THE APPLICATION OF EBSD AND MELTS MODELLING TO PLUTONIC LITHICS: A NEW MODEL FOR MAGMATIC PROCESSES AND CRYSTAL DEFORMATION TIMESCALES

EBSD is a technique increasingly applied to analysis of igneous fabrics. A small number of studies have applied EBSD to plutonic lithics and fewer to small, mafic systems. While EBSD research on quartz and mica is well established, initial challenges of indexing plagioclase have resulted in a limited database of such fabrics. Plagioclase, however, is key to understanding magmatic processes, evolution, and timescales. We examine plagioclase fabrics from plutonic lithics entrained and exhumed during emplacement of the Goat Rock lava dome of the Miocene-aged Akaroa Volcano, South Island, New Zealand.

The least evolved (45.35-47.52 SiO₂ wt%) of the lithics have very strong plagioclase CPOs with {001} and <100> alignment about a great circle and clustering of the {010}. Some crystals are distorted up to 10° and subgrain formation is evident. We can distinguish between a compaction CPO and foliation related to shear deformation because individual crystal long axes lie throughout the {001} and <100> girdles rather than in concentrated poles. This, and the lack of evidence of plastic deformation, indicates that no metamorphic overprint is present.

This magmatic CPO is characteristic of flattening (compaction) as opposed to stretching (directional flow). As these lithics are also the most compositionally primitive, we suggest that they represent the cumulate material of a small, shallow system. The preservation of this primary magmatic texture presents a unique opportunity to study magmatic processes.

We propose a new approach to crystal deformation in natural magmatic systems using solid state deformation equations and MELTS modelling. The resulting model examines the relationship between crystal column height and plagioclase deformation timescales. MELTS modelling determines a temperature at which compaction commences. Assuming only confining pressure from an overlying crystal mush 10s of meters thick, compaction beneath Goat Rock may have taken 1,000s to 10,000s of years. This timescale is likely an overestimate as more active compaction stresses were likely at play and future work will focus on constraining absolute pressure and melt fraction and composition. While speculative, these estimates allow us to begin constraining an otherwise blind magmatic system from textures in rare plutonic lithics.