Repeated hydrothermal quartz crystallization and cataclasis in the Bavarian Pfahl shear zone (Germany)

•We analyzed meso-to microstructures of hydrothermal quartz by optical microscopy and hot-cathode cathodoluminescence microscopy.•The complexity of these structures is a result of repeated processes such as fragmentation, fluid influx and silicification.•Fragmentation was partly accompanied by flow of suspensions of cataclastically deformed quartz (fluidized cataclasite).

Field and microstructural data of the Pfahl shear zone in north-eastern Bavaria (Germany) reveal the intimate spatial-temporal connection between fragmentation, fluid influx and quartz crystallization. These processes and their interaction led to complex-structured quartz units: (i) a dense network of early quartz veins, (ii) two domains of fine-grained reddish to grayish quartz masses, (iii) an extended central zone of massive white quartz, and (iv) late cross-cutting closely spaced parallel fractures and partly open quartz veins.The fine-grained quartz domains result from repeated and coeval cataclasis, fluidization and quartz precipitation. Material transport in these domains is at least partly governed by the flow of mobile fluid-quartz-particle suspensions. The complex internal meso-to microstructures of the massive white quartz are generated by repeated processes of fragmentation and grain growth. In general, the brittle part of the Pfahl shear zone represents a key example of cyclic dissolution/precipitation and fragmentation on large scale.