CONSTRAINING THE CONSTITUENTS OF THE NORTH ATLANTIC IGNEOUS PROVINCE: NEW PALAEOMAGNETIC DATA FROM THE ISLE OF MULL, SCOTLAND

Numerous palaeomagnetic data sets from the North Atlantic Igneous Province (NAIP) have been collected during the last 30 years but a discrepancy exists between new palaeomagnetic pole positions published from western Greenland and the Faroe Islands relative to older data from the British Tertiary Igneous Province (BTIP) (Riisager et al. 2002a,b). When restored back to the 60 Ma position relative to Europe for rocks of presumably similar ages, the different palaeomagnetic pole positions are often scattered, and may reflect that the time-averaged palaeomagnetic field is not well constrained, probably due, in part, to too few sampling points. Other issues such as partial to complete remagnetizations from the effects of hidden intrusives or from overlying flows, and lack of precise radiometric ages in much of the BTIP may add to this scatter. We conducted a new palaeomagnetic investigation from the Palaeogene lava sequence on the Isle of Mull in western Scotland where recently published, high-precision ⁴⁰Ar/³⁹Ar ages constrained the main lava eruption to lie between 60.56 ± 0.29 and 58.38 ± 0.19 Ma (Chambers & Pringle 2001), or within C26r. Our findings show a reversed polarity for the lava sequence, and the derived pole is in agreement with older published poles from the BTIP. Three intrusive gabbro complexes yield normal, mixed and reversed polarities, respectively, for the oldest to youngest centers, also in agreement with previously published results. Dikes intruding the lavas include both normal and reversed polarities, likely indicating that dikes were not intruded simultaneously. The similarity between our new pole from the Mull lavas and older poles from all of the BTIP lavas support the data quality from the older studies, but require explanation in light of the discordant data from Greenland and the Faroe Islands. New ⁴⁰Ar/³⁹Ar analyses on Mull and Antrim will be used to examine rates of lava eruption, timing of emplacement of intrusive complexes and different dyke generations in an effort to resolve the reasons for these discrepancies.