PLUTONIC LITHICS RECORD VARIATIONS IN THE MAGMATIC SYSTEM BENEATH THE AKAROA VOLCANIC COMPLEX, SOUTH ISLAND, NEW ZEALAND

Plutonic lithics from the Cretaceous Akaroa Volcanic Complex (AVC), New Zealand, have textures and compositions that record the dynamic magmatic processes that shaped this predominantly basaltic system. The AVC is a multi-vent system with multiple shallow magma bodies above a deep plutonic source. Plutonic lithics are crystallized rock fragments of the underlying magma chamber that are subsequently incorporated into the erupted materials. Such lithics are essential to understand magmatic processes that occurred in regions, such as AVC, with limited exposure of the plutonic body. Lithics imbedded in a lava layer at Paua Bay document some of the compositional and textural variation present within the Akaroa magmatic system. The 30 in-situ lithics observed have an asymmetric size distribution ranging from 3 to 15 cm (mean 5 cm) and are typically coarse-grained fragments with varying roundness. Twenty lithics were drilled out for analysis using Scanning Electron Microscope (SEM) techniques of Energy Dispersive Spectrometry (EDS), Cathodoluminescence (CL), and Electron Backscatter Diffraction (EBSD).

Paua Bay lithics exhibit wide variation in mineral abundances: plagioclase (25-70%), diopside (20-70%), ilmenite-spinel (5-10%). Mineral orientation data suggests that plagioclase is not a dominant fabric-forming mineral for most of these lithics. Full thin-section CL maps do show significant textures such as zoned plagioclase with resorbed cores, former melt pockets of fine-grained material, and micron-width highly luminous material between grains interpreted as crystallized interstitial melt. Plagioclase have compositions that vary from An₅₈ to An₇₀ and show both normal and reverse zoning patterns. The reverse zoning supports previous interpretations for multiple magmatic recharging events. Normal and reverse zoning within single lithics suggest crystal populations with different growth histories may have come together from disparate parts of the magma chamber. Analysis of the Paua Bay plutonic lithics reveals significant variations that may have resulted from dynamic variations within a single magma chamber or from crystallization in magma bodies at different crustal levels.