Mafic dykes (Groups A–D) intruded into Mesoproterozoic basement
amphibolites, gneisses, and
granitoids of the Cape Meredith Complex on the southern tip of West Falkland,
provide an important record
of at least three periods of lithospheric extension during Palaeozoic and
Jurassic times. Group A dykes are
calc-alkaline lamprophyres that were generated by partial melting of an
enriched subcontinental lithospheric
mantle in Cambro-Ordovician times. Group B dykes are Ordovician dolerites
derived from an
asthenospheric mantle source, perhaps during the same extensional episode
as
Group A dykes. Group C
dykes were also derived from an asthenospheric source and are possibly
of
Silurian age. The youngest,
Group D, dykes are part of the widespread Jurassic Gondwana province. This
group contains an oceanic
island basalt-like sample and an enriched sample similar to both Group
A
lamprophyres and to the Jurassic
Ferrar province in Antarctica. These correlations have interesting implications
for the composition and
evolution of mantle sources through time; the co-existence of Cambrian
lamprophyres and Jurassic Ferrar-type
magmas in the Cape Meredith Complex demonstrate for the first time that
the enriched lithospheric
mantle source postulated for the Ferrar magmas existed as far back
as Cambrian times.