Fissure Metasomatism in Volcanic Rocks Adjacent to the Shap Granite, Westmorland

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Abstract
Summary: Localizod alterations in wall-rocks of a pre-granite joint system, and other fissures, in thermally metamorphosed andesites, rhyolites and tuffs have resulted in either epidotization or sericitization. Epidotization is often associated with the development of Ca-Fe garnets within the fissure and, more rarely, in the wall-rock. Later minerals, such as quartz, calcite, magnetite, haematite and metallic sulphides, occur filling cracks in the garnet in the centro of these garnet-bearing veins. The formation of epidote is attributed to silicification of the wall-rock followed by the introduction of Ca and OH from the granite. Al, Fe, Mg and some Ca migrated toward the fissure, where under higher temperatures in conditions of iron enrichment Ca-Fe garnets formed. Subsequently, movement of the granite mass cause deformation of the country rocks and also opened the garnet-bearing veins to fluids which precipitated to suite of hydrothermal minerals. Sericitization is associted with the formation of cordierite and is restricted to rocks adjacent to pyrite veins. Al, K, S and OH ions appear to have diffused into the wall-rocks, causing this alteration.

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