Final cooling of magmas; textures of plutonic rocks

1.      Crystal nucleation in addition to growth

A.   How crystals shape in addition to grow

Crystal grow past times progressively adding ions to an existing grain. However, the initiation of a grain is a to a greater extent than energy-demanding procedure (you accept to seat together a small-scale let out of ions, hence posibely creating unbalanced ionic structures, amongst high surface/volume ratio (and hence high surface energy). Small nucleus are hence really unstable; the amount of crystals inward a organisation is strongly controlled past times 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 dissimilar mineral;
          Pre-existing crystalline faces (“epitaxis”).

B.   Competition betwixt growth in addition to nucleation in addition to the textures of rocks

Both growth in addition to nucleation rates modify every bit a business office of the temperature of the magma (more precisely, of the score of under-cooling, below the melting point).
          For of import undercooling (=fast cooled rocks, volcanic), nucleation charge per unit of measurement > growth rate; lot of small-scale crystals (microgranular texture).

          For moderate undercooling, growth > nucleation. Few, large crystals (plutonic textures).
Note: bimodal dstributions (porphyritic lavas): 2 stages of crystal growth amongst dissimilar degrees of undercooling!

c.   Water in addition to growth rates

Water presence increases both nucleation in addition to growth rate. This results inward fairly unpredictable textures, amongst really coarse in addition to really fine-grained rocks coexisting inward closed vicinity (aplite-pegmatite association)

2.      Textures in addition to relations related to crystallization order

As nosotros discussed previously, crystals shape inward a specific sequence that depends on the magma initial composition. Crystal growing inward the melt develop their ain crystalline shapes (euhedral), whereas crystals developing at a latter stage are probable to move influenced past times pre-existing grains, in addition to shape intersticial or engulfing grains.
On the other hand, fast growing grains tin every bit good include slower forming minerals, creating poekilitic grains, to a greater extent than or less euhedral.
Being euhedral is hence non an absolute criteria (engulfing reactions are).

A.   Inclusion relations

An included grain is older than the surrounding. This allows to suggest a sequence of crystallization (can move oft interpreted past times looking at the relevant stage diagram –see illustration inward lecture W5L1). Just move careful of “pseudo-inclusions” (2D sections of 3D structures!).

B.   Simultaneous growth

On eutectic or joints, when the crystallization reactions hit several mineral species simultaneously. Intergrowth (or, sometimes, mutually engulfing crystals), e.g. granophyric or graphic textures.

3.  Textures in addition to relations related to chemic development during cooling

A.   Normal zoning

During cooling, the magma composition evolves. Minerals that shape enterprise solution every bit good accept changing composition (see Fo-Fa or Ab-An diagrams from G214). Crystals typically are zoned, amongst a heat nub in addition to a lower temperature rim. This reflects exclusively normal cooling in addition to should move expected.

b.   “Anormal” zonings, resorptions, etc.

In to a greater extent than or less case, the zoning does non obey to this uncomplicated development (e.g., apparentely low-temperature cores). Or it is to a greater extent than complex, amongst mayhap several cycles, or overgrowth. Or to a greater extent than or less crystal resorption appears (truncated zoning, etc.).
All this indicates that the crystal had a complicated history in addition to in all likelihood cooled inward a changing (chemical) environement: it was carried to to a greater extent than or less other magma (cf. enclaves in addition to magma mixing), or the magma sleeping room was refilled past times a to a greater extent than primitive melt, etc. Study of crystal zoning (sometimes, inward theory) allows to speak over the details of the development inward the magma chamber.

4.  Textures related to deformation (syn-tectonic emplacement)

Plutonic rocks (granites, especially) are unremarkably syn-tectonic. They emplace in addition to cool during deformation, in addition to they tape the strain they exercised at dissimilar stages:
          As a liquid amongst few crystals floating;
          As a largely crystallized organisation amongst to a greater extent than or less liquid remaining;
          After consummate solidification.
Modern petrology (1990-onwards) interprets a lot of textures inward granites (mostly outcrop or hand-specimen scale) every bit related to deformation inward a partially molten “mush”. The n
otion of RCMP discussed previously every bit good applies here…

A.   Crystals orientation (flow figures)

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

B.   Crystal-liquid separation

 In a largely crystalline organisation (below RCMP). Evidence for circulation of late, residual magmatic liquids betwixt crystals; “dykes” in addition to “pipes” of magma tin sometimes move observed, together amongst crystal accumulations.

c.   Sub-solidus deformation

After consummate cooling. High-temeprature, solid-state deformation (commonly quartz sub-grain amongst ondulose extinction, sometimes feldspars fracturation or fifty-fifty orthogneissic textures). Difficult to translate every bit syn-plutonic deformation (as opposed to a latter tectonic event), unless y’all accept adept context (typically a whole sequence of deformation from magma-dominated to sub-solidus textures).