Geochemical and isotopic constraints on the nature and age of basement rocks from Rockall Bank, NE Atlantic

Abstract

An isotopic and geochemical reappraisal of samples from Rockall Bank, NE Atlantic, indicates that the granulites and granites present on the Bank have little anity with the Lewisian of NW Scotland, with which they have been previously associated. New Pb, Rb-Sr and Sm-Nd data, together with existing K-Ar data, indicate that the rocks formed at about 1625 Ma. Although this is contemporaneous with Late Laxfordian events in NW Scotland, the high model p value of 8.03 indicates that the rocks were not derived by metamorphic reworking of ancient Lewisian crust, but largely represent pristine mantle contributions at this time. In this sense, the Rockall terrane is comparable with the Islay terrane, which has a model p value of 7.9 but is apparently somewhat older (c. 1800Ma). The Rockall basement rocks therefore represent a pre- viously unrecognized crust formation event in the North Atlantic region, and reconstructions of Proterozoic geology will need to take into account the presence of this crustal block of predominantly juvenile mantle-derived material. The Rockall area may also be considered a likely source for the Torridonian sediments of NW Scotland. Our knowledge of the geology of the Rockall Plateau (NE Atlantic, Fig. 1) has not advanced significantly since the review published by Roberts (1975). Subsequent work on the eastern NE Atlantic area has largely concentrated on the development of the Rockall Trough which marks the eastern limit of the Plateau, as summarized by Smythe (1989) and on the development of the volcanic passive margin that marks the