Provenance of late Proterozoic Dalradian tillite clasts, Inner Hebrides, Scotland

A geochemical and isotopic study was made of granitoid clasts taken from the c. 660 Ma Dalradian tillites of Islay and the Garvellach Islands. Objectives were to investigate tillite provenance as a means of helping to characterize the ages, compositions and distributions of basement rocks near and beneath the Dalradian Supergroup. Most clasts have closely similar trace element and REE compositions, implying that they were derived from source rocks produced in similar, but not necessarily coeval, geotectonic settings. The clasts are petrographically similar to the syenites of the Rhinns Complex, the c. 1.8 Ga alkalic igneous suite exposed locally in the Hebridean region SE of the Great Glen Fault. Most clasts, however, are geochemically distinct from the Volcanic Arc-type Rhinns Complex. The isotopic model ages of the clasts overlap those of the Rhinns Complex, but the c. 1.57 Ga age of one clast is substantially younger. Although most clasts were not eroded from the Rhinns Complex of the type presently exposed, it is possible that they were derived from plutonic rocks closely associated in space and time with the Rhinns Complex.