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Final cooling and textures of igneous rocks (mostly plutonic)

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Presentation on theme: "Final cooling and textures of igneous rocks (mostly plutonic)"— Presentation transcript:

1 Final cooling and textures of igneous rocks (mostly plutonic)

2 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

3 1- Growth and nucleation Textures related to the growth rate of crystals

4 Nucleation and growth Many nuclei Few nuclei

5 Growth and nucleation rates are a function of the degree of undercooling Strong undercooling = Nucleation >> growth (fine texture) Moderate undercooling = Growth >> nucleation (coarse texture)

6 Plutonic and volcanic textures

7 Glass, groundmass Groundmass= microcrystals Glass= No crystals

8 Porphyritic textures 2 Grain-size populations = 2 growth events? (magma chamber & eruption)

9 Porphyroid textures Faster growth, or earlier crystals?

10 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

11 A complex pegmatite body

12 2- Textures related to the crystallization order

13 1274 Di An T o C Anorthite + Liquid Liquid Liquidus Diopside + Liquid Diopside + Anorthite Wt.% Anorthite

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

15 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.

16 Poekilitic texture Crystallization sequence Biotite > Feldspar

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

18 Micrographic textures

19 Graphic texture: coeval growth of quartz and K-spar

20 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.

21 3- Textures related to the evolution of the magma during cooling

22 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.

23 Zoned K-spar (Hercynian granite, France)

24 Binary diagrams with complete solid solution 1118 Ab An T C o Plagioclase Liquid plus L i q u i d u s S o l i d u s Weight % An Plagioclase The crystals formed change composition as the liquid cools (and changes its composition too)

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

26 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. Complex zonings

27 Plag sieving

28 Crystal resorption

29 Everything is not chemical effects!! Fast ascent can also dissolve crystals…

30 4- Textures related to deformation of a partially molten system Movements in a partially molten « mush » Syn-plutonic deformation

31 Magmatic flow

32 Late magma movement Leucocratic magma expulsed from the cooling « mush »

33 « ellipsoids », « snail structures », « diapirs »

34 Pipes of late magmatic liquids in the mush

35 K-feldspar accumulation (flow segregation?)

36 Rheology of partially molten systems

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

40 MagmaticSub-solidus Micro-structures

41 Quartz subgrains

42 Qz grain-size reduction

43 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

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