The age of the granitic injection complex of Harris, Outer Hebrides

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Abstract
Synopsis: The age of the granitic injection complex of Harris has been determined from an eight point Rb-Sr whole-rock isochron to be 1750 ± 34 m.y. An independent determination of the age of the complex of 1715 ± 20 10 m.y. was found from U-Pb measurements on eight zircon size fractions from two of the granite samples. These ages, from two independent methods, are the same within the uncertainties. The results confirm the Laxfordian age of the injection complex and establish a minimum age for the main Laxfordian metamorphism in the region. The variation in the initial Sr 87 /Sr 86 ratios of the whole rocks suggests that the granite intrusions were derived from an inhomogeneous source or sources. The Rb-Sr mineral ages are inconsistent and mineral data-points do not plot on the whole-rock isochron, indicating that the minerals have been open systems to the movement of Rb-Sr isotopes at some time after the emplacement of the granites.