Petrology and geochemistry of basaltic rocks from the Balleny Is, Antarctica

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Abstract
Rock samples obtained from Sabrina and Sturge Is in the Balleny Group, Antarctica during the 1978 Lewis Expedition are ocean island basanite, verging on hawaiite. A noteworthy geochemical feature is a consistently low Zr/Nb ratio (∼ 3.5) compared with a typical value of 5.8 (range 4.5–6.6) for other ocean island alkaline rocks of basanitic to alkaline basaltic composition. Sturge samples are partly cumulative, with conspicuous clinopyroxene and olivine phenocrysts. All rocks are derivative from primary mantle melts, and evidence for fractionation at moderate pressure (probably corresponding to mantle depths of 15–20 km) is provided by the presence of aluminous orthopyroxene, aluminous (high AlVI) clinopyroxene, kaersutitic amphibole and olivine with complex minor element zoning patterns. Oxide phases present are varied, from chrome spinel included in olivine and pleonaste included in plagioclase, to microphenocrysts of ilmenite and titanomagnetite. The Balleny volcanism appears to represent typical hot spot activity and may be part of a hot spot trace extending from the Balleny Is northwards to the Cascade Plateau and eastern Tasmania. There is no convincing evidence that it is related to magmatism through a ‘leaky’ transform fault linked with the conspicuous Balleny Fracture Zone.