Metamorphic Rocks (Chp 7)

Metamorphic Rocks (Chp 7)

1. Metamorphic Rock: protolith undergo changes in texture and mineralogy

   • protolith: parent rock
   • index mineral: helps determine the pressure and temp combination under which a rock recrystallized, and can help estimate the conditions present during metamorphism.
     • Andalusite: P and T were relatively low
     • Kyanite: high P, but a higher T requires a higher P to form Kyanite
     • Sillimanite: high T, found in contact rocks near intrusions and deep regional rocks
   • a parent rock may recrystallize into many different rocks depending on P and T during metamorphism
     • Basalt can turn into Greenschist (low T and P, schistose, contains chlorite, actinolite, and sodium rich plagioclase) or Greenstone (similar minerals, but not foliated, not strong tectonic forces) or Amphibolite (hornblende, plagioclase feldspar, garnet)

2. Types of metamorphism

   • Regional Metamorphism: metamorphism occurring over large regional extent and considerable depth in crust
     • associated with mountain building (orogenic belts) at convergent plate margins
     • almost always foliated, indicating differential stress during recrystallization
       • differetial stress based on tectonism; constant movement and squeezing of crust during mountain building
     • 300 C to 800 C based on geothermal gradient of region. subduction zones will have steeper gradients
       • geothermal gradient: profile of increasing temperature with increasing depth
     • Basalt -> Amphibolite (Hornblende, plagioclase, garnet); Shale -> Mica schist (biotite, muscovite, quartz, garnet); Quartz sandstone -> Quartzite (quartz); Limestone or dolomite -> Marble (calcite or dolomite)
   • Contact metamorphism: occurs from thermal effects of a nearby igneous intrusive body. Associated with high temperature.
     • Grade decreases as distance increases from contact aureole (zone of contact)
     • Pressure and Differential Stress is rarely significant because most contact occurs close to surface (10 km)
       • Most rocks will be nonfoliated
   • Metasomatism:

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