Page 209 - Microtectonics

7.4 · Classification of Porphyroblast-Matrix Relations 199

Fig. 7.14. Hornblende crystal in quartz-albite-mica schist, with straight inclusions. S makes an angle with S of about 60°. If S and S are
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interpreted as resulting from the same deformation phase, the porphyroblast is syntectonic and must have grown relatively rapidly after
a foliation had formed and before ongoing deformation produced the relative rotation of the crystal with respect to S . Alternatively the
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crystal may be intertectonic between D and D n+1 that caused the relative rotation. In this particular example the second interpretation
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is most likely because phyllites from the same area preserve a crenulation cleavage with intertectonic biotite porphyroblasts similar to
those shown in Fig. 7.12. Example of case c1 in Fig. 7.9. Kittelfjäll, Västerbotten. Central Sweden. Width of view 4 mm. Polars at 45°
are surrounded by a matrix affected by a later deforma- blasts grew (Barker 1994). The most characteristic syn-
tion phase that did not leave any record in the porphyro- tectonic porphyroblasts form when growth and strain
blast (c and d in Fig. 7.9; Figs. 7.12, 7.13, ×Videos 7.9c, rates are of the same order of magnitude (Figs. 7.15, 7.32,
9.7d). Porphyroblasts with a straight S that is oblique to ×Photo 7.15). Inclusion patterns are generally curved
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S can also be named oblique-S porphyroblasts and are in syntectonic porphyroblasts, and random or straight
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discussed in Sect. 7.6.8. (c1 in Fig. 7.9; Fig. 7.14, in pre- and intertectonic porphyroblasts. S can be
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×Photo 7.14). symmetrically arranged with respect to S , (e3, f3
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in Fig. 7.9, ×Video 7.9f) or show oblique-S , sigmoi-
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7.4.4 dal or ‘spiral’ geometry (Sect. 7.6.8; e1 and e2 in Fig. 7.9).
Syntectonic Porphyroblast Growth The latter is particularly common in garnet (e.g. Schone-
veld 1979; Bell and Johnson 1989; Johnson 1993a,b;
Syntectonic porphyroblasts have grown during a Williams and Jiang 1999; Fig. 7.39). Included folds in
single phase of deformation D and are the most fre- porphyroblasts (c3, f and h in Fig. 7.9) are known as
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quently encountered type of porphyroblasts in nature. helicitic folds. Porphyroblasts with oblique, sigmoidal and
This is probably due to the fact that deformation has spiral-shaped patterns have sometimes been loosely re-
a catalysing effect on mineral nucleation and dif- ferred to as rotated porphyroblasts (Sect. 7.6.8) but use
fusion rates (cf. Bell 1981; Bell and Hayward 1991). A large of this term should be discouraged. The geometry may
variety of microstructures can form in this group (Prior indicate relative rotation of porphyroblasts with respect
1987; Figs. 7.9, 7.15–7.19, 7.32, ×Photos 7.15, 7.17, 7.18, to S but determination of movement of either fabric el-
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7.19a–c). The principal controlling variables are finite ement in an external reference frame is more problem-
strain, the ratio of growth rate to strain rate, and the stage atic (Ramsay 1962; Bell 1985; Johnson 1993a,b, 1999b;
of progressive deformation during which the porphyro- Sect. 7.6.8).