Page 217 - Microtectonics

7.6 · Problematic Porphyroblast Microstructures 207
Fig. 7.24. a Spool-shaped andalusite porphyroblast with symmet- shortening normal to an earlier foliation (e.g. Bell 1986;
▼
ric distribution of deflection folds on both sides. b Schematic draw- Bell et al. 1986a). Strain shadows are not always accom-
ing of the structure in a. c Enlargement of the lower half of a, show- panied by deflection of S .
ing the deflection folds. Trois Seigneurs Massif, Pyrenees, France. e
Length of porphyroblast in a 2.9 mm. PPL Large, isolated elongate mineral grains such as micas
and amphiboles which lie parallel to a foliation and which
lack inclusion patterns can also be difficult to date rela-
tive to deformation (Fig. 7.26b and d). They may be inter-,
syn-, or post-tectonic. Syntectonic porphyroblasts are, if
elongate, usually well aligned with the foliation. Intertec-
tonic porphyroblasts may have rotated towards the folia-
tion plane and may be recognisable by slight but consist-
ent obliqueness to the foliation and evidence of internal
deformation or replacement along the edges (b in
Fig. 7.26). Post-tectonic porphyroblasts are difficult to
recognise if they have grown mimetically parallel to a
pre-existing foliation. However, their post-tectonic na-
ture can occasionally be recognised if some of the crys-
tals overgrew the foliation obliquely (Figs. 7.21, 7.26d).
7.6.2
Shape and Size of Inclusions compared to Matrix Grains

In many cases where a porphyroblast overgrew a struc-
ture without later deformation, the size and shape of in-
clusions differs little from those in the matrix (Figs. 7.1,
7.3, 7.6, 7.8, 7.27a). This is especially notable in the case of
opaque minerals; structures like grain shape preferred
orientation, crenulation cleavage and compositional lay-
ering can be perfectly preserved in this way as inclusion
patterns. Since many porphyroblasts behave as rigid bod-
Fig. 7.25. a Deformation partitioning in bulk non-coaxial progres- ies during later deformation, the inclusion patterns, once
sive deformation. b Deformation partitioning around a porphyro- incorporated, can remain unaffected by later deformation
blast (centre) in bulk non-coaxial deformation. Millipede and de-
flection-fold structures are thought to develop by overgrowth of or modification by grain growth, dynamic recrystallisa-
such partitioning patterns. 1 no strain; 2 coaxial progressive de- tion and transposition that may affect the matrix. In this
formation; 3 non-coaxial progressive deformation. (After Bell 1985) way porphyroblasts often preserve stages in the tectono-
metamorphic evolution that would otherwise be lost (e.g.
7.6 Figs. 7.4, 7.5). In some cases, inclusions in porphyroblasts 7.6
Problematic Porphyroblast Microstructures may have another size and shape than matrix grains of
the same mineral (Fig. 7.27b,c). Commonly, the inclusions
7.6.1 are smaller and somewhat more rounded in shape than
Inclusion-Free Porphyroblasts similar grains in the matrix (e.g. Fig. 7.6). This may be caused
by partial diffusion, or by a reaction involving the included
The presence or absence of inclusions in porphyroblasts mineral. A decrease in size of included grains from core
cannot be used to date them with respect to other fabric to rim in a porphyroblast may reflect a gradual change in
elements (Fig. 7.26). If porphyroblasts do not have in- diffusion rate or porphyroblast growth rate. More com-
clusions, it is difficult or even impossible to date their mon is an increase in size from core to rim often accom-
growth with respect to deformation. The relative age can panied by a sharp contrast with much larger matrix min-
in some cases be determined by the intensity of de- erals. This can be explained by progressive coarsening of
flection of S or from the presence of strain shadows the matrix during and after porphyroblast growth, as a
e
(Fig. 7.26). If there is no deflection of S , porphyroblasts consequence of temperature increase and accompanying
e
may be post-tectonic (c in Fig. 7.26); if there is deflec- grain boundary area reduction (GBAR; Fig. 7.27b;
tion of S or if strain shadows are present, porphyroblasts Sect. 3.10). The presence of large inclusions compared to
e
are pre-, inter or syntectonic (Figs. 7.26a and b, 7.46 at finer-grained matrix minerals, generally indicates grain
right, ×Photo 7.46). Care has to be taken since deflec- size reduction due to deformation in the matrix (Fig. 7.27c).
tion may be caused by later deformation phases and late This is particularly common in mylonites (Sect. 5.3.4).