Isotopic evidence for the age and origin of the “grey gneiss” complex of the southern Outer Hebrides, Scotland

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Abstract
Rb-Sr and Pb/Pb whole rock isochrons for the “grey gneiss” complex yield typical Scourian ages of 2690 ± 140 m.y. and 2640 ± 120 m.y. respectively, probably dating Scourian metamorphism. Subsequent events (< 2100 m.y.) produced complete or partial loss of radiogenic 40 Ar from minerals, but left the whole rock samples closed systems to Rb, Sr, U and Pb. The suggestion of an early Laxfordian granulite facies event followed by regional retrogression to amphibolite facies is thereby rendered most unlikely. Consideration of the initial 87 Sr/ 86 Sr ratio for the grey gneiss complex of 0.7014 ± 0.0007, as well as of the Pb isotope ratios, strongly suggests that the precursors of the “grey gneiss’ complex were derived from upper mantle source regions not more than 100–200 m.y. prior to the Scourian metamorphism, and rule out the possibility that the complex represents the reworking of a significantly older gneissic basement complex with anything like normal crustal Rb/Sr ratios.