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.
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