Metamorphism and deformation of slate

After the deposition of the sediments, compaction and diagenesis of this water-rich mud take place. This processes transform the mud into a solid mudstone: the porosity will be dramatically decreased and the water will be squeezed out. The diagenesis is related to a rearrangement of minerals especially of clay minerals which turn into phyllosilicates (mica). The transition from diagenesis to the metamorphosis is hardly to fix because the different processes are almost not distinguishable (Yardley, 1997).

Roofing slates form at temperatures between 390-470 °F (200-300°C) and pressure conditions of about 2-5 kbar and thus are developed during a low-grade metamorphism (Winkler, 1976). The foregoing rearrangement of minerals during the diagenesis is followed by folding during a mountain building process. The results are a stronger rearrangement and planar orientation of minerals, leading to a fracture cleavage. Depending on the mineralogical composition and the degree of deformation the nature of a fracture cleavage can be very different.

A very low deformation such as in the Rheinic slates can result in a very irregularly developed fracture cleavage. In contrast, the roofing slates of Thuringia and most of Galicia show a well developed fracture cleavage with a strong alignment of the mica layers.

Pictures

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  Fig. 1.: A second or further events of deformation, resp., can overprint the fracture cleavage S₁ of the first deformation. A result can be a crenulation foliation S₂ which can more or less fold the first cleavage S₁. This S₂
  can lead to a wrinkled cleavage surface and lower the bending strength, depending on the intensity of the deformation.

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  Fig. 2.: Pyrite with small fibres on the edges.

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  Fig. 3.: Pressure solution due to compentence contrasts, leading here to strain caps related with a local increase of the mica layers.

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  Fig. 4.: Different ore minerals with strain fringes as a result of simple shear deformation.

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  Fig. 5.: This picture show a pyrite framboid with fibrous strain fringes. The shape of this strain fringes reveal simple shear with a dextral shear sense (detail from Fig.4).

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  Fig. 6.: Ore mineral with strain fringes (detail from Fig.4).

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  Fig.7.: strain marker: one can see the strain caps above and below
  the marker as well es the strain shadows left and right of it.

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  Fig. 8.: Chlorid porphyroblast as strain marker: right and left of it
  one can see the non-fibrous strain shadows which are another
  type of strain shadow (comp. Fig. 5)