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Metamorphosed lava flows, and their related minor intrusions and volcanogenic sediments, are frequently found interspersed with metamorphosed sedimentary rocks. Larger intrusive bodies are sometimes also metamorphosed, especially where old crystalline crust is caught up in a later orogeny and remobilised (subjected to later metamorphism and/or deformation). This chapter will outline the metamorphic changes that take place in igneous rocks of basic composition, such as basalts and gabbros, to produce metabasites. Metabasites differ from the metapelites described in Chapter 4 in that their mineral assemblages tend to vary progressively, often without distinct zones marked by index minerals. Despite this we shall see that their assemblages continue to vary with metamorphic conditions over a very wide range of pressures and temperatures. In particular, amphiboles are stable in metabasites over a very wide range of conditions, and we shall see how the composition of the amphibole varies considerably with pressure and temperature.
Deformation of some sort often accompanies metamorphism and, when it does, it is possible to learn more about the metamorphic processes by also considering the accompanying deformation. Detailed analysis of the deformation of metamorphic rocks is beyond the scope of this book, but further reading is suggested at the end of the chapter. Here, we will look at how rocks can deform during metamorphism in general terms, and then focus on three aspects of the relationships between metamorphism and deformation.
Much of the material in this book so far has skirted around the problem of how quickly metamorphic rocks form and how long metamorphism lasts. Regional metamorphism can be thought of as taking place in a metamorphic cycle, involving burial, heating, exhumation and cooling. The question of ‘how long’ a metamorphic cycle takes from start to finish may be determined either directly or indirectly. Indirect methods involve calculations of how long it takes for rocks to heat up, cool down, or be buried or exhumed. Based on the thermal properties of rocks, indirect approaches have been used for many years to calculate the rates of heating and cooling associated with igneous intrusions.
Metamorphic rocks derived from carbonate-rich sediments, such as limestones and marls, also reflect the temperatures and pressures of metamorphism like metapelites and metabasites, but there is an additional factor that influences their mineralogy. Carbonate minerals release carbon dioxide during metamorphism and the mineral assemblages that form are influenced by the balance between water and carbon dioxide in the metamorphic fluid. In this chapter we will investigate the way in which minerals and fluids interact as limestones undergo metamorphism.
In the previous chapters, the emphasis has been on the attainment of chemical equilibrium in metamorphism, because it is only by identifying assemblages of minerals that have co-existed together in equilibrium that the pressures and temperatures of their formation can be determined. Equilibrium studies alone tell us only about the P–T conditions prevailing when a particular assemblage formed, they cannot tell us anything about the rock’s history before or after the assemblage grew.
A metamorphic rock is one that has been changed from its original igneous or sedimentary form: it has grown new minerals in response to new physical or chemical conditions. A wide variety of processes can cause changes to the mineralogical composition of rocks, including heating, burial, deformation, fluid infiltration or shocks caused by meteorites hitting the Earth’s surface. Most of these processes, and thus the formation of the vast majority of metamorphic rocks on Earth, take place near tectonic plate margins. As a result, metamorphic rocks provide us with a record of the ambient crustal conditions as rocks get buried, deformed, transformed into new varieties and then transported back up to the surface by a combination of tectonic and surface processes.
Some of the most important outcomes that emerge from the study of metamorphic rocks are the insights they provide into the past thermal structure and tectonic behaviour of the Earth. In order for metamorphic rocks to form, their protoliths must have become buried and heated. In order for us to be able to study them at the surface today, they must have been brought back to the surface in such a way that they preserve the mineralogical evidence for their history. Together, the evidence for these events document how the Earth’s crust has operated at different periods of geological time.
The fundamental assumption underlying most studies of metamorphic rocks, including the examples described in Chapter 1, is that their mineral assemblages reflect the physical conditions of pressure and temperature which prevailed at the time when they grew. This assumption is grounded in numerous field studies that have shown that the mineral assemblages in any particular rock type vary systematically and predictably across an area. Additionally, rocks of the same composition always appear to develop the same mineral assemblage when they are subject to the same metamorphic conditions. These observations are the basis for applying the theory of chemical equilibrium to metamorphic rocks. This chapter outlines the information that can be gleaned by treating rocks as equilibrium chemical systems along with the limitations and pitfalls of so doing.