The Field Description of Metamorphic Rocks. Dougal Jerram
Чтение книги онлайн.
Читать онлайн книгу The Field Description of Metamorphic Rocks - Dougal Jerram страница 7
Название: The Field Description of Metamorphic Rocks
Автор: Dougal Jerram
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781118618677
isbn:
Dougal is particularly grateful to the people who showed me some of the classic metamorphic terrains, intrusive contacts, and regional geology, where I learned much about these systems. Wes Gibbons, Dave Prior, John Wheeler, Lee Mangan, Bob Hunter, Mike Cheadle, and Henry Emeleus introduced me at the early stages of my career to some of the classic Scottish locations, and more recently the likes of John Schumacher and Torgeir Andersen introduced me to some of the more exotic and incredible metamorphic textures I have seen. My colleagues in Oslo such as Trond Torsvik, Henrik Svensen, Sverre Planke, Olivier Galland, Bjørn Jamtveit, François Renard, Stephanie Werner, Karen Mair, Brit Lisa Skjelkvåle, Bernd Etzelmüller, Carmen Gaina, and the whole of the CEED team over the last 10 years have shown great support, particularly to my book writing efforts and research collaborations, and further afield my “brother” Breno Waichel is thanked for exposing me to the South Atlantic Margins and to many Brazilian colleagues. Jo Garland and Izzy Jerram are thanked for their ongoing support, and particularly Jo for proof reading and figure commenting at various stages. Finally, I would like to personally thank all those that have helped in discussions in the field all over the world where complex hard rock relationships have been made clearer by great collaborations (you are soooo many, and you know who you are, cheers!).
Mark would like to thank Alan, Mike, Nigel, Tim, Jon, and the other great mentors he has met along the way. He only knows about many of the outcrops photographed in this book thanks to the generosity of friends and colleagues such as Eric Reusser, John Schumacher, Filippo Schenker, and Bob Tracy. The current and former members of the Metamorphic Processes group at Virginia Tech, and the students of VT’s GEOS 2024, 3704, and 4964, have always provided the best reasons to go back out and teach in the field, and I’m particularly grateful to those of you whose fingers, arms, and feet crept into some of the photos in this book – you know who you are! Thanks to Christiana Hoff for commenting on earlier versions of some chapters. Finally, thanks to my wife, Kristie, who read parts of the text, commented on many of the figures, and had the good grace to remain patient with me throughout this whole process.
1 INTRODUCTION AND OCCURRENCE
A classic old metamorphic map, the 1833 map of the geology of Massachusetts, from maps associated with Edward Hitchcock's ‘Report on the Geology, Mineralogy, Botany, and Zoology of Massachusetts’ (Amherst, Mass.: Press of J. S. and C. Adams, 1833).
Metamorphic rocks form a substantial proportion of the material that makes up the Earth's crust, and metamorphic processes have been almost continually occurring throughout geological time since the origin of that crust. Metamorphism can be defined simply as the process by which sedimentary or igneous rocks are transformed (metamorphosed) by re‐crystallisation due to changes in pressure, temperature, or fluid conditions. To complicate matters somewhat, metamorphism can of course also act on rocks that have already been metamorphosed previously, building layer upon layer of complexity into those rocks that record field evidence of some of Earth's most dynamic processes. Our understanding of metamorphism is somewhat limited by the fact that we are unable to directly observe it happening to the rocks. As you read this, metamorphism is in action all around the planet, in all aspects of the Earth's plate tectonic system (e.g. Figure 1.1), but we cannot directly see it (generally because it happens at depth and very slowly). In order to understand the processes and products of metamorphism and alteration in rocks, detailed fieldwork, petrography, experimental studies, and numerical modelling are required. It is important to note, however, that the very origin of metamorphic petrology (the science of understanding the distribution, structure, and origin of metamorphic rocks) is rooted in a tradition of careful and systematic field observation, and that this remains an absolute cornerstone of the discipline today. Since the late nineteenth century, Earth scientists have strived to develop an understanding of metamorphic processes by identifying the different types of key minerals, mineral assemblages, and structures present in the metamorphic rocks. Using these observations and knowledge of some fundamental principles, mineral reactions can be calculated and/or experimentally derived to help explain and understand the process by which the original rock was metamorphosed into its current state. These rocks often encode evolving conditions at tectonic plate boundaries, so deciphering their mineralogical history may be thought of as a window into the crustal‐scale processes that form, modify, and stabilise Earth's crust. Underpinning all of this is the petrologist's ability to identify, describe, relate, and collect metamorphic rocks in the field, and it is these skills which this book aims to explore and impart, by its use in the field description of metamorphic rocks.
1.1 The Importance of Fieldwork in Metamorphic Terrains
In many ways, metamorphic geology requires you to be skilful in most aspects of the Earth sciences. 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 СКАЧАТЬ