Gondwana break-up related magmatism in the Falkland Islands

M. J. Hole, R. M. Ellam, D. I. M. MacDonald, S. P. Kelley

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Abstract

Jurassic dykes (c. 182 Ma) are widespread across the Falkland Islands and exhibit considerable geochemical variability. Orthopyroxene-bearing NW-SE oriented quartz-tholeiite dykes underwent fractional crystallization > 1 GPa, and major element constraints suggest that they were derived by melting of pyroxenite-rich source. They have εNd182 in the range -6 to -11 and 87Sr/86Sr182 >0.710 and therefore require an old lithospheric component in their source. A suite of basaltic-andesites and andesites exhibit geochemical compositions transitional between Ferrar and Karoo magma types, and are similar to those seen in the KwaZulu-Natal region of southern Africa and the Theron Mountains of Antarctica. Olivine-phyric intrusions equilibrated at < 0.5 GPa, and have isotopic compositions (εNd182 1.6-3.6 and 87Sr/86Sr182 0.7036-0.7058) that require limited interaction with old continental lithosphere. A suite of plagioclase-phyric intrusions with 87Sr/86Sr182 c. 0.7035 and εNd182 c. +4, and low Th/Ta and La/Ta ratios (c. 1 and c. 15 respectively) also largely escaped interaction with the lithosphere. These isotopically depleted intrusions were probably emplaced synchronously with Gondwana fragmentation and the formation of new oceanic lithosphere. Estimates of mantle potential temperature from olivine equilibration temperatures do not provide unequivocal evidence for the presence of a plume thermal anomaly beneath the Falkland Islands at 182 Ma.
Original languageEnglish
Pages (from-to)108-126
Number of pages19
JournalJournal of the Geological Society
Volume173
Issue number1
Early online date27 Oct 2015
DOIs
Publication statusPublished - Jan 2016

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