The Coire Uaigneich Granophyre, Skye: a study in crustal reworking

Models of crustal growth must take account of areas of old crust which have been reworked in addition to the volume of juvenile crust generated. In this case study the Coire Uaigneich Granophyre in south-central Skye is viewed as a microcosm of the process of crustal reworking. Here we show that the granophyre is the product of the partial melting of Neoproterozoic Torridonian sandstone, triggered by the thermal energy from the Cuillins intrusion. Our evidence is based upon a study of areas of partial melting within xenoliths of Torridonian arkose in the granophyre and from the presence of restitic Proterozoic and Archean zircons in the granophyre. However, we find that the difference in composition between partial melts of fused Torridonian xenoliths and the granophyre itself suggests that the granophyre is the product of the mixing of two melts – one fused Torridonian and a more mafic melt, probably derived from the Cuillins intrusion. This view is supported by trace element modelling of the partial melting of Torridonian sandstone and the presence of a younger generation of euhedral, igneous-looking zircons in the granophyre which have a weighted average 206 Pb/ 238 U age of 58.92 ± 0.06 Ma, identical to that of the Cuillins intrusion.