The Field Description of Metamorphic Rocks. Dougal Jerram

circulate seawater and initiate mineral reaction. Depending on the temperature of the crust during this circulation and on the availability of water, the result can vary from almost pristine basalt with a little carbonate veining, through to highly ‘serpentinised’ rocks in which primary olivine in a mantle rock is thoroughly replaced by serpentine group minerals (see Figure 1.8).

      The metamorphic rocks exhibit considerable chemical changes that are often termed ‘metasomatic’, with the loss of calcium and silica, and the relative gain of magnesium and sodium. In modern settings the occurrence of ‘black smokers’ and ‘white smokers’ on the sea floor are direct evidence of the hydrothermal cells in action. In the rock record, examples of obducted oceanic crust in the form of ophiolites display this hydrothermal metamorphism and they can also be associated with rich economic metal sulphide mineralisation.

Figure 1.8 Serpentinised ocean crust from the Troodos Ophiolite, Cyprus. A fibrous serpentine vein is visible in the centre of the photo

       (photo Dougal Jerram).

1.4 Summary

As we have introduced, a number of key settings exist that lead to the wide variety of metamorphic rocks and metamorphic associations that we find in the field (summarised in Figure 1.9; see also Figure 1.1). This field descriptions book is organised and laid out such that it introduces metamorphic rocks in terms of how they may be recorded in the field, making comments about sample collection and how work might be followed up in the lab and through additional studies (e.g. under the microscope). It is important to understand the various classification schemes and the different grades of metamorphism that can affect the rocks, but it is also equally important to be aware of the many difficulties often faced in identifying metamorphic rock textures and assemblages. Chapters 2 and 3 provide advice and information on some of the key field skills and metamorphic outcrop description (Chapter 2), as well the main classification criteria (Chapter 3). Chapters 4 and 5 focus in more detail at key structures in metamorphic rocks such as cleavage, shistosity, and isolated bodies such as boudains, augen, and sheared entities. Chapter 6 looks at contacts with igneous rocks, veins, and reaction zones. Chapter 7 looks at the structural control of faults and fault zones in metamorphic rocks and shear zones including shear sense indicators. In Chapter 8, there are key tables and details for reference by way of a summary, as well as some advice for those who are undertaking a more detailed mapping dissertation or project.

Figure 1.9 Summary of the two main types of tectonic settings with an outline of the metamorphic grades

       (Source: partly redrawn from Hefferan and O'Brien (2010)).

       The metamorphic grades: Very Low Grade VLG, Low Grade LG, Medium Grade MG/MG2, High Grade HG/HG2 and Very High Grade VHG, are indicated on panels (a) and (b) (also refer to Table 3.1 and Chapters 3 and 8)

       (Source: Rock picture inserts from the figures in this chapter).