The Field Description of Metamorphic Rocks. Dougal Jerram

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photo Mark Caddick).

Photo depicts (a) Blueschist facies, Syros, Greece (Mark Caddick for scale) with inset figure highlighting lawsonite porphyroblasts, (b) Eclogite facies, Alps.

       (photo a Mark Caddick, photo b Hans Jørgen).

      

      1.3.3 Contact metamorphic rocks

Photo depicts highly folded metamorphic carbonate turbidites, Namibia.

       (photo Dougal Jerram).

Photo depicts contact metamorphism.

       (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.

      1.3.4 Hydrothermal metamorphic rocks

      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.

Photo depicts serpentinised ocean crust from the Troodos Ophiolite, Cyprus.

       (photo Dougal Jerram).

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