Petrografi Batuan Metamorf 1
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Transcript of Petrografi Batuan Metamorf 1
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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
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Syllabus
Thin section observation metamorphic textures
Crystal in metamorphic rocks
Microstructures and Deformation Mechanisms
Principal textures of metamorphic rocks
Metamorphic petrography in Indonesia
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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.
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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
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Metamorphic texture
1. Produced by metamorphic reaction (disequilibrium).
Symplectite, corona, reaction rim, zonation
2. Produced by deformation during metamorphism.
Pressure shadow, foliation, helical
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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
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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)
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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.
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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).
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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.
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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]
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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
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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.
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Order of crystallization
Grt equilibrium with Qz and develops pressure shadow as secondary minerals.
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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.
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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]
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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
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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.
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Reaction of microtexture D. Atoll garnet
[honey comb]
[trabecular]
[chain]
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Figure 23.27. Proposed mechanisms for the development of foliations. After Passchier
and Trouw (1996) Microtectonics. Springer-Verlag.
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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.
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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
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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.
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Helecitic syn-kinematic
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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.
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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
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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
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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.
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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.
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Figure 23.48a. Interpreted sequential development of a polymetamorphic rock.
From Spry (1969) Metamorphic Textures. Pergamon. Oxford.
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Shear Zone
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Crustal Level Mechanism
In general, from upper to lower crust: Brittle
Brittle-ductile transition
Ductile
Brittle: displacement variation discontinuous
Ductile: displacement variaton continuous.
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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
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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.
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Figure 23.9. Typical textures of
contact metamorphism. From
Spry (1969) Metamorphic Textures.
Pergamon. Oxford.
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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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Thank you