Kinds of metamorphism by gegeshandong

VIEWS: 127 PAGES: 29

									        Kinds of metamorphism
Lecture based on
http://csmres.jmu.edu/geollab/Fichter/MetaRx/MetaKind.html
And PowerPoint lecture of Dr. J.D. Winter
http://www.whitman.edu/geology/winter/
    The Types of Metamorphism
Different approaches to classification

1. Based on principal process or agent
   – Dynamic Metamorphism
   – Thermal Metamorphism
   – Dynamo-thermal Metamorphism
2. Based on setting (P-T conditions/tectonic)
   – Contact Metamorphism
      • Pyrometamorphism
   – Regional Metamorphism
      • Orogenic Metamorphism
      • Burial Metamorphism
      • Ocean Floor Metamorphism
   – Hydrothermal Metamorphism
   – Fault-Zone Metamorphism (cataclastic or
     mylonitic in your text)
   – Impact or Shock Metamorphism
   Types of metamorphism based on
  pressure and temperature conditions




http://csmres.jmu.edu/geollab/Fichter/MetaRx/MetaKind.html
Blueschist metamorphism


 Geothermal gradient:
 Indicates the average increase
 of temperature with depth




• HIGH pressure/Low temperature
• Typically associated with subduction
  zones
• Example: Blueschist (from glaucophane)
                                      1mm

                     Glaucophane in a blueschist (ppl).


http://www.union.edu/PUBLIC/GEODEPT/COURSES/petrology/met_minerals.htm#Amphiboles
Eclogite metamorphism




• VERY HIGH pressure/moderate
  temperature
• Happens in the mantle
• Example: Eclogite (pyroxene/garnet)
                 Eclogite (xpl)
Eclogite (ppl)
Hydrothermal metamorphism




• Low pressure/low temperature
• Hot, chemically active waters react with
  the surrounding rocks
• Example: Serpentinite, soapstone
Hydrothermal metamorphism




• Usually involves METASOMATISM
  (metamorphism plus exchange of ions from
  an external source)
• Difficult type to constrain: hydrothermal
  effects often play some role in most of the
  other types of metamorphism
Contact metamorphism




• Low pressure/HIGH temperature
• “Country” rocks baked by igneous
  intrusion (pluton)
• Example: Hornfels, skarn (baked
  carbonate rocks), quartzite, marble
Contact Metamorphism
 Most easily recognized where a pluton is
  introduced into shallow rocks in a static
  environment
(Pluton=intrusive igneous body, like a batholith)

     Hornfelses (granofelses) commonly
     with relict textures and structures
Spotted hornfels




http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/spottedhornfels.X.html
Pyrometamorphism
A minor type of contact metamorphism
Very high temperatures at very low
 pressures, generated by a volcanic or
 subvolcanic body
Also developed in xenoliths (pieces of solid
 rocks carried up by magma)
Pyrometamorphism may be accompanied
 by various degrees of partial melting
Barrovian metamorphism

Also referred to
as “regional”
metamorphism



• Intermediate pressure/Low-high
  temperature
• Example: Slate-phyllite-schist-gneiss,
  also quartzite and marble
Regional Metamorphism sensu lato:
 metamorphism that affects a large body of
 rock, and thus covers a great lateral extent
  Three principal types:
  – Orogenic metamorphism
  – Burial metamorphism
  – Ocean-floor metamorphism
  The term, “regional metamorphism” is
  often used synonymously with
  “orogenic metamorphism”
  (OROGENY=mountain building)
Orogenic Metamorphism is the type of
 metamorphism associated with
 convergent plate margins
 • Dynamo-thermal: one or more episodes
   of orogeny with combined elevated
   geothermal gradients and deformation
   (differential stress)
 • Foliated rocks are a characteristic
   product
                                                         Foliation




Slate (ppl)

                                        Phyllite (xpl)

                                  Quartz

                                  Muscovite




Quartz-muscovite schist (xpl)

     http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/metamicro.html
Crenulation cleavage in a muscovite-biotite-garnet schist (xpl)




  http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/metamicro.html
                                  Gneiss under a microscope (xpl)

                                       Biotite               Feldspar




                                                 Quartz
Gneiss in hand specimen




     http://www.geolab.unc.edu/Petunia/IgMetAtlas/meta-micro/metamicro.html
               Calcite (note cleavage)




Marble (xpl)
Burial metamorphism = low-grade
metamorphism in sedimentary basins
  – Metamorphic effects attributed to increased
    temperature and pressure due to burial
  – Occurs in areas that have not experienced
    significant deformation or orogeny
  – Mild deformation, no igneous intrusions
    discovered
Restricted to large, relatively undisturbed
  sedimentary piles away from active
  plate margins
  – Term coined by Coombs (1961) based on
    Southland Syncline in New Zealand: thick
    pile (> 10 km) of Mesozoic
    volcaniclastics
  Modern examples:
  – The Gulf of Mexico?
  – Bengal Fan?
Ocean-Floor Metamorphism affects the
oceanic crust at ocean ridge spreading
centers
•A wide range of temperatures at relatively
low pressure
•Seawater penetrates down fracture
systems, where it becomes heated, and
leaches metals and silica from the hot
basalts
•Considerable metasomatic alteration,
notably loss of Ca and Si and gain of Mg and
Na
Highly altered chlorite-quartz rocks-
 distinctive high-Mg, low-Ca composition
Black smokers - Another example of
 hydrothermal metamorphism
Fault-Zone and Impact Metamorphism
 – High rates of deformation and strain
   with only minor recrystallization


 (a) Shallow fault zone with fault breccia
 (b) Slightly deeper fault zone (exposed
 by erosion) with some ductile flow and
 fault mylonite
Width of view=4mm
Fault-Zone and Impact Metamorphism
 – High rates of deformation and strain
   with only minor recrystallization

  – Impact metamorphism at meteorite (or
    other bolide) impact craters
  – Both correlate with dynamic
    metamorphism, based on process

								
To top