RbSr and zircon study of ~2800 Ma Lewisian silicic gneisses from the Torridon Inlier of NW Scotland: Dyke intrusion and an open system

Abstract

The Torridon Inlier of the Lewisian of Scotland contains ~30 sq. km of granodioritic gneisses. A coordinated study of this inlier, including detailed mapping, geochemical analyses and geochronological work, has indicated that Rb-Sr whole rock analyses yield “ages” that are functions of the local structural setting and of the chemistry and mineralogy of the rock, as well as of the overall geological history of the region. This Archean region has been invaded by a swarm of northwest striking dykes of Inverian (2400 to 2240 Ma) age, accompanied by new foliations and major structures. However, several structurally distinct areas, termed “pips”, 1–2 km 2 in extent, are free of Inverian foliation and largely free of Inverian dykes. The 38 samples analysed for Rb and Sr scatter widely about a 2.66 Ma isochron; but give meaningful results only when subdivided according to their structural setting and lithology. A subset of silicic gneiss samples from the area most strongly overprinted by Inverian structures yields an age of 2240 ± 70 Ma with an initial 87Sr 86Sr of 0.7098 ± 18. A subset of samples from “pips” further north yields an age of2790± 100 Ma (initial 87Sr 86Sr = 0.7020± 10 ), identical to the 2780± 70 Ma U-Pb age obtained from zircon fractions separated from two of the samples. Samples from this northern district, located outside the “pips” in areas showing weak Inverian foliation, or within 50 m of Inverian dykes, or both, have grossly disturbed Rb-Sr systematics. The apparent ages of the felsic gneisses have been increased, while those of their mafic enclaves have been decreased (to as low as 1380 Ma). Field and geochemical evidence relate the increase directly to Rb loss and/or radiogenic Sr gain in the immediate vicinity of major dykes, which themselves have gained Rb and K by comparison with dykes elsewhere in the Lewisian. In terms of Rb-Sr work on complex terranes, these results must caution against direct interpretation of such data, especially in the absence of detailed field knowledge.