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

摘要: The Torridon Inlier of the Lewisian Scotland contains ~30 sq. km granodioritic gneisses. A coordinated study this inlier, including detailed mapping, geochemical analyses and geochronological work, has indicated that Rb-Sr whole rock yield “ages” are functions local structural setting chemistry mineralogy rock, as well overall geological history region. This Archean region been invaded by a swarm northwest striking dykes Inverian (2400 to 2240 Ma) age, accompanied new foliations major structures. However, several structurally distinct areas, termed “pips”, 1–2 km2 in extent, free foliation largely dykes. 38 samples analysed for Rb Sr scatter widely about 2.66 Ma isochron; but give meaningful results only when subdivided according their lithology. subset silicic gneiss from area most strongly overprinted structures yields an age ± 70 with initial 87Sr86Sr 0.7098 18. “pips” further north of2790± 100 (initial = 0.7020± 10), identical 2780± U-Pb obtained zircon fractions separated two samples. Samples northern district, located outside areas showing weak foliation, or within 50 m dykes, both, have grossly disturbed systematics. apparent ages felsic gneisses increased, while those mafic enclaves decreased (to low 1380 Ma). Field evidence relate increase directly loss and/or radiogenic gain immediate vicinity which themselves gained K comparison elsewhere Lewisian. In terms work on complex terranes, these must caution against direct interpretation such data, especially absence field knowledge.