Lecture Eighteen - Metamorphic processes and occurrences II Flashcards
How is the intensity of metamorphism measured?
We need a common term or framework to describe how intensely metamorphism has affected rocks.
E.g. Rocks close to the Earth’s surface are exposed to lower temperatures and pressures than those deep under mountain ranges.
Higher P and T often corresponds to greater degrees of metamorphism.
We use metamorphic grade to (semi‐) quantify this intensity of metamorphism.
Metamorphic grade:
Metamorphic grade is expressed by the grain sizes of the new minerals (neocrystallisation) and recrystallisation of existing minerals that occur.
This depends (and is controlled) mainly by temperature.
Low grade = 200-320 degrees C.
High grade = >600 degrees C.
The concept of metamorphic grade doesn’t apply to high temperature, low pressure metamorphism.
I.e. Subduction zones where there is low T but increasing P.
What are the primary sources of heat in the Earth?
Radio active decay of U, K and Th.
Crystallisation of the Earth’s inner core.
Mantle oxidation.
Heat from these primary sources is redistributed by radiation, convection and by magma migration.
The rate of increase in temperature with depth is known as the geothermal gradient, which averages ~30 degrees C per km, but can be as high as 60 and as low as 10 degrees per km depending on the tectonic setting.
Which different metamorphic grades yield which mineral groups?
We can use the presence of absence of these mineral groups in rocks to estimate the metamorphic grade.
What is the sequence of metamorphic rocks starting from shale to gneiss, and what are the characteristics of these rocks?
1) The clay flakes in the shale lie parallel to bedding.
2) Slate - Clay flakes are larger and align parallel to cleavage.
3) Phyllite - Higher metamorphic grade procudes new crystals of muscovite (white mica), chlorite and quartz.
4) Schist - Intermediate grade, new minerals of garnet, biotite, muscovite (large crystals). Water is released.
5) Gneiss - High grade, water free minerals (quartz, feldspar, pyroxene, biotite, garnet. Lack of micas so no schistocity, but gneissic banding/layering develops.
Explain the difference between metamorphic grade and metamorphic facies.
Metamorphic grade - An informal way of describing intensity of metamorphism (gives approcimate temperature).
Metamorphic facies - A more complete indication of intensity of metamorphism (mineral asseblage in the rock).
The specific mineral assemblage in a rock on P and T and protolith composition.
So a basalt metamorphosed at 400 degrees C and 6kbar under the Himalayas will have the same mineral assemblage as a basalt metamorphosed at 400 degrees C and 6kbar under the European Alps.
So metamorphic rocks with different mineral assemblages can be represented as distinct fields (or facies) on a P-T diagram.
Note that there are wide bands/gaps between facies as they are gradational and approximate.
Thus when describing facies it is okay to say a metamorphic rock is for example upper greenschist to lower amphibolite.
General rule:
1) Zeolite to lower greenschist facies = low grade.
2) Upper greenschist to lower amphibolite = medium grade.
3) Upper amphibolite to granulite = high grade.
Note the ‘wet granit melting’ line is where there is partial melt and the line between metamorphic and igneous rock is blurred. This is where migmatites are formed.
Describe the greenschist to amphibolite facies.
Lots of chlorite (hydrous mineral - contains -OH groups in its grystal structure).
Chlorite = (Mg,Fe)3(Si,Al)4O10(OH)2.(Mg,Fe)3(OH)6.
At ~500 degrees C chlorite starts to break down to form hornblend (an amphibole) = Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2.
Chlorite (12% water) to hornblend (2% water) reaction releases a lot of water.
Describe the amphibolite to granulite facies.
At ~750°C hornblende starts to break down to form pyroxene (anhydrous) and melt.
Pyroxene = (Ca,Mg,Fe)2Si2O6.
Hornblende breakdown releases H2O that is immediately consumed in a new silicate melt (partial melting -> migmatite).
Pyroxene granulite -> anhydrous.
Describe the blueschists and eclogites facies.
A common type of metamorphism that occurs in the accretionary prisms at subduction zones is low T and high P (geotherm 5).
The oceanic crust getting subducted tends to metamorphose to blueschist.
At higher pressures (when the rock gets buried deeper), metamorphism can produce ECLOGITES.
Note that the roots of very thick stable continental crust (geotherm 4) may also transform to eclogite (often denser than upper mantle rock).
What is prograde and retrograde metamorphism?
Prograde = when a rock gets buried deeper.
Reterograde = When a rock gets shallower/uplifted.
For retrograde metamorphism to occur water must be added back into the rock (to form the hydrous micas and amphiboles).
Exhumation can produce retrograde metamorphism as the weight/pressure of overlying rock is stripped away.
What are index minerals?
Certain minerals can indicate the approcimate metamorphic grade of a rock.
When geologists approach an area known to consist of metamorphic rocks, they can look at the change in minerals in rocks in different areas.
On geological maps, they can produce a line where an indec mineral first appears = ISOGRAD.
All points along the isograd have the same metamorphic grade.
This createes metamorphic zones between the isogrades.
