The volcanic origin of a number of major tonstein horizons has been established from mineralogical compositions and from trace elements that are quantitatively retained during diagenesis. Discriminant function analysis using the variables Ti/Al, Cr/Al, Zr/Al and Ni/Al allows the tonsteins to be classified according to original composition. Two main groups are recognized amongst the British tonsteins. Those formed from acid volcanic ash are comparable in composition with French and German tonsteins and a common source is postulated, whereas tonsteins formed from basic volcanic ash contain variable amounts of detrital sediment and are thought to originate from local eruptions. Possible lateral equivalents between tonstein horizons in Britain and elsewhere in Europe are suggested.

Geochemical investigations of doterite cores from four intrusions in the recently discovered Faeroe Shetland sill complex have established that the sills are of transitional (T) mid-ocean ridge basalt (MORB)-type composition. Some uncertainty surrounds the age of the complex, but there is no doubt that it is, at least in part, of Tertiary age. Comparisons with previously proposed models for the development of sill-sediment complexes during initial stages of seafloor spreading suggests that the Faeroe-Shetland sills may represent an intrusive episode associated with a spreading axis that eventually produced oceanic crust W of the Faeroes.During the last ten years, hydrocarbon exploration N and W of the Shetland Isles has revealed the existence of horizons of igneous rock within the Mesozoic and Cainozoic sediments of the Faeroe-Shetland and More Basins. There is little doubt that many of these igneous rocks are intrusive since they thermally metamorphose and are chilled against the overlying sediments. They are parts of an extensive 'belt' of intrusions which we refer to as the Faeroe-Shetland sill complex. This occurs below, and protrudes beyond, the SE subcrop limit of the Faeroes Lower Series lavas (Fig. 1). The limits of this complex are known from drilling and seismic investigations but, because of the seismic opacity of the lavas, little is known about how far it extends to the NW. This paper reports on the geochemistry of four tholeiitic sills recovered as core from two wells, 2t9/20-1 and 208/21-1, and discusses possible models for their origin. Structure of the sill complexThe exact location of individual sills can only be determined by drilling but the approximate positions of the uppermost members of the complex can be seen on seismic reflection profiles. A NNW-SSE seismic profile across the SE edge of the sill complex in the general vicinity of Well 219/20-1 is shown in Fig. 2b along with a geological interpretation (Fig. 2a). It is evident from Fig. 2 that, whatever the form of the complex below the upper level of sills (which seismically obscure the lower levels), its top is 'transgressive'. We have examined a number of simitar seismic profiles and all demonstrate that the top of the sitl complex is intruded at stratigraphically higher levels to the NW.A number of commercial wells have intersected the Faeroe-Shetland sill complex. In Well 208/ 15-1A (see Fig. 1 for locations of wells) eight intrusions, ranging in thickness from 2 m to more than 340 m, were penetrated in the depth interval 1935-3123 m. The distributions of igneous horizons in Wells 208/21-1 and 219/20-1 are given in Fig. 3 but with arbitrary depth-scale zeros to preserve the confidentiality of the true depths. The lateral dimensions of individual sills are unknown, but it appears from the seismic data that they can extend for several kilometres.It appears from the seismic and well data available to us that the complex is at least 1500 m thick in places, 'feathers out' to the SE and its upper surface descends to lower stratigraphic lev...

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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.

The composite dyke at An Cumhann, Arran consists of a central unit of quartz-feldspar porphyry flanked by narrow marginal dolerites containing xenocrysts of quartz, plagioclase and alkali feldspar identical to the phenocrysts in the porphyry. The chemistry of the marginal dolerites indicates that they formed by the crystallization of a hybrid magma produced at depth by the incorporation of the porphyritic acid magma in a basic liquid. During the intrusion of the hybrid magma, flow caused a differential distribution of the xenocrysts across the initial basic dyke. Subsequent intrusion of the quartz-feldspar porphyry magma along the still unconsolidated centre of the basic dyke produced the wide central unit. A similar origin is proposed for other composite intrusions of the same type in the region. Whilst it is unlikely that the coexistence of the highly contrasting magmas necessary for the formation of these intrusions is entirely coincidental there is little likelihood that the two liquids were related as members of the same fractionation series.