Compacted cumulates revealed by electron backscatter diffraction analysis of plutonic lithics

E.M Bertolett,D.M Gravley,D.J Prior,B.M Kennedy,S.J Hampton

doi:10.1130/g45616.1

E.M Bertolett, D.M Gravley + Show 3 more

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https://doi.org/10.1130/g45616.1

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Journal: Geology	Publication Date: Mar 21, 2019
Citations: 15	License type: cc-by

Affiliation: University of Canterbury, University of Otago

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.

Highlights

Plutonic lithics are sampled from magma bodies by volcanoes and, form the important and elusive connection between magmatic and volcanic systems
Strong crystallographic preferred orientations (CPOs) and the persistence of rotation axes indicative of uniaxial compression make it clear that Goat Rock plutonic lithics were subject to a compacting stress throughout both their organization and subsolidus deformation
Plagioclase CPOs and rotation axis distortion occur in the horizontal plane and define a foliation

Summary

Introduction

Plutonic lithics are sampled from magma bodies by volcanoes and, form the important and elusive connection between magmatic and volcanic systems. EBSD enables mapping of crystallographic orientations (Prior et al, 1999), which in the last decade has been applied more frequently to igneous rocks and has illuminated processes that include magmatic flow, cumulate and glomerocryst formation (i.e., synneusis versus crystal growth), and melt extraction (Žák et al, 2008; Beane and Wiebe, 2012; Satsukawa et al, 2013; Ji et al, 2014; Graeter et al, 2015; Fiedrich et al, 2017; Cheadle and Gee, 2017; Holness et al, 2017).

Results

Conclusion