Download Lecture W5-L13-14

G314 Advanced Igneous Petrology
2007
Week 5 – Lectures 13--14
Cooling of magmas, textures, sub-solidus evolution
See Winter, chap. 3 and 4
1. Final cooling of magmas; textures of plutonic rocks
1.1. Crystal nucleation and growth
How crystals form and grow
Crystal grow by progressively adding ions to an existing grain. However, the initiation of a grain is a
more energy-demanding process (you have to put together a small number of ions, therefore posibely
creating unbalanced ionic structures, with high surface/volume ratio (and therefore high surface energy).
Small nucleus are therefore very unstable; the amount of crystals in a system is strongly controlled by the
amount of crystal nuclei.
Crystal forms around…
- Spontaneous nuclei (possible but difficult);
- Small pre-existing crystals (“seeds”), of the same specie, or of a different mineral;
- Pre-existing crystalline faces (“epitaxis”).
Competition between growth and nucleation and the textures of rocks
Both growth and nucleation rates change as a function of the temperature of the magma (more precisely,
of the degree of under-cooling, below the melting point).
- For important undercooling (=fast cooled rocks, volcanic), nucleation rate > growth rate; lot of
small crystals (microgranular texture).
- For moderate undercooling, growth > nucleation. Few, big crystals (plutonic textures).
Note: bimodal dstributions (porphyritic lavas): two stages of crystal growth with different degrees of
undercooling!
Departement of Geology, Geography and Environmental Studies
G314 Advanced Igneous Petrology
2007
Water and growth rates
Water presence increases both nucleation and growth rate. This results in fairly unpredictable textures,
with very coarse and very fine-grained rocks coexisting in close vicinity (aplite-pegmatite association)
1.2. Textures and relations related to crystallization order
As we discussed previously, crystals form in a specific sequence that depends on the magma initial
composition. Crystal growing in the melt develop their own crystalline shapes (euhedral), whereas
crystals developing at a latter stage are likely to be influenced by pre-existing grains, and form intersticial
or engulfing grains.
On the other hand, fast growing grains can also include slower forming minerals, creating poekilitic
grains, more or less euhedral.
Being euhedral is therefore not an absolute criteria (engulfing reactions are).
Inclusion relations
An included grain is older than the surrounding. This allows to propose a sequence of crystallization (can
be often interpreted by looking at the relevant phase diagram –see example in lecture W5L1). Just be
careful of “pseudo-inclusions” (2D sections of 3D structures!).
Simultaneous growth
On eutectic or joints, when the crystallization reactions produce several mineral species simultaneously.
Intergrowth (or, sometimes, mutually engulfing crystals), e.g. granophyric or graphic textures.
1.3. Textures and relations related to chemical evolution during cooling
Normal zoning
During cooling, the magma composition evolves. Minerals that form solid solution also have changing
composition (see Fo-Fa or Ab-An diagrams from G214). Crystals typically are zoned, with a high
temperature core and a lower temperature rim. This reflects only normal cooling and should be expected.
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Departement of Geology, Geography and Environmental Studies
G314 Advanced Igneous Petrology
2007
“Anormal” zonings, resorptions, etc.
In some case, the zoning does not obey to this simple evolution (e.g., apparentely low-temperature cores).
Or it is more complex, with maybe several cycles, or overgrowth. Or some crystal resorption appears
(truncated zoning, etc.).
All this indicates that the crystal had a complicated history and probably cooled in a changing (chemical)
environement: it was carried to another magma (cf. enclaves and magma mixing), or the magma chamber
was refilled by a more primitive melt, etc. Study of crystal zoning (sometimes, in theory) allows to
discuss the details of the evolution in the magma chamber.
1.4. Textures related to deformation (syn-tectonic emplacement)
Plutonic rocks (granites, especially) are commonly syn-tectonic. They emplace and cool during
deformation, and they record the strain they exercised at different stages:
- As a liquid with few crystals floating;
- As a largely crystallized system with some liquid remaining;
- After complete solidification.
Modern petrology (1990-onwards) interprets a lot of textures in granites (mostly outcrop or handspecimen scale) as related to deformation in a partially molten “mush”. The n
otion of RCMP discussed previously also applies here…
Crystals orientation (flow figures)
In a liquid dominated system. Alignment of early crystals reflecting either magmatic flow or tectonic
stress.
Crystal-liquid separation
In a largely crystalline system (below RCMP). Evidence for circulation of late, residual magmatic liquids
between crystals; “dykes” and “pipes” of magma can sometimes be observed, together with crystal
accumulations.
Sub-solidus deformation
After complete cooling. High-temeprature, solid-state deformation (commonly quartz sub-grain with
ondulose extinction, sometimes feldspars fracturation or even orthogneissic textures). Difficult to interpret
as syn-plutonic deformation (as opposed to a latter tectonic event), unless you have good context
(typically a whole sequence of deformation from magma-dominated to sub-solidus textures).
2. Sub-solidus textures
After the final solidification of a plutonic rock, its evolution continues! The solidus is at 700-900 °C,
which (for upper crustal intrusions at least) is far from the surrounding thermal conditions. Therefore,
mineralogical changes will occur (retrograde metamorphism into greenschist facies, first).
2.1. Mineral transformations
Transformation affecting one single minerals: no reactions, but just re-adjustement of the crystalline
system.
Departement of Geology, Geography and Environmental Studies
G314 Advanced Igneous Petrology
2007
Annealing (Ostwald reapening)
Minimization of surface energy tends to migrate grains boundary and produce homogeneous grain size
distribution of relatively large grains. Affects particularily fine-grained rocks (lavas) or even glasses
(devitrification). Low temperatures help to preserve the volcanic textures.
Polymorphic transformation
.. e.g., quartz polymorphs (or feldspars). Little evidence preserved.
Polymorphs
Several minerals (e.g., silica) can form different minerals (quartz, cristobalite, etc.) in different parts of the
P-T field. Cooling will cause such changes (normaly very discrete, nearly impossible to see).
Exsolution
Minerals such as feldspars (or pyroxenes) form solid solutions only at high temperatures. At lower
temperatures, the minerals become immiscible and form separate phases (solvus).
A high-temperature grain will therefore exsolve the minor component into perthites.
2.2. Secondary minerals
“Autometamorphic” processes as the rocks cools down, effectively retrograde metamorphic reactions.
Reactions involving hydratation of igneous minerals (the most common) are called deuteric reactions.
This include, commonly:
- successive replacements of pyroxenes -> amphiobles -> biotite;
- chlorite replacing any mafic mineral (biotite, very often);
- sericite (=fine grained white micas) as alteration products of feldspars or feldspathoids
(feldspathoids more than feldspars, and K-feldspars more than plagioclase)
- saussurite (an epidote) replacing Ca-plagioclase
- iddingsite or serpentine replacing olivine
Such replacements are commonly in-situ, resulting in pseudomorphs or in more or less altered crystals
(commonly, secondary minerals form along the cleavages).
It is also possible to form veins of secondary minerals (commonly calcite + white micas/clays), pointing
to (limited) movement of elements.
2.3. Fluid expulsion and movement
A typical melt can contain 2-4 wt% of water (even more for low temperature granitic melts). Biotite
contains also about 2 wt% of water (and is the most hydrous common mineral in a granite); therefore,
crystallizing a granite will yield lots of excess water.
The hot fluids are a very potent chemical agent and can dissolve significant amounts of components such
as Si, Na or K.
In addition to the magmatic water, meteoric fluids infiltrating in the ground will also “feed” the
hydrothermal system (and, actually, contribute to a large part of the amount of water present – even more
than magmatic fluids in general).
Pegmatitic (and aplitic) veins

Hydraulic fracturation, resulting in a network of veins reflecting the strain pattern (occasionally it
is possible to prove that it’s s the same strain pattern that existed during previous deformation).
Departement of Geology, Geography and Environmental Studies
G314 Advanced Igneous Petrology
2007

As P and T decrease, precipitation of the dissolved elements in veins => pegmatites and aplites.

Changes in pH can also cause precipitation => pegmatites and ore bodies on the contacts between
contrasted rock types.
Mineralized veins
Some very incompatible trace elements tends to be concentrated in the last liquids, and then in the fluids
(large cations such as Au, U, Sn…). They will eventually precipitate in pegmatitic veins.
Hydrothermalism
Hydrothermal, mineral-rich fluids can react with solid rocks to form new (metasomatic) rocks. They also
tend to be good ore deposits.


In the pluton:
Simple leaching of the Si out of the rock (high pH conditions): a quartz-depleted rock called
“episyenite”.
Complex reactions, forming metasomatic rocks (endoskarns). “Argilitization” of the feldspars
into micas/clays (low pH, requires H+). Can lead to kaolinite+quartz rocks (“greisens”).
Around the plutons:
(exo)skarns. They develop best in carbonates, which supply Ca ±Mg while the metasomatic fluids supply
Si, Na, K. This forms calc-silicate rocks, typically rich in (clino)pyroxene and garnet, and a lot of weird
minerals (with Mn, Sn, U, Cu, …)
All these processes create mineral ore (cf. Economic Geology).
Departement of Geology, Geography and Environmental Studies