Читать книгу The Field Description of Metamorphic Rocks - Dougal Jerram - Страница 17

In a subduction zone setting (see Figure 1.1), the relatively fast burial of one of the cold plates leaves insufficient time for it to heat up substantially until it is at significant depth. It takes time for the subducted rocks to heat up (generally by conduction from the hotter rocks around them at depth), but application of pressure during burial is instantaneous. Thus in subduction zones the conditions of high pressure – low/moderate temperature metamorphism occur (e.g. the numbered trend 2 in Figure 1.2), and rocks exhumed from these settings record evidence for having been in the blueschist facies. Again, these rocks are characterised by certain indicator minerals and mineral assemblages, and blueschist rocks often appear blue (hence their name) because of the prevalence of a blue amphibole mineral called glaucophane. Exposures of blueschist facies rocks are relatively rare, most obviously because it is difficult to exhume them from the subduction zone to the Earth's surface, but they are an important record of plate tectonics on Earth and will be described in more detail in Chapter 3. An example of a blueschist is given in Figure 1.5a. When the most extreme pressures and moderate to high temperatures are reached, a group of rocks termed the eclogite facies form. Exposure of these on Earth's surface is again relatively rare, but they are generally easily identified, characterised by a pale green pyroxene (sodic rich called omphacite) and a deep red garnet (almandine‐pyrope), an example of which is given in Figure 1.5b (see Chapter 3 for more detail).

Figure 1.4 Examples of classic (Barrovian) regional metamorphic rocks

(slate photo Jim Talbot, phillite, schist, and gneiss photos Dougal Jerram, migmatite photo Mark Caddick).

Figure 1.5 (a) Blueschist facies, Syros, Greece (Mark Caddick for scale) with inset figure highlighting lawsonite porphyroblasts, (b) Eclogite facies, Alps

(photo a Mark Caddick, photo b Hans Jørgen).

The regionally metamorphosed rocks are often also characterised by having many structures associated with deformation. The rocks are put under pressure from all sides, but often this pressure is not the same from all sides. This leads to asymmetry in the pressure distribution and the alignment of new metamorphic minerals, rotation of existing and newly growing ones, and faulting and folding of the rocks during their metamorphism. These textures will be touched on in detail in Chapters 4 and 5, but banding, cleavage, folding, and dislocation structures are commonplace in regional metamorphic areas (e.g. Figure 1.6).