Transcript LECTURE W6-L2 - Cooling and Textures

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=
No crystals
Groundmass=
microcrystals
Glass, groundmass
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
1600
1553
Liquid
1500
T oC
1400
Anorthite + Liquid
1392
1300
Diopside + Liquid
1274
1200
Diopside + Anorthite
Di
20
40
60
Wt.% Anorthite
80
An
Poekilitic texture
Crystallization sequence Biotite > Feldspar
Simultaneous growth
Classical 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
1557
1500
Liquid
Plagioclase
1400
o
T C
plus
1300
Liquid
Plagioclase
1200
1118
1100
Ab
20
40
60
Weight % An
80
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
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
C/S
structures
Shear
zones
filledwith
withlate
aplites
and
pegmatites
Magmatic
foliation
«melts
Proto-shear
zone »
Closepet granite, south India (2.5 Ga)
Micro-structures
Magmatic
Sub-solidus
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