Petrography of metamorphic rocks 1

Petrografi Teknik Geologi UGM (TKG2211)

Nugroho Imam Setiawan, Ph.D Optical Geology Laboratory Dept. of Geological Engineering Gadjah Mada University @2014

Syllabus

Thin section observation metamorphic textures

Crystal in metamorphic rocks

Microstructures and Deformation Mechanisms

Principal textures of metamorphic rocks

Metamorphic petrography in Indonesia

Text book

Microtextures of igneous and metamorphic rocks, Bard. J.P., 1987

Atlas of metamorphic rocks and their textures, Yardley et al., 1990.

Igneous and metamorphic petrology 2nd Ed., Best, 2003.

Principles of igneous and metamorphic petrology, Winter, 2010.

Petrography, An introduction to the study of rocks in thin sections 2nd Ed., Williams et al., 1982.

How do we study petrography and microstructures

Observation Microscopy optical thin section analysis SEM: Scanning Electron Microscope observes surfaces only TEM: Transmission Electron Microscope observes thin foil

samples Cathodoluminescence (CL) displays trace chemical variations

Quantitative analysis Crystallographic Preferred Orientations (CPO) Grain Shape Foliations (GSF) Grain Size Distributions AVA (mapping crystallographic orientations across a thin

section) Stable isotope studies

Metamorphic texture

1. Produced by metamorphic reaction (disequilibrium).

Symplectite, corona, reaction rim, zonation

2. Produced by deformation during metamorphism.

Pressure shadow, foliation, helical

Theory of crystal growth origin

1. Temperature and pressure condition

2. Chemical heterogeneities

3. Presence of interstitial fluids (H2O, CO2, etc)

4. Porosity of the medium

5. Chemical composition of the solid state

6. Relative orientations of boundary surfaces between newly formed minerals.

7. Time

Crystal form

Idiomorphic (euhedral): minerals exhibit simple geometric forms regulated by the symmetries of their cristaline lattice.

Hypidiomorphic (subhedral)

Xenomorphic (anhedral): crystal do not display any well defined external form (they can have flat, rounded, lobed, lenticular, etc)

The Crystalloblastic Series

Most Euhedral

Titanite, rutile, pyrite, spinel

Garnet, sillimanite, staurolite,

tourmaline

Epidote, magnetite, ilmenite

Andalusite, pyroxene, amphibole

Mica, chlorite, dolomite, kyanite

Calcite, vesuvianite, scapolite

Feldspar, quartz, cordierite

Least Euhedral

Differences in development of crystal form among some

metamorphic minerals. From Best (1982). Igneous and

Metamorphic Petrology. W. H. Freeman. San Francisco.

Interaction growth mechanism

1. Free growth: where crystals develop in a liquid or solid matrix which does not noticeably interfere with their growth.

2. Disturbed growth: where the form of the minerals is regulated by that of neighboring crystals that ere formed at the same time (i.e. syngenetic)

3. Corrosion: where minerals exist as unstable or metastable relict not yet entirely eliminated by the processes leading to their disappearance.

4. Low-temperature mechanical ruptures: tectonic origin, able to fragment the minerals or displace their borders (brittle fracture).

5. High-temperature mechanical deformations: causing lattice distortions and migration of punctual an linear dislocation defects (ductile deformation).

Blastesis

P-T conditions are favorable for metamorphic mineral to grow, nucleation can start.

The number of nuclei and their survival rate determines whether many small or a few large porphyroblasts form.

The number depends on: The availability of favorable

nucleation sites. The driving force for the

metamorphic reaction. Transport rate of elements that

form new mineral and elements that have to be removed to make space available.

Inclusion Trails

1. If reaction C and E are both fast enough, no inclusions are incorporated.

2. If E slow out and reaction C fast enough, inclusions of E are incorporated of the minerals.

3. If reaction C and E are to slow, crystal A and B might be incorporated in inclusions.

[1] [2] [3]

Order of crystallization

Pre-existing (previous) phases: minerals which were formed before the event.

Contemporaneous (simultaneous) phases: those that occur at the time of the event.

Later (postgenetic) phases: minerals which later than the event

Order of crystallization Namely as mineral [1] and mineral [2].

A. [1] is older than mineral [2]. [1] is either partly or completely included in [2]. [1] form close to equilibrium developed in the early stages of [2].

B. [1] is distributed random in [2] without any observable connection between the individual crystals [1]. [1] can be very old.

C. [1] is distributed in [2] maintaining crystallographic and optical continuity. [1] can be very old or relict minerals pseudomorphic.

D. [1] surrounds [2], which two crystals are of the similar age. The orientation of thin section can make confusing of interpretation.

Order of crystallization

Grt equilibrium with Qz and develops pressure shadow as secondary minerals.

Isograde reaction

A(OH) + B C + D + H2O

C E

[continuous reaction]

The newly formed minerals may partially (or totally) surround the reacting materials which the get the appearance of corroded parent phases.

Reaction of microtexture

A. False rim: exsolution reaction of solid solution (unmixing of some phase B within a phase A may lead to the formation of an typical rim

B. Kelephytic textures: formed by un-mixing of surrounding minerals (pre-existing minerals at higher temperature or pressure. It may formed a symplectite. [symplectite] [kelephytic]

Reaction of microtexture

C. Complex coronitic textures: reaction rim found around minerals by an alternation of mineralogically different layers that prevent any contact between the mineral and matrix. The textures are definitely characteristic of a return equilibrium in earlier associations.

Ol Pl H2O Opx Cpx Spl Grt Hbl

Figure 23.54. Portion of a multiple coronite developed as concentric rims due to reaction at what was initially the contact between an

olivine megacryst and surrounding plagioclase in anorthosites of the upper Jotun Nappe, W. Norway. From Griffen (1971) J. Petrol.,

12, 219-243.

Reaction of microtexture D. Atoll garnet

[honey comb]

[trabecular]

[chain]

Figure 23.27. Proposed mechanisms for the development of foliations. After Passchier

and Trouw (1996) Microtectonics. Springer-Verlag.

Diagram showing that structural and fabric

elements are generally consistent in style and

orientation at all scales. From Best (1982).

Igneous and Metamorphic Petrology. W. H.

Freeman. San Francisco.

a. Bent crystal with

undulose

extinction

b. Foliation

wrapped around

a porphyroblast

c. Pressure shadow

or fringe

d. Kink bands or

folds

e. Microboudinage

f. Deformation

twins

Figure 23.34. Typical textures of pre-

kinematic crystals. From Spry (1969)

Metamorphic Textures. Pergamon.

Oxford.

Pre-kinematic crystals

Post-kinematic crystals a. Helicitic folds b. Randomly oriented crystals c. Polygonal arcs

d. Chiastolite e. Late, inclusion-free rim on a poikiloblast (?)

f. Random aggregate pseudomorph

Figure 23.35.

Typical textures

of post-

kinematic

crystals. From

Spry (1969)

Metamorphic

Textures.

Pergamon.

Oxford.

Helecitic syn-kinematic

Figure 23.37. Si characteristics of clearly pre-, syn-, and post-kinematic crystals as proposed by Zwart (1962). a. Progressively

flattened Si from core to rim. b. Progressively more intense folding of Si from core to rim. c. Spiraled Si due to rotation of the matrix

or the porphyroblast during growth. After Zwart (1962) Geol. Rundschau, 52, 38-65.

Analysis of Deformed Rocks

Deformational events: D1 D2 D3 Metamorphic events: M1 M2 M3 Foliations: So S1 S2 S3 Lineations: Lo L1 L2 L3 Plot on a metamorphism-deformation-time

plot showing the crystallization of each mineral

Tectonites: Rocks that are pervaded by foliation and/or lineation- flowed in solid state

S: Schistosity (foliation) only due to flattening- no lineation

L: Lineation only, due to unidirectional stretching/ constriction

LS: Foliation and Lineation, related to noncoaxial strain- shearing

Analysis of Deformed Rocks

Figure 23.42. (left)

Asymmetric

crenulation

cleavage (S2)

developed over S1

cleavage. S2 is

folded, as can be

seen in the dark

sub-vertical S2

bands. Field width

~ 2 mm. Right:

sequential analysis

of the development

of the textures.

From Passchier and

Trouw (1996)

Microtectonics.

Springer-Verlag.

Figure 23.46. Textures in a hypothetical andalusite porphyryoblast-mica

schist. After Bard (1986) Microtextures of Igneous and Metamorphic

Rocks. Reidel. Dordrecht.

Figure 23.47. Graphical analysis of the relationships between deformation

(D), metamorphism (M), mineral growth, and textures in the rock illustrated

in Figure 23.46. Winter (2010) An Introduction to Igneous and Metamorphic

Petrology. Prentice Hall.

Figure 23.48a. Interpreted sequential development of a polymetamorphic rock.

From Spry (1969) Metamorphic Textures. Pergamon. Oxford.

Shear Zone

Crustal Level Mechanism

In general, from upper to lower crust: Brittle

Brittle-ductile transition

Ductile

Brittle: displacement variation discontinuous

Ductile: displacement variaton continuous.

Principal textures of the metamorphic rocks

A. Granoblastic texture

1. Isogranular

2. Polygonal, mozaic (numerous triple junction)

3. Heterogranular

B. Mineral constituents habits

4. Lepidoblastic: sheet mineral

5. Nematoblastic: needle-like or elongated prismatic minerals.

6. Porphyroblastic: numerous poikiloblasts with helicitic inclusions. Poikiloblast: large crystal contain numerous, irregularly scattered inclusions but still preserve their idiomorphic shape.

7. (1), (2), or (3) + (4): grano-lepidoblastic

8. (1), (2), or (3) + (5): grano-nematoblastic

9. (1), (2), or (3) + (6): grano-porphyroblastic

10. Sheaf texture

11. Rosette texture

C. Spherical or spheroidal minerals

12. Nodular texture

13. Vermicular texture: symplectite or lobed minerals

14. Reaction corona

15. Augen texture

Porphyroclast vs Porphyroblast

Porphyroclasts: large grains that remained large while their surrounding matrix became fine grained. Ex: Feldspar augen in a recrystallized fine-grained quartz + feldspar matrix are common and typical examples.

Porphyroblasts: new-grown metamorphic minerals that grow over pre-existing minerals.

Figure 23.9. Typical textures of

contact metamorphism. From

Spry (1969) Metamorphic Textures.

Pergamon. Oxford.

Classification of tectonite textures A. Rock without foliation

1. Cataclastic texture: max 30% of clasts inferior in size to 0.2 mm.

2. Protoclastic texture: igneous rocks displaying broken, deformed or granular minerals included within the last crystals to have crystallized in magma

B. Foliated rock (mylonite)

3. Protomylonitic texture: 50-95% clasts included in a finely grained matrix (mortar or small recrystallized crystals; ribbon quartz is found frequently)

4. Augen mylonitic: 10-50% lens-shaped mono or polycrystalline porphyroclasts larger than 0.2 mm.

5. Ultramylonitic texture: 0-10% porphyroclasts smaller than 0.2 mm included in and molded by a finely grained, foliated or banded granoblastic matrix.

6. Blastomylonitic texture: 5-30% more or less recrytallized porphyroclasts moulded by a granoblastic matrix of recrytallized synkinematic and/or new minerals.

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