Themes of the Day:

• Conditions of Metamorphism (continued)
• Settings of Metamorphism

Conditions of Metamorphism

• Conditions that affect the textural and compositional characteristics of metamorphic rocks
  • composition of the protolith
  • temperature and pressure (T,P)
  • effects of tectonic forces (directed stress)
  • fluids
  • time/rates of change
• Temperature - geothermal gradient - may be perturbed by rising magmas, hydrothermal circulation
• Pressure - difference between confining pressure and directed stress - What happens to a styrofoam cup under deep sea pressures? before & after
• Metamorphic Facies - regions of P,T space - characteristic mineral assemblages (depends on protolith composition and P,T)
• Mineral Zones - ways to map isograds ("equal grade")

Settings of Metamorphism

• Contact - local, often High T, Low P (adjacent to a pluton; occasionally High P, moderate T (shock metamorphism)
• Regional - large areas, tectonic controls
  • Burial - sedimentary basins
  • Dynamothermal - mountain belts - most often characterized by deformation (directed stress)
• Hydrothermal (involving circulating hot water) occurs in both situations - very important at mid-ocean spreading ridges
• Progressive metamorphism describes increasing P,T effects on a single rock type
  • Regional progressive metamorphism in New England
  • P-T diagram with corresponding metamorphic rocks
  • Index minerals
  • Metamorphic Facies diagram with various geothermal gradients
  • Plate tectonic setting block diagram
• Prograde vs. Retrograde metamporphism - two different P-T-t paths
• Plate Tectonic Settings of Metamorphism
  • Mid-Ocean Ridge - hydrothermal, Low P, high T metamorphism
  • Subduction Zones
    • Near Trench - High P, Low T - blueschist facies
    • Shallow Volcanic Arc - High T, Low P - contact metamorphism - hornfels facies
    • Deep Volcanic Arc - High T, High P - granulite facies