18O-depleted igneous rocks from the Tertiary complex of the Isle of Mull, Scotland

New oxygen isotope analyses have been obtained on 31 rocks and minerals from the Tertiary ring-dike complex on the Isle of Mull, Scotland. Whole-rock δ ¹⁸O values (SMOW) of basalts range from +9.1 to -6.5, and systematically decrease toward the central plutonic complex. The δ ¹⁸O values of the intrusive igneous rocks systematically increase with successively younger ages of emplacement, presumably because the meteoric-hydrothermal systems decreased in intensity with time, as a result of waning igneous activity and mineral deposition within the conduit fractures. Whole-rock δ ¹⁸O values of intrusions around the Beinn Chaisgidle center show a narrow range from -3.9 to -1.5, whereas those about the later Loch Ba center can be divided into two groups, the earliest with δ ¹⁸O = -3.3 to +0.5, and the latest with δ ¹⁸O = +1.0to+3.4. All of the rocks within the "limit of pneumatolysis", a zone of hydrothermal contact metamorphism that lies 4-6 km outward from the central intrusions, are depleted in ¹⁸O, some by as much as 13‰. These effects are observed over an area of 500 km ², and were produced by meteoric-hydrothermal convective circulation of early Tertiary ground waters, which apparently had initial δ ¹⁸O values of about -11 to -12.