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

dolerite dykes of central West Falkland provided the first palaeomagnetic e vidence of the near-180 degrees rotation of the islands during the break-up of Gondwanaland. Here we present the results of a petrological and geochem ical 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 recognit ion of their dominant strike. E-W dykes have compositional affinities with magmas such as the Rooi Rand dolerites of SE Africa whose geochemical chara cteristics (e.g. Sr-87/Sr-86<0.704) are suggestive of predominantly astheno spheric 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 fractio nal crystallization (AFC), most likely at crustal levers, leading to initia l Sr-87/Sr-86=0.708-0.712, but there is no compelling evidence for initial Sr-87/Sr-86<0.708, perhaps suggesting an enriched lithospheric mantle sourc e. Dykes from Lively Island and Mount Alice appear to be correlatives of ot her Karoo magma types. All the Falklands dykes are low-Ti in character. How ever, within this one small area a variety of Karoo low-Ti magma types coex ist with the Ferrar magma type of Antarctica. This overlap in space of diff erent low-Ti magma types resembles the situation in Coats Land, Antarctica and enables us to propose an extension of the boundary of the Ferrar provin ce parallel to the subducting margin of Gondwanaland.