Crustal Forensics at Pūtauaki (Mt. Edgecumbe), New Zealand reveal the influence of deep crustal arc processes on magma evolution in the Taupo Volcanic Zone

Our understanding of magmatism in the New Zealand North Island subduction system is limited by sparse knowledge of the structure and composition of the deep arc crust, particularly in the northern Taupo Volcanic Zone (NTVZ), where there are multiple major tectono-magmatic transitions impacting the crust. Herein, we use detailed textural and micro-chemical analyses from multiple crystalline phases and melt inclusions to probe the magmatic processes occurring in the lower crust that ultimately control the early evolution of magmas beneath Pūtauaki (Mt. Edgecumbe) in the NTVZ. Here, two-pyroxene-plagioclase glomerocrysts in dacite lavas have textures, compositions, and phase associations consistent with a lower crustal cognate cumulate origin. Importantly, the glomerocrysts record a multi-stage evolution in the lower crust. Elevated 87 Sr/ 86 Sr isotope ratios (0.7058–0.7062) in high-An (up to An 92 ) plagioclase, along with relatively low Mg/Fe ratios in olivine and pyroxene within the glomerocrysts, record the deep assimilation of isotopically enriched lower crust and the significant fractionation of Mg-rich phases, near the base of the crust. The evolved melts appear to have then ascended into the lower crust and subsequently crystallized the gabbroic mineral assemblage preserved in the xenoliths. Two pyroxene thermobarometry indicates that crystallization/equilibration occurred at temperature between 926 and 1001 °C and pressure between 4 and 7 kbars, consistent with crustal depths of 15–26 km. Melt inclusions hosted within pyroxenes in glomerocrysts are rhyodacitic to rhyolitic (68–74 wt% SiO 2 ) indicating that fractionation of the glomerocryst assemblage resulted in the production of melts. We propose that these represent melts that ascend into the upper crust and fractionate to produce rhyolites that erupt from the large calderas of the central TVZ (CTVZ). Our findings also indicate that crustal architecture and magma pressure–temperature–time pathways beneath the NTVZ are similar to magmatic systems in the STVZ, but distinct from those beneath the large rhyolitic systems in the central TVZ. These differences may arise from variations in mantle flux and rates of extension along the arc.