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

Abstract

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 δ18O values (SMOW) of basalts range from +9.1 to −6.5, and systematically decrease toward the central plutonic complex. The δ18O 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 δ18O 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 δ18O = −3.3 to +0.5, and the latest with δ18O = +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 in18O, some by as much as 13‰. These effects are observed over an area of 500 km2, and were produced by meteoric-hydrothermal convective circulation of early Tertiary ground waters, which apparently had initial δ18O values of about −11 to −12.