Abstract
Synopsis A Tertiary tholeiitic dyke intruded into sedimentary rocks of Lower Carboniferous age is locally rich in xenoliths of siliceous and argillaceous rocks possibly of Upper Old Red Sandstone age. The xenoliths and the country rocks have suffered thermal metamorphism with the production of glass, tridymite, cordierite, pyroxenes and biotite in the former and tridymite, cordierite, biotite and lime-garnet in the latter. From considerations of the stability fields of tridymite, melt-products and the solidus of dolerite, it is estimated that the PT conditions under which the presently exposed levels of the dyke crystallised were approximately 500 bars and 1,000° C, possibly representing a cover in Tertiary times some 6,200 ft (1,900 m) thick.
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