Metamorphic rocks from the southern end of the Rockall Bank

Abstract

T wenty one cobbles of granulite-facies metamorphic rocks of Lewisianoid aspect have been dredged from the southern end of the Rockall Bank, North Atlantic Ocean. At the same locality two large fragments of Flandrian conglomerate contain petrologically similar heavy minerals and pebbles. We argue that these rocks have a local origin and therefore confirm that Rockall Bank has a continental composition as well as a continental seismic structure. Rockall Bank (Fig. 1) is the shoalest part of the extensive Rockall Plateau that is now known to have a continental structure from a variety of geophysical studies (a, Roberts 1970b ; Scrutton 1970 ; Scrutton & Roberts 1971 ; Scrutton in press ). However, geological evidence of continental granitic and metamorphic rocks has been lacking although a geochemical study of the aegirine-granite exposed on Rockall Islet suggests such rocks may underlie the Bank at depth ( Sabine 1960 ; Moorbath & Welke 1969 ). Previous geological sampling has been confined largely to the northern part of the Bank and the area immediately around Rockall Islet ( Cole 1897 ; Lacroix 1921; Roberts & Wilson 1971; J. B. Wilson personal communication). These dredge hauls have yielded sparse metamorphic rocks and quantities of both basic intrusive and extrusive rocks or their derivatives that are probably coeval with the ring complex centred near the Lower Eocene granite of Rockall Islet ( Sabine 1960 ; Miller & Mohr 1965 ; Roberts 1969 ). Although some of this material may be ice-rafted, magnetic anomaly interpretation ( Roberts 1969 ; Roberts & Jones in