Metamorphism and metamorphic rocks |
Metamorphism is the recrystallization of minerals in pre-existing, parent, igneous or sedimentary rock (protolith). Takes place in response to changes in P and T as well as to fluids and deformation. This can happen because the lithosphere is dynamic and rocks may be buried or dragged to great depths by the motion in the lithosphere and then brought back up again
Causes of metamorphism
Heat and pressure are main causes.
Heat: As we go down in the Earth the temperature increases geothermal gradient- this is because of the internal heat of the Earth. Ranges from 20-60C/km, 25-30 a good average
Pressure: Useful unit is the bar (~atmospheric pressure) Pressure increases 270 bars/km say 300. Kbar is better unit so better to say 0.27 or 0.3kb/km.
Note that pressure is not always equal- directed pressure (=stress)
[ Note pressure does not cause the heat!!]
Thus at 10km, T= 250-300C, P= 2.7-3kb (3000 atmospheric pressure)
Metamorphic grade: Low P/T low grade high P/T high grade
Fluids in metamorphism: Fluids (H2O, C02 mainly) important, even though not much there, because promote the break down of pre-existing minerals and formation of metamorphic minerals. Also reason that metamorphic minerals contain water (eg, amphiboles, mica, chlorite)
Chemical changes is not usually a change in chemical composition. But sometimes if a lot of fluids, changes can occur - metasomatism
Types of metamorphism
Depends what causes change in T and P
Regional metamorphism The most widespread - affects huge volumes of rock. Caused by lithospheric movements, therefore nearly always accompanied by deformation of the rocks as well as mineralogical change.
Contact (thermal) metamorphism Heat is from contact with magma. Adjacent to plutons
Hydrothermal metamorphism Sea floor, can also be around plutons
Shock metamorphism - meteorite impacts, transitory high pressures
Metamorphic Minerals
Mafic - chlorite, hornblende, biotite, garnet, epidote
Felsic - quartz, muscovite, kyanite-andalusite-sillimanite
Many are platy, or rod like or needle - like
Metamorphic textures
Foliation (also called schistosity; slaty cleavage when fine grained).
Caused by common alignment of platy or rod like grains). Usually limited to regioanally metamorphosed rocks.
Foliated rocks are classified according to grain size (which increases with increasing grade):
Slate - phyllite - schist - gneiss
(Fine grained) - - - - - -
- - - - - - - - (Coarse grained)
Mylonite - banded rock that has beed sheared, rather than flattened
Large grain textures - large grains on smaller matrix -
Unfoliated rocks
Greenstones, granulite, some marbles, hornfels
Contact metamorphic rocks
Hornfels
Aureole Greater the size of pluton, greater the thickness; hotter the pluton, greater the thickness. Therefore mafic plutons have thicker aureoles than granites
Metamorphic grade
The actual grade is determined by the minerals present in the rock.
With increasing grade we have the following
-in sediments: chlorite - musc biotite - garnet - staurolite kyanite
- in mafic igneous: zeolites- chlorite - epidote+al-poor amphiboles - hbl+garnet - pyroxene
Groupings of rocks of different compositions formed under similar P-T conditions.
Plate tectonics and Metamorphism
Blueschists and eclogites - high pressure, low temperature - formed in subduction zones
Island arcs and paired metamorphic belts
Continental collision --> burial and intraplate metamorphism because of crustal thickening