LINKING LITHICS TO LAVAS: INSIGHTS FROM PLUTONIC LITHICS AT THE AKAROA VOLCANIC COMPLEX, BANKS PENINSULA, NZ

Plutonic lithics provide a connection between processes occurring in magmatic and volcanic systems. On Banks Peninsula, very little is known about the upper level magmatic systems and the plutonic lithics exposed in the Goat Rock Dome provide insight. Goat Rock is a mugearite dome on the eastern flanks of the Miocene Akaroa Volcanic Complex that contains abundant plutonic lithics. Detailed observations of 258 insitu lithics (all >1 cm) show a normal size distribution, ranging from <1 cm to 17 cm, and reveal diverse textures including fine- and coarse- equigranular and schlieren-like fabrics. In comparison, the host rock is porphyritic with abundant phenocrysts. All lithics are surrounded by radiating cooling fractures that propagate into the host rock, and their shapes range from jagged to well rounded. Optical microscopy, SEM and EDS analyses were performed on five sampled lithics. The mineral assemblage is consistent but the abundances vary between lithics: feldspar (67%-87%), olivine (10 -20%), and clinopyroxene (0-3%). Lithic plagioclase compositions range from An₁₀ to An₂₉, while host rock plagioclase has a uniform composition of ~An₄₀. Patchy zoning occurs within plagioclase in both the lithics and host rock.

Our observations and analyses suggest a complex relationship between the lithics and their host. Sharp boundaries and cooling fractures between the lithics and the groundmass indicate that they were solidified when they were incorporated into the host magma. In addition, the variably rounded boundaries of the lithics suggests they may have been exposed to more than one melt body through their lithic history, which is also supported by patchy zoning of plagioclase in the lithics. The range of mineral abundances and anorthite compositions within the lithics is consistent with multiple magma batches that may be linked by fractionation processes, with the host rock being the most evolved end-member with respect to anorthite. In addition, lithic textures (equigranular to schlieren-like) indicate a range of processes responsible for their formation, from crystallization to settling or compaction of already formed crystals within the magmatic system. To further develop these preliminary hypotheses, more compositional (whole rock and mineral) and textural analyses (SEM, EBSD) need to be performed.