Plutonic lithics record dynamics in the magmatic system beneath the Akaroa Volcanic Complex, New Zealand
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Restricted Access Thesis
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Earth and Oceanographic Science
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 essential to understand magmatic processes that occurred in regions with limited exposure of the plutonic body. Lithics imbedded in a lava layer at Paua Bay document some of the compositional and textural variation in the AVC. Twenty lithics were analyzed 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%). Full thin-section CL maps 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 compositions vary from An58 to An70 and show both normal and reverse zoning patterns. These textures support previous interpretations for multiple magmatic recharging events. Highly luminescent material is composed of K-feldspars due to partial melting from the rejuvenation events. Analysis of the Paua Bay plutonic lithics reveals significant variations that may have resulted from dynamic variations within a single chamber or from magma bodies at different crustal levels. Paua Bay lithics can be placed in a broader framework of other plutonic lithics found in the AVC; these lithics were most likely formed in the shallow magma chambers.
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