The Field Description of Metamorphic Rocks. Dougal Jerram

Чтение книги онлайн.

СКАЧАТЬ 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 pressure increase, the grade of metamorphism progressively increases accordingly until, at temperatures of 600–800 °C (or greater), the rocks themselves begin to melt and we start to enter the realm of igneous petrogenesis. These fields and the main ways in which we classify metamorphic rocks will be discussed in detail in Chapter 3, and as you go along you will see that the P/T of the rocks can be displayed in a variety of diagrammatic forms.

1.3 Mode of Occurrence of Metamorphic Bodies

Because metamorphism is a response of pre‐existing rocks to changes in temperature and pressure, it may be expected that metamorphism is restricted to major zones of deformation in the Earth, such as convergent (destructive) tectonic plate margins. Clearly where major tectonic forces act, such as at subduction/collision zones, the crust undergoes deformation, and rocks will experience changing pressure and temperature upon burial as the crust is thickened. However, metamorphism is not restricted to these environments of the Earth. Extreme temperature changes can be achieved through the contact of molten igneous bodies (sills, dykes, magmas chambers) with country rocks. Also, in certain settings, the wholesale circulation of fluids through the crust can lead to alteration and metamorphism (such as at mid‐ocean ridges). Rocks that are metamorphosed in subduction/collision zones undergo metamorphic changes over broad zones, and can record evidence of passage from one metamorphic grade to another as they journey through different depths. These form the most common types of metamorphic rocks, termed the Regional Metamorphic Rocks. Where rocks are metamorphosed due to contact with hot igneous bodies they are referred to as Contact Metamorphic Rocks. Finally, where alteration and metamorphism occur due to fluids, the rocks are called Hydrothermally Altered Metamorphic Rocks. Some more exotic and rare examples of metamorphic rocks include those specific to fault zones (Cataclastic Metamorphism) occurring as a result of mechanical deformation when two bodies of rock move past one another, and Shock Metamorphism (Impact Metamorphism), where rocks are metamorphosed due to impact from an extraterrestrial body, such as a meteorite or comet.

Figure 1.2 The P/T diagram: (a) the classic fields of metamorphism of mafic rocks (the so‐called matamorphic facies) in P/T space, and (b) the routes that certain tectonic systems take through the P/T space which give rise to different metamorphic rocks. This will be expanded on in more detail in Chapter 3.

СКАЧАТЬ