final cooling and textures of igneous rocks (mostly plutonic)
TRANSCRIPT
Final cooling and textures of igneous rocks
(mostly plutonic)
• Growth and nucleation
• Textures related to the crystallization sequence
• Textures related to the chemical evolution of the magma during cooling
• Textures related to deformation in a partially molten system
• Textures related to sub-solidus deformation
• Sub-solidus textures
1- Growth and nucleation
• Textures related to the growth rate of crystals
Nucleation and growth
Many nuclei
Few nuclei
• Growth and nucleation rates are a function of the degree of undercooling
• Strong undercooling = Nucleation >> growth (fine texture)
• Moderate undercooling = Growth >> nucleation (coarse texture)
Plutonic and volcanic textures
Glass, groundmassG
roun
dmas
s=m
icro
crys
tals
Gla
ss=
No
crys
tals
Porphyritic textures
• 2 Grain-size populations = 2 growth events? (magma chamber & eruption)
Porphyroid textures
Faster growth, or earlier crystals?
Aplites & pegmatites• Close association
of (very) coarse pegmatites and (very) fine aplites
Water influences both nucleation and growth rates => complex, highly variable grain size associations
A complex pegmatite body
2- Textures related to the crystallization order
1274
Di 20 40 60 80 An
1200
1300
1400
1500
1600
T oC
Anorthite + Liquid
Liquid Liquidus
Diopside + Liquid
Diopside + Anorthite
1553
1392
Wt.% Anorthite
Poekilitic texture
Crystallization sequence Biotite > Feldspar
Simultaneous growthClassical eutectic diagram.
•First minerals are either Qz or K-spar
•Then, at the eutectic…
• Graphic texture: coeval growth of quartz and K-spar
Figure 3-9. a. Granophyric quartz-alkali feldspar intergrowth at the margin of a 1-cm dike. Golden Horn granite, WA. Width 1mm. b. Graphic texture: a single crystal of cuneiform quartz (darker) intergrown with alkali feldspar (lighter). Laramie Range, WY. © John Winter and Prentice Hall.
3- Textures related to the evolution of the magma during cooling
Igneous Textures
Figure 3-5. a. Compositionally zoned hornblende phenocryst with pronounced color variation visible in plane-polarized light. Field width 1 mm. b. Zoned plagioclase twinned on the carlsbad law. Andesite, Crater Lake, OR. Field width 0.3 mm. © John Winter and Prentice Hall.
Zoned K-spar (Hercynian granite, France)
Binary diagrams with complete solid solution
1118
Ab 20 40 60 80 An
1100
1200
1300
1400
1500
1557
T Co
PlagioclaseLiquid
Liquid
plus
Liquidus
Solidu
s
Weight % An
Plagioclase
The crystals formed change composition as the liquid cools (and changes its composition too)
Complex zoning
A complex sequence of cryst. Andmagma chamber « refill »
Figure 3-6. Examples of plagioclase zoning profiles determined by microprobe point traverses. a. Repeated sharp reversals attributed to magma mixing, followed by normal cooling increments. b. Smaller and irregular oscillations caused by local disequilibrium crystallization. c. Complex oscillations due to combinations of magma mixing and local disequilibrium. From Shelley (1993). Igneous and Metamorphic Rocks Under the Microscope. © Chapman and Hall. London.
Core Rim
30
20
10
0
10
0
20
10
0
a
b
c
Ch
ang
e in
An m
ol %
Complex zonings
Plag sieving
Crystal resorption
Everything is not chemical effects!!
Fast ascent can also dissolve crystals…
4- Textures related to deformation of a partially molten system
• Movements in a partially molten « mush »
• Syn-plutonic deformation
Magmatic flow
Late magma movement
Leucocratic magma expulsed from the cooling « mush »
« ellipsoids », « snail structures », « diapirs »
www.earth.monash.edu.au/~weinberg
Pipes of late magmatic liquids in the mush
K-feldspar accumulation (flow segregation?)
Rheology of partially molten systems
Magmatic foliation« Proto-shear zone »Shear zones with late meltsShear zones filled with aplites and pegmatitesC/S structuresOrthogneissification
Outcrop-scale structures
Closepet granite, south India (2.5 Ga)
Magmatic Sub-solidus
Micro-structures
Quartz subgrains
Qz grain-size reduction
Continuous sequence of textures
• Feldspar alignment/accumulation• Expulsion of late melts• Strain partitionning on the latest melts• C/S movement on weak planes
(phyllosilicates)• Ductile deformation of quartz (sub-grains,
etc.)• Orthogneissification,
deformation/recrystallization of all minerals