The Field Description of Metamorphic Rocks. Dougal Jerram

Chapter 3 provides a more detailed and systematic overview of how rock texture and mineralogy change in rocks of various compositions as metamorphic grade increases.

      1.3.2 Subduction zone rocks

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).

      

      1.3.3 Contact metamorphic rocks

Igneous rocks can be emplaced into the crust at exceedingly high temperatures. Granites will crystallise at around 700+ °C, and basic rocks such as gabbro may intrude around 1200 °C, establishing a marked temperature gradient between the molten rocks and the host into which they intrude (commonly termed the ‘country rocks’). Along the contact zones between the igneous bodies and their host rock, metamorphic reactions are commonly driven by heat from the cooling magma. This leads to a group of rocks called the contact metamorphic rocks. The contact or ‘baked’ zone around the igneous body can contain a variety of different metamorphic grades that are typically only seen over a relatively short distance as the effects of the hot igneous body diminish rapidly with distance from the magma. This zone of contact metamorphism is called the ‘aureole’ and is typically meters to tens of meters in thickness. Pressure tends to have little effect in contact metamorphism, as it is the act of emplacing the hot igneous body and not a change in burial that makes the metamorphic aureole. Fluid flow during the metamorphism can substantially modify the wall rock composition, a process called metasomatism that is described more in Section 1.3.4, and can increase the footprint of the metamorphic effects by carrying heat further from the magmatic source (a process known as advection).

Figure 1.6 Highly folded metamorphic carbonate turbidites, Namibia

       (photo Dougal Jerram).

As with regional metamorphic rocks, different assemblages of minerals occur depending on the grade (mainly defined by the amount of heat) that the country rock reached, and depending on the type of country rock. With silliciclastic sediments like sandstones and shales the sequence may consist of chlorite, andalusite, and corderite hornfels, with silimanite and K‐feldspar at very high temperature, and garnet if the crust was at sufficient depth during intrusion (e.g. a contact metamorphic overprint in a regional metamorphic setting). In limestone host rocks, marble is commonly formed, with tremolite, diopside, wollastonite, and forsterite as common minerals if the original carbonate was ‘impure’ (e.g. contained some Si). A schematic contact aureole with some examples is given in Figure 1.7 (further detail can be found in Chapter 6).

Figure 1.7 Contact metamorphism. (a) schematic of contacts around a granite body. (b) Examples of Andalusite (chiastolite form with graphite intergrowths) and cordierite hornfels from the Lake District, UK

       (photo Dougal Jerram).

      A major difference between contact metamorphic rocks and the regional metamorphism discussed here is that contact metamorphism is generally quite static, with far less deformation during mineral growth. This means that the newly formed minerals are not typically as strongly aligned as they are in regional metamorphism, and an irregular orientation of fine grained minerals is typical of a ‘hornfels’, a classically diagnostic rock of relatively high temperatures of contact metamorphism.

      1.3.4 Hydrothermal metamorphic rocks

The third major set of metamorphic rocks are formed through hydrothermal circulation of fluids. In hydrothermal alteration/metamorphism, the host