Читать книгу The Field Description of Metamorphic Rocks - Dougal Jerram - Страница 18

Igneous rocks can be emplaced into the crust at exceedingly high temperatures. Granites will crystallise at around 700+ °C, and basic rocks such as gabbro may intrude around 1200 °C, establishing a marked temperature gradient between the molten rocks and the host into which they intrude (commonly termed the ‘country rocks’). Along the contact zones between the igneous bodies and their host rock, metamorphic reactions are commonly driven by heat from the cooling magma. This leads to a group of rocks called the contact metamorphic rocks. The contact or ‘baked’ zone around the igneous body can contain a variety of different metamorphic grades that are typically only seen over a relatively short distance as the effects of the hot igneous body diminish rapidly with distance from the magma. This zone of contact metamorphism is called the ‘aureole’ and is typically meters to tens of meters in thickness. Pressure tends to have little effect in contact metamorphism, as it is the act of emplacing the hot igneous body and not a change in burial that makes the metamorphic aureole. Fluid flow during the metamorphism can substantially modify the wall rock composition, a process called metasomatism that is described more in Section 1.3.4, and can increase the footprint of the metamorphic effects by carrying heat further from the magmatic source (a process known as advection).

Figure 1.6 Highly folded metamorphic carbonate turbidites, Namibia

(photo Dougal Jerram).

As with regional metamorphic rocks, different assemblages of minerals occur depending on the grade (mainly defined by the amount of heat) that the country rock reached, and depending on the type of country rock. With silliciclastic sediments like sandstones and shales the sequence may consist of chlorite, andalusite, and corderite hornfels, with silimanite and K‐feldspar at very high temperature, and garnet if the crust was at sufficient depth during intrusion (e.g. a contact metamorphic overprint in a regional metamorphic setting). In limestone host rocks, marble is commonly formed, with tremolite, diopside, wollastonite, and forsterite as common minerals if the original carbonate was ‘impure’ (e.g. contained some Si). A schematic contact aureole with some examples is given in Figure 1.7 (further detail can be found in Chapter 6).

Figure 1.7 Contact metamorphism. (a) schematic of contacts around a granite body. (b) Examples of Andalusite (chiastolite form with graphite intergrowths) and cordierite hornfels from the Lake District, UK

(photo Dougal Jerram).

A major difference between contact metamorphic rocks and the regional metamorphism discussed here is that contact metamorphism is generally quite static, with far less deformation during mineral growth. This means that the newly formed minerals are not typically as strongly aligned as they are in regional metamorphism, and an irregular orientation of fine grained minerals is typical of a ‘hornfels’, a classically diagnostic rock of relatively high temperatures of contact metamorphism.