Hydrothermal activity along strike-slip faults: cyclicity of movements and fluid flow

Fault zones serve as pathways for large quantities of hot fluids that may change rock composition and structure, generating ore deposits and geothermal fields. This typically leads to quartz precipitation reflecting cyclicity of deformation and fluid-flow. The dextral strike-slip Pfahl shear zone (Germany) represents a fault zone with frequent pulses of deformation and fluid-flow. Polarizing microscopy and cathodoluminescence analyses reveal several stages of fluid flow, quartz crystallization and fragmentation: (i) At least three early stages of silicification and kaolinization of granitoid basement rocks result in µm-sized quartz matrices with variable amounts of kaolinite. The matrices are transect by an isotropic network of µm-thin quartz veins and show quartz overgrowth. (ii) Fragmentation during shear-zone activity and fluid flow leads to mm-cm wide veins roughly parallel to the fault zone and filled with blocky 100-500 µm-sized quartz grains. (iii) Brittle deformation in a central fault-parallel zone causes a massive quartz dyke with complex patterns of fragmentation and quartz overgrowth. (iv) Continued dextral shearing produces a set of steep, N-S oriented and cm-dm spaced fractures that locally form mm-cm wide quartz veins with voids, indicating decreased fluid flow. (v) µm-thin quartz veins with overgrowth textures represent the final stage of fragmentation coupled with silicification. The Pfahl shear zone, characterized by brittle deformation during fluid flow, represents long-term activity of a large-scale hydrothermal system. It is an excellent example of cyclic stress and strain-rate variation, fluid flow and mineralization. Fig. 1: Overview photograph of the study area (Waschinger quarry 1 km W of Regen/Germany), with mainly massive white quartz in the center and brownish altered wall rocks on the right side

Shearing of magma along a high-grade shear zone: evolution of microstructures during the transition from magmatic to solid state flow

Syntectonic plutons may record short-lived geological events related to crustal melting and deformation of the continental crust. Therefore, interpretation of microstructure and orientation of fabrics is critical in order to constrain space/time/temperature/deformation relationships during pluton crystallization. Here we describe the transition from magmatic to solid-state deformation in the late-Variscan Diorite-Granite Suite (DGS) emplaced along the Santa Lucia Shear Zone. The systematic collection of meso-, microstructural and quartz axis data allow us to examine the spatial distribution and the mode of superposition of different fabrics. We identify three magmatic microfabric types, thought to reflect the microstructural evolution at decreasing melt content during pluton crystallization. Our data suggest that diffusion creep, dislocation creep and grain-scale fracturing cooperated in accommodating the shearing of the partially molten quartzofeldspathic aggregate. We suggest that the switch from upward to horizontal magmatic flow occurred at melt fractions gt; w0.40, and that most of the hypersolidus fabrics formed during horizontal flow, reflecting the stress field imposed by the shear zone, and preserving no memory of the ascent stage

Shearing of magma along a high-grade shear zone: Evolution of microstructures during the transition from magmatic to solid-state flow

Syntectonic plutons may record short-lived geological events related to crustal melting and deformation of the continental crust. Therefore, interpretation of microstructure and orientation of fabrics is critical in order to constrain space/time/temperature/deformation relationships during pluton crystallization. Here we describe the transition from magmatic to solid-state deformation in the late-Variscan Diorite-Granite Suite (DGS) emplaced along the Santa Lucia Shear Zone. The systematic collection of meso-, microstructural and quartz < c > axis data allow us to examine the spatial distribution and the mode of superposition of different fabrics. We identify three magmatic microfabric types, thought to reflect the microstructural evolution at decreasing melt content during pluton crystallization. Our data suggest that diffusion creep, dislocation creep and grain-scale fracturing cooperated in accommodating the shearing of the partially molten quartzofeldspathic aggregate. We suggest that the switch from upward to horizontal magmatic flow occurred at melt fractions ~0.40, and that most of the hypersolidus fabrics formed during horizontal flow, reflecting the stress field imposed by the shear zone, and preserving no memory of the ascent stage