| Abstract |
An aeromagnetic survey that allowed an improved discrimination of the principal dyke swarms of the Falkland Islands has been augmented by additional radiometric dates and geochemical analyses of representative specimens. Most of the dykes previously described as forming a 'north-south' swarm of Jurassic age are associated with a set of NE-SW linear magnetic anomalies that are entirely separate from another set of N-S to NNW-SSE anomalies, The NE-SW Jurassic dyke swarm occurs mostly in West Falkland but extends sparsely into East Falkland; a separate E-W dyke swarm, also of Jurassic age, is restricted to the southern part of West Falkland. The newly discovered N-S swarm spans West and East Falkland, and the offshore area to the south-east and has proved to be Early Cretaceous in age. The different dyke swarms are petrographically and geochemically distinct with, in particular, the Cretaceous dykes having a much higher Fe content than the Jurassic dykes. Ar-Ar age dating of East Falkland dykes has confirmed an Early Jurassic, ca 184-178 Ma, age for the NE-SW swarm, and has provided an Early Cretaceous age range for the N-S orientated dyke swarm of ca 135 Ma (Valanginian-Hauterivian) to ca 121 Ma (Aptian); although it is apparently robust, on regional grounds the validity of the Aptian age may be questionable. The onshore Jurassic dykes, and a possible correlative dolerite body forming seismic basement in the offshore Falkland Plateau Basin, are associated with the regional Karoo-Ferrar magmatism, linked to the initial break-up of Gondwana. The Early Cretaceous onshore magmatism has the same age as that assigned from seismic interpretation to sills and/or lavas within the Falkland Plateau Basin sedimentary succession. Both the onshore and offshore Early Cretaceous magmatism is likely to be associated with the later extension phases of the Falklands Plateau and rifting of the North Falklands Basin as the South Atlantic Ocean initially opened. |
