Abstract

Well 209/3-1, located close to the centre of the Tertiary Erlend complex, north of the Shetland Isles, contains an unusual volcanic sequence in which thick basalts of N- to T-type MORB affinity overlie highly peraluminous, cordierite-bearing dacites. The dacites from the Erlend complex are almost identical in terms of their petrography and major-element, trace-element and isotope geochemistry to those of dacites from two previously-described localities in the North Atlantic, and indicate the formation of the dacites by anatexis of aluminous upper crustal sediments, which were probably pelitic in character. The discovery of graphite in the Erlend dacites suggests that carbonaceous shales may be implicated as parent material. The geological settings of the Erlend volcanic sequence, and the other examples in the Rockall Trough and on the Vøring Plateau, correspond to intra-continental, pre-break-up rift systems underlain by thinned continental crust, occurring close to the continental margin. A possible petrogenesis for the dacites involves the partial melting of upper crustal sediments as a result of the extensive contemporaneous basic magmatism documented to have occurred within a wide area of the North Atlantic continental margins during the initial stages of continental break-up. This mechanism suggests that the observed volcanic sequence may be more common than is obvious from the present, very limited, drilling evidence.

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