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Metamorphic Rock
The University of Qatar
Department of Geology
Prof. Dr. Sobhi Nasir
Graphical Representation
ACF and A’FK have been constructed for several metamorphic zones. They indicate different pasragenesis in rocks of different composition when metamorphosed at specific conditions. The diagram are based on the condition that SiO2 is presnet in excess in the form of quartz.
ACF Diagram
The scheme for calculating the ACF ratios (after necessary corections for magnetite, ilmenite and sphene) is as follow:
A=(Al2O3+Fe2O3)-(Na2O+K2O)
C=CaO-3.3*P2O5
F=MgO+MnO+FeO
These values are recalcul;ated to 100%.
A’FK Diagram
The general calculation scheme is :
A’=(Al2O3+Fe2O3)-(Na2O+K2O+CaO)^2
K=K2O
F=FeO+MgO+MnO
A’+K+F=100
AFM Diagram
A=(Al2O3-3*K2O)/((Al2O3-3*K2O+MgO+FeO)
M=MgO/(MgO+FeO)
F=FeO-Fe2O3-TiO2
TEMPERATURE LIMITS OF METAMORPHISM
The lower temperature limit, at metamorphism takes place, marks the boundary between metamorphism and diagenesis. Lower T = 100-150°C - excludes any diagenetic changes. Some workers consider any changes not occurring under atmospheric conditions to be the result of metamorphism. >br>
Metamorphic Reactions
If a mineral persists beyond the conditions in which it is at equilibrium iton the styability field of another mineral, it is said to be metastable and the reaction have been overstepped. Metamorphic reactions take place uusually close to equilibrium and at a specific temperature and pressure. The assemblages consisting of reactants and products has only one degree of freedum at equilibrium. This type of reaction is univariant (discontineous). Reactions that take place progressively so that reactants and products can coexist over an interval of pressure and temperature (divariant) are known as contineous reactions. Reactions that involves change in the compostion of the phases are known as cation exchange reactions. Solid-Solid reactions occurs among solid phases e.g., transformation of andalusite to silliminaite and kyanite. Dehydration reactions involve libration of water. Decarbonation reactions involve libration of CO2. Oxidation-reduction reactions involve minerals containing iron.
Regional Metamorphism
Corresponds to progressive changes, increases, in Temperature & Pressure, due to burial. Commonly this type of metamorphism is associated with orogenic events and processes. •Regionally metamorphosed rocks occur in very large belts, 10 - 100's km wide by 100 - 1000's km long.
Within each belt will be preserved at least one thermal axis, representing the maximum temperature of metamorphism within the belt. Temperatures will increase towards the thermal axis. Associated granitic and/or ultramafic masses intruded into the terranes generally, but not always near the thermal axis.
Metamorphism of Basic Rocks (Metabasites)
Metabsites are mostly the metamorphic products of basalt and andesites. IF water has had access to the rock, the metamorphic products formed with increasing temperature are chlorite schist (or glaucophane schist, depends on pressure), green schist and amphibolite. IF water has not had access but load presure was very high, then eclogite will be formed. The main constituents of basalts and andesites are plagioclase (An40-70) and clinopyroxene with some hypersthene and olivine. The main changes of basaltic and andesitic rocks during metamorphis, where amphibolites are formed are:
1-Formation of hornblende and garnet if pressure is high .
2- Formation of less anorthite-rich plagioclase (albite, oligoclase, andesine and labradorite) and clinozoisite or epdiote.
3-At low grade, actinolite, chlorite, quartz and clinozoisite/epidote are additional constituents to albite and oligoclase (greenschist).
Basalts are often included in sedimentary piles within orogenic belts, thus mineral assemblages in metabasites can be correlated with assemblages in associated pelites as both formed under similar pressure and temperature conditions. Basic rocks rich in MgO, FeO, CaO, & Al2O3, metamorphic minerals produced include:
Chlorite, actinolite, epidote, at low temp greenschist
Hornblende, plagioclase in amphibolite
Hornblende, plag & garnet, opx & cpx in granulite
Major Characterisitics and Mineral Assemblages
Very low grade:
a- High pressure: Lawsonite/pumpellyite-glaucophane
b-Medium pressure: Lasonite/pumpellyite-albite-chlorite
Low Grade:
a-Lower temperature part: Albite-actinolite-chlorite zone
b-Higher temperature part:Albite/oligoclase-hornblende-cholrite zone
Medium - and High-grade
Amphibolite
Metamorphism of Ultramafic Rocks
Ultramfic rocks consist of olivine and pyroxenes. MgO, FeO and CaO are the major components.
Lab. Problems
1- Identify the rock specimens provided by your instructor. Proceed as follows:
a- Determine whether the rock is foliated or non-foliated.
b- If the rock is foliated determine the type of foliation.
c- Determine the mineral composition
d- Determine the rock name.
e- On the basis of texture and mineralogy determine the parent rock type.
2- Why are marble and quartzite rarely foliated?
3- What original rock type changes to form slate.?
4- What is a metamorphic rock?.
5- List 5 agents of metamorphism
6- List 3 types of metamorphism, typical rocks formed and expected textures
7- What is the opposite of retrogressive?
8-What is the main mineral in shale?
9-How can you recognize metamorphic grade in field studies?
10- What is the difference between gneiss and granite?
-11 What is the mineral and textural changes that occur in the following example of progressive metamorphism:?
Shale-----------Low grade-----------Medium grade--------------High grade
12- What is the parent rock for schist, marble, hornfels, serpentie?
13- What is the metamorphic equivalent for basal at low, medium and high grade?
14- What is the metamorphic equivalent for hornfles at low and high grades?
Microscopy
a- Determine the optical properties for the common metamorphic minerals: muscovite, biotite, chlortie, hornblende, actinolite, pyroxene, epidote,quartz, orthoclase,plagioclase,garnet, staurolite, cordierite, sillimantie, kyanite, andalusite, tourmaline.
Examine one sample from each group of pelites and metabasites giving your own remarks
Examine the minerals and textures on different samples provided by your instructor. Write yoru remarks. Estimate the metamorphic grade, facies. Determine the parent rock. Draw ACF, AFM or AKF diagram for the sample. Determine the P-T conditions.
