The Field Description of Metamorphic Rocks. Dougal Jerram

As metamorphic rocks can be formed from any original rock (the parent rock henceforth being called the protolith), an ability to identify and be familiar with the wide variety of minerals and textures of sedimentary and igneous rocks is a general requirement for any budding metamorphic geologist. Additionally, as the very processes involved in metamorphism are commonly associated with deformation, a keen understanding of structural geology and tectonics is also needed. In many ways, the metamorphic scientist needs to be a jack of all trades and a master of one!

Due to the potential complexity within metamorphic rocks, the importance of careful fieldwork cannot be overstated. The different types of observation that can be made at various scales in metamorphic terrains allow the student/researcher to build up a list of clues, like in a forensic study, which can be used to help derive the type of metamorphic rock, its protolith, and the range of processes that it has undergone to reach its present state. The map‐scale distribution of metamorphic rocks can reveal the processes that formed them, but as we discuss in the following chapters, the correct interpretation of even the smallest parts of a field area are rooted in good field observations. This book aims to help build you skills in this area! Careful identification of rocks and structures is all the more important when taking samples from the field back to the laboratory for further study and analysis. The record of structures within and around the rock mass may ultimately help you to better interpret features you subsequently see down the microscope or the data that you receive from laboratory analysis.

Figure 1.1 Schematic of the plate tectonic settings where metamorphism is occurring around the world (see also Figure 1.2).

      Describable features which can be observed in metamorphic rock masses include:

      1 Pre‐metamorphic – e.g. bedding and other sedimentary features, contact relationships between batches of melt, or even fossils (though in most cases the features may be altered beyond normal recognition).

      2 Metamorphic – relating to local mineral changes due primarily to changing temperature and pressure.

      3 Metasomatic – involving the chemical transport and mineral change associated with fluids.

      4 Structural – relating to and recording the rock's deformation at any point in its history.

      Limitations exist as to how much information one can record regarding any of these features without the need for microscopic and chemical measurements, which is the realm of specialist study that will be touched upon within this book but is not our major theme. With good field observations of mineralogy, texture, and structure, one should still be able to adequately describe the rock masses in terms of their types and occurrence, hopefully also being able to build up an inference of the evolving conditions of their formation. Such description is particularly appropriate for the production of geological maps, logs, and recordings of outcrop structures, which will be covered in more detail in Chapter 2.

      This book forms a companion to the other texts in the geological field guide series, e.g. The Field Description of Igneous Rocks, Sedimentary Rocks in the Field, and The Mapping of Geological Structures, and as such does not cover in detail the pre‐metamorphic features of sediments and igneous bodies that may sometimes be preserved in metamorphic rocks. We do, however, show many examples of these in cases where they can either be shown to help in the identification of the protolith rock or reveal something fundamental about the metamorphism itself (e.g. that it happened in the presence or absence of deformation). There is substantial overlap between the skills required to be a metamorphic geologist in the field and those considered to be the realm of a structural geology, at least in terms of fieldwork measurements/observations, and particularly when mapping in metamorphic terrains. As such, this text will aim to provide as much help in terms of structural description, as will be necessary to get the most out of your metamorphic rocks. The reader will need to make an assessment as to what level of understanding of sedimentary, igneous, and structural geology might be best suited for the problem at hand, and where needed can supplement this guide with an appropriate partner guide. For example, if you are mapping a metamorphically altered igneous region, then additional help from The Field Description of Igneous Rocks may be useful. In a thrust zone, the structural guide may provide some vital additional assistance, and so on. However, we have tried, wherever possible, for this book to be a stand‐alone guide to achieve success in the field description of metamorphic rocks. Ultimately, we aim for this handbook to provide the required information on how to observe metamorphic rocks in the field, from the outcrop to the hand specimen scale, and to tie these observations into basic interpretations of how the metamorphic rocks formed. This also necessitates comments on sampling strategies for projects in which fieldwork is the start of a wide‐reaching study. As such, before we take on metamorphic rocks in the field it is useful to consider how metamorphism relates to regional and global tectonics and the main occurrence of metamorphic rocks.

1.2 Understanding Metamorphism; Pressure/Temperature Relationships

      Rocks undergo metamorphic and metasomatic changes as they are subjected to different pressure and temperature conditions, or are infiltrated by chemically reactive fluids. Indeed, a fundamental building block to a deeper understanding of metamorphism is a good grasp of pressure, temperature, and time (it takes time for metamorphic reactions to take place, evidence of which may be preserved in the field in the form of incomplete reactions). In this sense, it is very useful from the onset of your training as a metamorphic Earth scientist to become familiar with the ranges of pressure and temperature experienced in the Earth and the key metamorphic mineral associations (assemblages) that are found within these ranges. One of the main ways in which we consider this is through what is known as a P/T diagram, in which changing aspects of a rock are plotted as a function of pressure (P) and temperature (T). This allows one to highlight various aspects of metamorphism and question how they might be represented in the field. P/T diagrams will appear throughout this text to help understand the types and styles of metamorphism, and will feature specifically in Chapter 3 in relation to the main classification of metamorphic rocks, and in associated tables within the reference Chapter 8.

      At this introductory stage it is useful to consider the basic P/T diagram in relation to the relative intensity of metamorphism, as this forms a good basis for understanding under what conditions the different types of metamorphic rocks are formed. Figure 1.2 shows a P/T diagram (with approximate depths included) that expands on the key ‘facies’ concept (originally described by Pentti Eskola in 1915), namely that rocks of a similar composition will, when subjected to the same P/T conditions, form the same mineral assemblages. You can also see how this relates to the main tectonic settings by referring the numbers on the trends to the locations on Figure 1.1. The fields in Figure 1.2 thus map out the P/T stabilities of major mineral assemblages that could form in a metamorphosed mafic rock (e.g. a basalt) as a general reference. A far more detailed and subtle record of mineral reactions almost certainly occurs in most rocks and will be discussed in subsequent chapters, but the reactions at the boundaries of these fields are significant enough that the metamorphic facies (and thus approximate metamorphic P/T conditions) of a mafic rock can generally be identified in the field. Generally speaking, Figure 1.2 suggests that low grade metamorphism starts around 150–200 °C and ~3 kbar (300 MPa, or ~10 km depth). As temperature and