Texture, Classification and Phase Rules in Metamorphic Rocks

Classification of Metamorphic Rocks

• metamorphic rocks are classified on the basis of texture and composition (either mineralogical or chemical)

• unlike igneous rocks, their names are simple and flexible

• may coose some prefx-type modifiers to attach to names

Foliated Metamorphic Rocks

Foliation: any planar fabric element

lineation: any linear fabric elements

• they have no genetic connotations

• some high-strain rocks may be foliated, but they are treated separately

Cleavage

• traditionally: the property of a rock to split along a regular set of sub-parallel, closely-spaces planes

• a more general concept adopted by some geologists is to consider cleavage to be any type of foliation in which the aligned play phyllosilicates are too fine grained to see individually with the unaided eye.

Schistosity

• a preferred orientation of inequaint mineral grains or grain aggregates produced by metamorphic processes

• aligned minerals are course grained enough to see with unaided eye

• the orientation is generally planar, but linear orientations are not excluded

Gneissose structure

• either a poorly-developed schistosity or segregated into layers by metamorphic processes

• gneissos rocks are generally coarse grained

Slate vs Phyllite

Slate: compact, very fine-grained, metamorphic rock with a well-developed cleavage. Freshly cleaved surfaces are dull

Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite/phengite and/or chlorite), although rarely coarse enough to see unaided, impart a silky sheen to the foliation surface. Phyllites with both a foliation and lineation are very common

Non-Foliated Metamorphic Rocks

• simpler than foliated ones

• this discussion and classification applies only to rocks that are not produced by high-strain metamorphism

Granofels:

Granofels: a comprehensive term for any isotropic rock (a rock with no preferred orientation, granofelsic texture)

Hornfels

Hornfels: is a type of granofels that is typically very fine-grained and compact, and occurs in contact aureoles. Hornfelses are tough

Marble

metamorphic rock composed of predominately calcite or dolomite. The protolith is typically limestone or dolostone

quartzite

a metamorphic rock composed of quartz. the protolith is typically sandstone.Some confusion may result from the use of this term in sedimentary petrology for a pure quartz sandstone.

Skarn

a contact metamorphosed and silica metasomatized carbonate rock containing calc-silicate minerals, such as grossular, epidote, tremolite, vesuviantie, ect. Tactite is a synonym

Contact Metamorphism

Adjacent to igneous intrusions
• Thermal (± metasomatic) effects of hot magma
intruding cooler shallow rocks
• Occurs over a wide range of pressures, including
very low
• Contact aureole

Greenschist/Greenstone:

a low-grade metamorphic rock that typically contains chlorite, actinolite, epidote, and albite. The first three minerals are green.

Greenschist if foliated, greenstone if not.

Protolith is either mafic igneous rock or graywacke (dirty sandstone with higher proportion of feldspars, volcanic rock fragments, silt and clay)

Amphibolite

metamorphic rock dominated by hornblende + plagioclase. Amphibolite may be foliated or non-foliated. The protolith is either a mafic igneous rock or graywacke

Blueschist

A blue amphibole (glaucophane)- bearing metamorphosed mafic igneous rock or mafic graywacke. This term is so commonly applied to such rocks that is is even applied to non-schistose rocks

Eclogite

a green and red metamorphic rock that contains clinopyroxene and garnet (omphacite + pyrope). the protolith is typically basaltic

Granulite

a high grade rock of pelitic, mafic, or quartzo-feldspathic parentage that is predominately composed of OH-free minerals. Muscovite is absent and plagioclase and orthopyroxene are common

migmatiite

a composite silicate rock that is heterogeneous on the 1-10 cm scale, commonly having a dark gneissic matrix (melansome) and lighter felsic portions (leucosome). Migmatites may appear layered, or the leucosomes may occur as pods or form a network of cross-cutting veins

Serpentinite

an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine

Serpentinization, an important water-rock interaction process, is an exothermic metamorphic hydration reaction of upper mantle mafic to ultramafic rocks

Additional Modifying Terms: Porphyroblastic

Porphyroblastic means that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others. Each individual crystal is a porphyroblast

Some porphyroblasrs, particularly in low-grade, contact metamorphism, occur as ovoid “spots”

If such spots occur in a hornfels or a phyllite (typically as a contact metamorphic overprint over a regionally developed phyllite), the terms spotted hornfels, or spotted phyllite would be appropriate

auge

Some gneisses have large eye-shaped grains
(commonly feldspar) that are derived from pre-
existing large crystals by shear (as described in
Section 23.1). Individual grains of this sort are
called auge (German for eye), and the (German)
plural is augen. An augen gneiss is a gneiss with
augen structure (Fig. 23-18).

Additional Modifying Terms: Ortho and Para

Ortho- a prefix indicating an igneous parent, and

Para- a prefix indicating a sedimentary parent

The terms are used only when they serve to dissipate doubt. For example, many quartzo-feldspathic gneisses could easily be derived from either an impure arkose or a granitoid rock. If some mineralogical, chemical, or field- derived clue permits the distinction, terms such as orthogneiss, paragneiss, or orthoamphibolite may be useful

High Strain Rocks

The Phase Rule in Metamorphic Systems

If F greater than or equal to 2 is the most common situation, then the phase rule may be adjusted accordingly:
F = C - P + 2 Greater than or equal to 2
P less than or equal to C (Eq 24.1)
Goldschmidt’s mineralogical phase rule, or simply the mineralogical phase rule

Consider the following three scenarios:

C=1

P=1 common

P=2 rare

P= 3 only at the specfic P=T conditions of the invarient point (0.37 GPa AND 500 c)

Suppose we have determined C for a rock. Consider the following three scenarios:

1. P=c

1. P=C

   The standard divariant situation

   The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone

2. P<C

   Common within mineral systems that exhibit solid solution

3. P>C

   A more interesting situation, and at least one of three situations must be responsible

   1. F<2

      The sample is collected from a location right on a univariant reaction curve (isograd) or invariant point

   2. Equilibrium has not been attained

      The phase rule applies only to systems at equilibrium, and there could be any number of minerals coexisting of equilibrium is not attained.

   3. We didnt choose the # of components correctly

Some guidelines for an appropriate choice of C

• Begin with a 1-component system, such as CaAl2Si2O8 (anorthite), there are 3 common types of major/minor components that we can add

  a. Components that generate a new phase

  Adding a component such as diopside results in an additional phase: in the binary Di-An system diopside coexists with anorthite below the solidus

  b. Components that substitute for other components

  Adding a component such as albite to the 1-C anorthite system would dissolve in the anorthite structure, resulting in a single solid-solution mineral (plagioclase) below the solidus

• Fe and Mn commonly sub for Mg

• Al may sub for Si

• Na may sub for K

  C. Perfectly mobile elements

  Mobile components are either a freely mobile fluid component or a component that dissolves readily in a fluid phase and can be transported easily

• The chemical activity of such components is commonly controlled by factors external to the local rock system

• They are commonly ignored in deriving C for metamorphic systems

How do you know which way is correct?

Rocks should tell you:

• Phase rule= interpretive tool, not predictive

• If only see low-P assemblages → some component may be mobile

• if many phases in an area it is unlikely that all is right on univariant curve, and may require the number of components to include otherwise mobile phases, such as H2O or CO2, in oredr to apply the phase rule correctly

Chemographics

Chemographics refers to the graphical representation of the chemistry of mineral assemblages

A simple example: the plagioclase system as a linear c=2 PLOT:

Chemographic Diagrams

What happens if you pick a composition that falls directly on a tie-line, such as point (f)?

“compositionally degenerate”

Valid compatibility diagram must be:

must be references to a specific range of P-T conditions, such as a zone in some metamorphic terrane, because the stability of the minerals and their groupings vary as P and T vary

solid solution chemograph

What is the “right” choice of components? How to pick

1. Simply “ignore” components

• trace elements

• elements that enter only a single phase (we can drop both the component and the phase without violating the phase rule)

• perfectly mobile components

2. Combine components

• components that sub for one another in solid solution (Fe + Mg)

3. Limit the types of rocks to be shown

• only deal with a sub-set of rock types for which a simplified system works

4. use projections

• ill explain this in this in the next lecture

The ACF Diagram

• illustrate metamorphic mineral assemblages in mafic rocks on a simplified 3-C triangular diagram

• concentrate only on the minerals that appear or disappeared during metamorphism, thus acting as indicators of metamorphic grade

The three pseudo-components are all calculated on an atomic basis:

A= Al2O3 + Fe2O -Na2O - K2O

C= CaO - 3.3P2O5

F= FeO + MgO + MnO

A= Al2O3 + Fe2O -Na2O - K2O Why the subduction?

• Na and K in the average mafic rock are typically combined with Al to produce Kfs and Albite

• in the ACF diagram, we are interested only in the other K-bearing metamorphic minerals, and thus only in the amount of Al2O3 that occurs in excess of that combined with Na2O and K2o (in albite and K-feldspar)

• Bc the ratio of Al2O3 to Na2O or K2O in feldspars is 1:1, we subtract from Al2O3 an amount equivalent to Na2O and K2O in the same 1:1 ratio

The AKF Diagram

because pelitic sediments are high in Al2O3 and K2O, and low in CaO , Eskola proposed a different diagram that included K2O to depict the mineral assmblages that develop in them

Three of the most common minerals in metapelites: andalusite, muscovite, and microcline, al plot as distinct points in the AFK diagram

In the AFK diagram, the pseudo-components are

In the AFK diagram, the pseudo-components are

• A= Al2O3 + Fe2O3 - Na2O -K2O - CaO

• K= K2O

• F = FeO +MgO +MnO