Alteration of Ballantrae lavas, Ayrshire

Abstract

The Pinbain Block of the Ballantrae Complex has suffered a very low grade metamorphism, with the formation of albite-chlorite-sphene mineral assemblages. The base of the succession also has prehnite and pumpellyite, indicating the prehnite-pumpellyite fades, while the top is in the zeolite fades, and contains analcime. The temperature of the transition, based on mineral stabilities, is <190°C. The metamorphism affected the sedimentary cover of the Ballantrae Complex, and it occurred without penetrative deformation, therefore it was a burial metamorphism. Stable isotope studies show that the rocks were altered by large quantities of sea water, at a temperature of 150-170°C. This alteration resulted in the mobilisation of many of the chemical elements in the Pinbain Rocks, in a manner similar to that seen in experiments in which basalts are altered by sea water. Titanium, zirconium and yttrium remained immobile, and these elements can be used to identify the rocks as within-plate basalts. Fluid inclusion studies show that some of the veins in the Pinbain Block were formed by a high-temperature hydrothermal fluid, which probably formed the ascending leg of a hydrothermal cell, which was replenished by downwards percolation of sea water. The metamorphism probably occurred at the time of deepest burial, at the end of the Silurian, when Caledonian orogenesis may have raised the geothermal gradient and triggered the hydrothermal circulation.