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CET/UWA Microstructure Course

A history of a mountain belt or other tectonically active area often includes both metamorphic and deformation events. The combined analysis of deformation textures and metamorphic textures is essential to reconstruct a full geological history, in particular the relative timing of deformation events with respect to metamorphic events. Porphyroblasts are the main structures used to study deformation in relation to metamorphism.

Metamorphic petrology -> P-T-t path

+ deformation events sequence -> P-T-D-t path

Porphyroclast <-> porphyroblasts

Porphyroclasts and porphyroblasts are both relatively large crystals in a finer grained surrounding (matrix).

Porphyroclasts are large grains that remained large while their surrounding matrix became fine grained (clasis = breaking). Feldspar augen (=eyes) in a recrystallised fine grained quartz+fledspar matrix are common and typical examples.

Porphyroblasts are new-grown metamorphic minerals that grow over pre-existing minerals (blasis = growing).

Some terms for the shape of porphyroblasts

There are several terms that describe the shape of porphyroblasts:

idioblastic: porphyroblast which has grain boundaries controlled by its own crystallography

xenoblastic: porphyroblast which does not have grain boundaries controlled by its own crystallography.

Blastesis

Once P-T-etc. conditions are favourable for a metamorphic mineral to grow, nucleation can start. The small nuclei have a relatively high surface energy, which forms an energy barrier for their growth. The number of nuclei and their survival rate determines whether many small or a few large porphyroblasts form. This number depends on:

• the availability of favourable nucleation sites;
• the driving force for the metamorphic reaction (overshoot of PT-conditions)
• transport rate of elements that form new mineral and elements that have to be removed to make space available
  
  

Inclusion trails

To form and grow a new metamorphic mineral grain:

(a) the right mix of elements that form the mineral must get to the grain

(b) other elements have to be taken away from the grain

• If (a) and (b) are both fast enough, no inclusions are incorporated
• If (b) can't keep up with (a), inclusions are incorporated of minerals that do not contribute to the metamorphic reaction
• If (a) is too slow, even inclusions of minerals that do contribute to the reaction are incorporated.

Some porphyroblasts are full of inclusions. These are called poikiloblastic. The example here is of big cordierite crystal full of quartz and biotite inclusions.

Inclusion trail - foliation relation ships

The usual rigidity of the porphyroblasts protects the inclusion trail pattern from further deformation. Porphyroblasts with inclusions thus provide a frozen-in picture of the foliation at the time of their growth. This allows determination of the timing of growth (phases) relative to deformation or tectonic phases.

pre-tectonic

• The inclusion pattern is random, indicating no foliation at time of blastesis
• The younger foliation may be deflected around the porphyroblast

inter-tectonic

• The inclusions are aligned according to a foliation that was overgrown by the blast
• The inclusion pattern bears no relation with the foliation outside the porphyroblast
• The younger foliation may be deflected around the porphyroblast
• Inclusion trails and outside foliation are discontinuous

syn-tectonic

• Inclusion trails and outside foliation are continuous
• Inclusion pattern and outside foliation are similar, but inclusion trails may preserve outside foliation in an early stage of development.
• Gradual transition of pattern and orientation of inclusion trails from core to rim of porphyroblast
• Orientation of inclusion trails in core of porphyroblast may have different orientation due to rotation of porphyroblast during its growth (eg 'snow-ball garnets')
• Possible deflection of foliation outside porphyroblast

post-tectonic

• Inclusion trails and outside foliation are completely continuous and similar
• No deflection of foliation outside porphyroblast

See Passchier & Trouw 1996, pp 153-168 for more pictures of different classes of porphyroblasts

Complications

Rotating / non-rotating porphyroblasts

Rigid objects may rotate when deformation is non-coaxial. This can explain the spiralling or oblique inclusion trails in syntectonic porphyroblasts (as in snowball garnets).

The rotation of rigid objects is however inhibited if the object deflects the deformation around a lens-shaped region. This partitioning of strain can lead to 'millipede structures', which can give the appearance that the porphyroblast rotated during growth. The figure shows an example of the formation of a millipede structure by two shortening events at right angles.(see Bell et al. 1992, Passchier et al. 1992 and Passchier & Trouw 1996 for discussion of this controversial topic)

false inclusion trails

Care should be taken that 'false' inclusion trails are recognised. Such inclusions are usually crystallographically controlled and of course say little or nothing about the relation between growth and foliation development.

• exsolution features (perthite, antiperthite)
• healed cracks
• growth inclusions
• retrogressive alteration
  

Not all porphyroblasts have inclusion trails. Also, porphyroblasts can sometimes incorporate inclusions during some of their growth stages or only in crystallographically determined sectors (sector zoning and hour-glass zoning).

In the absence of inclusions, the relative timing of blastesis and foliation development can often be determined by the deflection of foliation around porphyroblasts