Metamorphic

• Metamorphism - a change in the form of pre-existing rocks (igneous, sedimentary or metamorphic) caused by an increase in pressure, temperature, or a combination of both.
• Limits of Metamorphism:

  • lower limit for temperature is 200$^o$C (below this diagenesis and lithification take place; sedimentary process)
  • upper limit for temperature is 800$^o$C (above this rocks melts; igneous proccesses occur)
  • no lower limit for pressure, but minimal metamorphism at depths <2km
  • no upper limit for pressure, but metamorphism unlikely to occur beyond depths of 45-50km (rocks melted at this depth)
• Pressure that causes Metamorphism:

  • Confining Pressure - the effect of the mass of the overlying rocks; it acts equally in all directions and increases with depth.
  • Directed Pressure - the effects of tectonic forces acting at destructive plate margins (pressure in two directions).

  A) confining

  B) directed

  
• Metamorphic Processes

  • metamorphic rocks may be formed from all types of rock; loose sediments (mud/silt/sand) become sedimentary rocks by compaction/cementation of the grains
  • cement is mostly composed of chemicals dissolved in water trapped in pore spaces between grains of sediment
  • if sedimentary rock is affected by heat of magma rising nearby, it may be recrystallised into a metamorphic rock; this is thermal/contact metamorphism (from increased heat alone)

  A) sedimentary

  B) low

  C) medium

  D) high

  E) regional

  F) contact

  G) igneous

  
• Metamorphic Processes

  • sedimentary rock can be metamorphosed by being squeezed from the side as two plates collide, and become more deeply buried/heated by higher temperatures several km down in crust
  • this is regional metamorphism; varies from low to high grade depending on temperature/pressure - in extreme conditions the rock may melt to form magma helped by presence of water in rock (lowers melting point)

  
• Underlying Principles:

  • metamorphism involves recrystallisation of an original rock without melting taking place
  • metamorphism may be caused by increased temperature and/or pressure (mostly from plate tectonic forces)
  • quartzite/marble formed by increased heat and/or pressure, but directed pressure needed to form slate/schist /gneiss
  • flaky minerals in a mud-rock/clay recrystallise into other flaky minerals (micas) to lie perpendicular to forces on the rock
  • larger, more equidimensional grains (sandstone/limestone) recrystalline together in roughly hexagonal shapes
• Effects of Metamorphism:

  • destruction/distortion of fossils
  • destruction of sedimentary structures
  • hardening of the rock
  • colour changes to the rock
  • growth of new metamorphic minerals
  • alignment of new minerals in the rock
• Metamorphic Grade - the extent to which the pre-existing rocks have been changed in form/altered; low grade = slight alteration, medium grade = significant alteration, high grade = extensive/total alteration.
• Metamorphic changes are isochemical; the bulk composition of the parent rock and the new metamorphic rock are the same (contain same % of elements) - only loss from system is water as hydrous clay minerals are dehydrated by rise in temperature + recrystallisation takes place in the solid state during metamorphism (no melting).
• Contact/Thermal Metamorphism

  • changes due to an increase in temperature alone - associated with major igneous intrusions (plutons/batholiths)
  • the volume of country rock altered by heat from the intrusion is the metamorphic aureole - recrystallisation occurs with random orientation of minerals (non-foliated)
• Dynamic Metamorphism

  • changes due to an increase in pressure alone - associated with major fault planes (San Andreas Fault/Moine Thrust)
  • relatively localised/small scale (limited to few metres either side of fault plane) + rocks crushed and ground into angular fragments under intense shear pressure (directed stress)
• Regional Metamorphism

  • changes due to an increase in both temperature/pressure - associated with destructive plate margins and orogenies
  • occurs on a large (regional) scale and involves thousands of km of rock + subducted ocean floor sediments undergo regional metamorphism from 5-50km depth
  • regional rocks have a foliated texture with minerals being aligned parallel to each other
• Susceptibility to Rocks to Metamorphism:

  • argillaceous sediments (clay/mudstone/shale) undergo greatest mineralogical change + clay minerals (chlorite/illite/kaolinite) only stable at low temperatures/pressures at or close to surface
  • clay minerals chemically complex and a range of mineral types can form by recrystallisation under different temperature/pressure conditions
• Slate - Low Grade Regional Metamorphism

  • forms 5-15km depth at high pressure and low temperature (<300𝑜oC) conditions
  • clay minerals recrystallise into chlorite/biotite mica + coarser grained than original clay minerals; chlorite/biotite mica are platy minerals that have long axes aligned at right angles to principle stress direction to form slaty cleavage
  • slate coarser/more crystalline than parent rock shale, but crystals fine-grained (<1mm)
  • relic sedimentary structures may be preserved (bedding planes or laminations) + fossils may be present but deformed

  
• Schist - Medium Grade Regional Metamorphism

  • formed under higher temperatures (400-500$^o$C) and at 15-25km depth - medium grained (1-3mm) + crystalline + shiny
  • higher temperature results in larger crystals and growth of new minerals (garnet, quartz and mica); garnet crystals are porphyroblasts (<5mm) and distort foliation
  • coarser foliation known as schistosity (schistose texture); formed by alignment of micas due to stress

  
• Gneiss - High Grade Regional Metamorphism

  • formed under higher temperatures (500-700$^o$C) and at 25-40km depth - mineral composition similiar to granite + coarse grained (>3mm)
  • higher temperatures result in larger crystals over 2mm and the growth of new minerals (feldspar, quartz and mica)
  • minerals segregated into discontinous layers to produce coarse foliation known as gneissose banding

  
• Migmatite - Very High Grade Regional Metamorphism

  • temperature above 700$^o$C and 40-50km depth; results in gneiss starting to melt - beyond 50km depth and temperatures >800$^o$C, the migmatite melts to form magma (forms granite when cools)
  • migmatite ('mixed rock') comprises of a banded gneissose component and a non-foliated granite component
• Phyllite; produced by higher temperatures/pressures than slate

  • it is fine grained (<1mm) and crystalline + often has a silky sheen due to parallel orientation of platy minerals (recrystallied mica/chlorite) and contains quartz + phyllite is fissile/foliated and usually grey/pale greenish-grey in colour.
• • Foliation - the texture found in metamorphic rocks, formed by the preferred alignment of flat, platy minerals.
  • Porphyroblasts - large crystals that have grown during recrystallisation in a metamorphic rock and surrounded by finer grained groundmass of other crystals.
  • Granoblastic - texture found in metamorphic rocks that contain interlocking equidimensional crystals.
• Slaty Cleavage - the texture in fine grained rocks formed by low grade regional metamorphism; platy minerals recrystallise perpendicular to the direction of stress applied during metamorphism, so that the rock splits into thin sheets.
• Schistosity - the texture in medium and coarse grained metamorphic rocks formed by the preferred alignment of flat/tabular minerals; the alignment is perpendicular to the direction of stress applied during metamorphism (no traces of original bedding remains).
• Gneissose Banding - the segregation of light- and dark-coloured minerals into layers or bands (vary in thickness); the light band is normally granoblastic and the dark band normally shows schistosity.
• Contact Metamorphism (ranges from 200$^o$C to over 1,000$^o$C) occurs when country rock is affected by heat from large igneous intrusion; as temperature differences between the country rock and the intruded magma are greater at shallow levels, contact metamorphism has high temperatures/low pressures.

  • high temperatures lead to formation of altered, recrystallised, unfoliated rocks in metamorphic aureole
  • effects of contact metamorphism are greatest near the contact and decrease with distance
• Metamorphic Aureole - the volume of rock that has undergone recrystallisation due to heat from the intrusion; zone of recrystallisation has to be at least 50m wide (otherwise it is a baked margin).
• Factors controlling size of Aureole:

  • Size of intrusion - larger volume intrusions take longer to cool and produce larger aureoles (minor intrusions like dykes/sills produce baked margins)
  • Magma Composition - mafic magma is 1100-1200$^o$C but has low volatile content + felsic magma is 800-850$^o$C and rich in volatiles (CO2 and water vapour)
• Factors controlling size of Aureole:

  • Characteristics of the country rock - mineral composition, porosity and permeability, pore water content and thermal conductivity of the country rock
  • Dip of the contact - steep/vertical dip produces a narrow aureole + gently dipping contact produces a wide aureole + if sides of intrusion dip at different angles an asymmetrical aureole will be produced
• Spotted Rock - Argillaceous Low Grade Contact Metamorphism

  • temperature 200-350$^o$C (partial recrystallisation)
  • new minerals form (cordierite/iron oxides) as oval spots 2-5mm in diameter
  • spots have overgrown and included grains of the original argillaceous rock (poikiloblastic/sieze texture)
  • relic structures such as bedding/laminations + fossils may be evident

  
• Chiastolite Rock - Argillaceous Medium Grade Contact Metamorphism

  • temperature 350-500$^o$C produces a coarser grained rock (extensive recrystallisation) + groundmass mainly micas + no relic structures are evident
  • chiastolite needles develop (porphyroblasts) up to 2cm long and 3mm wide with square cross sections often with small iron inclusions; needles show random orientation (crystallised in absence of directed pressure)

  
• Andalusite Hornfels - Argillaceous High Grade Contact Metamorphism

  • temperature 500-650$^o$C + coarser grained >2mm with andalusite porphyroblasts
  • hornfelsic texture; tough, fibrous and splintery looking rock with crystalline texture + no evidence of any relic structures

  
• Marble

  • parent rock is limestone (composed of calcite; stable over wide range of temperatures/pressures)
  • metamorphism causes original calcite crystals to be larger /coarser; calcite grains/fossil fragments recrystallise forming an interlocking mosaic of equidimensional calcite crystals (no foliation)
  • marble has granoblastic texture (all crystals more or less the same size), but calcite crystals look sugary (saccharoidal texture)
• Marble

  • grain size increases with grade; low (<1mm), medium (1-2mm) and high (>2mm) + fossils/older structures are destroyed in recrystallisation
  • limestones are chemically simple rocks comprising of calcite (calcium carbonate); no new minerals form (only calcium, carbon and oxygen atoms present) + only recrystallises into coarser form
  • clay minerals in limestone form garnet + reaction between calcite and quartz (from sand grain) produces wollastonite + marble from pure limestone is white (impurities give coloured streaks)
• Metaquartzite

  • parent rock is orthoquartzite; type of sandstone composed of quartz grains held together by quartz cement + resistant to weathering
  • quartz grains recrystallise forming interlocking quartz crystals with irregular boundaries; quartz crystals are equidimensional so there is no foliation (granoblastic texture)
  • chemically simple rocks comprising mainly quartz; no new minerals form from pure sandstone (only silicon/oxygen present) + quartz recrystallises into coarser form
  • grain size increases with grade; low <1mm, medium 1-2mm, high >2mm + sedimentary structures/fossils destroyed
• • Recrystallisation with an absence of directed pressure results in a non-foliated texture.