final cooling and textures of igneous rocks (mostly plutonic)

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