Early Tertiary igneous activity west of the Outer Hebrides, Scotland — Evidence from magnetic anomalies and dredged basaltic rocks

Abstract

Much of the continental shelf off the Outer Hebrides, Northwest Scotland, is underlain by a basement platform which has not undergone the subsidence typical of many parts of the margin west of Britain. Although seismic data suggest that the platform is largely an extension of the Precambrian Lewisian gneiss of the Outer Hebrides the pattern of magnetic anomalies indicates that other lithologies are present. Dredging operations in an area of basement outcrops between St. Kilda and the Flannan Isles have resulted in the recovery of a collection of basaltic rocks in addition to granitic gneisses. Analysis of the freshest basaltic samples reveals that they are olivine tholeiites, typical of “within-plate” basalts, which are chemically distinct from basic intrusions on St. Kilda and other Tertiary volcanic rocks nearby. Whole-rock K-Ar dating yields a mean age of 43.4 ± 1.0 Ma. The association of the basaltic material with one of several prominent linear belts of magnetic anomalies crossing the shelf points to an extensive invasion of the margin by basic igneous intrusions during the Early Tertiary. The dredged basalts appear to represent a late-stage volcanic episode in the evolution of the Tertiary igneous province of Britain. One block of gneiss in the dredge haul has given ages of 404 ± 9 and 431 ± 7 Ma, perhaps indicating reworking of the Precambrian basement by Caledonian tectonic events on the Lewisian foreland. However, further work is necessary to establish the provenance of this gneissic material.