Connecting the Dots: the Lava Domes' Perspective of Magmatism Related to an Ignimbrite Flare-Up

Abstract

Abstract Dome eruptions associated with rhyolitic calderas offer an important insight into how extremely large (> > 10 km3), rhyolitic magma systems are constructed through time. We focus on rhyolitic calderas in the central Taupō Volcanic Zone leading to, during, and immediately following the 350- to 240-ka ignimbrite flare-up. We identified 103 dome eruptions that are dated between ca. 650 and 150 ka and collated 239 literature whole-rock compositions from these domes. For each composition, we modelled the pressure of magma extraction from the magma mush and the mineral assemblage of the mush using the rhyolite-MELTS geobarometer. We calculated extraction temperatures using zircon saturation geothermometry. We show that magmas are extracted from typically quartz-bearing magma mush at a wide range of depths (~50–425 MPa, ~2–16 km) and temperatures (~750°C to ~850°C). Throughout the central TVZ, there are two dominant extraction pressure modes at 1) 150–175 MPa and 2) 250–325 MPa, consistent with 1) the depth of the brittle–ductile transition (~6 km) and just below typical pre-eruptive storage depths of other TVZ magmas (100–150 MPa, ~4–6 km); and 2) partial melt regions imaged below ~8 km by previous geophysical studies. In some regions, there is a clear correlation between crustal structures, the depth of magma extraction, and the composition of the magmas. In the Whakamaru caldera, the domes erupted inside the caldera following caldera collapse are extracted from ~225 to ~350 MPa at ~810°C and have orthopyroxene-bearing compositions dissimilar to the caldera-forming eruption. These domes are aligned along normal faults, suggesting that rifting creates pathways for magma extraction from a deeper mush rejuvenated by recharge. The domes erupted along the structural margins of the Whakamaru caldera have very evolved, hornblende-bearing compositions, similar to the caldera-forming eruption and shallow, colder extraction from ~100 to ~200 MPa at ~770°C, suggesting the mush feeding these domes is a remnant of the older caldera-forming magma system mobilized along the caldera-bounding faults. Two structural levels of magma extraction at ~6 km and 9 to 12 km are persistent throughout the flare-up period and across the central TVZ region, demonstrating the need for further investigation into the factors controlling the depth of mush development.