Middle Jurassic within-plate granites in West Antarctica and their bearing on the break-up of Gondwanaland
Five post-tectonic granitic plutons isolated within the central Ellsworth-Whitmore mountains crustal block in West Antarctica form a distinctive geochemical suite. All have some characteristics of S-type granites and are atypical of active continental margins. They range in composition from a within-plate granite (WPG) end member, with the lowest 87Sr/86Sr initial ratio (0.707), to granites with a much more marked crustal signature and high initial ratios (0.722). The granitic suite was emplaced over a restricted Middle Jurassic time interval at the same time as the extensive Ferrar-Karoo-Tasman mafic suite and just prior to the disintegration of the supercontinent Gondwanaland. Petrogenetic modelling suggests that the WPG end member could have been derived entirely by differentiation of the enriched mantle-derived Ferrar magma, and the end member with the highest initial ratio by partial melting of a crustal source. Low initial 143Nd/144Nd ratios and Proterozoic model ages are compatible with a Precambrian crustal component but may alternatively, as in the case of Ferrar Supergroup magmas, reflect partial inheritance from enriched lithospheric mantle geochemically coupled to the lower crust since Precambrian differentiation. Data from these granites are consistent with large-scale underplating of mafic magma and crustal melting in response to a thermal disturbance in the Gondwanaland lithosphere related in some way to break-up of the supercontinent.