0000000000422038
AUTHOR
Dirk J. Wiersma
showing 7 related works from this author
Protomylonite, Mylonite and Ultramylonite
2009
The objective of this chapter is to show how variation of strain intensity can be judged in thin section. Usually this kind of variation can best be observed in low-grade mylonites where the percentage of porphyroclasts decreases progressively with strain intensity. However, the percentage of matrix is highly dependent on mineralogical composition (e.g. quartz and biotite tend to convert to matrix readily). Compositional banding in gneiss can therefore result in mylonitic banding of apparent strain variation, which in fact only reflects variation in composition of the parent rock. Several examples of ultramylonite are derived from quartzitic rocks that tend to form few or no porphyroclasts …
Shear Sense Indicators
2009
Many geologists study mylonites with the exclusive aim to determine the sense of shear. Obviously this is an important aspect, but it is important to study shear zones first, before shear sense determination is attempted. In order to deduce the correct sense of shear we recommend the following procedure (Fig. 9.1)
Low-Grade Mylonites
2009
The temperature range for these mylonites is thought to be roughly between 250 and 500 °C. There is a gradual transition between cataclasites and low-grade mylonites. Whereas many feldspar porphyroclasts in low-grade mylonites still show fracturing by cataclasis, the quartz is usually deformed by crystal-plastic processes as shown by its change in shape and by undulose extinction. At increasing temperature bulging recrystallisation starts to manifest itself along the lobate contacts and eventually recrystallisation by subgrain rotation takes over (Chapter 10).
Crystal-Plastic Deformation, Recovery and Recrystallisation of Quartz
2009
As stated in the introduction, this chapter is included because of the special importance of quartz to estimate metamorphic conditions during and after mylonitisation. The theory behind crystal-plastic deformation is treated elsewhere (e.g. Passchier & Trouw 2005). The main optical expression of crystal-plastic deformation is smooth, non-patchy undulose extinction. Elongated grains with such undulose extinction, sometimes accompanied by deformation lamellae, are indicative for low-temperature deformation. At slightly higher temperatures recovery produces subgrains and recrystallisation tends to substitute the old deformed grains by small new ones. Three types of recrystallisation can be dis…
High-Grade Mylonites
2009
High-grade mylonites are formed at temperatures above 650 °C. They are relatively uncommon, probably because their conservation is problematic. Most mylonites formed under these conditions would tend to fully recrystallise which destroys and masks the mylonitic structure. Mylonitic features are only preserved if grain growth is somehow inhibited in the rock, e.g. by its polymineralic nature.
Mylonites Derived From Parent Rocks Other Than Granites and Gneisses
2009
Most mylonites shown in this atlas are derived from granites and gneisses. This is not a coincidence; the mineralogy of these rocks favours the formation of mylonites because of the contrasting behaviour of quartz and biotite on the one hand (forming matrix) and feldspar and muscovite on the other hand (forming porphyroclasts). Another group of rocks that readily forms mylonites are impure quartzites in which resistant minerals tend to form fish-like structures, again, by strong contrast in rheological behaviour.
Medium-Grade Mylonites
2009
The temperature range for the formation of this group of mylonites is approximately 500 to 650 °C. In medium-grade mylonites quartz is usually fully recrystallised, mainly by subgrain rotation, and has grown to a polygonal crystalloblastic fabric of strain free grains with an average grain size exceeding about 50 micrometers.