Melt evolution and crystal cargo uptake at Mount Ruapehu: a magma plumbing system characterized by narrow dikes and sills

Abstract

Abstract To assess the magma plumbing architecture of Mount Ruapehu, an active stratovolcano in the Taupō Volcanic Zone (New Zealand), we combine whole rock, groundmass and crystal geochemistry of lava samples from the south-west flank, focusing on the Mangawhero (50–15 ka) and Whakapapa (< 15 ka) formations. Carrier melts show nearly identical compositions, suggesting homogenous melt production and differentiation through time. Variations in whole rock MgO compositions are attributed to accumulation of mafic crystals (ortho- and clinopyroxene). The crystal cargo (plagioclase and pyroxene) is relatively small (typically < 100 μm, maximum 2 mm), however compositional variability, crystal zoning and disequilibrium with carrier liquids suggest transcrustal differentiation involving fractional crystallization, magma mixing and interaction with crystalline bodies. The most recent lavas of Mount Ruapehu (< 26 ka), exhibit SiO 2 versus K 2 O reverse trends overlapping with earlier Mangawhero Formation compositions. We interpret this trend, observed in whole rock and groundmass compositions, as the result of variable crustal assimilation and fractional crystallization, consistent with previous studies based on trace elements, isotope data, and crustal xenoliths. We propose a plumbing system of stacked, narrow magma bodies (likely dike- or sill-geometry) connected by a dike transport network, where ascending melts incorporate glomerocrysts and crystal fragments from previously emplaced crystalline bodies within the crust, modulating bulk rock MgO signatures. Rapid transport of melts and crystal cargo is favored by re-injection through the dike network, enhanced by rifting geodynamics.