The geology of the Cairnsmore of Fleet granite and its environs, Southwest Scotland

Abstract

The thesis is divided into three parts relating to (i) the sedimentary rocks, (ii) the granite and minor intrusive rocks and (iii) the Fleet orefield. The Sedimentary successions around the Cairnsmore of Fleet pluton are divided into two new formations; the Craignell and Knockeans Formations. The former is sub-divided into three facies based upon a geochemical classification of greywackes undertaken using cluster analysis. Facies boundaries within the Craignell Formation and interformational boundaries between this formation and the underlying Moffat Shales and the younger Knockeans Formation are all probably diachronous. Tectonically derived structures within the sedimentary rocks are correlated with those reported from areas elsewhere within the Southern Uplands. Certain structures are, however, unique to this area and some may be related to the emplacement of the Fleet pluton. Mineralogical zones within the extensive thermal aureole surrounding, the granite are described and correlated with previously published facies of contact metamorphism: albite-epidote hornfels, hornblende hornfels and K-feldspar-cordierite hornfels facies. Petrological facies within the granite are described and the classification thus produced endorsed geochemically with the aid of cluster analysis. Chemical trends in separated minerals, are correlated with bulk chemical variation in the granite which is outlined using correlation, regression and trend surface analyses. Geo-chemical and petrological data are compared with published experimental mineralogical studies. Minor intrusive rocks are similar in composition to those occurring throughout Galloway with two notable exceptions of more basic composition. The chemistry of the minor of the Fleet pluton are described the western margin to the Fleet pluton are described in detail for the first time. A distinct mineralogical zoning pattern is spatially related to the granite and consists of an inner zone within the pluton and generally close to the contact in the country rocks, containing dominant chalco-pyrite, pyrrhotite, pyrite and high temperature ores such as pentlandite and arsenopyrite with a quartzose gangue. Outer zones contain veins of sphalerite and mixed carbonate and quartz gangue, and give way to an outermost zone of galena with carbonate and barytes gangue. Geochemical studies on separated ore minerals has enabled the distinction of ores from particular zones and give an indication of their relative temperatures of formation. Wallrock alteration is described and is generally consistent in its characteristics with the changing mineralogy of the vein deposits. The ore deposits are related to the geophysically predicted granite batholith beneath the Southern Uplands, but more specifically to the Fleet pluton. The mineralogy and geochemistry of the deposits are consistent with this, but geochronological studies from other areas suggest that the ores may be at least 40 my younger than the granite. A hypothesis for the origin of the hydrothermal deposits is presented and is based upon data outlined above.