A TEXTURAL AND GEOCHEMICAL ANALYSIS OF GOAT ROCK DOME XENOLITHS

Banks Peninsula is the site of remnant Cenozoic volcanism comprised primarily of two main phases: the Lyttelton (12.4-9.7 Ma) and less-evolved Akaroa (9.4-8.0 Ma) phases. During these phases multiple mafic domes were emplaced, some of which contain xenoliths that can provide insight in to the magmatic evolution of volcanism on the peninsula. This study focuses on Goat Rock: a pelean dome intruded during Akaroa volcanism that contains trachy-basaltic xenoliths (45-47 wt.% SiO₂ compared to 49-50 wt. %SiO₂ of the host material) exhibiting schlieren textures. The xenoliths, ranging from 5-25 cm, consist of aligned plagioclase, augite, olivine, apatite, and iron oxide minerals.

Crystallographic preferred orientations (CPOs) give insight into igneous processes and petrogenesis. Electron backscatter diffraction (EBSD) was used to collect CPO data from five samples with grain sizes up to a few mm. EBSD maps of entire thin sections were made.

Plagioclase CPO in four samples are characterized by very strong clustering of the {010} with the {001} and <100> distributed in a broad great circle to the {010} cluster. <100> has a distinct maximum within the broad great circle. In the fifth sample, the CPO is similar except the {010} is spread around a great circle. In all samples, individual plagioclase grains are distorted up to 10 degrees: {001} and <100> lie on segments of great circles that contain the {010} cluster. Augite and olivine CPOs are less well defined (fewer grains): both phases have weak, non-random CPOs that share symmetry with the plagioclase CPO. In the best example, olivine {010} is clustered parallel to the plagioclase {010} cluster, with olivine {001} matching the plagioclase <100>.

The results of this preliminary analysis suggest that the fabric was formed through settling/flow and compaction involving physical rotation of grains and internal distortion by dislocation mechanisms. We suggest the samples represent the initial cumulate material formed from the extraction of more-evolved melt in a shallow magma reservoir. In addition, the composition of Goat Rock dome fits the gaps in the fractionation spectrum represented by underlying lava sequences. This study provides evidence of the physical processes that occurred to produce the magmas erupted during Akaroa volcanism.