INSIGHTS INTO MAGMA PRODUCTION THROUGH QUARTZ GROWTH HISTORIES

Plutonic lithics entrained within ignimbrites provide insight into subsurface magmatic processes. The ~ 50 ka Rotoiti Caldera ignimbrite contains entrained granitoid plutonic lithics which we investigate to interpret the magmatic processes that drove eruptions within the Okataina Volcanic Complex, Taupo Volcanic Zone, New Zealand. Previous geochemical research suggested the lithics were partially crystalized within an isolated magma chamber in the shallow crust, unrelated to the Rotoiti magmatic system and consistent with a model of decoupled volcanic and plutonic processes. However, a growing body of research suggests dynamic linkages between volcanic and plutonic process within the Okataina Volcanic Complex. In this “mush model” volcanic products are the result of high-silica melt extraction from a shallow sub-solidus crystal mush.

This study uses cathode luminescence (CL) imaging of quartz to examine the crystallization history of the lithics and test the feasibility of the two magmatic models for the Okataina Volcanic Complex. Quartz zoning patterns imaged by CL suggest a history of temperature variations with significant resorption. The presence of quartz clusters with disparate crystallization histories indicates that crystals were mobile and may have come together in a cumulate. Some quartz grains show pressure induced textures and/or undulatory extinction that may be evidence of compaction within a cumulate.

The existence of the zoning patterns, resorption, and variable quartz clustering suggests that Rotoiti Plutonic Lithics were subjected to dynamic processes of melt, crystallization, and recharge, consistent with a “mush model”. Thus, we interpret that these lithics formed as part of a sub-solidus crystal mush which was periodically rejuvenated and partially remelted through periodic thermal inputs prior to entrainment within the erupting Rotoiti ignimbrite during caldera collapse.