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. INTRODUCTION Plutonic lithics are sampled from magma bodies by volcanoes and, hence, form the important and elusive connection between magmatic and volcanic systems. Geochemical studies have dominated research in this field, beginning with Bowen (1928). Instead, we use microstructural analysis as a powerful, quantitative tool that directly relates physical crystal relationships within volcanic deposits to magmatic processes. This study is the first to link microstructural deformation within the magmatic system to erupted deposits via plutonic lithics, which provides a more complete picture of the evolution of magma from the plutonic to the volcanic realm. Plutonic lithics (xenoliths, cumulates, enclaves) may represent the bulk liquid or the crystal residue of magmatic bodies that reached solidor near-solid-state conditions. Crystal phases in plutonic lithics accumulate by crystallization, crystal settling, magmatic flow, and/or compaction (Sewell et al., 1993; Burt et al., 1998; Bacon et al., 2007; Graeter et al., 2015). In concert with these processes of crystal accumulation, interstitial melt can be extracted and stored separately (cf. mush model; Bachmann and Bergantz, 2008), and ultimately erupt. In this study, gabbroic plutonic lithics were sampled from Goat Rock Dome, a trachy andesitic lava dome on the flank of the extinct Akaroa Volcanic Complex, South Island, New Zealand (Fig. 1A). Lithics were discovered following a rockfall event during the Canterbury Earthquake Sequence (September 2010). The lithics exhibit a strong foliation defined by framework-forming plagioclase (40%; Fig. 1B) and, to a lesser extent, pyroxene (13%) and olivine (14%). We use a combination of crystallographic preferred orientations (CPOs) and microstructures to determine the magmatic processes responsible for plagioclase accumulation and alignment. ELECTRON BACKSCATTER DIFFRACTION ANALYSIS The microstructures of lithics with the most conspicuous plagioclase alignment (e.g., Fig. 1B) were quantified using electron backscatter diffraction (EBSD). 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). The application of EBSD to plagioclase in plutonic lithics is limited but critical in determining how magmas separate and erupt—a problem that geochemistry alone is unable to solve without this textural context. Goat Rock plutonic lithics contain elongate plagioclase crystals up to 10 mm long that define a planar foliation (all data that follow refer to plagioclase). EBSD maps were collected from entire polished thin sections of five samples (see the GSA Data Repository1) using a step size of 50 μm, with one sample (GR8b) remapped at a higher resolution (5 μm step). All CPOs are characterized by point maxima of the {010} perpendicular to foliation and great circle girdle distributions of {100} (Fig. 2A). Generally, poles to {001} are also distributed about a great circle. The following data in this paper refer to the representative sample GR8b. Misorientations are differences in the crystal orientation of different points in a crystal lattice (Fig. DR2 in the Data Repository). Intragrain misorientation transects reveal that individual plagioclase grains are continuously distorted by as much as 10°, corresponding to bent twin boundaries and undulatory or patchy extinction (Fig. 1C). Boundaries between plagioclase grains are irregular and in some cases lobate (Fig. 1C). Plagioclase crystals impinged on by other grains are common and are visibly bent (Fig. 1C). The conventional approach of plotting mis orientations of neighboring pixels gives large errors on rotation axis orientations, as GSA Data Repository item 2019169, supplemental figures and tables, is available online at http:// www .geosociety .org /datarepository /2019/, or on request from editing@ geosociety .org. CITATION: Bertolett, E.M., et al., 2019, Compacted cumulates revealed by electron backscatter diffraction analysis of plutonic lithics: Geology, v. 47, p. 445–448, https:// doi .org /10 .1130 /G45616.1 Manuscript received 19 September 2018 Revised manuscript received 26 February 2019 Manuscript accepted 26 February 2019 https://doi.org/10.1130/G45616.1 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 21 March 2019 Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/5/445/4680681/445.pdf by guest on 07 June 2019 446 www.gsapubs.org | Volume 47 | Number 5 | GEOLOGY | Geological Society of America misorientation angles between neighboring pixels are very small (Prior et al., 1999). Therefore, we plotted the orientation of rotation (misorientation) axes calculated from randomized pairs of points within individual twins in the sample reference frame (Fig. 2B; Michels et al., 2015). Rotation axes from individual twins generally form tight point clusters (Fig. 2B), although some have double clusters or more smeared distributions (see the Data Repository). Importantly, when all twin sets from all grains are taken together, rotation axes align along a shared great circle perpendicular to the maximum in {010} (Fig. 2C). ALIGNMENT AND DEFORMATION DURING UNIAXIAL COMPRESSION Crystal Organization in Magmatic Fabrics EBSD studies on magmatic fabrics have focused on minerals such as quartz, olivine, and biotite (Romeo et al., 2007; Žák et al., 2008; Beane and Wiebe, 2012; Graeter et al., 2015), whereas studies on magmatic plagioclase CPOs are rare (Satsukawa et al., 2013; Ji et al., 2014; Holness et al., 2017; Fiedrich et al., 2017). Observations of orthogonal cuts of Goat Rock lithics in hand sample are consistent with elongate plagioclase shapes in three dimensions (i.e., a > b > c). Plagioclases are mostly elongate normal to {010} (see the Data Repository) and have average axial ratios, measured in the thin section plane, of 2.7. This is likely an under estimate of the ratio of the long and intermediate axes in the plane perpendicular to {010}. Lithics have very distinct plagioclase CPOs (Fig. 2A) that are characteristic of elongate crystals that have been mechanically rearranged to form a foliation via uniaxial shortening (Fig. 3A; Axial-B of Satsukawa et al. [2013] and Type A of Ji et al. [2014]). Weak lineation fabrics may also have this CPO; however, elongate crystals in a flowing medium are generally characterized by clusters in all three directions ({100}, {010}, and {001}) orthogonal to each other (Fig. 3B). Goat Rock CPOs are best explained by elongate crystals that were free to arrange themselves within a foliation plane with no constraint on a preferred direction (Fig. 3A). Rotation axis orientations provide a means of definitively distinguishing CPOs that may be attributed to compaction or weak magmatic flow. Magmatic Compaction versus Flow in the Solid State An important distinction can be made between the organization of crystals under liquid-rich conditions and the intra-crystalline deformation that occurs with continuing stress after rheological lockup (Arzi, 1978; Paterson et al., 1989; Philpotts et al., 1999). The former is within the presence of melt, and any further crystal growth will reflect this stress (i.e., chemical zoning will by asymmetric; Holness et al., 2017), while the latter refers to the kinematics of crystallographic deformation in the solid or near-solid state. 8 km AVC A