Low δ18O magma, Isle of Skye, Scotland:: Evidence from zircons

Zircons in Tertiary granitic rocks from the Isle of Skye, Scotland were resistant to measurable oxygen isotope exchange during intense hydrothermal activity in the subvolcanic environment. Five granite bodies from the Western Red Hills complex were investigated; four have O-18(Zrc) = 4.1 +/- 0.2 parts per thousand (VSMOW) while the fifth, which intruded before major cauldron subsidence, is 2 parts per thousand lower. Zircons from Lewisian gneiss are visually and isotopically distinct indicating that zircons in the granite are not xenocrysts from the Lewisian basement. Analysis of different magnetic and size fractions of zircon shows no significant correlation to delta(18)O supporting the conclusion that these values represent the true magmatic compositions and that the Western Red Hills granites intruded as low delta(18)O magmas with a whole rock delta(18)O less than or equal to 6.0 parts per thousand. Quartz separates from four granites are close to the calculated magmatic values in delta(18)O, but in the fifth, values are 5 parts per thousand lower indicating variable exchange of quartz with low delta(18)O heated meteoric waters. These results might be explained by magmatic interaction with hydrothermally altered crust early in the evolution of the granites, before crystallization of zircon, followed by interaction with later hydrothermal fluids. Alternatively, independent evidence for low delta(18)O mafic magmatism in NW Scotland suggests the presence of large quantities of low delta(18)O subducted ocean crust in the subcontinental lithosphere that could contaminate or be the source of low delta(18)O mantle-derived melts. Copyright (C) 1997 Elsevier Science Ltd.