FUNDAMENTAL CONCEPTS IN METAMORPHIC PETROLOGY
Study of metamorphism deals with the physico-chemical conditions of recrystallization. The study of metamorphic rocks gives us very important information about 1) the pressure and temperature conditions of tectonic processes and 2) the nature of fluid flow in the deep crust. Changes in P, T, and fluid compositions result in reactions among minerals to produce new minerals. Therefore, by studying mineral assemblages in metamorphic rocks we can deduce the conditions of metamorphism.
I. Types of Metamorphism
1. Contact metamorphism – is the result of intrusion of magmas into colder, upper crustal rocks. The zone of metamorphism is called the contact aureole. The change in metamorphic grade, as expressed in change in the mineral assemblage, is generally concentric to the intrusion. Contact metamorphism can be thought of being isobaric at a given crustal level. It involves fairly rapid changes temperature.
(Time interval is 200,000 years.)
Factors influencing nature of a contact aureole:
- Size of igneous body. The larger the body, the larger the aureole.
- Temperature difference between magma and wall-rocks. The larger the difference, the larger the apparent effect of metamorphism.
2. Regional metamorphism
- Orogenic (most important)
- related to formation of orogenic belts
- hundreds to thousands km2
- T up to 800° C observed
- geothermal gradient > normal (30-50°C/km)
- metamorphism is dynamothermal, meaning it is associated with deformation (get foliated rocks)
B.Burial regional metamorphism
- result of simple burial
- T is normal geothermal gradient (max ~400°C)
C.Ocean floor metamorphism
- involves the alteration of the ocean crust by seawater while the crust is still hot
- typical minerals include serpentine, chlorite, epidote, zeolites, etc.
3. Hydrothermal metamorphism
- mineral reactions are controlled by fluid flow and compositions of fluids
- is usually associated with all other types of metamorphism
- metasomatism involves the movement of chemicals, such as Si, Cl, Na, metals, by hydrothermal fluids
4. Fault-zone (cataclastic) metamorphism
- involves crushing and grinding by friction during movement along faults
- metamorphism is local in extent
- low temperature; high pressure
- causes formation of fault breccia, or fine-grained, foliated rock called mylonite
5. Impact (shock) metamorphism
- occurs during impact of meteorites and other extraterrestrial objects
- involves very high pressures and temperatures generated by energy of the impact.
- involves the formation of high-pressure polymorphs of minerals (e.g. coesite and stishovite) or loss of internal structure of minerals (e.g. feldspars)
II. Pressure and Temperature Regimes in the Crust
1. Pressure
Many metamorphic reactions are dehydration or decarbonation reactions. An often made assumption in metamorphic petrology is that the fluid pressure equals the total pressure on the rock system. However, in the upper few kilometers in the crust where pores in rocks are filled with fluids and the pore spaces are connected, the fluid pressure in pore-spaces is that exerted by overlying column of the fluid. Thus the fluid pressure (Pfluid) is hydrostatic. It is given by the weight of the overlying column of the water.
Pfluid = fluidgh
where fluid is the density of the fluid, g is gravitational constant, and h is depth. Because the fluids in metamorphic rocks are typically not pure H2O, to total
Pfluid = pH2O + PCO2 + ... .
However, at depths >~6 km, fluid pressure becomes equal to the lithostatic pressure:
Pfluid = Plith,
because at the high pressures the fluid cannot keep pore-spaces open anymore as the minerals push against each other. Lithostatic pressure is given by the weight of the overlying rocks:
Plith = rockgh .
2. Metamorphic field gradients (metamorphic series)
In a given regional metamorphic terrane, a given prograde metamorphic sequence usually indicates an increase in both temperature and/or pressure. However, the prograde sequence may not necessarily represent an originally vertical section through the crust at the time of metamorphism. Therefore, the apparent P-T gradient is not indicative of the geothermal gradient that existed in the crust during metamorphism. However, because the gradient is observed in the field, it is called the metamorphic field gradient.
A) Hornfels series – isobaric contact metamorphism. However, it can occur at higher pressures than indicated.
B) Buchan series – low-P regional metamorphism, usually cause by intrusion of large volume of magmas into the upper crust.
C) Barrovian series – occurs in orogenic terranes by dynamothermal metamorphism.
D) Blueschist (Franciscan) series – high-P/low-T series occurs in subduction zone environments.
III. Metamorphic Pressure-Temperature-Time Paths
Assume that a several km thick section of a sedimentary pile, represented by samples a-c, gets buried during crustal convergence in an orogen and then gets exposed at the surface:
The pressure on the rocks increases faster than they get heated up. Therefore, the three samples undergo clockwise P-T-t paths:
Metamorphic Field
Gradient
Thus, metamorphism of rocks needs to be viewed as a dynamic process in which pressure and temperature conditions change. Note that the maximum P on a given path does not correspond to the maximum T. If the three samples were collected in a metamorphic terrane, mineral assemblages corresponding to the maximum T reached by each sample would define a metamorphic field gradient. (Typically, near-maximum T conditions are preserved in mineral assemblages of metamorphic rocks because retrograde reactions are energetically difficult.)
IV. Nomenclature of Metamorphic Rocks
The best classification of metamorphic rocks is the faciesclassification, in which mineral assemblages are used to define ranges of P-T conditions. We will discuss metamorphic facies later. However, names are also given to metamorphic rocks that may be indicative of their morphology, mineralogy, or bulk chemical composition.
1. Structural (morphological) classification
Hornfels - non-schistose, fine-grained rock with 'granoblastic' fabric (mosaic of small mineral grains). May have porphyroblasts. Hornfelsic texture is usually produced by contact metamorphism where there is no deviatoric stress.
Slate - Fine-grained rock with perfect foliation usually defined by sericite and chlorite.
Phyllite - Fine-grained foliated rock but coarser-grained than slate. Muscovite imparts sheen to it.
Schist – Medium to coarse grained foliated and commonly lineated rock in which most individual mineral grains can be recognized without a hand-lens.
(Slate, phyllite, schist most commonly develop in pelitic rocks and tend to indicate a progression in grade.)
Gneiss - Medium to coarse-grained rock that is discontinuously banded, with banding generally separating feldspar/quartz from biotite/hbld/px. The production of some gneisses may also involve partial melting.
2. Bulk chemical composition (indicates the protolith of a metamorphic rock)
Pelitic - derived from aluminous sediments (shales). Micas are abundant as are other aluminous minerals, including the aluminosilicates, pyralspite garnets, staurolite, and cordierite.
Quartzo-feldspathic - derived from sandstones, tuffs, granites. The principal minerals are quartz and feldspars.
Calcareous - derived from calcareous rocks. The principal minerals are either calcite or dolomite.
Calc-silicate- derived from mixed calcareous and pelitic protoliths. Calcite, dolomite, muscovite, chlorite, and biotite are common at low grades, whereas diopside, tremolite, grossular garnet, wollastonite, and vesuvianite are common at high grades.
Basic - Derived from mafic igneous rocks. Typical minerals are chlorite, hornblende, plagioclase, epidote, pyroxene.
Magnesian - derived from ultramafic rocks (peridotites). Serpentine, talc, magnesite, and brucite common.
The metamorphism of the last two groups is essentially retrograde.
3. Traditional names indicating the dominant mineralogy
Marble - non-foliated or weakly foliated rock composed dominantly of calcite or less commonly dolomite.
(Meta-)Quartzite - recrystallized sedimentary quartzite (sandstone).
Amphibolite - composed essentially of hornblende and plagioclase. It is often foliated and lineated.
Eclogite - Dominantly omphacite clinopyroxene and garnet (±kyanite).
omphacite - (Ca,Na)(Mg,Fe2+,Fe3+,Al)Si2O6
garnet - pyrope-almandine solid solution.
4. Modifiers and prefixes
Modifiers may be placed in front of the above names, e.g. :
tremolite marble
hornblende gneiss
garnet-biotite schist
staurolite-andalusite schist
Common prefixes:
Meta - Used to describe an originally igneous or sedimentary rock that was metamorphosed to indicate the protolith. E.g. meta-basalt, meta-graywacke, meta-gabbro, meta-arkose
Ortho - Indicates that the metamorphic rock was originally igneous. E.g. orthogneiss, orthoamphibolite
Para - Indicates that the metamorphic rocks was originally sedimentary, e.g. paragneiss, paraamphibolite.
V. Concept of Metamorphic Facies
Terms such as low grade, medium grade, or high grade are only useful for comparing the metamorphic grade in a given area. These terms are worthless when comparing different terranes that may follow different metamorphic field gradients. For example, a blueschist series rock may be metamorphosed at high pressures but fairly low temperature, whereas a contact-metamorphic rock may be metamorphosed at high temperature but low pressure. Much more useful is the concept of metamorphic facies, where each facies defines a certain range of P-T conditions. The facies concept was proposed by Turner in 1968:
"A metamorphic facies is a set of metamorphic mineral assemblages, ... , such that there is a constant and therefore predictable relationship between mineral composition and chemical composition.”
Explanation of the definition:
- Each facies defines a restricted set of P-T conditions.
- To belong to the same facies, rocks of the same chemical composition must have the same mineral assemblage. However, rocks of different composition may have a different assemblage at the same facies. For example, in the greenschist facies:
basic composition: plagioclase, chlorite, actinolite, epidote
pelitic composition: quartz, chlorite, muscovite, garnet, biotite.
- Facies boundaries may not be shown by all compositions: