Gondwana break-up related magmatism in the Falkland Islands

• Published in: Journal of the Geological Society Vol. 173; no. 1; pp. 108 - 126
• Main Authors: Hole, M. J, Ellam, R. M, Macdonald, D. I. M, Kelley, S. P
• Format: Journal Article
• Language: English
• Published: London, UK Geological Society of London 01-01-2016; The Geological Society of London; Geological Society Publishing House
• Subjects: absolute age; Africa; alkaline earth metals; andesites; Antarctica; Ar/Ar; Atlantic Ocean Islands; basalts; crystal fractionation; Crystallization; dates; diabase; dikes; Falkland Islands; Ferrar Group; fractional crystallization; Geology; Geophysics; Gondwana; heterogeneity; igneous and metamorphic rocks; igneous rocks; intrusions; isotope ratios; isotopes; Jurassic; Karoo Supergroup; KwaZulu-Natal South Africa; large igneous provinces; Lithosphere; Magma; magmatism; major elements; mantle; Melting; Mesozoic; metals; neodymium; paleogeography; Petrology; plutonic rocks; pyroxenite; rare earths; solid Earth (tectonophysics); South Africa; South America; Southern Africa; Sr-87/Sr-86; stable isotopes; strontium; Temperature; trace elements; Transantarctic Mountains; ultramafics; volcanic rocks; Weddell triple junction; Falkland Islands; PSW, South Africa, KwaZulu-Natal; PS, Antarctica; South Africa, Karoo; PSW, South Atlantic, Falkland Is
• ISSN: 0016-7649; 2041-479X
• DOI: 10.1144/jgs2015-027

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 at >1 GPa, and major element constraints suggest that they were derived by melting of a 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.