Mesozoic dolerite dykes of the Falkland Islands: petrology, petrogenesis and implications for geochemical provinciality in Gondwanaland low-Ti basaltic rocks

Abstract

The dolerite dykes of central West Falkland provided the first palaeomagnetic evidence of the near-180° rotation of the islands during the break-up of Gondwanaland. Here we present the results of a petrological and geochemical study of those dolerites. Most of the dykes sampled can be assigned to one of two suites named north-south and east-west respectively in recognition of their dominant strike. E–W dykes have compositional affinities with magmas such as the Rooi Rand dolerites of SE Africa whose geochemical characteristics (e.g. 87 Sr/ 86 Sr<0.704) are suggestive of predominantly asthenospheric mantle derivation. N–S dykes, which are the most numerous, resemble the Ferrar magma type of Antarctica and the rare Hangnest and Kraai River magma types of the Karoo. The N–S dykes evolved by assimilation and fractional crystallization (AFC), most likely at crustal levels, leading to initial 87 Sr/ 86 Sr=0.708–0.712, but there is no compelling evidence for initial 87 Sr/ 86 Sr<0.708, perhaps suggesting an enriched lithospheric mantle source. Dykes from Lively Island and Mount Alice appear to be correlatives of other Karoo magma types. All the Falklands dykes are low-Ti in character. However, within this one small area a variety of Karoo low-Ti magma types coexist with the Ferrar magma type of Antarctica. This overlap in space of different low-Ti magma types resembles the situation in Coats Land, Antarctica and enables us to propose an extension of the boundary of the Ferrar province parallel to the subducting margin of Gondwanaland.