PETROGRAPHIC INVESTIGATION OF MAGMA BATCHES AND CYCLING OF THE AKAROA VOLCANIC COMPLEX

A volcanic complex consists of multiple calderas, cinder cones, lava domes, and craters, which form due to multistage eruptive events. Eruptive events are well recorded in the textures of mineral grains in igneous rocks because as magmas ascend, magmas entrain crystals from a variety of depths and from magmas in various ages and states of crystallization. Detailed petrographic analysis of individual phenocryst can provide details of magmatic and eruptive processes. This study analyzes a stratigraphic sequence of lava flows of the Akaroa Volcanic Complex, and investigates plagioclase textures to interpret the magmatic processes, and further discuss these in the context of magmatic evolution and geochemistry.

Two transects through a series of eighteen lava flows, were examined, as this sequence has geochemical analysis which indicates eruptive cycles and evolving magma batches. To further investigate this trend, textures within plagioclase phenocrysts and creation of a petrographic guide for plagioclase textures allow interpretation on magmatic processes and events. Primary plagioclase phenocryst textures observed are sieved, resorption surface, and melt inclusions. Sieved rim, patchy cores, zoning, swallow-tailed, synneusis, glomerocrysts, broken crystals and crystal clusters were also observed to a lesser extent within the transects.

Evolutionary paths of phenocrysts were constructed for each thin section, extracted from their petrographic textures, with most of the samples have experienced multiple magma recharge events and decompression. Through broadscale observation and petrographic guide interpretation of magmatic processes, combined with geochemistry data and magma batches information, a conceptual model of an effusive feeding system of the AVC is constructed.