Black Phyllites of the Greywacke Zone

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These intensely schistose rocks originated from dark, clayey sediments of the deep sea. The dark, sometimes reddish color is due to often-present organic components and iron compounds. However, none of the original clay minerals remain. The current mineral composition is characterized by metamorphism (rock transformation) during a mountain-building process under extreme conditions (temperature ≈ 350°C - 550°C, pressure ≈ 200 - 1000 MPa), resulting in mica, quartz, sometimes feldspar, chlorite, and iron oxide. The conspicuous reddish-brown spots are referred to as "Brandenzonen" or "Brandenschiefer," an old miner‘s term for rust-brown weathering ore horizons.

Conceptual sketches of the formation.
1. Alternation of dark clays with sandstones and/or volcanics: Deposition in an ocean basin with coastal influence, concurrently with active volcanism.
2. Formation of an initial foliation due to subsidence. From temperatures of about 300°C (at a depth of approximately 10 km), the mobilization of quartz and the formation of light mica start perpendicular to the direction of pressure.
3. Constriction during mountain-building processes and formation of folds perpendicular to the direction of constriction.
4. Further constriction and formation of a new foliation plane parallel to the axial plane.