Читать книгу The Field Description of Metamorphic Rocks - Dougal Jerram - Страница 19

The third major set of metamorphic rocks are formed through hydrothermal circulation of fluids. In hydrothermal alteration/metamorphism, the host rocks can be involved in wholesale chemical changes upon interaction with chemically reactive fluids. There exist large parts of the Earth's oceanic crust and upper mantle that are almost entirely made of hydrothermally metamorphosed rocks, formed as oceanic rocks interacts with large hydrothermal cells that circulate seawater and initiate mineral reaction. Depending on the temperature of the crust during this circulation and on the availability of water, the result can vary from almost pristine basalt with a little carbonate veining, through to highly ‘serpentinised’ rocks in which primary olivine in a mantle rock is thoroughly replaced by serpentine group minerals (see Figure 1.8).

The metamorphic rocks exhibit considerable chemical changes that are often termed ‘metasomatic’, with the loss of calcium and silica, and the relative gain of magnesium and sodium. In modern settings the occurrence of ‘black smokers’ and ‘white smokers’ on the sea floor are direct evidence of the hydrothermal cells in action. In the rock record, examples of obducted oceanic crust in the form of ophiolites display this hydrothermal metamorphism and they can also be associated with rich economic metal sulphide mineralisation.

Figure 1.8 Serpentinised ocean crust from the Troodos Ophiolite, Cyprus. A fibrous serpentine vein is visible in the centre of the photo

(photo Dougal Jerram).