EXPLORING BULK ROCK GEOCHEMICAL CONTROLS ON FORMATION OF VOLCANIC “BENCHES” AT PIGEON BAY PEAK, BANKS PENINSULA NZ

Distinct mineralogical, textural and geochemical variation is present among separate stacked lava flows from the Miocene-aged Akaroa Volcanic Complex on New Zealand’s South Island. The complex, known as Banks Peninsula (BP), displays geomorphological features referred to as ‘volcanic benches’ above the bays. The factors that controlled the formation of these ‘benches’ are not fully understood by geologists studying this volcanic peninsula, as there is no known evidence for tectonic uplift since volcanism ceased. The features appear as large scale, flat, step-like surfaces that break the steep slope of the hillsides and form at the contact between stacked lava flows. This study aims to evaluate the theory that there is a relationship between the bulk geochemistry of separate basalt lava flows and the formation of these geomorphic ‘benches’. Geochemical analyses using Portable X-Ray Fluorescence (XRF) of separate lava flow transects from Eastern Pigeon Bay (EPB) and Pigeon Bay Peak (PBP) show variation in elemental composition between separate flows, suggesting that primary magmatic bulk rock geochemistry influences bench formation on the peninsula. Preliminary textural analysis suggests that more resistant bench-forming flows may be more fine-grained (aphyric) and uniform-textured than inter-bench flows. Additional whole rock XRF analysis, scanning electron microscopy (SEM), and lab reactions between rainwater and crushed rock samples compared geochemistry of fresh and weathered basalt to better understand chemical mineral weathering. Comparison of these data sets showed significant differences between Mg:Ca and Mg:Na ratios for whole rock and rainwater chemical compositions between fresh and weathered basalts. These observations suggest that Mg-Fe bearing minerals (olivine and clinopyroxene) are more susceptible to weathering than the main Na-Ca bearing mineral present in the basalts, plagioclase feldspar (An₅₀). Greater understanding of chemical weathering processes for these basalts will lead to more insight into the erosional process and formation of the bench features found on Banks Peninsula.