Initial Sr⁸⁷/Sr⁸⁶and possible sources of granitic rocks in southern British Columbia

Whole-rock strontium isotopic analysis of twenty-two post-Jurassic granitic rocks from southern British Columbia, selected from about eighty field localities, gives a range of values of Sr87Sr86 from 0.705 to 0.710. The age correction is insignificant for samples of very low Rb/Sr, and for the remainder it is based on published K-Ar biotite ages (45 to 86 m.y.). The corrected ratios are significantly higher than the means (0.704 to 0.705) established for mantle-derived oceanic and continental basalt. The ratios are, on the other hand, so much lower than those for the average continental crust (0.725 + .005) and composite shales (0.7204, 0.7226) that a source for the granitic magma in the upper crust is believed to be unrealistic. The magma is therefore assumed to have originated either from a gabbroic layer at the base of the crust or in the upper mantle itself, and to have been slightly enriched in Sr87 as it passed upward. A plot of uncorrected Sr87Sr86 and Rb87Sr86 shows one isochron of moderate quality (58±6 m.y.), which is in general agreement with K-Ar biotite ages for many of these intrusives.