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

The Western Rockall Plateau: Stratigraphy and Structural Evolution

“…More recent geophysical studies have suggested that another continental fragment may underlie the Paleocene lavas of the Faeroe Islands (Bott et al, 1971;Casten, 1973;Bott et al, 1974;Casten and Nielsen, 1975). Laxfordian and Grenvillian granulites dredged and drilled on Rockall Bank have proved the presence of continental rocks (Roberts, Matthews and Eden, 1972;Roberts, Ardus, and Dearnley, 1973;Miller, et al, 1973). The presence of oceanic magnetic anomalies ( Figure 3) and a typical oceanic seismic structure suggests that oceanic crust underlies the deep basins to the west and southwest of Rockall Plateau (Ewing and Ewing, 1958;Gaskell et al, 1958;Vogt and Avery, 1974).…”

Evolution of Volcanic Rifted Margins: Synthesis of Leg 81 Results on the West Margin of Rockall Plateau

“…and Grenvillian (1000 m.y.) granulites have been dredged and drilled on Rockall Bank, while refraction studies have found continental seismic velocities and mantle depths of 33 km beneath Rockall Bank (Scrutton, 1970;Scrutton and Roberts, 1971;Roberts, Matthews, and Eden, 1972;Scrutton, 1972;Roberts, Ardus, and Dearnley, 1973;Miller et al, 1973).…”

Sites 403 and 404

“…More recent evidence of continental crust beneath the Tertiary lavas of the Faeroe Islands may indicate a northward extension of the Rockall Plateau (Bott et al, 1971;Casten, 1973;Bott et al, 1974). Laxfordian and Grenvillian granulites dredged and drilled on the Rockall Bank have proved the presence of continental rocks previously inferred from geochemical evidence (Roberts et al, 1972(Roberts et al, , 1973aMiller et al, 1973;Moorbath and Welke, 1969). Studies of the planated intrusive complex around Rockall Island suggest phases of igneous activity at 52±9 m.y.…”