Fluid inclusion planes in selected granitic rocks of the British Isles
When hydrothermal fluids flow through microfractures in quartz, they effectively heal (anneal) them to produce Fluid Inclusion Planes (FIPs). These FIPs are viewed as a three dimensional plane or array of secondary fluid inclusions and may be used to relate brittle deformation associated with tectonic, thermal and hydraulic stresses to contemporaneous hydrothermal events. In this study, FIPs have been interpreted in granitic quartz from various geological settings. Analysis of the FIPs includes recording their orientation, the morphological characteristics of both the FIPs and the inclusions contained within them, together with the thermometric properties of the inclusions. Results show that FIPs may possess a preferential orientation over a wide area, similar to the trend of macrostructures observed in the field. However, local deviations in the stress field, related to the proximity of possible second order faults, may cause local deviations in the orientation of the FIPs. The presence of high FIP abundances has also been linked to the widespread, localised development of kaolinisation (possibly linked to tectonic stresses) and greisenisation (possibly linked to thermal and hydraulic stresses). These phenomena have been identified at several localities in the field areas. It is presumed that intense microfracture networks will allow the wide-spread movement of hydrothermal fluids allowing pervasive alteration. A classification scheme for FIPs has been devised, whereby FIPs may been classified as tensile, dilatant (mode 1) and dilatant shear (mode 2) fractures. This classification is based upon the orientation of the FIPs (to macro structures), their proposed origin and their morphology (both the FIP and the inclusions contained within them). Variation in inclusion morphology has been recorded within selected FIPs. Inclusions have been observed to occur parallel and perpendicular (rare) to the length of the FIP. The presence of a preferred shape orientation in the inclusions has been attributed to a combination of fracturing and subsequent healing process, with controlling parameters including the temperature and chemistry of the fluids (affecting crack lifetimes) together with the geometry of the fracture network.