causing mineral grains in preexisting rocks to develop parallel alignments.
  • e.g. parallel alignment of platy minerals thru rotation, recrystallization and flattening of mineral grains or pebbles.
• Metamorphic Textures

  • Foliated Textures
  • Rock/Slaty, Cleavage

  > rocks that split into thin slabs when hit with a hammer.

  > develops in various rocks but best displayed in slates that exhibit an excellent property called splitting cleavage.

  > evidence of sedimentary beds is preserved.

  > develop at an angle to the sedimentary beds.

  > slates split across bedding surfaces.
• Metamorphic Textures

  • Foliated Textures
  • Schistosity, Gnessic: Banding
  • when platy minerals are large enough to be discerned, they exhibit planar or layered structures called schistosity.
  • rocks are termed as schist.
  • schist often contain deformed quartz and feldspar crystals that have been flattened and are embedded among mica grains.
• Metamorphic Textures

  • Nonfoliated Texture

  > rocks that do not develop a layered or banded appearance

  > form in environments where:

  • deformation is minimal
  • parent rockis composed of minerals that develop equidimensional crystal rather tabular.
  • e.g. limestone > metamorphosed > calcite grains recrystallize > marble with intergrown calcite crystals with no banding.
  • e.g. shale and mudstone > intruded by magma body > no differential stress so rocks are baked with alignment to its platy grains > hornfels
• Metamorphic Textures

  • Porphyroblastic Textures

  > develop when minerals in the parent rock recrystallize to form new minerals.

  • e.g garnet develop a small number of large crystals. in contrast, muscovite and biotite form a large number of grains. with this, garnet embedded in a finer grained biotite and muscovite.
• Metamorphic Rocks (Foliated)

  1. Slate

  > very fine grained (less than 0.5 mm)

  > composed of minute chlorite and mica flakes

  > contain tiny quarts and feldspar crystals

  > appears dull and resembles shale

  > excellent rock cleavage & break into flat slabs.

  > generated by low grade metamorphism of shale

  > color: black (carbon), red (iron oxide), green (chlorite)
• Metamorphic Rocks (Foliated)

  2. Phyllite

  > degree of metamorphism between slate and schist

  > platy minerals are larger than those in slate but not large enough to be identified by unaided eye.

  > can be distinguished from slate by its glossy sheen and waxy surface.

  > rock cleavage
• Metamorphic Rocks (Foliated)

  3. Schist

  • medium to caorse-grained in which platy minerals are dominant
  • flat components include biotite and muscovite
  • smaller amounts of quzrts, feldspar and muscovite
  • parent rock: shale, undergone medium to high grae metamorphism during a major mountain bldg episode
• Metamorphic Rocks (Foliated)

  4. Gneiss

  • medium to coarse-grained banded metamorphic rocks in which granular and elongated minerals predominate.
  • quartz, potash feldspar and plagioclase feldspar
  • some split along layers of platy minerals but most break in an irregular fashion.
  • during high grade metamorphism, light and dark components separate which results to a layered or banded appearance.
  • thus, gneisses contain alternating bands of white or reddish feldspar rich zones and layers of dark ferromagnesian minerals.\
  • banded gneisses exhibit evidence of deformation like folds and sometimes, faults.
• Metamorphic Rocks (Non-Foliated)

  1. Marble

  • produced by metamorphism of limestone/dolostone
  • pure marble is white and composed of calcite.
  • when exposed to acid, undergoes chemical weathering.
  • when marbles forms from limestone interbedded with shales, it appears banded and has foliation.
  • when deformed, these banded marbles may develop highly contorted mica-rich folds.
• Metamorphic Rocks (Non-Foliated)

  2. Quartzite

  • very hard metamorphic rock from quartz sandstone
  • under high metamorphism, quartz grains in sandstone fuse together.
  • cross bedding are preserved and gives a banded appearance.
• Metamorphic Rocks (Non-Foliated)

  3. Hornfels

  • fine-grained, has a variable mineral composition
  • parent rock: shale, another clay-rich rock that has been baked by an intruding magma body.
  • grey to black and has conchoidal fracture
• Metamorphic environments

  1. Contact/Thermal metamorphism

  • occurs in upper crust (lower pressure), when rocks immediately surrounding a molten igneous body are "baked"
  • does not involve stress, so resulting rocks are not foliated.
• Metamorphic environments

  1. Contact/Thermal metamorphism

  • alter rocks in a discrete zone adjacent to the heat zone called aureole.

  > emplacement of dikes and sills typically form aureoles only a few cm thick.

  > by contrast, large molten bodies that form batholiths produce aureoles that extend outward for several kilometers.

  • often consist of distinct zones of metamorphism
  • close to the magma body, high temperature minerals can form such as garnet
  • whereas far away, low-grade metamorphism produce minerals such as chlorite
• Metamorphic environments

  1. Contact/Thermal metamorphism

  • roof pendant

  > type of aureole that consist of metamorphosed host rock that are in contact with upper part of light colored igneous pluton.

  > implies that the rock was once the roof of a magma chamber.
• Metamorphic environments

  2. Hydrothermal metamorphism

  • occur when hot, iron-rich water circulates thru pore spaces or fractures in a rock.
  • a chemical alteration
  • contribute to metamorphism by enhancing the recrystallization of existing minerals.
  • occur at low pressure (shallow depth) with low to medium temperature.
  • water that drives hydrothermal metamorphism can:

  > be groundwater that has percolated down the surface, where it is heated and circulates upward.

  > may arise from igneous activity

  • widespread occurrence occur along the axis of mid ocean ridge system
• Metamorphic environments

  2. Hydrothermal metamorphism

  • hydrothermal solutions circulating thru the seafloor also remove large amounts of metals such as iron, cobalt, nickel, silver, gold, copper from the newly formed crust.
  • these hot, metal-rich fluids rise along fracture and gush from the seafloor at 350°C generating particle filled clouds called black smokers.
• Metamorphic environments

  3. Burial and Zone metamorphism

  • Burial metamorphism
  • occur where massive amounts of sedimentary or volcanic material accumulate in a subsiding basin.
  • low grade metamorphic conditions may be reached within the deepest layers.
  • confining pressure & heat drive recrystallization of constituent minerals, changing texture and mineralogy of the rock, without deformation.
• Metamorphic environments

  3. Burial and Zone metamorphism

  • Subduction Zone metamorphism
  • cold, dense oceanic crust and sediments which are conductors of heat are subducting rapidly enought that pressure increases faster than temperature.
  • differs from burial metamorphism becauser rather from confining pressure, differential stress plays a major role in deformation.
• Metamorphic environments

  4. Regional metamorphism

  • common widespread type associated with mountain building, where large segments of crust are intensely deformed by collision of 2 continental crust blocks.
  • The general thickening of the crust that occurs during mountain building results in a buoyant lifting in which deformed rocks are elevated high above sea level.
  • Crustal thickening also results in the deep burial of large quantities of rock as crustal block is thrust beneath another.
• Deep burial


  Elevated temperatures caused by deep burial are responsible for the most intense metamorphic activity within a mountain belt
• Magma production

  1. Deeply buried rocks become heated beyond their melting points, producing magma

  2. When these magma bodies grow large enough to buoyantly rise, they intrude the overlying metamorphic and sedimentary rocks.

  • The cores of mountain belts consist of folded and faulted metamorphic rocks often intertwined with igneous bodies
  • Over time, these deformed masses are uplifted and erosion removes the overlying material to expose the igneous and metamorphic core of mountain range
• Regional metamorphism

  1. Shale is metamorphosed to produce the sequence of slate, phyllite schist and gneiss

  2. Quartz sandstone and limestone are metamorphosed into quartzite and marble
• Metamorphism along Fault Zones

  
  

  • Rock behaves like a brittle solid, and movement along a fault zone fractures and pulverizes rock, resulting in a loosely coherent rock called fault breccia, composed of broken and crushed fragments.
  • Much of deformation occur at great depth and high temp.
  • Preexisting rock minerals deform by ductile flow, forming elongated grains that give the rock a foliated/linear appearance
  • Rocks formed in these zones of intense ductile deformation are termed mylonites
• Impact/shock metamorphism

  Occurs when meteorites (fragments of comets or asteroids) strike Earth's surface at high speeds
• Impact of a rapidly moving meteorite

  1. Energy transformed into heat energy

  2. Shock wave passes through surrounding rocks

  3. Results in pulverized, shattered and sometimes melted rock
• Impactiles

  • The product of these impacts, including mixtures of fused fragmental rocks plus glass-rich ejecta that resemble volcanic bombs.
  • Existence of high-pressure minerals provides evidence that pressure and temperature in impacts can be as great as those in the upper mantle
  • Very dense form of quartz (coesite) and minute diamonds are found in some cases.
• Migmatites


  • A rock type wherein under high-grade metamorphism, felsic minerals in a gneiss may begin to melt, while the mafic minerals remain solid
  • If the melt rolls out, migmatite will contain light colored igneous rock intermixed with metamorphic rock composed of mafic minerals
• Migmatites illustrate that some rocks are transitional and do not fit neatly into any of the 3 basic rock groups