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, groundmass Groundmass= microcrystals No crystals Glass=

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

Liquid Anorthite + Liquid Diopside + Anorthite T C Wt.% Anorthite 1600 Liquid 1553 Liquidus 1500 T o C 1400 Anorthite + Liquid 1392 1300 Diopside + Liquid 1274 1200 Diopside + Anorthite Di 20 40 60 80 An Wt.% Anorthite

Figure 3-7. Euhedral early pyroxene with late interstitial plagioclase (horizontal twins). Stillwater complex, Montana. Field width 5 mm. © John Winter and Prentice Hall.

Figure 3-8. Ophitic texture Figure 3-8. Ophitic texture. A single pyroxene envelops several well-developed plagioclase laths. Width 1 mm. Skaergård intrusion, E. Greenland. © John Winter and Prentice Hall.

Poekilitic texture Crystallization sequence Biotite > Feldspar

Simultaneous growth Classical eutectic diagram. First minerals are either Qz or K-spar Then, at the eutectic…

Micrographic textures

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 C o Plagioclase Liquid plus L i q u d s S l Weight % An The crystals formed change composition as the liquid cools (and changes its composition too)

Complex zoning A complex sequence of cryst. And magma chamber « refill »

Complex zonings 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.

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

Outcrop-scale structures Orthogneissification Shear zones with late melts Magmatic foliation C/S structures Shear zones filled with aplites and pegmatites « Proto-shear zone » Closepet granite, south India (2.5 Ga)

Micro-structures Sub-solidus Magmatic

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