Palaeogene igneous rocks reveal new insights into the geodynamic evolution and petroleum potential of the Rockall Trough, NE Atlantic Margin

Data acquired from petroleum exploration well 164/7-1 drilled in the UK sector of the Rockall Trough have yielded fresh insights into the igneous and thermal history of this frontier region. The well targeted a large four-way dip closed structure of presumed Mesozoic age named 'The Dome Prospect'. The structure is now known to have a magmatic, rather than a purely structural origin, which was the preferred pre-well interpretation. The well encountered 1.2 km of Palaeocene age basaltic lavas, overlying Late Cretaceous mudstones which were intruded by over 70 dolerite sills ranging from <1.5- to 152-m thick. 40Ar/39Ar dating of the dolerite intrusions indicates an Early Palaeocene age (63-64+/-0.5 Ma), which are among the oldest 40Ar/39Ar dates recognised in the North Atlantic Igneous Province. Radiometric dating of the overlying basaltic lavas proved unsuccessful, because of excessive alteration. Biostratigraphic dating of underlying and overlying sedimentary strata was utilised to constrain the age of the lavas to Late Paleocene to Early Eocene age (similar to55 Ma). Despite being related to two distinct events separated by similar to8 Ma, the intrusives and extrusives are compositionally similar. The basaltic rocks from well 164/7-1 possess Sr-Nd isotopic, major and trace-element geochemical compositions similar to other volcanic and intrusive rocks of the British Tertiary Igneous Province and represent partial melts of both lithospheric and asthenospheric mantle associated with the proto-Icelandic mantle plume head. Joint consideration of thermal maturity, potential fields and 3D seismic data indicate a deeper igneous body in addition to the sills encountered in well 164/7-1. Jack-up and arching mechanisms associated with both scales of intrusive body are believed to have developed the dome structure. The preferred interpretation is of a mafic laccolith, 17 km in diameter, similar to7 km thick, intruded at 64.5 Ma, situated similar to2.5 km below the bottom of the well. 3D thermal modelling suggests that all of Tranche 52 was thermally affected by the intrusion of the magmatic body. The thermal aureole, between 27 and 51 km in diameter, is not thought to play an important role in the hydrocarbon prospectivity of the surrounding Tranches in the NE Rockall Basin. Results show that hydrocarbon exploration prospects that are circular in map view should be interpreted with caution on volcanic continental margins. In sedimentary basins, where salt domes and shale diapirs are absent and igneous rocks prevalent, periclinal structures such as 'The Dome Prospect' should undergo a thorough multi-disciplinary risk assessment.