Low-O¹⁸ Igneous Rocks from the Intrusive Complexes of Skye, Mull, and Ardnamurchan, Western Scotland

Abstract

Oxygen isotope analyses have been obtained on rocks and coexisting minerals from the Tertiary stocks, ring-dikes, and cone-sheets in Ardnamurchan (18 samples), Skye (41 samples), and Mull (18 samples); these include a few samples of the plateau basalt country rocks. Almost all of the rocks in the vicinity of the central ring complexes (within 2 mi. of an intrusive contact) are strongly depleted in O18 relative to ‘normal’ igneous rocks from other areas. The rocks in Skye (≈ 150 sq. mi.) and Mull (≈ 150 sq. mi.) have suffered an overall O18 depletion of about 6 to 7 per mil, and those in Ardnamurchan (≈ 30 sq. mi.) about 3 to 6 per mil. These data indicate that very large hydrothermal convection systems involving heated low. O18 meteoric ground waters were established in these areas at the time of igneous intrusion. The heated ground waters exchanged with the gabbros, granities, and basaltic lavas, locaclly lowering their O18/ O16 rations by at least 12 per mil. Much, if not all, of this exchange occurred after crystallization of a given igneous rock was essentially complete, inasmuch as feldspars invariably have undergone much more depletion in O18 than has coexisting quartz or pyroxen. The meteoric-hydrothermal process and the presence of an aqueous gas phase may possibly be responsible for the widespread epidote-chlorite alteration, the turbid or cloudy feldspars, the abundant feldspathic veins, the felsitic and granophyric textures, the miarolitic cavities, and much of the explosive volcanic activity and brecciation that are found in these areas.