Textural and isotopic evidence on the fluid source and transport mechanism of antitaxial fibrous microstructures from the Alps and the Appalachians

Abstract

The source of fluid-forming veins is of great importance in order to understand the hydraulic system acting in the earth's crust. The study of syntectonic antitaxial veins is one of the few methods by which the opening history can be deduced from rocks, and thus these veins are of primary importance in determining rock kinematics. Antitaxial veins were taken from black shales in two different tectonic settings in the Helvetic Alps, Switzerland, and the Taconic Appalachians, New York State. These syntectonic extension veins are regularly spaced and are oriented sub-normal to bedding. The vein microstructure displays a symmetry around the median line in the centre of the vein, and a symmetry in cathodoluminescence banding parallel to the vein-wall interface, which suggests transport along bedding-parallel dissolution planes from both vein-walls. Antitaxial veins nucleated in transgranular fractures, but evidence for ongoing multiple crack-seal increments is lacking; rather, veins grew continuously keeping close contact to the vein-wall. Radiogenic 87 Sr/ 86 Sr ratios are higher in the surrounding matrix than in the vein, and higher than the corresponding seawater data in all samples. Variations are small and calcite in both the vein and the host rock were derived from the same source of fluid in the Helvetic samples. Mass balance of Sr suggests that the amount of calcite is too small in the surrounding host rock to be derived locally. Stable oxygen compositions are heavier in the host rock than in the veins, with overall low variation in both d 18 O and d 13 C values in the Mesozoic Helvetic samples. Data point to a rock-buffered system, the precipitate most likely derived from an external source. The lower Palaeozoic Appalachian veins have lesser d 18 O values than the host rock, similar to the Helvetic veins. Radiogenic 87 Sr/ 86 Sr data and a large heterogeneity in stable isotope values indicate an open system. Microstructural and isotopic evidence suggests that the antitaxial veins were formed by pervasive fluid flow, with the solute at least partly derived from an external source.

Genesis of Syntectonic Hydrothermal Veins in the Igneous Rock of Teschenite Association (Outer Western Carpathians, Czech Republic): Growth Mechanism and Origin of Fluids

“…The texture of younger V1 veins suggests the transport of fluid along an open fracture resulting in a blocky vein microstructure combined with a decrease of growth rate and increase of the grain size towards the center of the vein (Hilgers et al 2004;Hilgers & Sindern 2005). The observed orientation of veins corresponds to the major system of faults in this area that formed during the Late Miocene ( Fig.…”

“…The regular course of older composite V2 vein suggests that the vein formation was initiated by brittle fracturing of rock (Cosgrove 1993;Hilgers & Sindern 2005). The tension veins initially formed parallel to the maximum shortening direction which corresponds to the direction of the vein (Bons 2000).…”

“…A detailed description of internal vein microstructure refining the paleostress analysis of veins presents an important part of investigation of hydrothermal veins. Especially fibrous veins can record the opening trajectories of veins in greater detail (Hilgers & Sindern 2005).…”

The relevance of vein texture in understanding the past hydraulic behaviour of a crystalline rock mass: reconstruction of the palaeohydrology of the Mecsekalja Zone, south Hungary

“…advective, fracture-channelised-flow versus percolation through the host rock) have been brought into focus (Bons 2000;Hilgers & Urai 2002a). Hilgers & Sindern (2005) and Barker et al (2006) combined stable-isotope data with radiogenic isotopes (Sr 87 ⁄ Sr 86 ) and trace and rare earth elements to resolve the fluid source and flow path of antitaxial vein parent fluids. Rye & Bradbury (1988) found that vein calcites with the same fluid source but different textures display different isotope compositions.…”

“…Applications include assessment of fluid-rock interaction (Hilgers & Sindern 2005) and the evolution of mineralising fluids during fluid-rock interaction (Cox 2007). Bottomley & Veizer (1992) and Rye & Bradbury (1988) assessed recrystallisation of a pre-existing vein system and fluid-rock interaction during nappe stacking, respectively.…”