Compacted cumulates revealed by electron backscatter diffraction analysis of plutonic lithics

Abstract

Cumulates, exposed as plutonic lithics in a volcanic host, provide insight into the storage conditions, evolution, and eruptibility of an otherwise invisible magmatic system. Here, we present electron backscatter diffraction analysis of plagioclase-rich cumulates erupted from the Akaroa Volcanic Complex in New Zealand. Plagioclase 010 is clustered normal to foliation with girdle distributions of 100 and 001. This crystallographic preferred orientation does not definitively distinguish magmatic compaction from flow. However, the rotation axes of distortion for plagioclase observed in this study lie in the foliation plane, indicating that compaction drove both crystal organization and further deformation in the solid state. As such, we propose that these lithics represent cumulates formed first from uniaxial compression involving alignment of shaped grains by rigid rotation in magma, followed by grain distortion by dislocation creep and accompanying grain boundary migration associated with melt expulsion. Petrographic evidence of decreasing glass abundance with increasing fabric strength further confirms melt extraction. Our quantitative microstructural analysis on the preferred orientation and deformation of plagioclase grains in erupted gabbroic lithics is an important complement to more traditional geochemical approaches and improves our understanding of how crystal mush evolution is physically linked to melt extraction and, possibly, volcanic eruption.