METAMORPHIC ROCKS

					METAMORPHIC ROCKS
                METAMORPHISM
• Alteration of any previously
  existing rocks by high pressures,
  high temperatures, and/or
  chemically active fluids.
             Agents of Metamorphism
– high temperatures -
   • most important agent because it provides energy to drive
     chemical reactions
   • geothermal gradient (25o C /km) & compressional heat
   • proximity to magmatic intrusions
– high pressures -
   •   compressive stresses during mountain building
   •   increases with depth
   •   can cause rock to flow or bend (deform plastically)
   •   sliding (shearing) can pulverize rock
– chemically active fluids -
   • water is in pore spaces of most rocks
   • many minerals are hydrated
   • hot water contains ions in solution that form new minerals
  Textural, Structural & Mineralogical Changes
• Degree of metamorphism is reflected by
   – mineral composition
   – texture of rocks
• Rocks become more compact (denser)
• Minerals recrystallize and grow larger in solid state
• Some new minerals form
• Preferred (structural) mineral orientations develop
   – perpendicular to stress
      • platy minerals
      • elongated minerals (needle-like)
   – causing foliation, a layered structure
• chemical mineral segregation into light and dark bands
                           Foliation
• Rock cleavage (slaty cleavage)-
   – low-grade metamorphism (low pressure, low temp.)
   – clay minerals recrystallize to minute mica flakes
   – develops parallel fracturing
   – forms slate
• Schistosity
   – high-grade metamorphism
   – mica crystals become visible (scaly appearance)
   – forms schist
• "banding"
   – highest-grade metamorphism
   – silicate minerals segregate into light & dark bands
   – forms gneiss
  Development of Rock Cleavage (Slaty Cleavage)
• High pressure, associated with folding, develops parallel
  alignment of growing microscopic mica crystals (in a former
  clayey shale) that are perpendicular to the stress direction
           Rock Cleavage in Outcrop
• Rock cleavage enhances
  erosion
               Development of Banding
• Light and dark minerals segregate out into parallel layers, such as
  in a gneiss, that are perpendicular to the stress direction
  Low-Grade Foliated Metamorphic Rocks
• Shale metamorphoses to slate
     • Clay minerals alter to
       microscopic mica crystals
     • Rock cleavage develops as the
       mica crystals align parallel to
       each other and perpendicular
       to the main stress direction
     • Breaks into parallel sheets
     • Once used for school
       blackboards
• Slate metamorphoses to phyllite
     • Microscopic mica crystals
       enlarge, giving the rock a glossy
       sheen
  High-Grade Foliated Metamorphic Rocks
• Phyllite metamorphoses to
  schist
   – Mica crystals become
     easily visible to the naked
     eye
Highest-Grade Foliated Metamorphic Rocks
• Schists metamorphose to
  gneiss
   – Minerals alter to form
     abundant quartz &
     feldspar
   – Light and dark minerals
     separate out into bands
Further Metamorphism Produces Migmatites
• When the temperatures
  during metamorphism are
  high enough, low-melting-
  point minerals melt and
  the hot fluid separates
  from the still-solid, high-
  melting-point minerals
• Forms bands of
  alternating igneous and
  metamorphic rock
   – light bands = igneous
   – dark bands = meta.
       Non-Foliated Metamorphic Rocks
• Foliation will not develop in rocks that are
   – Equant grained (equidimensional in all directions)
   – Monomineralic (e. g., all quartz, all calcite, etc.)
• Example #1: Quartz sandstone metamorphoses to a
  quartzite which may look the same whether it forms
  under low-grade to high-grade metamorphic conditions
      Non-Foliated Metamorphic Rocks
• Example #2: Limestones metamorphose to marble
  – Equant grained
  – All calcite
       Metamorphism Along Fault Zones
• Movement along a fault zone may break up and grind
  the adjacent rocks, due to shear, to form a breccia that
  is called a "fault breccia" near the surface where rocks
  are brittle.
• The fragments in a breccia are very angular
• A high pressures deeper in Earth, rocks flow & don't
  form breccias
             Contact Metamorphism
• Magma comes into contact with cooler country rock
   – base of thick lava flows
   – near-surface dikes and sills
   – around deep stocks & batholiths
• Heat and fluids from magma alter country rock
• Forms concentric zones of varying degrees of mineral
  alteration
• Hydrothermal solutions from the magma can form
  metallic ore deposits in the country rocks
              Regional Metamorphism
• Intense metamorphism and deformation during mountain
  building associated with lithospheric plate collisions
• increases towards the core or major mountain ranges
• most common type of metamorphism
Mineral Changes with Increasing Metamorphism

				
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posted:11/2/2011
language:English
pages:18