The Pen-y-gader dolerite sill, in the Cader Idris area south of Dolgelly, northwest Wales, was intruded into Ordovician strata. Thermal metamorphism of the strata followed by latestage metasomatism produced spilosites and adinoles, with the introduction of Na, B, and F into the sediments. The petrochemistry of the dolerite, roof sediments, and floor sediments, including 21 chemical analyses, is discussed.

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