Pyrometamorphosed Otago Schist xenoliths cause minor contamination of Dunedin Volcanic Group basanite

Abstract

We examine the pyrometamorphic changes and resulting crustal contamination that occurred when fragments of Otago Schist were entrained and pyrometamorphosed in Dunedin Volcanic Group (DVG) basanite. Otago Schist greenschist facies quartz-albite laminae were replaced by fine-grained quartz grains (inverted from β-quartz or tridymite) with orthopyroxene coronas and dacitic-rhyolitic glass. Former muscovite + chlorite + epidote ± garnet segregations were converted to plagioclase (An30-80), olivine, spinel, cordierite, ilmenite, pigeonite and trachytic to trachy-andesitic glass. Minerals largely formed by mineral replacement and then by crystal nucleation from a quenched liquid as protolith minerals passed their liquidi, with the latter process generating swallow-tailed, skeletal, dendritic and ladder-structured crystal morphologies. Orthopyroxene-, olivine- and plagioclase-liquid geothermometry indicates xenoliths reached at least 940°C. Despite the molten state of the xenoliths, discernible chemical and isotopic (Sr, Nd) contamination of the basanite was inhibited by its enriched trace element budget. The only evidence for crustal contamination comes from basanite Pb isotopes, which have less radiogenic compositions within several cm of the xenoliths. The subtle variation in DVG Sr and Nd isotopes may therefore be due to different mantle source components rather than crustal contamination by the underlying Otago Schist.