U-Pb zircon, monazite and Rb-Sr whole rock systematics of granitic gneiss and psammitic to semi-pelitic host gneiss from Glenfinnan, Northwestern Scotland

Abstract

New U-Pb zircon data from a segregation pegmatite in the granitic gneiss at Glenfinnan yield discordant points which appear to be aligned along a chord on a concordia diagram with upper and lower intersection ages of 1,517±30 Ma and 556±8 Ma, respectively. The results are similar to published U-Pb zircon data from the granitic gneiss but the lower intersection age does not correspond to concordant ages of 455±3 Ma obtained for monazites from the segregation pegmatite and from paragneiss which hosts the granitic gneiss. The apparent U-Pb zircon chord also gives no indication of a 1,030±50 Ma (large sample) Rb-Sr whole rock isochron age for the granitic gneiss (Brook et al. 1976). A traverse of adjacent 5–8 cm thick slabs in the paragneiss yields a Rb-Sr ‘errochron’ of 455±60 Ma which also does not agree with the U-Pb zircon lower intersection age. The scale of this Sr whole rock diffusion (ca. 10 cm) is not at variance with existing thermal, temporal and experimental constraints.