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Constraints on the origin of mafic alkaline volcanics and included xenoliths from Oberon, New South Wales, Australia

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Abstract

Basanites and alkali basalts from Oberon, NSW, Australia contain variable abundances of small Cr-diopside lherzolite xenoliths. Despite a limited range in (metamorphic) textures and modal mineralogy, there is significant variation in mineral chemistry. Mineral thermometric data applied to the geotherm of O'Reilly and Griffin (1985) suggests equilibration over a narrow pressure interval corresponding to depths of 30–45 km. These data show that significant compositional variations exist over a small depth interval in the subcontinental mantle.

Basaltic host rocks show near-primary chemical characteristics. Mildly and strongly incompatible element (i.e. D< 1 and D≪1 respectively) concentrations have been used to constrain the modal amounts of clinopyroxene and garnet in a presumed garnet peridotite mantle source. Estimated proportions of (ol+opx)=73%; cpx=16%; gar= 11% closely resemble source compositions for other basaltic rocks of eastern Australia. Batch partial melting of this source in the range F=9.5–15% applied to the available REE data suggests the source is enriched relative to chondrite 8–10 × La, 2.1–2.4 × Tb and 2.5–3.7 × Yb.

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  1. Department of Geology and Geophysics, University of Sydney, 2006, NSW, Australia

    Paul A. Morris

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Morris, P.A. Constraints on the origin of mafic alkaline volcanics and included xenoliths from Oberon, New South Wales, Australia. Contr. Mineral. and Petrol. 93, 207–214 (1986). https://doi.org/10.1007/BF00371322

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