Final Cooling of Magmas; Textures of Plutonic Rocks

1. Crystal nucleation and growth

A.   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”).

B.   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!

c.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)

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).

A.   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!).

B.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.

3.  Textures and relations related to chemical evolution during cooling

A.   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.

b.   “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.

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 hand-specimen scale) as related to deformation in a partially molten “mush”. The n
otion of RCMP discussed previously also applies here…

A.   Crystals orientation (flow figures)

In a liquid dominated system. Alignment of early crystals reflecting either magmatic flow or tectonic stress.

B.   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.

c.   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).

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